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Figure 1. Emission spectrum: Atlas Model RM 65 Xenon arc lamp (with WG320 filter), showing UVR and visible light regions.

When describing the chemical and biological consequences of a relatively broad spectral band exposure, any reference to the total energy involved (e.g., irradiance in W/m2 or radiant exposure in J/m2) is of limited relevance. That is because the efficacy of photochemical interaction per incident quantum and the photobiological effects per unit radiant exposure vary substantially with wavelength. A quantitative plot of such spectral variation, usually normalized to unity at the most effective wavelength, is technically known as a "weighting function"; photobiologists frequently use the term "action spectrum".1'3

As one example, visual acuity is much greater at wavelength 550 nm than at 450. Thus, the weighting function for the part of the electromagnetic spectrum that activates the human retina ("visible light") assigns a greater value to photons around 550 nm. When the "photopic vision" weighting function is applied to the measured irradiance, the result ("brightness") is expressed as luminance (in lumen/m2, i.e., lux). In brief, a measure of W/m2 cannot tell us how bright an illuminated surface will look, but the number of lux can.

A second example involves ultraviolet radiation (UVR) and the skin response known as threshold erythema (the first perceptible pinkness following exposure to sunburning UVR). The radiant exposure producing the threshold response could be 200 J/m2 or 2000 J/m2, depending on the wavelength or mix of wavelengths involved. When the "erythema action spectrum" weighting function is applied to the measured or calculated radiant exposure, the result ("erythema effectiveness") is expressed as "exposure dose" in J/m2e (where J/m2e means "producing the response that would be associated with the same J/m at the peak of erythema effectiveness, wavelength 297 nm").4-5 In brief, a broad-spectrum measure of J/m2 cannot tell us whether a given surface dose will produce erythema in skin, but a figure of J/m2e can. (Note that under specific circumstances, "fluence" is preferable to "exposure dose". The circumstances include measurement of radiation, including back scatter, from all directions).1'6

The erythema-effective exposure dose (in J/m2e) may be calculated from a measured (unweighted) radiant exposure, or it may be estimated directly by a radiometer designed for the purpose (i.e., with a wavelength-dependent detector).

Wavelength-independent (thermal) radiometers incorporate detectors that have a flat spectral response over a wide range of wavelengths. Such thermal detectors operate on the principle that incident radiation is absorbed by a receiving element, and the temperature rise of the element is measured, usually by a thermopile or a pyroelectric detector.

Wavelength-dependent radiometers have spectral responses that vary widely depending on the types of detector and filters that may be incorporated. Detectors can be designed to have a spectral response that matches a particular action spectrum for a photobiological endpoint. A case in point is the Robertson-Berger (Solar Light™) meter which incorporates optical filters, a phosphor and a vacuum phototube or photovoltaic cell.7'8 This device measures wavelengths in the global UV spectrum with a spectral response that rises sharply with decreasing wavelength. The R-B meter has been used to monitor natural UVR continuously at several sites throughout the world, and also to monitor exposures to artificial sources of UVR in the laboratory.

2.3 Recording Exposure Data

The GLP environment favors on-line capture of radiation dosimetry data. Control CB™ is an example of accessory hardware and software that provides multi-channel display and recording of exposure data in real time. Virtually any type of electronic detector can be used with such software as part of a "virtual instrument" installation. In contrast, chemical actinometry has little to commend it in this application.

Spectroradiometry is used to characterize a source of UVR in terms of its spectral power distribution. The output, as indicated above, can be used to calculate photochemically or biologically weighted radiometric quantities. Emission spectra illustrated in this paper were determined with an Oriel™ multidiode array spectrograph linked to a computer running Instaspec II software; tabular output was imported into an Excel™ spreadsheet to generate presentation graphics.

2.4 Simulated Sunlight as a Source for PPS Testing

■ At least qualitatively, the xenon arc lamp is generally accepted as the best available source of simulated optical radiation.9 Optical filters, such as those produced by Schott™, can be used to attenuate the shorter wavelength emissions and thus mimic the result of terrestrial exposures from various solar angles.10 With a WG 320 filter of appropriate thickness, the xenon lamp provides UVR similar to mid-latitude noon summer sunlight, with good color balance in the visible spectrum (Figure 1). Without the WG 320 filter, the emission spectrum includes the entire far-UVR component characteristic of extra-terrestrial sunlight (Figure 2). With an added WG 345 filter (Figure 3), the UVR component is typical of that found in winter sunlight, or in sunlight through window glass.

Atlas RM 65 Xenon Arc Lamp at 2.25 Meters (with and without WG 320 filter)

Wavelength (nm)

Figure 2. Emission spectrum: Atlas Model RM 65 Xenon arc lamp (with and without WG320 filter; reading taken at 2.25 m from source).

Wavelength (nm)

Figure 2. Emission spectrum: Atlas Model RM 65 Xenon arc lamp (with and without WG320 filter; reading taken at 2.25 m from source).

Table 1. Watt-hour conversion to Joules (J). From this relationship the unweighted UVR needed to achieve the ICH "200 W-hr/m2" would be 720 kJ/m2.

Watt-hour conversion to Joules (J)_

(example 1: ca. 9 hours c. bank of BL (TL10) lamps at 80 kJ/m2 in 1 hour.)

(example 2: ca. 72 hrs at 2m from Atlas model RM 65)_

The ICH draft guideline for PPS11 (ICH Expert Working Group, 1997) expresses UVR and visible light exposures in terms of Watt-hours and lux-hours, respectively. For laboratory workers accustomed to using more conventional international units of measure, the Joule (J)

is the appropriate energy unit for at least the UVR component (Table 1), provided that no effectiveness weighting is required.

Atlas RM 65 Xenon Arc Lamp at 1 Meter (with WG 345 filter)

Wavelength (nm)

Figure 3. UVR Emission Spectrum: Atlas Model RM 65 Xenon arc lamp (with WG345 filter to restrict emission to near ultraviolet and visible regions; reading taken at 1 m from source).

Wavelength (nm)

Figure 3. UVR Emission Spectrum: Atlas Model RM 65 Xenon arc lamp (with WG345 filter to restrict emission to near ultraviolet and visible regions; reading taken at 1 m from source).

Table 2. Visible light intensities as a function of distance from Atlas RM 65 xenon arc lamp, compared with mid-day sunlight, shown with duration required to achieve ICH "1.2 million lux-hours".

Atlas 6.5 kW xenon arc compared with mid-day sunlight (40° N. latitude)

Table 2. Visible light intensities as a function of distance from Atlas RM 65 xenon arc lamp, compared with mid-day sunlight, shown with duration required to achieve ICH "1.2 million lux-hours".

Atlas 6.5 kW xenon arc compared with mid-day sunlight (40° N. latitude)

distance

ft-candles

lux

duration (for ICH 1.2 million lux-hr)

(meters)

(lumens/ft2)

(lumens/m2)

(hours)

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