​Over the wavelength range that different lasers operate, the skin is a strong absorber, thereby protecting all the organs of the body except for the eyes.

A1.1 Injury mechanisms

There are a number of tissue damage mechanisms for laser radiation, including thermal, photochemical thermos-acoustic transients and non-linear effects. For supra-threshold exposures, the predominant injury mechanism is determined principally by a combination of laser wavelength and exposure / pulse duration. In general, photochemical mechanisms dominate in the ultraviolet region where thresholds for photochemical damage are generally lower than for thermal injury. For wavelengths between about 400 nm and 550 nm, both photochemical and thermal mechanisms occur with the dominant mechanism dependent on the timescale of exposure. At longer wavelengths mechanisms are predominantly thermal. As noted above, the injury mechanism also depends on the exposure duration. The predominant injury mechanisms for different exposure / pulse durations are summarised below.

A1.2 Ocular injuries

The site of injury is determined by the spectral transmission of ocular tissues. Broadly the dependence on wavelength is as follows:

Retinal eye damage from laser radiation can occur at very low power or energy levels due to the point source nature of most laser sources, which results in a very small image of the source being formed on the retina. As essentially, all the power / energy entering the posterior eye will be focussed onto this small spot, which typically has a diameter of 10 - 20 μm, the resulting retinal irradiance or radiant exposure may be very high. Even allowing for intraocular scattering and corneal aberrations, it can be estimated that the retinal irradiance will be around 200,000 times higher than the irradiance incident on the cornea. Immediately behind the photoreceptor layer is the retinal pigment epithelium, which is very rich in melanin and absorbs strongly throughout the visible and into the near infrared. The result of these two effects (focussing to a small spot and strong absorption) is that the retina is extremely sensitive to laser radiation at wavelengths between 400 nm and 1400 nm.

In a research laboratory there may be lasers operating at different wavelengths, fixed and variable, from the UV through the visible and IR. What may seem to be a steady beam may be a pulsed laser operating at a high pulse repetition rate with peak powers in the individual pulses that are more than a million times the average power of the beam.

Apparently weak blue or red beams may be operating at wavelengths at the extreme of the visual response range of the eye where its sensitivity is several thousand times below its peak sensitivity (green light). UVA and blue light transmission through the ocular media decreases with cumulative exposure, which is broadly related to age: UV does not penetrate to the retina in adults and consequently does not elicit a visual response. The long wavelength end of the visible spectrum is defined as 700 nm by convention. However, longer wavelengths still penetrate to the retina, but the response of photoreceptors decreases rapidly above 700 nm: most people can perceive wavelengths up to at least 850 nm if there are enough photons. This is a major safety concern as people perceive very intense beams at wavelengths between 700 nm and 850 nm as a dull red glow and are consequently misled into believing the beam power to be very low. Longer wavelengths in the near infrared will not elicit any visual response at all.

A1.3 Injuries to the skin

Skin hazards from exposure to laser radiation are simpler to assess than those for the eye as the complications arising from imaging of the source in the visible and near infrared spectral regions do not apply. Injuries are generally superficial, although the depth of penetration is wavelength-dependent, being maximal in the visible and near infrared and decreasing in both the ultraviolet and mid/far infrared. Ultraviolet radiation produces photochemical injuries, which can include erythema, oedema and blistering. As for other photochemical effects, exposures are generally cumulative adverse reactions may have delayed onset. As a result, it is important to consider exposure to scattered radiation as well as direct beam exposures. In addition, exposure can give risk to long-term adverse effects including skin ageing and an increased risk of skin cancer; neither the exposure limit values nor the maximum permissible exposures are intended to protect against these long-term effects.

Laser damage mechanisms for skin tissue are similar to those for the eye in terms of the effect of wavelength and exposure duration. As noted above, the eye is particularly sensitive to injury in the retinal hazard region (400 nm – 1400 nm). For shorter (ultraviolet) and longer (mid- and far-infrared) wavelengths the sensitivities of the eye and skin are similar and the limits are identical. Hence if eye protection is required outside the retinal hazard region, then skin protection will also be required