ation in sensitivity when calibrating the instrument in which the detector is used. Alternatively, a thermopile, bolometer or pyrometer might be used. The luminance or brightness is therefore constant with respect to Such detectors generate signals which can be related to the incident 0. The brightness of a diffuse source is the same, regardless of the power as a result of a change in temperature which is caused by angle from which it is viewed. absorption of the radiant energy. These detectors have an advantage that their response as a function of wavelength is almost flat (con- stant with wavelength) but are limited to the measurement of rela- 7.5 Instrumentation tively high intensity sources and normally at wavelengths greater than 1 um. A wide variety of commercial instruments is available for carrying The choice of a radiometer is usually defined by its application. A out optical radiation measurements. The terminology used to de- spectral radiometer is an instrument which disperses optical radia- scribe them can be confusing although in many instances the basic tion that is to say it employs an 'element' which is capable to some component make up is in fact very similar. extent of separating the constituent wavelength components of a polychromatic light beam. The 'element' referred to will be a prism 7.5.1 Photometers or diffraction grating. The instrument then analyses the radiation as a function of wavelength using one of the detector types described. From the definition of photometric units and its relation to eye A long list of names have been given to such instruments. Most response, photometric measurements will be appropriate when commonly found are spectroradiometers, spectrophotometers, evaluating a light source or lighting from the point of view of its spectrometers, spectrum analysers, spectrocolorimeters. effect on human vision. Typical measurements could be on room In essence all of these instruments employ similar basic compo- lighting, panel light indicators or warning light systems. For such