Double‐monochromator spectroradiometers are considered the “gold standard” for measurement of the spectral distribution of optical radiation. Unfortunately, double‐monochromator spectroradiometers are not suited to field industrial hygiene measurements because of their large size, delicate components, and time necessary to scan through the entire spectral range of interest in narrow bandwidth steps. Single‐monochromator diode array spectroradiometers (SMDASs) are portable and relatively inexpensive instruments that can measure radiation in multiple narrow bandwidths simultaneously. Performance criteria have been established for SMDASs for the measurement of solar UV (22). Numerical corrections for stray light may be applied to reduce spectral measurement error (24, 25), but in some cases, such corrections may yield anomalous results on the UV region (25, 26). Corrected SMDASs should be validated against double‐monochromator spectroradiometers using sources that are representative of those in the workplace (24).
4.2.2 Broadband Detectors and Dosimeters
Portable, easy‐to‐use broadband detectors for optical radiation are widely available. Electronic broadband detectors for UV, visible, and IR‐A radiation typically use a semiconductor photodiode that produces a current proportional to the photon flux at a given wavelength. The spectral responsivity of a photodiode may be defined as the ratio of current generated to radiant power incident on the detector. Based on Eqs. (1) and (2), the incident power is proportional to the photon flux and inversely proportional to the wavelength. The spectral responsivity also depends on the inherent quantum efficiency of the device, which is a function of λ, and on the spectral transmittance of any filters and input optics. Some UV detectors include a phosphor that absorbs UV and emits visible photons that are detected by the photodiode.
Detectors are commercially available with relative spectral responses that approximate the ACGIH relative spectral effectiveness function S(λ) for UV, the CIE erythema reference action spectrum Ser(λ), the ACGIH blue‐light hazard function B(λ), and the ACGIH retinal thermal hazard function R(λ). Photometers, which are used to measure illumination levels, have a spectral response matched to the photopic luminous efficiency function. Unweighted IR irradiance may be measured using thermopile or pyroelectric detectors, which give a nearly uniform response over a wide spectral range.
Performance characteristics for broadband UV radiometers have been described by the World Meteorological Organization for measurement of solar erythemal radiation (23) and by the CIE 220:2016 guideline for measurement of artificial sources (27). Radiometers with broadband detectors are typically calibrated to read out directly in irradiance units such as W m−2. Integrating radiometers can be set to measure radiant exposure received over time. An evaluation of the directional response of broadband UV detectors from seven different manufacturers with four different main types of input optic found that raised polytetrafluoroethylene (PTFE) dome diffusers showed the best directional response compared to recessed PTFE diffusers, quartz diffusers, or no diffuser (28). The raised PTFE diffusers had directional errors of 4–10% relative to an ideal cosine response. Modeling suggests that optimized raised planar diffusers and dome diffusers can have directional errors less than 2% (29), and some commercially available diffusers are reported to have directional error in this range. UV sources that subtend an angle greater than 80° need only be measured over a field of view of 80° (15). For assessments of retinal hazards, measurements of source radiance should be made using a detector equipped with input optics that narrow the field of view to 0.011 rad (30).
Personal UV dosimetry methods used in research on occupational or community UV exposure include small electronic dosimeters, UV‐sensitive chemical films such as polysulphone (34) and poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) (35), and UV‐sensitive biofilms (36, 37). A wearable sensor patch has recently been developed that uses UV‐induced color change in a dye, which is read using a smartphone app, to measure personal erythemal dose (38). Electronic personal dosimeters or dataloggers with spectral response approximating the CIE erythemal or ACGIH/ICNIRP UV‐hazard functions are commercially available.
4.2.2.1 Spectral Corrections for Broadband Detectors
Significant errors in measurement of effective irradiance or dose can result when the spectral response of a broadband detector is not well matched to the spectral weighting function for the biological effect of interest. Under these circumstances, a broadband detector that is calibrated using a monochromatic calibration source will give inaccurate readings when measuring broadband radiation. The deviation of the broadband detector reading from the true effective irradiance depends on the spectral distribution 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 Iλ 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
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 Iλ 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