ailable Exit slit in array form may not be as sensitive as say a photomultiplier tube. However, with detector cooling, quite acceptable performance can Mirror be achieved for many applications. Many array based instruments use a 256 pixel array to cover a region from 400nm to 600nm in wavelengths. Such a configuration results in about a 2nm resolution. Focusing Detector Signal Arrays at 1024 elements are available which can achieve about a processing 0.6nm resolution. electronics 7.5.4 Optical power meter Figure 7.6 Tradional spectroradiometer. (Grating rotates The term optical power meter is normally applied to an instrument to illuminate detector in discrete steps, wavelength by without a dispersing element and with a broad band sensitivity. wavelength) Typically such an instrument would be used to measure power or energy from a narrow band source such as a laser. Using a range of fraction grating which is rotated to illuminate the detector in dis- calibrated detector probes which would be typically pyroelectric crete steps. The function of the instrument can be broken down into silicon and thermopile it is possible to measure both energy and steps: Input-collection optics to assure proper collection geometry, power from pulsed and CW lasers from the UV to the far infrared. the dispersing element to break the light flux into its spectral constituents, a detector to measure the amount of flux and a means 7.5.5 Interferometers of processing and displaying the output signal. Important specifica- tions which will need to be assessed for a specific application will An interferometer is an instrument which can be constructed to be the wavelength range of the instrument and the resolution (the smallest resolvable spectral element which the instrument can de- measure changes in distance, refractive index or wavelength by tect). These will be determined by the slit width and the specifica- comparing the relative phase differences between two or more tion of the grating in use, plus the type of detector employed in the b