TYPE OF FILM As indicated in Figure 9-7 diffraction lines 



AND ITS POSITION (rings) can be produced over the entire region 



IN CAMERA from 0° to approximately 175° of the 20 angle. 



Some materials such as metals and inorganic 

 compounds produce patterns ranging over the entire region from 0° to 175°, 

 whereas other materials as organic compounds produce their entire visible 

 pattern at relatively small angles. Certain sections of the region from 0° to 175° 

 may be selected for detailed study, as for example in back-reflection work 

 or studies where extreme accuracy is involved, when the region from 130° to 

 175° is used. Consequently, the type of camera and film selected depends upon 

 the objective of the investigation. 



Most, if not all, X-ray film emulsions presently available were developed 

 primarily for radiographic work and consequently have a rather high degree 

 of contrast to reveal relatively small differences in absorption by the materials 

 studied. For diffraction work, especially for studying mixtures, a film showing 

 a straight-line function with a moderate slope over a considerable range, when 

 the line density is plotted against exposure, is desirable. Thus the choice of 

 film rests on a number of conditions. For rapid and only approximate identi- 



Bc-ck reflection , 

 film 



Flat film 



Cylindrical film-' 

 Figure 9-7. Film positions in various cameras used for powder studies. 



160 



