The ACGIH and ICNIRP guidelines for protection of the lens of the eye from UV‐A radiation is based on irradiance or radiant exposure that is not spectrally weighted. Ideally, measurements of UV‐A for purposes of assessing this hazard should be taken with a detector having a flat spectral response from 315 to 400 nm; such a detector might not, however, be available in practice. If the source spectral distribution is known, a correction factor can be calculated for a UV‐A detector using Eq. (22) with s(λ) set equal to unity for the entire range 315–400 nm and set equal to zero outside that range. An evaluation of two commonly used broadband UV‐A detectors found that correction factors of 1.27–1.41 were applicable when measuring a UV‐A phototherapy source (41).
4.3 Alternative Assessment Methods
Alternatives to direct measurement of optical radiation may be needed when reliable measurement instruments are not available or when installation of a source is being planned or designed. Alternative methods include calculations of radiometric values and use of lamp classifications. Other source‐related guidelines, such as the UV Index (UVI) for solar radiation and shade numbers for welding, are discussed in Section 6.
4.3.1 Calculation of Effective Radiometric Values
Lamp manufacturers should be able to provide spectral distribution data on potentially hazardous lamp output in the range 200–1400 nm expressed as spectral radiant power, spectral radiance, spectral radiant intensity, or spectral irradiance (42). The spectral radiant exitance of a blackbody source such as a furnace can be calculated using Planck's formula (Eq. (9)). The spectral distribution of the source can then be weighted by the appropriate spectral weighting function for the biological effect of interest, yielding an effective radiant power, effective radiance, effective intensity, effective radiant exitance, or effective irradiance, as the case may be.
If the relevant exposure guideline is expressed in terms of effective irradiance at the exposed skin or eye, that effective irradiance can be calculated based on the spatial characteristics of the source and the geometry of exposure. For a point source (i.e. an isotropic source with dimensions that are very small relative to the distance r between the source and the exposed surface), the effective irradiance can be calculated from the effective radiant power or effective radiant intensity using the inverse square law (Eq. (6)). For a flat Lambertian source of area As where r is at least five times greater than the longest dimension of the source, the effective irradiance may be calculated from the effective radiance using Eq. (8). Calculation of irradiance from a cylindrical source, such as a low‐pressure mercury‐vapor tube used for a germicidal lamp or a “black light,” involves more complicated geometric considerations. An example of such a calculation may be found in the American National Standards Institute/Illuminating Engineering Society of North America (ANSI/IESNA) Recommended Practice for Photobiological Safety for Lamps and Lamps Systems – Measurement Techniques (43).
If the source data are reported in terms of spectral irradiance, the manufacturer should provide information on the geometric configuration at which the spectral irradiance was measured. If the measurement distance exceeded five times the longest dimension of the source, then the following version of the inverse square law can be used to calculate the effective irradiance at the exposed surface of the skin or eye, Esurf:
where rsurf is the distance between the source and the exposed body surface, Eref is the effective radiance under the measurement conditions, and rref is the measurement distance.
TABLE 2 Illuminating Engineering Society of North America Risk Group classification criteria (44).
| Hazard | Exempt Group | RG‐1 very low risk | RG‐2 low risk | RG‐3 high risk |
|---|---|---|---|---|
| Exposure limit will not be exceeded within a period of | ||||
| Actinic UV hazard | 8 h | 10 000 s | 1000 s | Fails RG‐2 criterion |
| UV‐A hazard | 1000 s | 300 s | 100 s | Fails RG‐2 criterion |
| IR‐cornea/lens hazard | 1000 s | 100 s | 10 s | Fails RG‐2 criterion |
| Retinal thermal hazard (arc sources) | 0.25 s | n/a | n/a | Fails Exempt Group criterion |
| Blue‐light hazard | 10 000 s | 100 s | 0.25 s | Fails RG‐2 criterion |
| IR‐retinal hazard, non‐lighting sources | 810 s | 10 s | n/a | Fails RG‐1 criterion |
h, hour; s, second.
4.3.2 Lamp Classifications
Radiation‐emitting devices, including optical radiation sources, are subject to regulation in the United States by the U.S. Food and Drug Administration (FDA) through its Center for Devices and Radiological Health. The FDA has not established a classification system, analogous to its laser classification system, for noncoherent optical radiation sources.
In the absence of governmental regulations requiring safety‐related classification of all lamps, the IESNA has published a voluntary guideline, ANSI/IES RP‐27.3, Recommended Practice for Photobiological Safety for Lamps and Lamps Systems – Risk Group Classification and Labeling (44). This recommended practice applies to all electrically powered sources emitting radiation between 200 and 3000 nm, except for LEDs used in fiber optic communications systems and lasers. Laser‐driven broadband light sources are included under the standard.
Under the RP‐27.3 recommended practice, the lamp manufacturer should evaluate the lamp for potential to exceed exposure guidelines for UV hazards, blue‐light hazard, IR‐corneal/lens hazard, and retinal thermal hazards. The spectral weighting functions and exposure limits are similar to current ACGIH guidelines (13, 18). An exception is the retinal thermal hazard for IR sources with weak visual stimulus, where the RP‐27.3 recommended practice uses an unweighted radiance.