eams through changes in an interference pattern. The coherence output optics. Also, knowledge of the total beam angle or solid angle property of laser light leads to many interference related applica- of emission of the source under investigation is important since this tions. Laser interferometry (usually with He-Ne lasers) is used for parameter could determine the necessary field of view for the measuring the flatness, physical dimensions of objects and also measuring instrument. Scanning a spectrum with this type of instru- distance between objects. The dispersive action of the diffraction grating used in the spectroradiometer is due to multiple beam Entrance slit interference. For resolving power larger than that provided by a grating instrument the Fabry Perot etalon can be used. This is a Fabry Perot Interferometer with a fixed separation between its Input optics plates, see for example Hecht and Zajac (1974). Such an instrument is frequently used to examine the detailed structure of spectral lines and laser emissions. Figure 7.8 shows a set up for what is called the Grating central spot scanning technique. It is applicable when only a few wavelengths are present in a source SO that preliminary separation can be accomplished with a filter or grating. Light is collimated by lens L1 and after passage through the etalon is focused by lens L2. In the focal plane of lens L2 a circular fringe pattern is seen. A pin-hole is positioned in front of the detector in the centre of the pattern such that the flux density of the centre spot of the pattern is Array recorded. Purely monochromatic light generates a particular circu- detector lar fringe system. However, if two such monochromatic compo- nents were present then two superimposed ring systems would result. The etalon is commonly scanned by varying the plate separ- Signal ation. The condition for an interference maximum is as in Equation acquisition 7.6, where m is an integer called the order of interference, a is the wavelength, d is the plate separation and n is