Chapter 8

AEROSPACE OPHTHALMOLOGY

 Thomas J. Tredici, M.D.

INTRODUCTION

   Aerospace ophthalmology attempts to deal with the visual problems peculiar to the aviator. These problems are the result of human - machine integration and its interaction with the flying environment. This chapter addresses flight medicine topics that the flight surgeon may encounter. It does not address basic ophthalmology knowledge that normally is in the domain of medical schools, but rather the occupational issues that are important in flight medicine.

   The old problems of protecting the flyer's eyes from windblast, light, and trauma have become more difficult with supersonic aircraft and high-altitude and space flight. In addition, high intensity electromagnetic radiation and the flash from nuclear and laser devices are also problems facing the military aviator of today and tomorrow.

   Air Force Occupational Safety and Health (AFOSH) Standard 127-31 contains specific guidance concerning occupational vision in the industrial setting. It covers the following subjects: eye protection for personnel employed in hazardous environments; vision testing associated with the placement of personnel; prescribing safety glasses.

GENERAL EFFECTS OF ALTITUDE

   The temporary visual difficulties of the human organism at high altitudes are due to hypoxia, decompression, glare, empty visual fields, brightness, and the visual effects of light.

Visual Effects of Hypoxia

   Hypoxia can cause several changes in vision. These visual disturbances, and the ophthalmoscopically visible changes in the blood vessels which accompany them, are described in this section.

   The indifferent zone is the range from sea level to 10,000 feet, because ordinary daytime vision is not affected within this range. There is, however, a slight impairment of night vision, a fact which makes it imperative for night combat flyers to use oxygen from the ground up.

   The zone of adaptation ranging from 10,000 to 16,000 feet. At these altitudes, visual functions are impaired, but the flyer is able to carry out the mission. In this zone, the following changes occur, becoming more marked with increasing altitude:

a. the retinal vessels become dark and cyanotic
b. the diameter of the arterioles increase 10 to 20 percent
c. retinal blood volume increases up to four times
d. the retinal arteriolar pressure increases along with the systemic blood pressure
e. the intraocular pressure increases somewhat with the increase in blood volume
f. the pupil constricts
g. there is a loss (at 16,000 feet) of 40 percent in night vision ability
h. accommodation and convergence powers decrease
i. the ability to overcome heterophorias diminishes.
 All these changes return to normal by either the administration of oxygen or the return to ground level. Up to 16,000 feet, these effects remain latent, in the sense that physiologic compensation enables the flyer to continue basic tasks, unless this altitude is maintained for long.

   The zone of inadequate compensation is the region from 16,000 to 25,000 feet, because one, several, or all of the preceding changes become severe enough to produce interfering visual difficulties. Visual reaction time is slowed. Motor response to visual stimuli is sluggish; mental processes are all slowed. Heterophorias are no longer compensated by fusion and become heterotropias with resulting double vision. Accommodation is weakened and convergence lost, so that instruments become blurred and doubled.

   Dilation of retinal vessels, with the accompanying pressure changes, continues to increase until circulatory collapse intervenes. Visual acuity is impaired by diplopia, loss of accommodation, and general retinal and cerebral malfunction, and night vision is seriously impaired. All these changes are reversed by the use of oxygen or by returning to sea level.

   The zone of decompensation or zone of lethal altitude occurs above 25,000 feet. In this zone circulatory collapse occurs; there is a loss of both vision and consciousness. As result of the death of neurons from severe hypoxia and lack of circulation, the flyer may suffer permanent damage to the retina and brain.

Effects of Glare at High Altitudes

   Glare is caused by a difference in brightness between various parts of the visual field. The eye can be dazzled by a lighter object, when it is adapted for the darker portion of the field. Glare is then present.

   The pilot who flies at altitudes in excess of 40,000 feet encounters glare from the cloud layer below the aircraft. The human facial contour is not shaped to protect the eyes from glare coming from below, which can cause hazy vision. One investigator theorized that the cause of this subjective haze is probably the persistence of a positive after-image of the bright cloud floor.

   Other causes of this hazy vision have been suggested and investigated. These include fluorescence of the crystalline lens caused by the intensity of ultraviolet light at high altitude and intraocular scattering of light. However, the actual cause of this subjective haze has not been clearly established. Possible solutions to the problem have included the use of filters. These filters would, of necessity, vary in design due to the various aircraft configurations affecting visibility. Three techniques have been tried and involve:

a. Maximum absorption in the central portion with increasing transmission superiorly and inferiorly.
b. Maximum absorption in the superior portion with a gradual increase in transmission towards the center and inferior portion.
c. A self-attenuating variable density filter.
   The latter would seem to be the ideal protection, because the light transmitted would depend upon the intensity of light incident upon the filter. Thus, constant transmission at all times would occur. While photosensitive lenses are available commercially, the open state density is too dark and cycle time from the closed to the open state is too long for aviation. Darkening of the lenses is activated by ultraviolet light. However, this light is largely attenuated by windshields and canopies. Hence, the lenses cannot react as planned.

   The glare from below and the sides, in combination with the lack of light scatter in the environment at high altitudes, may cause a relative shadow on the instrument panel. Since the external environment is bright and a relatively small amount of light diffuses into the cockpit, the instrument panel may appear to be dark as the pilot's attention turns from outside the aircraft to the panel. One solution to this problem is the use of white light in the instrument panel. The brightness of the panel can, then, be equalized with the environmental lighting by a rheostat balancing light intensity.

Effect of Space Myopia (Empty Visual Field)

   At high altitudes, pilots may develop a physiological myopia due to the ciliary muscle tone when the eye is at rest. At these altitudes, one may not have a distant object on which to fixate. In such an empty visual field, a reflex accommodation often occurs, creating from 0.50 to 2.00 diopters of relative myopia. Theoretically, under this condition, an emmetropic individual would find it difficult to detect a target at the normal far point.

Sunlight and its Effect on the Eyes

   Light is part of the electromagnetic energy spectrum. The entire spectra extends from the extremely short cosmic rays, with wavelengths on the order of 10-12 centimeters, to the long radio waves several miles long. Visible light consists of a small portion of the spectrum, from about 380 nanometers (violet) to about 760 nanometers (red). A nanometer is a millionth of a millimeter. The neighboring portions of the visible spectrum, although not visible, can have effects on the eye and are, therefore, of interest (see figure 8-1).

   Wavelengths of 360 nanometers down to 200 nanometers are known as abiotic rays. Exposure of the eyes to this portion of the electromagnetic energy spectrum may produce ocular tissue damage; the severity depends upon the intensity and time of exposure. Wavelengths from 760 nanometers to the microwaves at about 1 mm are the infrared or heat rays. These rays, too, may cause ocular tissue damage, depending upon intensity and exposure time. The infra-red rays may affect all ocular tissues, whereas the ultraviolet have their effect chiefly upon the conjunctiva and the cornea. Continuing theoretical research suggests that long term exposure to UV-A (320 nm to 400 nm) and blue light (420 nm) may play a part in the formation of cataracts and/or retinal degeneration. However, clinical data to support these conclusions either does not exist or is uncontrolled.

   The daytime light intensity in extraterrestrial space above 100,000 feet is approximately 13,600 foot-candles. At 10,000 feet on a clear day, it is about 12,000 foot- candles, and at sea level on a clear day, it is about 10,000 foot-candles. The water vapor, dust particles, and air molecules in the atmosphere absorb light. In addition to absorbing light, water vapor also scatters light. This diffraction accounts for unexpected sunburns on overcast days. Furthermore, certain selective atmospheric absorptions occur. Ultra- violet light shorter than 200 nanometers is almost entirely absorbed by dissociated oxygen as high as 400,000 feet.

Figure 8-1. Visible portion of the radiant energy spectrum
Figure 8-1. Visible portion of the radiant energy spectrum.

   Ultraviolet light from 200 to 300 nm is absorbed by the ozone layers in the atmosphere at an altitude of about 125,000 feet , the height of second (upper) ozone layer. This absorption is very fortunate because these wave lengths are the most damaging to the eye. These are the wavelengths that produce the actinic keratitis welders receive when they fail to wear protective hoods. Above 125,000 feet, these ultraviolet wavelengths require consideration. Work done in the space program showed that the most abiotic rays have a wavelength of 270 nanometers. They must be filtered by protective visors or they will severely limit the time that astronauts can spend in extravehicular space activities(15). The rays of particular concern for the USAF aviator, therefore, are from 300 - 2,100 nm, with an intensity varying between 10,000 foot- candles at ground level to about 13,000 foot-candles at above 100,000 ft.

   Ultraviolet radiation produces its harmful effects mainly externally. These short rays are absorbed by the outer one-tenth of a millimeter of the eyeball. Hence, the effect of these rays is limited to this area of absorption. Ultraviolet light produces a painful epithelial swelling, accompanied by extreme sensitivity to light--photophthalmia, or the so-called snow blindness that one can experience in the Arctic. It is produced only after prolonged exposure to high-intensity sunlight, such as that reflected into the eyes by a snow field, water, or a desert. Ultraviolet burns usually do not produce permanent damage to the eye. The epithelium recovers completely, although the temporary pain can be severe.

   Both infrared and visible light are concentrated by the eye's optical system. If an individual looks directly at the sun with inadequate eye protection (all so-called sunglasses are inadequate for this purpose), the lens system of the eye will concentrate this energy 100,000 times and focus it to a point on the retina, like a burning glass, and produce an injury to the retina. This type of injury happens so frequently during observation of an eclipse of the sun that it is called "eclipse-blindness." It is a permanent eye injury and is clinically manifested by a foveo-macular scar. A corresponding central scotoma can be demonstrated on the Amsler grid or on visual field testing. Final visual acuity may vary from a barely discernible decrement to 20/70 or worse. Infrared radiation is also reputed to cause chronic redness of the eyes, chronic conjunctivitis, and pterygiums, but its role in these eye conditions has not been completely determined. Fortunately, most aircraft canopies and windscreens block almost all or all of the ultraviolet and some of the infrared light. USAF sunglasses and sunvisors block most of the rest.

Brightness of the Field of View

   The amount of light reflected back to the eye determines the brightness of the individual's field of view. Snow, for example, reflects back 85 to 90 percent of the light falling on it. White sand, coral, and white clouds may reflect as much as 75 to 80 percent of light. Grass and forests may reflect as little as 10 percent of the light. The apparent "coolness" of green fields probably depends as much upon the fact that they reflect low percentages of light as it does upon any specific psychological effect of the color.

   In so far as the feeling of brightness in sunlight is concerned, there are two factors, the amount of light falling on a surface and the amount of light reflected by the surface.

SPECTACLES AND OPHTHALMIC FILTERS

Basic Issues

   Standard crown glass has always been thought to give the clearest and least distorted vision. It blocks most of the short ultraviolet and infrared radiation. Thus, if an individual wears spectacles which have large lenses to prevent peripheral ultraviolet light from entering the eyes, they are protected to a great extent from snow blindness. Glare may be bothersome, but photophthalmia will not develop. Standard plastic lenses transmit ultraviolet light, unless an ultraviolet absorber is added in the manufacturing process. The plastic lenses provided to military personnel, CR-39, do contain ultra-violet absorbers, but they block very little infrared radiation. Hi-index plastic lenses are also available. Polycarbonate is a new light-weight, soft, very impact-resistant plastic that was developed out of the space program. It is difficult to work with, requires special manufacturing equipment, and requires careful handling to prevent scratching. Polycarbonate is an excellent blocker of ultraviolet light. All plastic lenses require special anti-scratch coatings to prolong their usefulness. All plastic lenses can warp under frame rim pressure and during manufacturing. This warpage may affect spatial perception. This is even more true for the soft polycarbonate lenses. There are other problems (weight, scratch resistance, annoying reflections, etc.), that are variously associated with each of these types of lenses.

Figure 8-2. Transmission Curve of Crown Glass
Figure 8-2. Transmission Curve of Crown Glass.
 
Figure 8-3. Transmission Curve CR-39
Figure 8-3. Transmission Curve CR-39.
Figure 8-4. Transmission Curve of Polycarbonate
Figure 8-4. Transmission Curve of Polycarbonate.

   All visible and infrared wavelengths of light pass through crown glass, with only about an 8 percent reduction. Magnesium fluoride coated anti-reflection lenses absorb only 4 percent of light and allow 96 percent to pass through. Other anti-reflection coatings are even more efficient. Flyers requiring prescription spectacles are authorized clear coated magnesium fluoride lenses, on request. In addition to reducing light loss in transmission, annoying reflections are significantly reduced, improving night vision compared with untreated lenses. Care in handling and cleaning these lenses is required, for scratched and scuffed coatings negate the optical advantage.

   All types of colored lenses filter light. There are four types of sun glass lenses in common use: colored filters; neutral filters; reflecting filters; polarizing filters. They all transmit only a certain percentage of the total amount of light. However, they produce this effect differently. The colored, neutral, and polarizing filters achieve this effect by absorbing some of the light and allowing the rest to pass. Such spectral filtering is usually achieved, in glass lenses, by adding specific chemicals to the melt. Thus, a through-and-through color neutral tint is observed in a side view of the lens or fragment. It is also possible to coat the glass lens surface. Plastic lenses are dipped into dye baths for desired color or neutral filtration, after they have been ground and polished, i.e. fabricated, in the clear state.

Colored Filters

   The reason that a green sunglass looks green is that it absorbs a higher percentage of the other colors than it does of the green. It allows the green to pass through. The same is true of other colored sunglasses. They permit different amounts of light of different wavelengths to pass. Yellow or amber sunglasses, for example, absorb all the blue light (blue-blockers) and most of the green and allow only red, orange, yellow, and a little of the green to reach the eye.

Neutral Filters

   On the other hand, neutral filters absorb approximately equal amounts of all wavelengths of light--as much of the red as of the green, the blue, or any other color in the visible spectrum. For this reason they appear gray. However, all gray appearing filters are not necessarily neutral. Neutral filters darken a scene without changing its color.

Reflecting Filters

   Reflecting filters allow a certain percentage of light to pass to the eye and reflect the remainder back in the general direction of the source. They act like partially silvered mirrors and, when worn, they resemble small mirrors. The silver-colored coating on the upper part of certain "graded density lenses" is such a filter. It is usually a thin coat of a mixture of chrome and nickel. As a rule these reflecting filters are nearly neutral, in that they reflect an approximately equal percentage of all wavelengths. This is not a valid assumption with other metallic oxide reflective coatings.

Polarizing Filters

   Polarizing filters transmit only light vibrating in a certain direction. They absorb light oscillating in other directions. They are not neutral, in that they pass more light of certain wavelengths than of others. Most spectacle lens polarizing filters pass about 30 percent of light overall, and absorb nearly all vibrations in one particular plane, the plane of polarization. For this reason they require combination with other types of filters to be effective as a general purpose sunglass. They have an additional disadvantage, in that polarization is achieved by a hermoplastic film matrix with herapthite crystals. This film is quite delicate. For protection, the film is usually placed between two layers of glass. Lamination such as this makes it expensive to produce hi-quality curved lenses, especially when a refractive correction is required. Delamination is not uncommon with wear, especially when mounted in a metal frame. The thermoplastic matrix material also has warpage/distortion problems, when exposed to heat.

Transmission

   Filters used for sunglasses have their density described in terms of the amount of light they transmit. Thus, a 15 percent filter will allow 15 percent of the visible light falling on it to pass through. If it is a neutral filter, this will be about 15 percent of each wavelength of visible light. If it is a colored filter it may be only 1 or 2 percent of one wavelength and as much as 30 or even 40 percent of another wavelength. Colored lenses do not "add yellow" or "add green" to the light. Filters cannot add anything. They only make things appear to be certain colors because they subtract other wavelengths of light by absorption or reflection.

   Ophthalmic filters can transmit rather large percentages of infrared radiation, especially the near infrared wavelengths. It is possible to obtain near infrared attenuation to the same degree as the visible spectrum. The acceptable transmittance for ultraviolet, visible, and infrared wavelengths for the standard military non-prescription aircrew sunglass filter is specified in military specifications (MIL-S-25948E). Reduction in the total amount of light may aid one's ability to see, when the total brightness is so high that one cannot adapt to it by the normal eye mechanisms. If retinal adaptation, a small pupil, and partially closed lids do not reduce the amount of light entering the eye sufficiently, the individual will be unable to see well, especially when flying just above a dense sunlit overcast, over snow, or over water into the sun. The use of a filter lens will reduce the overall brightness to a level that can be tolerated so that the individual can see comfortably.

Glare

   It is frequently stated that filter lenses reduce glare. This statement is usually scientifically incorrect. The usual filter lens reduces the brightness of all objects by the same amount; it does not change the ratio between the brightest and the darkest areas. Therefore, glare is still present.

   Polarizing filters can reduce glare when the bright area consists of polarized light, such as sunlight bouncing from pavement, snow, water, or similar surfaces. Polarizing lenses have certain disadvantages, some previously mentioned, which limit their use. Polarizing lenses are further limited in their usefulness because of stress patterns in aircraft transparencies due to tempering or the tension effects of the frame. Dark patches can appear on the transparency when viewed through polarizing filters. Partial polarization of sunlight also occurs by the atmosphere, and the flyer might see dark bands in searching the sky.

Color Perception

   It is quite obvious that colored sunglass lenses will distort color perception to varying degrees. The degree of distortion will depend upon the amount of various wavelengths absorbed by the lenses. Carefully designed experiments will show some degree of color perception error induced by any colored lens. This altered color perception has been shown to cause delays in information acquisition and processing and reaction times in pilots doing piloting tasks. Only with a true neutral filter is color vision normal.

Visual Acuity

   The ability to distinguish small objects at long distances is essential for the flyer. The amount of light available during the day exceeds that required for maximum acuity. For this reason it can be reduced considerably by a filter lens without reducing the ability to see detail distinctly. A lens of about 10 to 15 percent transmission has been shown to be the most acceptable and useful, provided that the lens is somewhere near a neutral lens. A slightly darker lens could be tolerated under conditions of extreme brightness, but the lens of 10 to 15 percent transmission is adequate. On sunlit days, this density will not reduce Snellen acuity. Prescriptions of high power, when made in 15 percent transmitting lenses, present a bull's-eye appearance. In order to reduce the noticeable density gradient across the lens and to keep the thickest sections of the lens from excessive transmission reduction, 31 percent lenses are used when the correction exceeds +4.00 and -5.50 diopters. Filtered lenses should be removed, when the illumination falls below bright sunlight, or acuity will be decreased. This is particularly true at dusk and dawn. Claims have been made that certain lenses increase acuity--especially the yellow or amber lenses. This statement is usually based on the fact that light is scattered by haze or fog. It is known that the shorter wavelengths (i.e., blue and blue-green) are scattered more by haze and fog than the longer wavelengths (i.e., red, orange, and yellow). Also, the refractive characteristics of the eye causes blue light to focus in front of the retina and red light to focus behind the retina, when compared to yellow light that is focused on the retina. In other words, when the eye is emmetropic for yellow light, it is myopic for blue light and hyperopic for red light. This difference in color refraction, known as the chromatic interval, causes some distortion of the image on the retina. On theoretical grounds, then, the elimination of the short wavelengths by a filter should increase the sharpness of an image. This would seem to be confirmed by the use of yellow filters in photographing distant scenes. Such filters absorb the short wavelengths and allow the long ones to pass. They do give sharper photographs of distant scenes.

   When worn, such yellow filters give a subjective sensation of increased brightness. This is a false impression, because the lens subtracts light, it does not add it. These yellow filters also give a subjective sensation of sharpening the image. However, all carefully controlled research experiments conducted to date (both civilian and military) fail to show a statistically significant increased ability to see in clear weather, haze, or fog, by marksmen and pilots using yellow filters. The difference between the effect on the eye and the effect on film is readily explained by the relative sensitivity to blue light of the photographic film on one hand and the retina of the eye on the other. Photographic film is extremely sensitive to blue light. Scattering of blue light, therefore, gives a marked haziness to pictures. On the other hand, the human eye has a very low sensitivity to blue light, so the scattering has less effect on ability to see in haze or fog. No ophthalmic filter exists which will appreciably increase the ability of the eye to see in haze or fog.

Sunsensor Lenses

   Photodynamic lenses now exist which vary in density in response to the ultraviolet content of incident light. This effect is the result of silver halide crystals embedded in the glass. They produce a neutral gray tint. Other colors are also available. PhotograyR was the original formulation and was termed a "comfort" glass, since transmission reached 45 percent at maximum. PhotosunR is a more recently available lens and has a 20 percent maximum transmission. The response is also temperature dependent, tint change being more rapid in colder environments. The percent values provided above were obtained at 77oF with a 2 mm thick lens. Some residual tint remains in the lenses after activation, and clearance to 85 percent transmission and 65 percent for Photogray and Photosun, respectively, is the best maximum open state. Other plastic photochromic, organic, colored filters are available. Since cockpit canopies and windscreens usually block ultraviolet light and reversal time from dark to light is relatively slow, photochromic lenses are not recommended for use by the military flyer. This recommendation is reinforced by the reduced open transmission of the lenses at night caused by the residual tint in these photodynamic lenses. Night flying should not be done with these types of lenses. The activity of photochromic lenses is temperature sensitive and not consistent over the life of the lens. Lenses of different thicknesses in one pair of spectacles (anisometropia) are tinted differently. This can create a visual motion parallax known as the Pulfich phenomenon.

Selection of a Sunglass for U.S. Air Force Use

   Selection of the best sunglass lenses for U.S. Air Force use has taken the above factors into consideration. After much research by the USA, USN, and USAF, it has been determined that a neutral-gray lens with 15 percent transmission is most suitable for the level of brightness encountered in flying. All invisible electromagnetic radiation is virtually eliminated by this lens. A transmission curve for the standard N-15 USAF sunglass is shown in Figure 8-5.

Figure 8-5. Transmission Curve of the Standard N-15 sunglass lens
Figure 8-5. Transmission Curve of the Standard N-15 sunglass lens.

   Glass N-15 lenses eliminate most of the abiotic wavelengths below 350 nanometers and approximately 80-85 percent of the longer UV between 350 and 400 nm. However, fluorescence of the crystalline lens may present a problem at high altitudes, when lenses are used which transmit light in the region of 360 nanometers. Infrared rays are much more effectively attenuated by the presently available neutral lens than by any of the colored lenses or the reflecting lenses. The ability to recognize colors without any impairment occurs only with neutral lenses--either absorbing or reflecting types. Colored lenses distort colors. The neutral absorbing lens is superior to the neutral reflecting lens because the reflecting lens transmits infrared rays and because the reflecting coat is susceptible to damage.

Goggles or Visors

   Flying goggles have lost their importance as protection against wind blast, oil droplets, flash fire, and so forth. Fighter pilots now use clear and sunglass visors attached to the helmet that can easily be pulled down or pushed up with one hand. In addition, the visor protects the eyes against wind blast in bailouts at less than supersonic speeds. At high speeds, the visor will be torn off by the wind blast.

   Special appliances (goggles, spectacles, visors) are available to protect against flashblindness and chorioretinal burns from nuclear detonations and lasers.

OTHER LIGHT HAZARDS

Nuclear

   Hiroshima and Nagasaki, in 1945, marked the beginning of atomic warfare. Nuclear weapon development since that time has resulted in devices that are many times more devastating than the 20 kiloton (KT) bombs detonated over Japan. Explosion of nuclear devices results in damage to the human body by concussion (blast), radiation, heat, and light. The concussion and radiation effects are limited to finite distances from the center of burst; these can be predicted from the yield and location of the detonation relative to the earth's surface. The radiant energy released, however, as UV, infrared, and visible light can cause damage to the human body at finite but potentially much greater distances.

   The eye is susceptible to injury at far greater distances than the other organs or tissues of the body, because the eye concentrates light energy by a factor of 100,000 and focuses it on one spot on the retina. An eye, having a pupil of a given size, and exposed to a nuclear detonation at a given distance, will have a certain amount of energy distributed over the image area on the retina. If the distance from detonation is now doubled, the amount of energy passing through that same size pupil will be one-fourth as great; the image area will also be one-fourth as large. Therefore, the energy per unit area will remain constant, irrespective of the distance, except for the attenuation due to the atmosphere and ocular media.

   Flash blindness and chorioretinal burns resulting from viewing nuclear fireballs have been of great concern to aircrew members and have created new problems for the flight surgeon. The chorioretinal burns that result from exposure to nuclear weapons detonations vary in size and severity, depending upon the distance from the center of burst. The more severe burns are sustained at positions closer to point of detonation. Individuals exposed to nuclear flash beyond the point where retinal burns occur may, nevertheless, be subject to flash blindness lasting several minutes. Although this flash blindness is temporary, it could result in an inability to complete a mission or loss of the aircraft.

   A permanent loss in visual acuity and central vision would result from a chorioretinal burn in the foveal or perifoveal area. A burn in the mid- periphery of the retina would result in a non-noticeable localized scotoma, if the burn were minimal, or in a segmental field defect, if the burn were severe. While crews can use shades or blinds or cover one or both eyes, these only have value as an offensive tactic.

   Adequate protection of the eyes from nuclear flash appears to be the only method of preventing flash blindness and chorioretinal burns. Research on filters and shutters that absorb or occlude the light released by nuclear fireballs and, yet, provide adequate visibility immediately before and after the detonation has been going on for years. Investigations continue in the development of a self-attenuating variable-density filter that can transmit a safe amount of visible radiation, regardless of the intensity incident on the filter. Some products have emerged. One filter available to crew members is made of crossed polarizers with an electroactive ceramic material between the polarizers. Application of voltage to the ceramic poly-crystalline lanthanum- modified lead zirconate/lead titanate (PLZT) lens appropriately rotates the polarized light, canceling the near opaque effect of the crossed polarizers and providing nearly 20 percent "open state" transmission. When a nuclear flash sensor is activated, the voltage to the PLZT is switched off, and the device "closes." The 50-100 microsecond response time provides the safety required in nuclear detonations (see figure 8-6).

Figure 8-6. Nuclear flash protection device
Figure 8-6. Nuclear flash protection device.

Lasers

    No less formidable a threat for aircrew's eyes is that of laser radiation. A laser (light amplification by stimulated emission of radiation) is a device that emits an intense narrow beam of light at discrete wavelengths which can range from the near-ultraviolet (invisible to the eye) through the color spectrum (visible) and into the far-infrared spectrum (also invisible).

   The rapid growth of laser science and engineering has resulted in the increased use of lasers by the military. Laser energy outputs are sufficient to produce significant interference with vision (glare and flashblindness), as well as eye injuries and loss of vision, even at distances of a kilometer or more. The areas of the eye affected can be the skin, cornea, or retina. (see
figure 8-8).

   Aircrews, protected by standard clear canopies and windscreens, are at risk from near-infrared and visible lasers, while other personnel, such as airbase ground defense forces, are additionally at risk from ultraviolet and far infrared lasers. Exposure to laser may be inadvertent ('friendly fire' from targeting lasers) or intentional offensive of defense acts.

Figure 8-7. The Radiant Energy (Electromagnetic) Spectrum
Figure 8-7. The Radiant Energy (Electromagnetic) Spectrum.
Figure 8-8. Adverse effects of lasers. (Adapted from Sliney and Wolbarsht, 1980)
Figure 8-8. Adverse effects of lasers. (Adapted from Sliney and Wolbarsht, 1980)

   Flight surgeons must concern themselves with the following issues regarding lasers: principles of laser energy; anatomy and function of the eye; biological effects of laser energy; laser effects on vision; symptoms of a laser injury; examination for a laser injury; physical findings in a laser injury; treatment of laser injuries; return to duty criteria (wartime versus peacetime and ground duty versus flying duty); evacuation criteria; prevention and protection; psychological impact; and reporting (peacetime versus wartime). These are all dealt with in the USAF Flying Safety Kit for flight surgeons called "Operational Hazards of Military Lasers," which includes the USAFSAM technical report #88-21 entitled, "Medical Management of Combat Laser Eye Injuries". Furthermore, AFOSH 161-10 contains safety procedures for the operation of laboratory and technical lasers.

Figure 8-9. Principal characteristics of common lasers
Figure 8-9. Principal characteristics of common lasers.

   Research into appliances that will protect the flyers eyes goes on apace. Products are brought forth and rapidly implemented. Currently dye absorption visors and spectacles are the most common method of protection for both laboratory and aircrew personnel. Unfortunately, these devices can be designed only to block specific wavelengths thought to be the threat wavelengths. If they blocked all wavelengths, one would see nothing. These devices are also not without their induced problems, e.g. altered color vision, distortion, and inability to use at night. Fast changing devices (open-closed systems) are being researched and may, ultimately, provide successful and complete protection. The ultimate point, however is that any device must be worn to be effective.

VISUAL PROBLEMS OF HIGH SPEED AND ACCELERATION

Introduction

   Aircraft speeds of 3,000 mph will be commonplace in the future. Obviously, many medical problems will arise when pilots are subjected to such speeds. Among these problems will be visual difficulties. Airflow, vibration, acceleration, temperature, and time lag in human visual perception will all be factors. First, however, it is necessary to discuss the physical conditions which exist at these speeds before considering the visual problems.

   At sea level, the speed of sound is approximately 760 mph--varying with the density of air, temperature, and other conditions. As the altitude varies, so does the speed of sound. However, at any altitude, the speed of sound is called Mach 1. At 40,000 feet, Mach 1 is roughly 660 mph. Speed ranges are subdivided as follows:

   Subsonic.........Up to Mach 0.8
   Transonic........Mach 0.8 to 1.3
   Supersonic......Mach 1.3 to 5.0
   Hypersonic......Over Mach 5.0

   The same characteristic which regulates the speed of sound produces the compressibility phenomenon. At subsonic speed, air particles are able to get out of the way of a moving body (see figure 8-10). Above Mach 1, air particles begin to pile up in front of it. As these particles of air bump against each other, they compress the air. This is the phenomenon that enables sound to be transmitted. A forward wave front is built before a body moving faster than Mach 1 (see figure 8-11). This forward wave front, lying rather far ahead of the moving body, is not very dense. The wave forms a more acute angle over the leading edge of the body, as speed increases and it becomes more dense. The leading edge of an aircraft is never able to pierce this compression wave (see figure 8-12).

Figure 8-10. Projectile moving at a speed of Mach 0.92 (700 mph)
Figure 8-10. Projectile moving at a speed of Mach 0.92 (700 mph)(2)
 
Figure 8-11. Projectile moving at a speed of Mach 1.31 (1,000 mph)
Figure 8-11. Projectile moving at a speed of Mach 1.31 (1,000 mph)(2)
 
Figure 8-12. Projectile moving at a speed of Mach 2.63 (2,000 mph)
Figure 8-12. Projectile moving at a speed of Mach 2.63 (2,000 mph)(2)

   In each speed range, the air behaves differently. For instance, air passing thorough a venturi tube at subsonic speeds has increased velocity but decreased pressure at the constriction in the tube. At supersonic speeds, there is increased pressure, as well as increased velocity at the constriction. Air flowing over the wing surfaces behaves differently in each of the different speed ranges. There is buffeting in the transonic range because of the mixture of the two types of airflow. Going through this speed range of mixed airflows is called "passing through the sonic barrier." Current high performance aircraft are not as affected by this buffeting, because of their design and great speed in passing through the sonic barrier (see figure 8-13).
 

Figure 8-13. Behavior characteristics of air at subsonic and supersonic speeds
Figure 8-13. Behavior characteristics of air at subsonic and supersonic speeds(2)

Figure 8-14. Visual effects produced by shock waves
Figure 8-14. Visual effects produced by shock waves(2).

Effect of Slanting Optical Surfaces

   To permit supersonic flight, an aircraft must be free from projections, and optical surfaces must be slanted to produce the least possible drag. A windshield of hardened plate glass with nesa coating can be slanted to a 70o angle without producing a measurable change in visual acuity or depth perception. However, curved windshields and canopies may produce undesirable visual effects. Distortion, light transmission loss, and multiple images have been reported in highly slanted and curved aircraft transparencies. Reflected images on the canopy bow of fighters during night flight is particularly noticeable and distracting. Problems with depth perception and eyestrain are additional aircrew complaints.

Visual Effects Produced by Shock Waves

   The air in shock waves is optically more dense than normal air, producing a deviation of light rays that can optically displace objects from their true position. This phenomenon varies with speeds between Mach 1 and Mach 4; it is entirely absent below Mach 1. The flow of air in the compression wave will obviously not be absolutely homogeneous and will probably produce mild rippling effects, such as one sees in heat waves (see figure 8-14).

Vibration

   No vibrations of intensities sufficient to harm human eyes have been produced by jets or rocket-propelled craft. Vibration can cause on the eye to resonate at a frequency of about 40 hertz (Hz). This effect is more likely to be produced by low frequency vibrations of 10 to 40 Hz or 60 to 90 Hz than by high frequency vibration.

Effects of Acceleration of the Eye

   Inflight, the pilot's vision can be affected by radial and rectilinear accelerations. These forces can have different physiological effects, depending upon the orientation of the pilot to the force vectors. The various principles involved are discussed in Chapter 4. When centrifugal force is applied in the head-to-seat direction, i.e., positive Gz, there is a progressive decrease in blood flow to the head and eyes. The result, if the onset is not too rapid, is a progression from full vision through gray-out, black-out, to unconsciousness.

   When the force vectors are in the foot-to-head direction, i.e., negative Gz, and prolonged, there is congestion of all vessels of the upper part of the body. Congestion of the face and violent headache may follow. A so-called "red-out" may occur. The actual cause of this phenomenon is unknown. It may be due to congestion of the retina, orbit, or cerebrum. Some investigators feel it may be caused by looking through lids forced closed by the acceleration.

Lag in Visual Perception

   The length of time between an occurrence and the time a person sees it depends upon two factors, the length of time required for light to reach the eye, and the conduction time in the visual pathways and brain tracts. Because of the incredibly high speed of light, it is a non-limiting factor. However, the lag in the visual mechanism is appreciable and, at supersonic speeds, is important.

   The latent period of perception varies with the individual, the state of attention, the part of the retina stimulated, and the intensity of the stimulus. It may vary from 0.035 to 0.300 seconds. Sensory conduction times are important at supersonic speeds, because of the distance traveled. For example, an aircraft flying 1,800 mph is traveling approximately a mile every two seconds. From the time an object appears in the peripheral vision until the object is seen by central vision, about 0.400 second will have elapsed and the aircraft will have traveled 1,042 feet. At that point, the person has only seen the object. It has not been recognized. Recognition time varies between 0.65 and 1.50 second, an average of 1.0 second, during which time an additional 2,640 feet will have been traveled. This means that, from appearance to recognition, the aircraft has traveled 3,683 feet. The time required to make a decision and the motor reaction time to move control surfaces is not included. Therefore, if two aircraft came out of the clouds 3,000 feet apart and were coming toward each other with a closure rate of 1,800 mph, they would collide before the pilots could do anything about it.

   To move the eye from clear distance vision to read a dial with recognition and return to clear distant vision takes about 2.39 seconds. During this time, the aircraft would travel 6,336 feet. The time of accommodation increases with age and is an important factor for pilots of high-speed aircraft. Some countries consider it a limiting factor. Because of this, it is important that instrument dials be maximally readable (see figures 8- 15(A) through (E)).
 

Figure 8-15(A). Latent perception time at 1,800 mph(2). If an object appears in the visual field, it will be roughly 0.1 sec before the pilot is aware that an object is present. During this period the aircraft will travel 264 feet
Figure 8-15(A). Latent perception time at 1,800 mph(2). If an object appears in the visual field, it will be roughly 0.1 sec before the pilot is aware that an object is present. During this period the aircraft will travel 264 feet

Figure 8-15(B). After perception of an object in the peripheral field, in order to see it centrally a motor reaction time of 0.175 sec is required to prearrange the eye movement. The eye movement itself requires 0.050 sec. Distance traveled 594 feet.
Figure 8-15(B). After perception of an object in the peripheral field, in order to see it centrally a motor reaction time of 0.175 sec is required to prearrange the eye movement. The eye movement itself requires 0.050 sec. Distance traveled 594 feet.

Figure 8-15(C). After the visual axis is fixed on the object to be viewed, foveal perception requires 0.070 sec. Total time from extrafoveal appearance to central perception is 0.400 sec. Distance traveled is 1,042 feet.
Figure 8-15(C). After the visual axis is fixed on the object to be viewed, foveal perception requires 0.070 sec. Total time from extrafoveal appearance to central perception is 0.400 sec. Distance traveled is 1,042 feet.

Figure 8-15(D). Speed of recognition varies from 0.65 to 1.50 sec, average is about one second.
Figure 8-15(D). Speed of recognition varies from 0.65 to 1.50 sec, average is about one second.

Figure 8-15(E). If two pilots emerge from clouds 3,000 feet apart on a collision course, they would crash before they could do anything about it. If the distance were only 500 feet, they would collide without either pilot clearly seeing the other.
Figure 8-15(E). If two pilots emerge from clouds 3,000 feet apart on a collision course, they would crash before they could do anything about it. If the distance were only 500 feet, they would collide without either pilot clearly seeing the other.

   Lag in visual perception is a very important factor in supersonic aircraft. The human visual apparatus is unable to cope with the demands placed on it by supersonic speeds. For this reason, current high performance aircraft and future aircraft, whether jet or rocket-propelled, require devices which can detect aircraft or other objects in space, before the unaided human eye can see them, and warn the pilot to take offensive or evasive action.

NIGHT VISION

Introduction

   There are two types of sensory receptors in the retina--rods and cones. According to the widely accepted duplicity theory of vision, the rods are responsible for vision under very dim levels of illumination (scotopic vision), and the cones function at higher illumination levels (photopic vision). The cones alone are responsible for color vision. This receptor system allows the human eye to function over an impressively large range of ambient light levels (Fig. 8-16). There is a common misconception, however, that the rods are used only at night and the cones only during the day. Actually, both rods and cones function over a wide range of light intensity levels and, at intermediate levels of illumination, they function simultaneously.

Figure 8-16. Range of ambient light levels, in millilamberts of luminance, over which the human eye can function. Ranges of photopic (cone) vision and scotopic (rod) vision are shown, along with the transition zone of mesopic vision.
Figure 8-16. Range of ambient light levels, in millilamberts of luminance, over which the human eye can function. Ranges of photopic (cone) vision and scotopic (rod) vision are shown, along with the transition zone of mesopic vision.

Mesopic Vision

   There is a transition zone between photopic and scotopic vision where the level of illumination ranges from about 1 to 10-3 millilamberts. Both the rods and cones are active in this range of light, and the perception experienced is called mesopic vision. Although neither the rods nor the cones operate at peak efficiency in this range, mesopic vision may be of great importance to the military aviator, because some low level of light is usually present during night operations. Below the intensity of moonlight (10-3 millilamberts), the cones cease to function and the rods alone are responsible for vision, i.e. scotopic vision. Scotopic vision is characterized by poor acuity resolution and a lack of color discrimination, but greatly enhanced sensitivity to light.

Brightness Thresholds

   The dimmest light in which the rods can function is about 10-6 millilamberts, which is the rod threshold. This is equivalent to an overcast night with no moonlight. The dimmest light in which the cones can function is about 10-3 millilamberts, the cone threshold, which is roughly equivalent to a night with 50% moonlight. Thus, a white light which can just barely be seen by the rods must be increased in brightness 1,000 times before it becomes visible to the cones.

Central Blind Spot at Night

   That portion of the retina responsible for the keenest visual acuity is the fovea, which corresponds to the center of the visual field. The fovea is used constantly to fixate objects. The fovea is devoid of rods and is composed entirely of cones. Therefore, at luminance levels below 10-3 millilamberts, a blind spot develops in the center of the visual field, because the ambient lights is below cone threshold. (see figure 8-17).
 

Figure 8-17. Area of the central blind spot under scotopic conditions. Because central vision cannot function in diminished illumination, any object an individual fixates directly, in dim illumination, will not be seen.
Figure 8-17. Area of the central blind spot under scotopic conditions. Because central vision cannot function in diminished illumination, any object an individual fixates directly, in dim illumination, will not be seen.

   Rods are present outside the foveal area and gradually increase in number, finally reaching a maximum concentration at a point some 17-20o from the fovea. Since the rods have a lower threshold than the cones, they are much more sensitive to light. Thus, a person attempting to see, in illumination dimmer than moonlight, has to depend entirely on rods. To best utilize the rods under such circumstances, the individual must look 17- 20o to one side, above, or below any object to see it. This is known as eccentrically fixating. Proper education and training is, therefore, essential for maximum use of vision at night. Individuals should be taught to fixate slightly above, below, or to either side of a night target and to employ a scanning technique. (see figures 8-18 and 8-19).

Figure 8-18. Eccentrically fixating.
Figure 8-18. Eccentrically fixating.
Left - The central blind spot present in very dim light makes it impossible to see an aircraft, if it is fixated directly.
Right - The aircraft can be seen in the same amount of light by looking below 17-20o, so that it is not obscured by the central blind area.
Figure 8-19. Dark adaptation.
 Figure 8-19. Dark adaptation.
 Left - View seen by a person who is not dark-adapted.
 Right - The same view seen by a dark-adapted person who is looking at a point above the aircraft.

Dark Adaptation

   Both the rods and cones contain photopigments which, on exposure to light, undergo a chemical change that initiates visual impulses in the retina. A reversal of this process occurs during dark adaptation, where there is regeneration of the photopigments. Intense light will transform the photoreceptor pigments fairly rapidly and completely; this reduces retinal sensitivity to dim light. In the fully dark-adapted eye, photopigment regeneration is complete and retinal sensitivity is at its maximal level. The rods and cones differ in their rate of dark adaptation. Rods require 20 to 30 minutes, or longer, in absolute darkness to attain their maximum sensitivity after exposure to bright light. Cones attain maximum sensitivity in about 5 to 7 minutes.

Photochromatic Interval

   Rods are not sensitive to wavelengths of light greater than about 650 nanometers, i.e., the red portion of the visible spectrum. However, rod insensitivity to red light is not present in the cones. This fact is easily demonstrated by slowly decreasing the intensity of a colored light, until the cone threshold is reached. This is the point at which the color will disappear, but not the sensation of light. When this procedure is performed with any color except red, for example blue light, the color will disappear at the cone threshold, but the light will still be perceived by the rods as dim gray.

   If the intensity is further decreased, until the rod threshold is reached, the light will disappear entirely. With red light, the color and sensation of light disappear at the same time. The difference between the level of illumination at which the color of a light disappears (the cone threshold) and that at which the light itself disappears (the rod threshold) is known as the photochromatic interval. There is a photochromatic interval for every color of the spectrum, except the longer wavelengths of red (see figure 8- 20).

Figure 8-20. Rod and cone spectral sensitivity.
Figure 8-20. Rod and cone spectral sensitivity.

Operational Aspects of Night Vision

   Contrast Discrimination: Visual acuity is reduced at night under low illumination conditions, and 20/20 vision cannot be sustained below a level of about one millilambert, the low photopic or upper mesopic range. Accordingly, objects are seen at night because they are either lighter or darker than their backgrounds, i.e. can be discriminated by a difference in contrast. These contrast differences may be reduced by light reflected from the following: windshields, visors or spectacles; fog or haze; scratched or dirty windshields, visors or spectacles.

   Because visual acuity is a function of small differences in the luminance contrast between objects and their backgrounds, any transparent medium through which the flyer must look should be spotlessly clean for night operations. Also, knowledge of the importance of contrast at night may be used by pilots to detect enemy planes, as well as to hide their own. Pilots should fly below the enemy, when flying over dark areas, such as land. They should fly above the enemy, when flying over white clouds, desert, moonlit water, or snow.

   Under conditions of low illumination, following other aircraft, either from above or below, rather than from directly behind will enlarge their the retinal images and lessen the likelihood of losing them in the darkness.

   Night Myopia: A person who does not normally wear spectacles (emmetropia) may have a shift toward low myopia under conditions of extremely reduced illumination. The exact cause of this night myopia, although controversial, suggests two components, ocular spherical aberration produced by the widely dilated pupils and slight involuntary accommodation. These components apparently vary in their importance with different people, but some people will have about 0.75 diopters of night myopia. This can occur in spectacle wearers corrected to emmetropia with spectacles, also. Night myopia is usually of relatively minor importance, as no visually resolvable target is visible, when it occurs. When a target does become visible, the eye rapidly readjusts. Problems may occur, however, in the initial detection of targets.

   Enhancing and Maintaining Dark Adaptation: For maximum utilization of scotopic vision, 20 to 30 minutes are required, in total darkness, to attain satisfactory dark-adaptation. A more practical alternative is to have the aircrew members wear red goggles to facilitate dark adaptation. Red goggles can be worn in normal illumination and do not interfere significantly with the ability to read maps, charts, manuals, etc. They block all light except red light, and red light does not simulate the rods, as we have seen.

   To understand why red filters can be used to achieve dark adaptation, it is necessary to examine the relative positions of the photopic and scotopic sensitivity curves in Figure 8-20. If a red filter with a cutoff at about 650 nanometers is worn, essentially no light is transmitted to the eye that can stimulate the rods. However, the cones are sensitive to the red light, and, thus, adequate visual acuity is permitted. By wearing red goggles for 30 minutes, the rods are almost fully dark adapted. Although the cones are not dark adapted, it only takes about 5 to 7 minutes, after a pilot steps into the dark, for the cones to adapt. Cone adaptation is relatively unimportant, since they are incapable of functioning in starlight illumination. There are, however, some drawbacks to wearing red goggles. For example, when reading maps, all markings in red ink on a white background may be invisible. In addition, red light creates or worsens near point blur in the pre-presbyopic or presbyopic pilot, as red light comes to a focus behind the retina and requires more accommodation to bring it into focus.

   Dark adaptation of the rods develops rather slowly over a period of 20 to 30 minutes, but it can be lost in a second or two upon exposure to bright lights. The night flyer must, therefore, be taught to avoid bright lights. Also, the instrument panel must be kept illuminated at the lowest level consistent with safe operation, and the flyer must avoid looking at flares, after-burner flames, or gun flashes. If light must be used, it should be as dim as possible and used for the shortest possible period.

   Dark adaptation is an independent process in each eye. Even though a bright light may shine in one eye, the other will retain its dark adaptation, if it is protected from the light. This is a useful bit of information, because a flyer can preserve dark adaptation in one eye by simply closing it.

   Cockpit Illumination: The use of red light (wavelength greater than 650 nanometers) for illumination of the cockpit is desirable, because it, like red goggles, does not affect dark adaptation. Red cockpit lighting has been traditional since World War II. The intent was to maintain the greatest rod sensitivity possible, while still providing some illumination for central foveal vision. However, red cockpit lighting did create some near vision problems for the pre-presbyopic and presbyopic aviators. With the increased use of electronic and electro-optical devices for navigation, target detection, and night vision, the importance of the pilot's visual efficiency within the cockpit has increased and new problems have been created. Low intensity, white cockpit lighting is presently used to solve those problems. It affords a more natural visual environment within the aircraft, without degrading the color of objects. Blue-green cockpit lighting is used in aircraft in which night-vision devices are used because, unlike the human eye, these devices are not sensitive to light at that end of the visual spectrum. In addition, blue-green light is the easiest for accommodative focus and is seen by the rods more readily than any other color. It is not seen as blue-green, however, but only as light. However, the enemy can easily see a blue-green light, under scotopic conditions, in any position of his peripheral field, whereas a low intensity red light would be invisible unless viewed directly.

   Drugs: The use of systemic drugs to improve normal night vision has been uniformly unsuccessful. Vitamin A has improved night vision only when there has been a chronic insufficiency of the vitamin and the stores in the liver are depleted.

   Smoking Tobacco: The effects on night vision of smoking tobacco products are somewhat controversial. Early studies showed a significant decrease in scotopic dark adaptation with smoking, which was attributed to the hypoxic effects of carbon monoxide. Later studies found that smoking actually improved night visual performance on some psychophysical tests. This was presumed to be a result of the stimulant effect of nicotine. More recent studies have reported that smokers have reduced mesopic vision when compared with non-smokers.

   Smoking should be discouraged. There is evidence that it degrades mesopic and/or night vision. The hypoxic effect of carbon monoxide is additive with high altitude hypoxia. Secondary smoke is a significant irritant for contact lens wearers, and many flyers could be wearing contact lenses. Smoke forms filmy deposits on windscreens, visors, and spectacles that can degrade contrast at night. The chronic, long-term effects of smoking are hazardous to one's overall health. A recent USAF directive prohibits smoking during night missions and three hours before.

   Hypoxia: The effect on night vision, of hypoxia at altitude, is primarily one of an elevation of the rod and cone threshold. The rise in foveal cone threshold, at 4,000 feet, is less than 0.05 log units and, at 8,000 feet, it is less than 0.1 log units. Since the pilot uses cone (central) vision for reading instruments, the actual decrement in central acuity from hypoxia is minimal. However, scotopic function, at altitude, can be significantly affected. It is reported that night vision capability is decreased by 5% at 1100 meters, 18% at 2800 meters, and 35% at 4000 meters without the use of supplemental oxygen. Thus, the use of oxygen, even at low pressure altitudes become very important at night.

   Further information on night vision and night vision devices can be found in AL-SR-1992-0002, "Night Vision Manual for the Flight Surgeon" and its revisions.

   Currently, several electro-optical NVD are available to improve flyers' vision at night. These include night vision goggles (NVG) and forward looking infrared systems (FLIR).

   Night vision goggles are like binoculars and are helmet-mounted in front of the eyes. They employ image intensifier tubes that are sensitive to some visible and short wavelength infrared (IR) radiation. NVG greatly enhance night vision over unaided scotopic mission, however, they do have significant limitations. These include a maximum best visual acuity of 20/40 to 20/50, a field of view of only 40 degrees or less, degraded depth perception/stereopsis, and a different sensitivity to light than the human eye. Thus, training and experience with NVG are critically important for flying safety.

   A FLIR device consists of a cockpit-mounted video monitor and external infrared sensor that is usually slaved to the nose of the plane. The sensor is sensitive to the long wavelength infrared (IR) wavelengths of light and provides excellent resolution. However, FLIR devices have a smaller field of view than NVG and no capability to look from side to side.

OCULAR DISEASES

   Ocular problems that can interfere with the completion of the mission, their safety, or the safety of others.

   Modern aircraft are equipped with sophisticated, automated equipment to reduce the pilot's workload. However, they have also increased the visual demands. Thus, flying remains largely a visual task. Nonetheless, some ocular conditions that formerly grounded an aviator can now, after careful ophthalmologic evaluation by aeromedical specialists, be waived. The USAF Surgeon General has sanctioned the creation of ten Ocular Study Groups to study and follow aircrew members with specific visual problems. They include the following disorders/diseases: idiopathic central serous chorioretinopathy; keratoconus; glaucoma/ocular hypertension; cataract/intraocular lens; retinal detachment; microtropia; ocular migraine; optic neuritis; contact lens; microwave exposure. Periodic reports on the results from these study groups are presented to the Surgeon General and published as journal articles and/or USAFSAM technical reports. Ultimately, the results can lead to changes in visual standards for selection and/or retention. The Ophthalmology Branch at USAFSAM should be contacted about the initial evaluation for and follow-up of any aviator with these diagnoses. Flight Surgeons can obtain the latest recommendations/results by contacting the Ophthalmology Branch at USAFSAM or HQ USAF/SGPA.

Glaucoma Study Group

   The largest of the study groups is that dealing with glaucoma/ocular hypertension. Glaucoma affects 2-3 percent of the general population and accounts for approximately 15 percent of the blindness in the United States. Open angle glaucoma accounts for 85 percent of all cases. Angle closure glaucoma comprises 5-10 percent of the cases; four other types of glaucoma, including one that is congenital, comprise the remainder.

  Over the past decade alone, almost 450 flyers have been evaluated for ocular hypertension (pressures greater than 22 mmHg) at the United States Air Force School of Aerospace Medicine (USAFSAM) consultation service. From this group, 200 have been diagnosed as having glaucoma. Of the 450, 65 percent have been deemed qualified to fly. Thus, these conditions represent a problem for the USAF aviator.(23)

   Prior to 1963, all personnel diagnosed with glaucoma were grounded. The only treatment available was the drug, pilocarpine. It was the secondary effects of the drug that caused the grounding. By 1970, 36 percent of all USAF aviators were over 40 years old and 55 percent were over 35. Above 40 glaucoma becomes increasingly manifest. Primary open angle glaucoma was the main type diagnosed, with only 3 of 450 aviators having angle closure glaucoma. The prevalence of glaucoma in the USAF aviator population seen by the Ophthalmology Branch at USAFSAM, in 1970, was 0.70 percent.(23) This prevalence closely approximates the lowest estimates in reported mass surveys.

   USAF Policy on Glaucoma: The current policy on glaucoma/intraocular hypertension, as established by the USAF Surgeon General, requires the following: no visual field defects should be present; the intraocular tension must be adequately controlled with or without medications; no visual or systemic effects from the medications should be present; frequent examinations are required for waiver continuation. The latest revision of the physical standards regulation contains specific guidelines.

   Ophthalmologic Drugs and Their Use: The largest number of flyers with intraocular hypertension are not treated at all. The USAFSAM Ophthalmology Branch has recommended levo-epinephrine (1-2%) in the initial treatment of flyers with glaucoma.(15) It is administered as topical eye drops. It does not have the drawbacks of miotic preparations, which consist of transient myopia, altered night vision, and reduced peripheral fields.(9) Some individuals become allergic to epinephrine and some develop adrenochrome deposits in the conjunctiva. These effects cease upon discontinuation of the drug.(23) Occasionally, individuals will notice their pupillary diameter being larger than normal from the epinephrine.

   A precursor of epinephrine, dipivefrin hydrochloride (Propine© ) is effective and with fewer side effects. This drug is administered in drops at a 0.1 percent concentration. It is absorbed well and converted intraocularly into epinephrine. The liberated epinephrine appears to exert its action by decreasing aqueous humor production and enhancing outflow.

   Beta adrenergic agents such as timolol, betaxolol, and levobunolol are effective in the treatment of glaucoma. They are administered as topical eye drops. However, they may reduce blood pressure, heart rate, and compromise pulmonary function. They do have secondary visual side effects, e.g. dilated pupils, in some individuals. The latest treatment for open angle glaucoma is laser trabeculoplasty. This procedure reduces intraocular pressure in approximately 85% of patients, but often patients must still use drugs. Studies are still ongoing as to how long the desired effects will last.

   Other treatment modalities such as the use of miotics (pilocarpine), carbonic anhydrase inhibitors, and osmotic agents are not used in aviators without grounding because of their side effects. However, they are useful in the treatment of acute glaucoma or postoperatively.

Contact Lenses

   Hard, polymethyl metacrylate (PMMA), contact lenses became popular during the 1960s and 1970s. USAFSAM began its research on contact lenses in 1959. The primary problem with PMMA lenses was that they did not transmit enough oxygen to the cornea and, thus, interfered with corneal physiology. Hydrogel (soft) contact lenses were introduced during the late 1960s and became the lens of choice in the 1980s. They require a more extensive disinfection and cleaning system and have a higher risk of corneal ulceration and other eye infections. Rigid, gas-permeable (RGP), contact lenses may be the lens of the future. They have the advantages of being oxygen permeable and giving sharper visual acuity in patients with astigmatism. However, new technology is pushing this area of optics along rapidly.

   Contact lenses correct refractive errors because the contact lens replaces the corneal surface as the primary refractive surface. PMMA and RGP contact lenses require a period of adaptation before they can be tolerated. PMMA lenses may cause corneal molding that usually reverts to normal after their use is discontinued. This molding change usually occurs within 48 hours, but may take 3 to 5 months to resolve once the contact lenses are stopped. RGP and soft lenses, due to their oxygen permeability, are not as apt to cause any corneal molding. Hard contact lenses may be dislodged from the eye and are prone to disabling subcontact foreign body incursions in dusty environments.

   Soft contact lenses are hydrophilic, oxygen permeable, flexible, not easily dislodged from the eye, and more comfortable to wear. The visual acuity, however, may not be as good, in patients with astigmatism, as with hard contact lenses or spectacles.

   The following are advantages offered by the contact lenses to the aviator:

   a. An increase in the size of the field of vision. e.g. checking six
   b. Good vision in inclement weather.
   c. No lens fogging.
   d. Elimination of reflection from the spectacle lens.
   e. Better interface with other optical instruments., such as helmet mounted target sights.
   f. More compatible with life support and chemical defense ensembles.
   g. No perspiration problems.
   h. Specific treatment for keratoconus, irregular astigmatism, aphakia, and anisometropia.
   The following are disadvantages, in addition to the ones already mentioned:
   a. Some individuals cannot tolerate contact lenses.
   b. Bubbles may form beneath hard lenses at altitude.
   c. High G forces may dislodge the lenses, especially hard ones with smaller diameters.
   d. Lenses may be dislodged or lost for other reasons.
   e. Contact lenses are more difficult and time consuming to fit and care for than spectacles.
   f. Professional eye care is needed for fitting and follow-up. This is an additional burden to the medical care system.
   g. Lens hygiene is difficult under field conditions.
   h. Corneal changes are possible with contact lenses that can reduce vision rapidly and for variable amounts of time.
   i. In a chemical environment, soft contact lenses initially act as barriers. Then, after the first hour, they may act as sinks to prolong chemical effects.
   j. With contact lenses there is no ocular protection from blunt trauma as with spectacles.
   k. There may be a decrease in visual acuity with spherical soft lenses in individuals with over 0.75 diopters of astigmatism.
   l. Many ocular complications can occur, some with devastating effects.
   Soft contact lens wear was approved for aircrew members on 21 June 1989. The flight surgeon is the focal point of the program. Each flight surgeon should have a copy of and know the soft contact lens implementation plan. Extended-wear soft contact lenses are being used to maximize the amount of oxygen delivered to the cornea. However, they are being worn on a daily-wear regimen, i.e., they must be removed, cleaned and sterilized at night. The soft lenses used must be approved by the FDA for extended-wear, not be tinted, be 55% or less water content and be approved for use by the Ophthalmology Branch at USAFSAM. Contact lens wear for medical indications (i.e., keratoconus, aphakia, etc.) still require a waiver by HQ USAF/SGPA and must be fitted by USAFSAM/NGO.

   Research is continuing at USAFSAM on contact lenses. Various style lenses are being studied for both the correction of simple refractive error and for the management of special eye pathology.

VISUAL ILLUSIONS

Introduction

   Spatial disorientation may arise from labyrinthine, proprioceptive, or visual mechanisms (17). This section will only address those illusions that are mainly attributable to vision. There are two forms of visual processing, foveal (central) and peripheral. Foveal vision is mainly concerned with object recognition, whereas peripheral vision deals mainly with spatial orientation. Runway illusions are the ones that are the best example of illusions produced by the foveal process. Other visual illusions are mainly produced by peripheral vision or a combination. (3,17).

Runway Aerial Perspective

   A runway that is sloped upwards 3 degrees will give an illusion that the aircraft is too high on the approach. If the pilot corrects the aircraft attitude, so that the visual image corresponds to one produced by a level runway, the glide path will be shallowed, resulting in a landing short of the runway. The opposite will occur with a downward sloping runway. A runway narrower than the one the pilot is used to will give the illusion that the runway is farther away and much longer. If corrective pitch changes are made, it may result in a late flare with a hard touchdown. The wider runway may be perceived as shorter and closer, resulting in a high flare with the aircraft then dropping down on the runway from a height.

   At night, if the runway lights are displaced laterally, it may give the illusion that the runway is closer, resulting in an early flare. The terrain near a runway may also give false impressions of height. If the terrain at the approach end is descending, the pilot may fly a shallow approach. If the terrain is sloping up to the runway, the pilot may fly a steeper than usual approach.

   Other hazardous illusions may be produced by the presence of snow, fog, smooth water nearby, size of buildings, size of vegetation, and differences in runway light brightness that may give false height perceptions. On practice bombing missions in the deserts of the southwest, the smaller size of the trees may cause the flyer to come closer to the ground, if the pilot is used to flying in an area where the trees are larger.

Visual Autokinesis

   The apparent wandering of an object or a light when viewed against a visually unstructured background is called autokinesis. A bright star may be seen as moving in a circle or moving linearly. During night formation flying, when only one running light of the lead aircraft is seen, other pilots may have trouble distinguishing the real movements of the aircraft. To counteract this illusion, it is useful to avoid staring at solitary lights for more than a few seconds and to establish a reliable reference to some structure in the aircraft, such as the canopy bow.

Linear and Angular Vection

   If a large structure nearby moves forward, there is an illusion that one is slipping backwards. The most familiar situation occurs when one is stopped at a traffic light and the nearby vehicle rolls forward. A false impression is created that your vehicle is rolling backwards. A person may also perceive a rotational sensation when the background surrounding an image rotates.

The Black Hole Approach

   The black hole illusion is produced during night landings, when there are no references except for the runway lights. This situation may be worsened when the lights of a city at the end of the runway make the approach look high and the horizon is not distinct. The natural tendency to lower the aircraft nose may cause a crash short of the runway. A similar situation is produced by blowing snow or when the runway and nearby terrain is covered by fresh snow.

Sloping Cloud Decks

   A sloping cloud deck may cause the pilot to adjust the aircraft attitude to what is perceived as the real horizon. This is particularly hazardous when flying near mountainous terrain. There is a strong tendency to accept the level appearance of the clouds as the true horizon, especially if it is indistinct. An unperceived angle of bank will lead to a loss of altitude, if it is not corrected.

Lean-on-the-Sun Illusion

   Terrestrial creatures are accustomed to seeing the brighter part of the horizon above and the darker ground below. When flying in weather, an attempt to position the brighter cloud layer above may result in an unexpected aircraft attitude. When flying in and out of cloud layers, the pilot will generally remember the relative bearing position of the sun in the sky before the weather was penetrated. It may cause the pilot to unconsciously seek the "correct" visual image, resulting in imprecise flight. During a formation flight this action can be hazardous.

False Horizon

   At night, approaching ground lights that are in a row may be mistaken for the horizon. Depending on their relative orientation to the real horizon, an illusion of aircraft tilt may be perceived. At high altitudes, the position of the horizon will be below the position of the horizon at a lower altitude. Resultant changes in pitch attitude and aircraft controls may be induced.

OPTICAL PRINCIPLES

   The flight surgeon should understand the components of the spectacle prescription and know how to transpose the prescription into its two configurations, minus and plus cylinder forms. This skill will enable the flight surgeon to make accurate determinations of the proper flying class category.

   Light is electromagnetic radiation that initiates the neurosensory process known as vision. Light propagates in the form of a wave front. Diopter is the term used to express the degree of vergence or deviation of the light as the front leaves an optical surface (see figures 8-21 and 8-22). This curvature may be divergent or convergent. A diopter is defined as the reciprocal of the focal point distance in meters (D = 1/m).

Figure 8-21. Divergence.
Figure 8-21. Divergence.
Figure 8-22. Convergence.
Figure 8-22. Convergence.

   The direction of light can be changed (refracted), when it passes obliquely through two media of different refractive indexes. The phenomenon of dispersion occurs in transparent materials when light of longer wavelengths travels through the material faster than the light of shorter wavelengths. The refractive power of a material is higher for shorter wavelengths. The classical example is the glass prism that breaks light into its component colors (see figure 8-23). Light is always deviated toward the base of the prism, but the image is deviated away from the base. A prism diopter is the measurement of the amount of deviation. The formula for this is:

PD = lateral deviation (cm) / distance (m).
 
Figure 8-23. Prism refraction.
Figure 8-23. Prism refraction.

   Positive or convex lenses can be thought of as two prisms base to base. Concave lenses may be thought as two prisms apex to apex. Where the prisms touch is the optical center (see figure 8-24).

Figure 8-24. Convex and concave lenses.
Figure 8-24. Convex and concave lenses.

A diopter in the measurement of power of the optical system. The formula is:

D = 1 / distance (focal length in meters)
 
where D is the power in diopters. Convex lenses are represented in plus diopters and concave in negative diopters.

A cylinder lens is a different type of lens that has maximum power in one meridian and zero power in the meridian perpendicular to the first. The optical cross is two perpendicular lines representing the meridians of least and most power in a spectacle. It goes from zero to 180 degrees (see figure 8-25). The axis meridian is the meridian with zero power in a cylinder lens (see figure 8-26).

Figure 8-25. The optical cross.
Figure 8-25. The optical cross.
Figure 8-26. The cylinder lens.
Figure 8-26. The cylinder lens.

   The combination of a simple sphere lens and a cylinder lens is called a toric or compound lens. The standard spectacle prescription is a toric lens formula. This prescription can be expressed as plus or minus cylinder. A plano lens has no cylinder or sphere power.

   To obtain the proper flying classification of individuals, it is vitally important that the proper procedures for obtaining cycloplegic refractions be followed. These are outlined in AFI 48-133, Physical Exam Techniques (to be released Summer 1996). Once these have been obtained and presented to the flight surgeon, it now becomes his duty to determine whether and for what flying class an individual is qualified.

   To tell whether some one has any myopia in any meridian and to what degree, simply write his cycloplegic refraction in plus and minus cylinder forms, i.e. transpose, and look at the first number. If either form has a first number with a minus sign and a value more than plano, he has some myopia. Next, look at the second number. This tells you how much astigmatism he has. It is the same in either plus or minus form.

   Transposition is a simple process. To change from a plus (minus) cylinder prescription to a minus (plus) cylinder form, use the following procedure:

   a. Add the sphere and cylinder powers, keeping the appropriate signs, and place that resultant number and sign as the new sphere value (first number).
   b. Change the sign of the cylinder (second number).
   c. Keep the same cylinder power (second number).
   d. Change the axis by 90o (third number).
   The following example illustrates these points. Convert +0.50 - 0.75 x 075o to the plus cylinder form.
   a. -0.25
   b. +0.75
   c. 0.75
   d 165o
   Thus, the plus cylinder form is -0.25 + 0.75 x 165o.Thus, you have determined that he has a quarter diopter of myopia, in one meridian, and three-quarters of a diopter of astigmatism.

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