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1.1.1 Wavelength Considerations

Wavelength Considerations

An overview of effects in the different wavelength ranges is shown in figure 3. In the wavelength range of about 400 nm to 1400 nm the laser beam is transmitted to the retina onto which it is focussed to a very small spot of the order of 10 - 20 µm. Consequently the irradiance on the retina is much larger than the irradiance on the cornea (up to a factor 100000) and therefore a comparatively small laser power is sufficient in this wavelength range (the "retinal hazard region") to damage the retina. It should be noted that the retinal hazard region comprises both the visible wavelengths (about 400 nm to 700 nm) and the near infrared region (700 nm to 1400 nm) where radiation is not visible but it is still focussed onto the retina (see figure 3).

The wavelength range of 400 nm to 700 nm is defined as "visible" in the field of laser safety. However, it should be noted that there is no sharp dividing line between visible and non-visible: the sensitivity of the eye in respect of the visual stimulus is highest in the green, i.e. around 550 nm, and falls off towards red on one side and blue on the other. Radiation of 810 nm can also be seen when the intensity is correspondingly high. Cynical colleagues argue that with sufficient intensity, also 1064 nm Nd:YAG laser radiation can be seen, "but not for very long".

Figure 3:

 The part of the electromagnetic radiation spectrum which is of interest in laser safety. Ultraviolet, mid- and far-infrared radiation is absorbed by the cornea and lens of the eye, while radiation in the wavelength range of 400 nm to 1400 nm is focussed onto the retina, which results in very high irradiance levels on the retina even for small laser powers. Exposure of the skin with sufficiently high exposure levels results in skin injury.

The ultraviolet and infrared region is further subdivided into wavelength regions which are relevant in terms of photobiological interactions:

Table 1:

 Wavelength bands as relevant for photobiology, following CIE notation

CIE Shorthand

Wavelength Range

Tissue Interaction

UV-C

100 nm - 280 nm

absorbed in uppermost cell layers of eye and skin; highly effective in producing photokeratoconjunktivitis ; germicidal. Radiation with wavelengths smaller than about 180 nm - 200 nm are heavily absorbed by the oxygen of the air and is also termed "vacuum ultraviolet". Vacuum UV usually need not be considered for hazard evaluation.

UV-B

280 nm - 315 nm

intermediate absorption depth; highly effective in producing photokeratoconjunktivitis and sunburn

UV-A*

315 nm - 400 nm

penetrates deep into eye and skin; possible damage to the lens

IR-A*

700 nm - 1400 nm

radiation focussed onto the retina, but not visible; deep penetration into the skin

IR-B

1400 nm - 3000 nm

radiation absorbed in volume of the eye

IR-C

3000 nm - 1 mm

radiation absorbed in uppermost cell layers of eye and skin

* It is noted that the visible wavelength range according to CIE is defined as 380 nm to 780 nm (and UV-A up to 380 nm and IR-A starting from 780 nm accordingly). In laser safety, the "visible" wavelength range, in the sense that a bright visual stimulus induces protective reflexes such as the blink reflex, is defined as the wavelength range between 400 nm and 700 nm.

The CIE shorthand notation for wavelength ranges in the UV and IR are closely related to the absorption depth of radiation in tissue, which is described by the absorption coefficient. A natural exponential absorption law, the Beer Lambert law, applies and therefore the inverse of the absorption coefficient is a representative value for the depth at which the irradiance has fallen to 1/e of the value at the surface. Hence a large absorption coefficient corresponds to a small penetration depth. The absorption coefficient for water and tissue as function of wavelength is shown in figure 4.

Figure 4:

 Absorption coefficient as function of wavelength for water and tissue. It can be seen that the tissue absorption is governed by the water content of the tissue for infrared wavelengths and the absorption characteristics of haemoglobin and other organic molecules in the visible and in the UV. Blood for instance has a strong absorption in the blue and green part of the spectrum, and penetration depths into the skin are greatest for the red and near IR wavelengths.

In the ultraviolet region (UV) (wavelengths below 400 nm) and in the mid- and far-infrared region (IR) (wavelengths above 1400 nm), the cornea and lens absorbs the incident radiation and the radiation is not transmitted to the retina. However, damage to the cornea or lens can occur when exposed to high power lasers and this can also result in serious vision loss.

Exposure of the skin can produce skin injuries, i.e. "sunburn" in the UV-region for long term exposure to low irradiance levels and burns in the visible and IR region. High power materials processing lasers can produce large and deep injuries.