target="_blank" rel="nofollow" href="#ulink_e9628e79-b1a7-5a8a-9406-e632b0cc32e3">FIGURE 7 Relative spectral effectiveness function S(λ) used for spectral weighting of incident UV radiation in the ACGIH and ICNIRP exposure guidelines (13, 14). S(λ) is dimensionless.
Source: From Ref. (15). Reprinted with permission of ACGIH.
The relative spectral effectiveness function S(λ) was derived from an action spectrum, measured in terms of radiant exposure, that was drawn to form an envelope around experimentally determined threshold doses for minimal erythema and barely detectable photokeratitis (14, 16). This action spectrum defines the harmonized ACGIH Threshold Limit Values (TLV®) and ICNIRP exposure limits for monochromatic radiation between 180 and 400 nm. S(λ) represents the ratio between the TLV for radiant exposure to monochromatic radiation at 270 nm (30 J m−2) and the TLV for radiant exposure to monochromatic radiation at wavelength λ. The TLV and S(λ) are thus inversely proportional to each other. The ACGIH/ICNIRP exposure limits include a safety factor that may be as low as 1.5–2.0 for acute photokeratitis at some wavelengths (16). The safety factor for erythema is higher because this effect has higher dose thresholds than photokeratitis (18). Dark pigmentation may confer additional protection against erythema; on the other hand, photosensitive individuals and individuals simultaneously exposed to photosensitizing chemicals may suffer erythema at effective doses that are below the ACGIH/ICNIRP exposure limits.
CIE has established an erythema reference action spectrum defined by a spectral weighting function Ser(λ) (17). The erythemal effective irradiance Eer can be expressed by Eq. (11).
The erythemal effective radiant exposure is the integral of Eer over time. CIE has also defined the standard erythema dose (SED) as an erythemal effective radiant exposure equal to 100 J m−2 (17). Unlike the TLV, the SED is intended to be used as a reference dose, not as an exposure limit. Nevertheless, it may be of interest to compare the TLV at wavelength λ with the erythemal effective radiant exposure that is equivalent to 1 SED for monochromatic radiation at wavelength λ:
(12)
The ACGIH/ICNIRP exposure limit and CIE SED curves are presented in Figure 8. At wavelengths longer than 300 nm, the two curves nearly coincide.
The exposure limits defined by the ACGIH/ICNIRP action spectrum are considered protective against photochemical effects leading to erythema and photokeratitis, but concerns remained that these limits might not protect the lens of the eye against thermal effects resulting from absorption of UV‐A at high dose rates over short time periods (16, 18). Therefore, the ACGIH established a TLV of 10 000 J m−2 for radiant exposures to UV‐A, without spectral weighting, over periods less than 1000 seconds. The TLV for unweighted UV‐A irradiance is 10 W m−2 for periods longer than 1000 seconds. This guideline is based on the premise that ambient outdoor irradiances of about 10 W m−2 due to solar UV‐A are not harmful to the eyes (16, 18).
4.1.2 Retinal Hazards
The ocular media are effectively transparent to visible radiation and transmit some IR‐A and UV‐A radiation as well. Transmitted optical radiation is focused by the lens onto the retina to form a retinal image. The irradiance received at the retinal image of an optical source is proportional to the radiance of the source. Photochemical or thermal damage may result from excessive irradiance at the retinal image.
ICNIRP (19) and ACGIH (20) have established guidelines for protection against retinal hazards. The relevant spectral weighting functions used by ACGIH are generally harmonized with those recommended by ICNIRP, but the exposure limits recommended by ICNIRP and ACGIH differ in some cases.
FIGURE 8 Comparison of the ACGIH TLV for narrowband radiation (15) with the erythemal effective radiant exposure (J m−2) equivalent to 1 SED (17) at wavelength λ.
4.1.2.1 Blue‐Light Hazard
High retinal irradiances from short‐wavelength optical radiation may cause photochemical damage to the retina, leading to loss of visual acuity. This hazard is referred to as the “blue‐light” hazard because the effect is greatest in the blue region of the optical spectrum between 435 and 440 nm.
The ACGIH TLV for protection against the blue‐light hazard (20) is similar, but not identical, to the ICNIRP exposure limits (19). The source spectral radiance Lλ is weighted by a blue‐light hazard function B(λ) over the range 305–700 nm:
(13)
for exposure time t less than 10 000 seconds, or
(14)
for time greater than 10 000 seconds. The blue‐light hazard function B(λ) is plotted in Figure 9. The ICNIRP blue‐light hazard weighting differs from the ACGIH function by including 300–305 nm radiation with a weighting factor of 0.01. For light sources that subtend an angle less than 11 milliradians (mrad) at the viewer's eye, the TLV is determined from the blue‐light‐hazard‐weighted irradiance incident at the eye. The blue‐light‐hazard‐weighted irradiance multiplied by the exposure time should not exceed 0.01 J cm−2 for exposure time less than 100 seconds. For exposure time greater than 100 seconds, the blue‐light‐weighted irradiance should not exceed 0.1 mW cm−2.
4.1.2.2 Aphakic Hazard
The natural lens of the eye is highly UV‐absorbing, and together with the cornea serves to shield the retina from UV radiation. Aphakia, the absence of a lens, is a rare condition that may be a result of cataract surgery or a congenital defect. If the natural lens is not replaced with a UV‐absorbing artificial lens, harmful levels of UV may reach the retina and cause photochemical damage. The ACGIH has established a TLV for exposure of people with aphakia to radiation in the range 305–700 nm (20). The exposure limits for the aphakic hazard are the same as for the blue‐light hazard, except that the source radiance (or the irradiance at the eye, in the case of a small source subtending less than 11 mrad) is spectrally weighted by the aphakic hazard function A(λ) instead of the blue‐light hazard function B(λ). A(λ) is plotted in Figure 9.
4.1.2.3 Retinal Thermal Hazard
Thermal damage to the retina may occur when the rate at which optical radiation is absorbed and converted to thermal energy (heat) exceeds the rate at which heat can be dissipated. Heat is transferred by conduction to tissues adjacent to the portion of the retina on which the image of the optical source is formed. Because dissipation of heat takes time, a dose of optical radiation received in a very short period has greater potential to do thermal damage than the same dose received over a longer period. A small retinal image dissipates heat more efficiently than a large retinal image. The size of