THE DIFFRACTION PLATE 87 



dilTraction plate or series of diffraction plates to be incorporated into a 

 given objective. At times it may be necessary to provide a diffraction 

 plate that does not belong to the category of general-purpose diffraction 

 plates in order to help solve a special problem. 



2. THE DIFFRACTION PLATE 



The first part of this section will describe briefly methods for producing 

 a diffraction plate in which the x'alue of the optical path difference be- 

 tween the conjugate and the complementary area is established for a 

 given wavelength and remains fixed, and in which the value of the energy 

 transmission of the conjugate area relative to that of the complementary 

 area is also fixed. Apparatus that permits a gradual variation of either 



Fig. III.2. Schematic drawing for tlie design of a noii-absorbing diffraction plate. 



the optical path difference or the relative energy transmission or both 

 by making use of the properties of polarized light will be discussed in 

 Section 9. 



Suppose that a diffraction plate is to be made with an optical path 

 difference equal to some fraction rf of a wavelength between the conjugate 

 and the complementary area, and suppose that there is to be no absorp- 

 tion in either area. It is simplest to delineate the case in which materials 

 are to be deposited on a support such as a flat, parallel, homogeneous 

 glass plate in order to produce the conjugate and complementary areas. 

 A design for a non-absorbing diff'raction plate is shown schematically in 

 Fig. III. 2. A thickness ti of material with a refractive index Ui forms 

 the conjugate area. Over the complementary area is a thickness ^2 of 

 a second dielectric material which has a refractive index M2. The coat- 

 ings on both areas are surrounded by a medium which has a refractive 

 index n. The optical path difference between the conjugate and com- 

 plementary areas is 



(ni - n)h - (no - n)t2 = d\. (2.1) 



It is usually most practicable to choose materials that will allow the 

 condition ti = (2 = t to he satisfied when the required optical path 

 difference is introduced. Here it is convenient to take the point of view 

 either that the embedding material with refractive index n becomes the 

 material that forms the complementary area and /i2 = n, or that, if a 



