92 INSTRUMENTATION 



between the refractive index of the dielectric forming the conjugate area 

 and the refracti\'e index of the dielectric forming the complementary 

 area, the smaller the thickness of the coating need be. With the tech- 

 nics generally used, some dielectric films which are evaporated on glass 

 tend to scatter and peel if the optical path through the film exceeds 

 1 ju. If materials are so chosen that a very thin e\'aporated layer of 

 dielectric substance produces an optical path difference of, say, 3^ wave- 

 length, the thickness of the deposition during the evaporation must be 

 more carefully controlled than if a greater thickness of material produces 

 the same optical path difference. Whenever a thin film is deposited on 

 a support that has a refractive index different from that of the film, or 

 if several films Avith different refractive indices are superimposed on a 

 support, then multiple reflections of light take place at all interfaces 

 at which there is a change in index of refraction. These multiple re- 

 flections are accompanied by interference phenomena which cause the 

 transmitted light to be colored. The color and its intensity will depend 

 on the thickness of the films, the refractive indices, and the order of 

 succession of the films if there are more than one. The superposition 

 of metal and dielectric films on the diffraction plate can result, therefore, 

 in a broad-band interference filter. For a given choice of materials, 

 such color effects can be minimized by properly arranging the order of 

 deposition of the films. If, for example, magnesium fluoride and Inconel 

 are to be superimposed and deposited on glass and if air is the other 

 dielectric medium adjacent to the coating, then least color is introduced 

 if the magnesium fluoride is evaporated directly on the glass and the 

 Inconel is deposited on top of the magnesium fluoride. Introducing 

 color by forming an interference filter on the diffraction plate is generally 

 not considered favorable. 



Plastic films are a versatile medium for making diffraction plates. 

 Thin films (e.g., about 6 jj. thick) can be produced by using a doctor blade 

 to spread a layer of the plastic in solution uniformly over a smooth, flat 

 surface such as a glass plate and allowing it to dry. Obviously, dif- 

 ferent thicknesses of the layer of the solution and different concentra- 

 tions of the solution will dry as films of different thicknesses. A punch 

 and die or a cutting blade can be used to form a section of the film in 

 the shape of either the complementary or the conjugate area. The 

 plastic sections are then cemented between two supporting surfaces such 

 as flat glass plates or lens components. The thickness of the plastic 

 film, the index of refraction of the plastic, and the index of refraction of 

 the cement can be changed according to Eq. 2.7 in order to introduce 

 the necessary optical path difference between the conjugate and com- 

 plementary areas. Dyes can be incorporated into the plastic solution 



