- F -


A measure of the relative aperture of a lens; its light gathering capacity. It is equal to the ratio of the focal length of the lens divided by the diameter of its limiting opening (aperture): f-number = focal length/iris diameter. Note that the number becomes smaller as the aperture grows larger and that it must be squared to directly measure the area which is the light gathering capacity. Also see numerical aperture (NA).

false Becke line

A second bright line which moves in the direction opposite to the Becke line. It is usually observed with thick particles or when the refractive index difference between particles and mountant is large. It appears as a concentration of reflected light at the high index side of the interface.

far point of eye

For the normal eye, the far point is at infinity. The rays of light from an infinitely distant point source are parallel and can be focused with the accommodation muscles of the eye entirely relaxed. See accommodation; near point of the eye.

fast ray

The fast ray or fast component for a crystal or fiber corresponds to the lower refractive index.


The area of the object that is seen when the image is observed. It may range in diameter from several millimeters to less than 0.1 mm. The magnification and the size of the diaphram opening in the eyepiece governs the diameter of the field of view.

field depth

The thickness of the object space within which objects focused by a lens will all appear in good simultaneous focus. Penetration is a synonym.

field diaphram

In a photomicrographic system particularly, an iris diaphram that is imaged in the field of view with Kohler illumination. This limits the extent of the illuminated field and eliminates much extraneous light. See diaphram.

field diaphram

The iris diaphram that is located in front of the collecting lens of the light source. With Koehler illumination, the condenser focuses the image of the field diaphram onto the image plane.

field-emission microscope

Either one of two kinds of point-projection microscopes, both invented by E. W. Muller: (1) The older device (1936) is a specialized cathode-ray tube, employing field-emission of electrons from a negatively charged tip of a very sharp needle in a vacuum, by point-projection of the image onto a positively charged, fluorescent screen. (2) A later device (field-ion-mission microscope, 1950) emits absorbed helium ions from an anode.

field lens

The lower lens in an ocular; the lens nearest the object field.

field of view

The extent of the visible image field that can be seen.

field planes

The set of planes in a microscope adjusted for Koehler illumination that are conjugate with the focused specimen. They include the plane of the specimen, the field diaphram, the intermediate image plane, and the image on the retina, photographic emulsion, or the faceplate of the video pickup device.


An electrically heated wire used as a source of radiation, such as electrons, or as a source of heat such as for the vaporization of a metal.

filar micrometer or filar eyepiece

an eyepiece equipped with a fiducial line in its focal plane that is movable by means of a calibrated micrometer screw in order to make accurate measurements of length.

filar micrometer ocular

A micrometer ocular with crossed lines which can be simultaneously focused in and moved across the field by means of a micrometer screw. The amount of displacement can be read in 0.01 steps on the micrometer drum head. This makes a very delicate measuring device, since calibration of the micrometer scale by means of a stage micrometer gives values for each interval on the drum head which are much less than the resolving power of the highest-aperture objectives.


Unwanted light in an optical instrument that arises by reflection at lens surfaces (including the observer's eyeglasses) and lens barrel, etc., and sometimes from lens aberration. Flare reduces image contrast and may form undesirable focused images and hot spots. Flare in a microscope is reduced by immersion of the condenser and objective lenses, anti-reflection

fluorite objective

Microscope objective lenses considerably better corrected than achromats but not quite as well corrected as the apochromats. By using fluorite crystals (which have lower dispersion) in place of some of the glass elements, a fluorite objective corrects for spherical aberrations in three wavelengths at considerably lower cost than the apochromats.


Dyes which have little coloring effect under ordinary lighting conditions but which fluoresce when irradiated with ultraviolet radiation.


The absorption of radiation to produce radiation of longer wavelengths, i.e., lower energy.

fluorescence microscope

A microscope illuminated by ultraviolet or blue light so that the object may re-radiate light of longer wavelengths. To protect the eyes, a W-absorbing filter should be provided if not built into the fluorescence microscope.

fluorescence recovery after photobleaching (FRAP)

A new technique in light microscopy using a pulse from a focused laser microbeam to deplete the fluorescence in a local region in a living cell. The subsequent recovery of fluorescence in the irradiated region is measured to establish the mobility of the molecules that carry the fluorescent tag.

Flourite objectives

Microscope objective lenses considerably better corrected than achromats but not quite as well corrected as the apochromats. By using fluorite crystals (which have lower dispersion) in place of some of the glass elements, a fluorite objective corrects for spherical aberrations in three wavelengths at considerably lower cost than the apochromatis.

flux, light

Sometimes called luminous flux; the visible portion of the radiant energy emitted by a light source. It is measured in lumens per solid angle. In electrical engineering, it is analogous to the lines of force in a magnetic field, spoken of as magnetic flux.

focal length

The distance from the optical center of the lens to the focal point
The focal length of an objective and its working distance are directly proportional.

focal point

The point on the principal axis of a lens at which impinging parallel rays converge to a common point or focus. The point is virtual for a negative lens. See virtual image.


A point at which rays, originating from a point in the object, converge.


The point or plane in which light rays or an electron beam form a minimum-sized spot that has the proper intensity distribution. Also the act of bringing light or electron beams to a fine spot.

focus, principal

the point at which a lens focuses an axial object point. Synonymous with focal point see focal point

focusing device (electrons)

A device which effectively increases the angular aperture of the electron beam illuminating the object, rendering the focus more critical.

focusing glass (light microscopy)

A hand magnifier, used at the focal plane of a camera, usually with the ground glass removed. Its purpose is to examine the image for critical focus. It is, as a rule, a low-power magnifier so mounted that it is in focus when supported on a piece of clear glass at the focal plane of the cam- era, for studying the image. It operates equally well on the aerial image.

focusing magnifier (electron microscopy)

A low-power light microscope, telescope, or simple lens used to observe the electron image formed on a fluorescent screen.

Fresnel fringes

A class of diffraction fringes formed when the source of illumination and viewing screen are at a finite distance from a diffracting edge. In the electron microscope these fringes are best seen when the object is slightly out of focus.

Fresnel lens

A lens built up, progressively, in zones or steps, each zone with its own individual radius. Considerable spherical correction is attained, and the weight of the lens is greatly reduced. Fresnel lenses were originally designed for lighthouses but they are now attainable for small spotlights, automobile headlights, and similar uses.

Fresnel reflection

Process by which radiant flux is reflected from an optically plane boundary between two transparent dielectric materials.

front lens

The front element of a compound lens system; the first lens element which the entering light encounters. See back lens.