546 Table 666 



PHOTOGRAPHIC EFFECTS OF X RAYS 



X rays affect a photographic plate (or film) in much the same way as does light, except 

 that that part, Dx, of the photographic density which is due to the radiation depends on 

 radiation intensity, I, and the time of exposure, /, in a simpler way. The relation for mono- 

 chromatic X rays is 



Dx = D — Do = k(i—e-*H), o<D<4 (O 



where e is the base of natural logarithms, and k and a are constant for a given plate and 

 given X-ray wave length. D and Do are the photographic densities of the exposed and 

 unexposed parts of the plate. These densities are measurable with a photometer, photo- 

 graphic density being defined as the common logarithm of the reciprocal of the transmission 

 T, i.e., density = logic (i/T), where T is the ratio (transmitted light)/ (incident light) for 

 a beam of light normal to the developed plate. 



The limits of applicability of formula (i) probably depend on the characteristics of the 

 plate used and on the development, as well as on the wave length of the X rays. Experi- 

 mental tests indicate that the relation holds within a few per cent up to a density of at 

 least 4, for X-ray plates, if the plate is fully developed and if the wave length of the 

 X rays is between 0.4 and 1.1 Angstroms. Effects due to intermittency, and to the failure 

 of the reciprocity law are negligible in the X-ray region. 



Smithsonian Tables 



