GENERATION, CONTROL, AND MEASUREMENT 201 



of the transmission band (Bausch & Lomb Optical Co., 1953; Buc and 

 Stearns, 1950; Greenland, 1952; Greenland and Billington, 1950). 



The reflection filter consists of two or three metal films separated by- 

 dielectric films, the first metal film on the glass forming a total-reflecting 

 mirror. The others are semitransparent, as in the case of the trans- 

 mission filter. These filters have characteristics similar to those of the 

 transmission filter. 



The dielectric-film filters yield band-pass filters, neutral filters, and 

 beam splitters. One or more films of transparent dielectric materials, 

 such as magnesium fluoride or zinc sulfide, are evaporated onto the optical 

 surfaces. Single films of thicknesses sufficient, in relation to the wave 

 length, to produce interference of the reflected rays are used as anti- 

 reflection coatings on optical components, such as telescope and camera 

 lenses. 



Multiple films of appropriate thickness and refractive index produce 

 band-pass filters of high peak transmittance. They are especially useful 

 for obtaining complementary colors (the transmitted ray is the comple- 

 ment of the reflected ray) and for removing broad regions from the radi- 

 ation of the main beam. "Cold" mirrors for projection equipment can 

 be made with high reflectance in the visible and low reflectance (high 

 transmittance) in the infrared. They do not have to dissipate so much 

 heat as glass filters, since the infrared is transmitted out the back of the 

 projection lamp. 



The frustrated total-reflection filter is a dielectric film deposited on the 

 hypotenuse of a total-reflecting prism. This system eliminates the metal 

 film of the Fabry-Perot filter. The incident rays entering a reflecting 

 prism are normally totally reflected at the hypotenuse if they are incident 

 at an angle greater than the critical angle. If a dielectric film with a 

 high refractive index and of the right thickness is deposited on the reflect- 

 ing surface, the rays penetrate into this layer and are partially trans- 

 mitted instead of being reflected; hence the name "frustrated" total- 

 reflection filter (Bilhngs and Pittman, 1949). These filters can be 

 designed for any spectral region where transparent materials are available. 



The birefringence, or polarization, filter was developed for the study of 

 solar prominences, where a very narrow band pass of less than an ang- 

 strom unit was needed to isolate the hydrogen line of the prominence 

 from the continuous background of the solar disk. This filter is the most 

 selective of all filter systems. A birefringent, or double-refracting, crys- 

 tal placed between crossed polarizers produces a "channel" spectrum in 

 which evenly spaced bands of wave lengths are missing. By the proper 

 selection of a series of alternate birefringent plates and polarizers, all but 

 a few widely separated pass bands are canceled out. These filters con- 

 sist of many expensive precision components and require precise collima- 



