24.5.1 Occupational Exposure Limits
Relevant MPEs or ELs for lasers of interest are given in Table 24.1, and are specified at the corneal plane. If the laser beam is less than 7 mm in diameter, it is assumed that the entire beam could enter the dark-adapted pupil, and one can express the maximal safe power or energy in the beam (in the 0.4-1.4 (m retinal hazard region); it is the EL multiplied by the area of a 7 mm pupil, that is, 0.4 cm2. For example, for the visible CW lasers, an exposure limited by the natural aversion response of 0.25 seconds is 2.5 W/cm2, and this EL multiplied by 0.4 cm2 results in the limiting power of 1.0 mW. This 1 mW value has a special significance in laser-safety standards, since it is the Accessible Emission Limit (AEL) of Class 2, that is, the dividing line between two laser-safety-hazard classifications: Class 2 and Class 3 [2-8]. A 3.5 mm aperture is applied with CW infrared lasers operating at wavelengths greater than 1400 nm, and a 1 mm aperture is applied in the UV spectral region for brief ocular exposures.