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

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СКАЧАТЬ of the incident radiation. Correction factors must, therefore, be developed that are specific to the exposure conditions to be assessed. If the relative spectral response function D(λ) of the broadband detector and a spectral distribution function I_λ of the optical radiation source (representing the normalized spectral intensity, spectral irradiance or spectral radiance) are known, then a correction factor f may be calculated as

(22)

      where s(λ) is the spectral weighting function for the biological effect and the summation is over the wavelength range to which the detector responds. It should be recognized that the detector response might not cut off sharply at the limits of the spectral weighting function for the biological effect.

Some practical difficulties with calculating correction factors should be noted. The source spectral distribution function must be obtained from the source manufacturer, and the detector response function must be obtained from the detector manufacturer. The functions s(λ), D(λ), and I_λ may each range over several orders of magnitude. Spectral features such as the tails of a curve, which might appear small when a spectral distribution or response function is plotted on a linear scale, may become important when multiplied by a high value in another function. For accurate calculations, spectral data should be obtained from the manufacturer in numerical form if possible. If numerical data are not available, semilogarithmic plots of spectral distribution or response function can be used to evaluate small peaks, tails, continuum levels, and other features. It should also be recognized that, due to variations in the manufacturing process, the spectral distribution function for the particular source that is actually present in the workplace may differ from representative data provided by the manufacturer. Shifts in wavelengths of emission peaks between the actual and the reported spectral distribution functions may affect the correction factor if a shift coincides with a spectral region where s(λ) or D(λ) is changing rapidly with wavelength.

      An alternative approach for determining correction factors for a broadband detector is to compare effective irradiance measurements from that detector to effective irradiance measurements obtained simultaneously using a more accurate method. This may be the only practical approach when the radiation source is not a manufactured lamp for which a spectral distribution function can be obtained. The correction factor will be affected by conditions that modify the spectral distribution of the source. For example, when polysulphone films used to measure erythemal effective dose from ambient solar UV radiation were calibrated against electronic radiometers and dosimeters that had spectral responses well matched to the erythemal reference action spectrum, the calibration curves showed variability related to atmospheric conditions that altered the ground‐level solar UV spectrum (39). Erythemal dose measurements of solar UV determined from polysulphone films are vulnerable to spectral variability in the incident radiation because the action spectrum for polysulphone dosimeters does not reproduce the shape of the erythemal reference action spectrum at wavelengths longer than 300 nm, where most solar UV occurs. Spectral mismatch effects in measurement of solar UV were also seen, to a lesser extent, in an assessment (40) of electronic dosimeters that used an aluminum gallium nitride photodiode detector with a response function that approximates the CIE erythemal reference action spectrum (32); the dosimeters, which had been calibrated in New Zealand in summer under high solar UV conditions, showed deviations of up to 30%, compared to the erythemally weighted irradiance determined from reference spectroradiometer measurements, when tested in northern Germany in autumn (40).

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 A_s 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, E_surf:

(23)

      where r_surf is the distance between the source and the exposed body surface, E_ref is the effective radiance under the measurement conditions, and r_ref is the measurement distance.

TABLE 2 Illuminating Engineering Society СКАЧАТЬ