336 T. V. Shcherbakova 



The coefficient of loss of contrast was determined from formula (3). 

 Determination of the coefficient of loss of contrast was conducted with 

 the same samples of clay suspension with which the transmission of light was 

 determined; the layer of clay suspension was obtained in the same way. 

 Measurements were made with monochromatic light of wavelength varying 

 within the limits of 430 to 730 m//. 



Curves shomng the dependence of the coefficient of loss of contrast K 

 on the thickness of the layer of suspension h for various wavelengths A 

 of light were constructed from the results of the measurements (Fig. 5). 



As can be seen from Fig. 5, the contrast falls sharply v,dth increase in 

 thickness of the layer and in specific gravity of the clay suspension. The 

 contrast also decreases, although comparatively slightly, "with decrease in 

 the wavelength of light. In practice variation of contrast with variation of 

 wavelength of the light can be neglected over the visible region of the spectrum. 



Curves of the dependence of K on h, when both quantities are plotted 

 on logarithmic scales, are quite close to straight lines and, consequently, 

 the dependence of K on the thickness of the layer h can be represented in 

 the following manner: 



K = Ch-"" (4) 



The coefficient m is approximately constant and equal to 0.54; the 

 coefficient C depends on the specific gravity of the clay suspension samples 



C=-7-^ (5) 



{y-l)P 



For the clay suspensions, on which the investigations were carried out, 

 p = 0.7 and D = 2.5. 



For clear reproduction of an object it is most important in photography 

 to convey its contrasts. However, in the passage of light rays through the 

 layer of clay suspension a reduction of contrast is observed. Loss of contrast 

 is the main factor obstructing the possibility of photography. Therefore 

 in examining the possibility of photography in a clay suspension it is first 

 of all necessary to get an idea of the loss of contrast. 



Starting from the data obtained, one must determine the permissible 

 thickness of the layer of drilling fluid between the viewing window of the 

 camera and the wall of the well, and the permissible turbidity of the drilling 

 fluid (Table 2). 



The main factor in the sharpness of the image is the resolving power of 

 the system, which specifies the number of lines in 1 mm that can be 

 distinguished on the photographic film. 



