Therefore, the retinal thermal hazard depends not only on the spectral radiance of the source but also on the exposure duration and potentially on the angular subtense of the source. The ACGIH TLV for protection against retinal thermal injury from a visible light source is defined in terms of a weighted radiance LR (20):
(15)
where Lλ is the source spectral radiance expressed in W cm−2 sr−1 nm−1 and R(λ) is the retinal thermal hazard function. R(λ) is plotted in Figure 9. Alternatively, for viewing durations less than 0.25 seconds, the TLV can be expressed as in terms of the time‐integrated radiance dose DLR, with units of J cm−2 sr−1. This allowable dose limit decreases with decreasing viewing duration (or pulse length) from 0.25 seconds down to 1 μs. For viewing durations greater than 0.25 seconds, the TLV is expressed solely in terms of the radiance LR, which for sources subtending an angle greater than 0.1 rad should not exceed 45 W cm−2 sr−1 (20). The ICNIRP exposure limit under these conditions is 28 W cm−2 sr−1 (19), owing to a modestly greater safety factor than that incorporated into the TLV. For viewing durations greater than 0.25 seconds and sources with an angular subtense less than 0.1 rad, the TLV or ICNIRP exposure limit may be increased by a factor equal to one‐tenth the inverse of the angular subtense in radians. For viewing times shorter than 0.25 seconds, more complicated conditions apply for when the exposure limit may be adjusted by the angular subtense, and these conditions differ between ICNIRP (19) and the ACGIH (20).
FIGURE 9 ACGIH retinal hazard functions (20). Dimensionless weighting factors for the blue‐light retinal hazard function B(λ), the aphakic hazard function A(λ), and the retinal thermal hazard function R(λ). Note that B(λ) and A(λ) coincide at wavelengths longer than 440 nm, and B(λ) and R(λ) coincide between 380 and 440 nm.
Source: From Ref. (20). Reprinted with permission of ACGIH.
IR heat lamps and other IR‐A sources may not emit sufficient visible radiation to stimulate the aversion response or pupillary constriction. A lower exposure limit is therefore recommended for extended viewing (>0.25 seconds) of such sources than is recommended for visible light sources. The TLV for protection against retinal thermal injury from an IR‐A source in the absence of strong visible light is given by the following weighted radiance in W cm−2 sr−1 (20):
for exposure times less than 810 seconds, where α is the angular subtense of the source in radians and t is the viewing duration in seconds. For exposure times longer than 810 seconds, the allowable weighted radiance is 0.6/α W cm−2 sr−1 (20). The ICNIRP exposure limit for IR sources with weak visual stimulus is a retinal‐hazard‐weighted radiance that is a factor of 1.6 lower than the value given by the right‐hand side of Eq. (16) for exposure durations up to 100 seconds and has value of 0.63/α W cm−2 sr−1 for durations greater than 100 seconds (19).
4.1.3 Infrared Thermal Hazard to Cornea and Lens
The ACGIH TLV (20) for protection against thermal injury to the cornea or lens from IR‐A and IR‐B radiation is
for t less than 1000 seconds, where the spectral irradiance Eλ at the eye is expressed in W cm−2 nm−1. For exposure periods greater than 1000 seconds, the total unweighted irradiance due to IR‐A and IR‐B radiation should not exceed 0.01 W cm−2. The ICNIRP exposure limit for protecting thermal injury to the cornea and lens is the same as the TLV but weights the spectral radiance between 780 and 1000 nm by a factor of 0.3 and does not include wavelengths shorter than 780 nm (19).
4.1.4 Thermal Hazard to Skin from Visible and Infrared Radiation
ICNIRP recommends the following exposure limit to prevent thermal burns of the skin from visible and IR radiation of wavelengths less than 3000 nm (19):
(18)
for t less than 10 seconds, where Hskin, the unweighted radiant exposure at the skin, is expressed in J cm−2. The sensation of pain is expected to cause avoidance of exposures longer than 10 seconds.
4.1.5 Numerical Values of Spectral Weighting Functions
Numerical values of S(λ), B(λ), A(λ), and R(λ) are available in tabular form in numerous publications 15, 16, 19–21.
Numerical values of the CIE erythema reference action spectrum can be computed using the following formulas (17):
(19)
for λ between 250 and 298 nm;
(20)
for λ between 298 and 328 nm; and
(21)
for λ between 328 and 400 nm.
4.1.6 Ozone Hazard from UV‐C Sources
UV radiation of wavelength shorter than 242 nm interacts photochemically with O2 to form ozone, a highly irritating and potentially lethal gas. Potential sources of ozone‐generating UV radiation include gas shielded arc welding, xenon lamps, mercury lamps (including some germicidal lamps), excimer lamps, and deuterium lamps. Ozone exposure should be controlled by means of local exhaust ventilation at the source.
4.2 Measurement of Optical Radiation
To measure broadband optical radiation in a way that is meaningful for assessing health hazards, it is necessary to take account of the spectral distribution of the radiation and apply an appropriate spectral weighting function. This can be done by either (i) using a spectroradiometer, which measures the amount of incident radiation in narrow wavelength bands, then mathematically weighting the measured spectral distribution using the relevant hazard function or (ii) using a broadband radiometer with a detector that has a spectral response function that closely matches the relevant hazard function.
For accurate irradiance or radiant exposure measurements, the input optics of the measurement instrument should have a directional response that is proportional to the cosine of the angle between the normal to the receiving surface and the line of sight from the receiving surface to the radiation source (22, 23).
4.2.1