A study is made on the illumination of viewing screen for radiography by the theory of vision. It is concluded that.
1) The min. discriminating difference of uniform objects which can be appreciated in radiograph decreases with increasing film density, film constant and illumination of the view-ing screen.
2) The subjective sharpness depends on the illumination and the film density or distribu-tion of densities, is almost always better than physical sharpness, and rises with decreasing illumination of the viewing screen.
3) The max. value of subjective contrast is influenced by the phenomenon of irradiation, and depends on the illumination and the film density, rises with decreasing illumination of the viewing screen.
4) The subjective contrast rises, if the shortwave-length light of the viewing screen is cut off by filter up to approximately 500mμ.
5) It is to be desired that the intensity of illumination of the viewing screen for radio-graphy be widely adjustable.
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