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Manufacturers have referred to lasers with operating wavelengths longer than 1400 nm (mid to far infrared) as eyesafe. Wavelengths in this region are absorbed in anterior portions of the eye (mainly cornea) and therefore never reach the retina. This is in contrast to the eye- hazardous portion of the optical spectrum of 400 - 1400 nm (visible and near infrared) where the anterior portions of the eye have high transmittance and refractive power. Irradiance levels are typically 5 orders of magnitude greater at the retina than at the cornea for visible and NIR wavelengths. Although wavelengths longer than 1400 nm do not interact with the retina, they can interact with the skin or cornea and cause a thermal injury. Specifiers of laser equipment have required that systems be eyesafe at a particular distance with or without optics. By this they mean that exposure at that distance for the naked eye or optically aided viewing through a specified power of optics should not exceed exposure limits. Some wavelengths in the visible and near IR are more 'eyesafe' than others because the safety factor between the level that would actually produce an injury (ED-50 value) and the exposure limit is greater. For example, the ED-50 value for a single Q-switched laser pulse for the ruby laser wavelength of 694.3 nm is 11.2 (mu) J into the eye and the Accessible Emission Limit (AEL) is 0.19 (mu) J, a factor of 59 between ED-50 and the AEL. The ratio for 1064 nm for a single Q- switched pulse is 99 (mu) J/1.9 (mu) J or 52. So, in a sense, exposure to 694 laser radiation at the AEL, is slightly more 'eyesafe' than exposure to 1064 nm radiation at the AEL. To the laser safety specialist, however, eyesafe, means only one thing, Class 1. Class 1 is a laser hazard classification that means the laser does not exceed specified limits for a reasonable, worst-case combination of distance, exposure duration, and collecting aperture diameter during its intended use. Determining whether a laser is Class 1 is usually a two-step process. The first step is determination of the AEL and the second is determining whether or not the laser in question exceeds this AEL. Exceeding the AEL is determined by calculation or measurement. If the AEL is exceeded under prescribed parameters then the laser is not Class 1. Differences in laser safety standards in both of these classification steps have caused confusion. Differences in exposure limits chosen by laser safety specialists based upon the same criteria have also caused confusion.
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