AS NZS IEC 60825.14:2011 pdf – Safety of laser products Part 14: A user’s guide.
5 Determining the maximum permissible exposure (MPE)
5.1 General remarks
Levels of maximum permissible exposure, which are based on values developed by the
International Commission on Non-lonising Radiation Protection (ICNIRP), are given in Tables
5. 6 and 7 as functions of the emission wavelength and exposure duration. These tables
should be used in conjunction with the correction factors given in Table 8.
Table 5 defines the MP for the eye under conditions of direct exposure to a single laser beam (and in all other cases where the apparent angular subtense of the laser source does not exceed 1.5 mrad, see 4.3.3). For exposure of the eye to laser radiation at wavelengths between 400 nm and 1 400 nm (the retinal hazard region) to larger apparent sources than would be the case for the direct viewing of a single laser beam (that is. for certain multiple or extended sources that subtend an angle at the eye greater than 1,5 mrad), a relaxation (increase) in the MPE is possible. This is because the eye cannot focus such non-point sources onto a small spot on the retina, and therefore the maximum safe power or energy entering the eye is larger. These relaxed MPEs are given in Table 6.
Table 7 specifies values of the MPE for the skin.
The exposure duration used in determining the MPE from Tables 5, 6 and 7 should be based on the maximum duration of accidental exposure that could reasonably be expected to occur. taking into account the wavelength of laser emission end the conditions under which the laser might be used. Under worst-case conditions of accidental exposure, 100 S may be used as the maximum duration of exposure for laser radiation at wavelengths above 400 nm, and 30 000 s for wavelengths below 400 nm where longer-term photocheniical effects may be initiated. This longer time-base is applicable in circumstances where repeated or prolonged exposure to ultraviolet radiation could occur without an immediate apparent effect, but is clearly not realistic in the case of direct accidental exposure to a high-power ultraviolet laser beam where immediate and obvious injury would be caused. For accidental exposure to visible laser radiation (400 nm — 700 nm) wtiere purposeful staring is not intended or anticipated, the aversion response time of 0,25 S may be used.
Further consideration of the exposure duration is included in the discussion of risk assessment given in 7.3.
5.2 RepetItively pulsed or modulated
Since there are only limited data on multiple pulse exposure criteria, caution must be used in the evaluation of exposure to repetitively pulsed radiation. The lollowing methods should be used to determine the MPE to be applied to exposures to repetitively pulsed radiation.
The NPE for ocular exposure for wavelengths from 400 nm to 106 nm is determined by using the most restrictive of requirements a). b). and C). Requirement C) applies only to the thermal limits and not to the photochemical limits.
The MPE for ocular exposure for wavelengths less than 400 nm and the MPE for skin exposure are determined by using the most restrictive of requirements a) and b).
a) The exposure from any single pulse within a pulse train shall not exceed the MPE for a single pulse.
b) The average exposure for a pulse train of exposure duration T 5hall not exceed the MPE given in Tables 5. 6. and 7 for a single pulse of exposure duration T. (T is the duration used in the assessment of exposure as discussed in 4.1.)
C) The average exposure from pulses within a pulse train shall not exceed the MPE for a single pulse multiplied by the correction factor C5.
ln some cases,this value may fall below the MPE that would apply for continuousexposure at the same peak power using the same exposure time. Under thesecircumstances, the MPE for continuous exposure may be used.