Patty's Industrial Hygiene, Physical and Biological Agents. Группа авторов

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      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 the retinal image depends on the angle subtended by the radiant source at the viewer's eye. 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 L_R (20):

      (15)

      where L_λ 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 DL_R, 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 L_R, 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):

(16)

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

(17)

      for t less than 1000 seconds, where the spectral irradiance E_λ 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)СКАЧАТЬ