**6. Spectroradiometry**

One of the basic characteristics of a radiant source is the spectral distribution of the radiant output. As we have seen, even the accurate determination of photometric and colorimetric quantities requires knowledge of the spectral distribution of the radiation. The spectral distribution of the radiation in each of the geometrical quantities that we have discussed may be determined. Since the spectral techniques will be similar in each case, we will illustrate the methods using spectral irradiance as the example. In addition, since spectral irradiance can be used for a large majority of the calibration requirements, it is the most commonly available facility in a laboratory. Several references available for more information concerning spectroradiometric measurements are (CIE 063-1984, Grum & Becherer, 1979, Kostkowski, 1997).

The basic instrument for spectral measurements is a spectroradiometer, which is an instrument for measuring radiometric quantities in narrow wavelength intervals over a given spectral region (CIE S017, 2011). An example of a spectroradiometer measurement configuration for spectral irradiance measurements is shown in Figure 15. The monochromator shown is a grating instrument with a prism predispersor input to remove the high-order wavelengths that would be passed by the grating components. There are two detector output ports with manual switching that allows several wavelength regions to be measured during one measurement set. Detectors such as photomultiplier tubes (PMT), and Si, Ge, InGaAs and InSb photodiodes are available to allow measurements covering the wavelength range from 200 nm to 2500 nm. Various manually interchangeable gratings with different dispersions and blaze are available for use in measuring the same wavelength range. The purpose of the conical radiation trap is to remove radiation emitted in the back direction from scattering back into the input. The aperture defines the spatial region at the spectral irradiance source from which radiation is accepted at the input to the integrating sphere.

Fig. 15. Spectroradiometer and spectral irradiance measurement

A spectroradiometric measurement system may be considered to be composed of seven basic components (Figure 16): i) the radiation source, ii) the input optics that couples the desired radiation into the monochromator, iii) the monochromator, iv) the output optics that couples the radiation from the monochromator into the detectors, v) the detector, vi) a measurement control system, and vii) some signal processing equipment.

Fig. 16. General configuration for spectroradiometric measurements

A spectroradiometric measurement system may be considered to be composed of seven basic components (Figure 16): i) the radiation source, ii) the input optics that couples the desired radiation into the monochromator, iii) the monochromator, iv) the output optics that couples the radiation from the monochromator into the detectors, v) the detector, vi) a

monochromator

optics focusing optics

aperture dispers er

Measurement Control

Data Processor

computer

output optics

detector

Fig. 15. Spectroradiometer and spectral irradiance measurement

input

optics optical

radiation source

measurement control system, and vii) some signal processing equipment.

limiting

collimating

Fig. 16. General configuration for spectroradiometric measurements

For simplicity, in Figure 16 and other figures, lenses (refractive elements) are used to show the focusing of radiation. In most cases, mirrors (reflective elements) are preferable since they will focus all wavelengths of radiation to the same physical spot, whereas lenses do not. For example, the monochromator in Figure 15 uses mirrors in the collimating and focusing optics.
