 nents. The methods of assembly, integration and output for these measurements a photometer is used. The eye itself is a very limited components can create significant variations in performance. Such photometer; the only photometric determination it can make is instruments have a wide range of applications. They can be used to whether two adjacent white areas are or are not equally bright. In monitor and measure absolute spectral emission from sources and the past a number of photometers were developed which were based the transmission or reflectance of materials. In addition to this, on visual measurements namely the matching of luminances. How- recent advances in detector array technology has opened up the ever, such systems are slow and tedious to use and many instruments capabilities to the analysis of time dependent optical emissions have now been developed where a photocell replaces the eye as a where rapid acquisition of spectral data is required. detector. In photometric measurement the luminance and illuminance of 7.5.3 Spectroradiometers sources with any spectral content and spatial distribution are re- quired. The spectral response of the photometer must be matched Figure 7.6 (Ruff, 1991) illustrates the basic components of the closely to that of the human eye. In addition, the geometrical traditional spectroradiometer. Here the dispersing element is a dif- Instrumentation 7/7 Entrance slit ment is relatively slow, usually of the order of several minutes. This is because of the need to rotate the grating in discrete increments, Input optics where the detector captures the output at each position. An alternative configuration is shown in Figure 7.7 (Ruff, 1991) Grating which uses a linear array detector. There are two advantages to this type of design. There are no moving parts and the elements of the array are exposed simultaneously to the entire spectrum. Signal acquisition from the spectrum in this case can be as short as 10-20 milliseconds. In this type of instrument the type of detector which is av