Conclusion

As seen previously, the great efficiency and directionality of laser beams is the source of an extremely high hazard for the eye. Moreover, the wavelengths responsible for damages on the eye (cornea excepted) at the lowest intensities are in the domain 400 to 1400 nm. Indeed, if one considers that the image of a collimated beam on the retina has a width of about 10 mm, and that the diameter of a dilated pupil is of about 7mm, then a pulse carrying a few µJ or a continuous He-Ne laser beam of 1 mW are powerful enough to create permanent lesions on the retina.

In order to find the spectral domain where the damage threshold on the eye is the highest, and therefore the hazard the lowest, one has to find a compromise between the absorption spectra of the different parts of the eye.

Thus, concerning a laser emitting between 1.5 and 1.55 µm, the absorption is negligible in the cristalline lens, 75% occurs inside the cornea, and 25% inside the aqueous humor. We see that most of the energy is absorbed inside the cornea. The absorption is distributed along the relatively high thickness of this optical element, and thus the energy absorbed per volume unit is too weak to provoke important damages. The cornea is as resistant to radiations as the skin. Moreover, it has a very high regeneration ability. But outside of the previously defined wavelength range, the eye faces high hazard levels...

Laser sources emitting in the so-called “ocular safety domain” (1.5 to 1.55 µm) can freely propagate without inducing laser hazard. These are so-called “ocular safety laser sources”. This spectral domain presents many other advantages : it is commonly used for telecommunications through optical fibers, because it corresponds to the minimum of the absorption spectrum of the silica, and is situated in a spectral domain transmitted by the atmosphere.