GENERATION, CONTROL, AND MEASUREMENT 199 



Sinsheimer and Loofbourow, 1947). The radiant flux is scattered from 

 the beam except at those wave lengths where the soHd and hquid have 

 the same refractive index ; radiant flux of this wave length passes through 

 the cell undeviated, and the mixture appears optically clear. At other 

 wave lengths the degree of scattering increases with the difference in 

 refractive index between solid and liquid. Since a small part of the 

 scattered energy always appears in the main beam, the general back- 

 ground transmission is appreciable. By using several filters in tandem, 

 with sufficient separation between them, the background can be reduced 

 to a low value. The refractive indexes of organic liquids have high tem- 

 perature coefficients; consequently the temperature must be controlled 

 very precisely. The region of maximum transmission can be shifted 

 slightly b}^ changes in temperature; large changes in wave length are 

 obtained by selecting different solid and liciuid mixtures. 



A very simple selective-scattering filter is used in infrared spectroscopy 

 to scatter visible and near-infrared energy out of the main beam. A 

 layer of very small particles of a transparent material such as powdered 

 quartz or zinc sulfide (Henry, 1948) on the surface of a rock-salt window 

 will scatter the shorter wave lengths but have little effect upon energy 

 of wave lengths much longer than the dimensions of the particles. A 

 diffraction grating can be used in a similar manner (White, 1947). 



2. Interference filter. Wave interference within thin films of optical 

 elements forms the basis for the selective spectral characteristics of the 

 interference filter. Energy absorption within the filter components plays 

 no constructive role in determining the spectral properties; it serves only 

 to attenuate the peak transmission or reflection. Since the undesired 

 regions are removed by reflection or transmission and are not directly 

 absorbed by the filter, heating from high-intensity sources is often less 

 of a problem than with the selective absorption filters. 



The four most common types of interference filters include (1) the 

 Fabry-Perot etalon filter of alternate dielectric and semitransparent metal 

 films, (2) the single or multiple dielectric-film filter, (3) the frustrated 

 total-reflection filter, and (4) the Lyot birefringent filter. All these 

 have been reviewed by Greenland (1952) and Turner (1950). 



The Fabry-Perot filter consists of a sandwich of two semitransparent 

 films of metal, usually silver, separated by a thin film of a dielectric such 

 as magnesium fluoride (Fig. 3-21). It derives its name from the Fabry- 

 Perot etalon interferometer, which consists of partially reflecting silver 

 mirrors separated by a thin air space, and the same basic ec^uations apply 

 to both the interferometer and the filter. To form the filter, a silver or 

 aluminum semitransparent film is evaporated in a vacuum on a plane 

 glass surface. Then a transparent dielectric film is deposited, followed 

 by a second transparent metal film. A protective cover glass is next 

 cemented on the surface. The thickness of the dielectric spacer film must 



