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Analyzing the Spectral Exposure

Spectral exposure relates to the range of wavelengths to which a surface is simultaneously exposed. Spectral exposure can be presented in a graph, chart, or a table.

The spectral distribution may be expressed in absolute terms, for example, mW/nm, in which the irradiance – usually the peak irradiance – is analyzed into 1 nm divisions or grouped into wider wavelength ranges, for example, 10 nm groups. Instruments that have pre-set bandwidth or selectable band integration will measure over different ranges. This can make comparisons with different instruments difficult.

There are three methods of determining the spectral exposure received by a surface.

1. Spectral emission of the bulb. This data is provided by the bulb manufacturer, and may be expressed in total watts output, in 1 nm or 10 nm divisions. Usually this data requires the use of a calibrated monochromator – equipment not often found outside of the bulb manufacturer. But, because these instruments can measure the very short (200-240 nm) ranges, and can be calibrated for total power output, they provide some information that is not available from other instruments. This data can be used to compare the emission of one bulb to another. It should be noted that this provides NO INFORMATION on irradiance, peak or profile, as those are a function of the optics (reflector) of the lamp.

By applying the spectral reflectance data for reflectors to the emission data, a good relative spectral distribution can be obtained. This can be used for spectral irradiance calculations for in-band and out-of-band of a radiometer with a known responsivity.

2. Spectroradiometers. These are miniature monochromators, and can make only static measurements – essentially at a single location. They can be calibrated to report irradiance in very fine (½ nm or 1 nm) divisions. Because these instruments require an integration time to record their measurements, they are not used for measuring the irradiance profile of exposure – only the spectral content of irradiance.

3. Multi-band filter-detector integrating radiometers. Radiometers that have a precisely defined set of wavelength bands can be used to evaluate the spectral distribution of exposure. Filter-detector instruments may have bandwidths that range from 10 nm to 100 nm. It is very important to know the center wavelength (CWL) and the bandwidth of any instrument used. These are dynamic instruments (pass through the exposure zone) so can record the irradiance – peak or profile depending on the instrument – and the energy delivered within each of its wavelength bands.