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Type of Interaction

The type of interaction of laser radiation with tissues of the eye and the skin which leads to injury can be either thermal or photochemical in nature. When the temperature of the tissue is increased above a critical temperature, proteins are denaturated and thermal damage occurs. If temperatures above 100 C are induced, water in the tissue begins to boil and further temperature increases lead to a carbonisation of the tissue. In the ultraviolet and blue end of the visible spectrum, photochemical damage can occur, as photon energies are sufficiently high to cause direct damage to macromolecules of cells such as to the DNA. It is typical for photochemical damage that the degree of damage depends on the radiant exposure, i.e. the accumulated energy in terms of J m-2, and a short duration exposure (seconds) with high irradiances has the same effect as a long term exposure (hours) with a correspondingly smaller irradiance. Also individual exposure doses add up over a period of several hours. Consequently photochemical damage mainly occurs for exposure to continuous wave lasers or repeated exposures to irradiance levels which are too small to cause a temperature rise sufficient to cause thermal damage. Thermal damage dominates for short time exposures, especially to single pulses or bursts of pulses.

When working with UV lasers such as Excimer lasers, due to the photochemical additivity, even faint stray light or reflections, for instance from lenses and walls, can cause damage when exposure occurs for a few hours. The resulting injury to the eye is called photokeratoconjunctivitis, i.e. an inflammation of the cornea and the conjunctiva, and if the skin is exposed, sunburn (erythema) can result. These injuries are, depending on the level of over-exposure, painful, but due to renewal of damaged cells within a few days, usually do not constitute permanent damage. Direct exposure of the eye to short pulsed UV laser radiation can cause ablation of the cornea (similar to the process which is used to correct farsightedness) or clouding of the lens (especially with wavelengths of about 320 nm to 360 nm), which are both thermal effects, and which may result in permanent serious reduction of visual acuity.

Similarly to thermal and photochemical damage mechanisms to the cornea or the lens in the UV spectral range, optical radiation in the blue end of the visible part of the spectrum, i.e. radiation with wavelengths of approximately 400 nm to 550 nm, can cause thermal or photochemical damage to the retina, however as the retina is not renewed after damage as the skin and the cornea is, both photochemical and thermal injuries to the retina may result in serious loss of visual function. When referring to photochemical retinal damage, this is also often called photoretinitis or "blue-light hazard".

When tissue is exposed to pulses with durations in the nanosecond regime or to even shorter pulses, "non-linear effects" occur, such as photoablation and photodisruption. In terms of eye safety, shockwaves which result in thermomechanical damage of tissue are of major concern, as in the retina they lead to rupture of tissue and possibly bloodvessels, and haemorrhage. For ocular exposure to ultrashort pulses with correspondingly high peak powers, effects such as laser induced breakdown and self-focussing occur. These effects result in a decrease of the level of exposure necessary to cause an injury.

Depending on the interaction duration and peak irradiance values, each interaction type can be assigned a general domain, as is depicted in figure 5.

Figure 5. The type of interaction of laser radiation with tissue will be determined by the interaction duration on the one hand and the irradiance on the other. For instance, photochemical interaction mechanisms are dominant for low irradiance, long term exposure, while non-linear effects only occur for short pulse, high irradiance exposure.

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