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HANDBOOK OF PHOTOGRAPHY 



arrangement of split-beam illumination. In Fig. 15, "two beams of light enter the 

 instrument through two circular apertures, and 0', each approximately 6 mm. in 

 diameter. Both of these beams are polarized by means of the Wollaston prism A, 

 which splits the light into two components, one of which is polarized in a plane per- 

 pendicular to that of the plane of polarization of the other. On the upper face of the 

 Wollaston prism is cemented a Fresnel biprism which forms the photometric field. 

 The analyzing prism C is of the Nicol type. The lens Li cemented to the lower face 

 of the Wollaston prism is a field lens, while L2 is the eyepiece lens. The analyzing 

 prism C is supported so that it may be rotated about the optical axis of the instrument, 

 its orientation being indicated by the index / reading on a scale *S, which remains in a 

 fixed position relatiA^e to the Wollaston prism A. The photometric field as seen by 

 the eye placed at the exit pupil of the ej^epiece is circular in shape, divided along a 



Fig. 16. — Section diagram showing construction of Martens polarization densitometer. 

 The essential parts are: 1, lens for forming an image of subject, M at apex of biprism; M, 

 light source diffusing plate; N , analyzing Nicol prism; P, photographic plate to be measured; 

 p, prism for reflecting light from M to eyepiece; q, prism for reflecting light from M through 

 photographic plate, P, to eyepiece. 



diameter bj^ an image of the apex of the biprism B. The field thus consists of two 

 juxtaposed semicircular areas, the relative brightness of the two being controlled by a 

 rotation of the analyzer C. One half of this photometric field is illuminated by light 

 which enters the aperture 0, while the other is illuminated by light which enters the 

 aperture O'. If the densities of the two beams entering the instrument are equal, the 

 two halves of the photometric field will be of identical brightness when the index / 

 reads at 45 degrees on the scale S or at a similar position in each of the other three 

 quadrants of the graduated scale S. If an absorbing material is placed over one of the 

 apertures (either or 0') the two parts of the field will no longer be of equal brightness 

 but by rotating the analj^zer C the brightness can be restored. The relative brightness 

 of the two parts of the photometric field for any specified angular relationship between 

 the position of the analyzer and the polarizer may be computed by means of the 

 tangent-square law. Hence for any setting which has been made to equalize the 

 brightness of the two fields, after the insertion of the photographic density in one of 

 the two beams, the magnitude of that density may be directly computed."^ 



1 Jones, L. A., "Photographic Sensitometry," Eastman Kodak Co. (1934). 



