564 



TABLE 605.— COMPARISON OF NUCLEAR AND OPTICAL EMULSIONS 



Nuclear track plates differ markedly in physical composition and general characteristics 

 from the ordinary photographic materials (optical type) as shown in the table, where a 

 number of properties of optical and nuclear emulsions are compared. 



Property Optical type Nuclear type 



AgBr: gelatin (wt) 47:53 80:20 



AgBr: gelatin (vol) 15:85 45:55 



Grain diameter 5 to 3/t 1 to Ay. 



Emulsion thickness lOfi 25 - 300/i 



Emulsion wt mg/cm s 2-4 10-80 



Sensitivity to light Very high Low 



Response to a-particles High Individual tracks 



Response to /3-particles Moderate Individual tracks 



Response to 7-rays Low Very low 



TABLE 606.— RESOLVING POWER AND EDGE GRADIENT VALUES 176 



Part 1. — Definitions 



Resolving power (R). — The resolving power of a photographic material is broadly 

 defined as the ability to record fine detail distinguishably. Any quantitative evaluation de- 

 pends on the type of detail, and for convenience parallel lines separated by spaces whose 

 width is equal to the common width of the lines are almost universally used. 178 Values are 

 usually given as the number of lines per millimeter that can be resolved visually under 

 adequate magnification. 



Resolving power increases with increasing exposure to a maximum and then decreases, 

 It is relatively unaffected by the type of developer, although developers that markedly 

 reduce the grain size improve resolution. As the development time increases from zero, 

 resolving power rises rapidly to a maximum, decreases slightly, and then remains sensibly 

 constant for all practical development times. It increases in a roughly exponential manner 

 as the contrast in the test object increases from zero, becoming substantially constant for 

 contrasts exceeding about 1 00 : 1 . Its dependence on wavelength is less well known, but in 

 general it increases as wavelength decreases because of the increasing opacity of the emul- 

 sion. Although resolving power tends to increase as granularity decreases, this is by no 

 means always the case. The values given in Table 608 apply when the ratio of brightness 

 of the light to the dark lines is 1000: 1 and the test object is photographed with an espe- 

 cially well-corrected //5 lens in tungsten light with the optimum exposure ; the materials 

 were developed for practical times in the developer for which the data are given in 

 Table 604. 



As thus specified, resolving power is a threshold phenomenon and is not a criterion of 

 the clearness with which gross details will be reproduced. Furthermore, it is of questionable 

 value when the image is to be scanned with a physical photometer because the effect of 

 granularity depends upon the design of the instrument. 



Edge gradient (G). — The appearance of sharpness produced by a photographic image 

 probably depends, among other factors, upon the rate of change of density across the edge 

 of the image with distance measured normal to the boundary. The curve of density vs 

 distance resembles the H and D curve, and its gradient, called edge gradient to distinguish 

 it from the gradient of the H and D curve, passes through a maximum with respect to 

 distance. The values of this maximum for the respective materials in density units per 

 micron are given in Table 608. These values were determined with a test object consisting 

 of an extremely sharp, clear line in an opaque background on a high-resolution plate. This 

 test object was pressed firmly against the sample with a contact liquid between and the 

 combination was exposed to light from an f/5 lens. The resulting image was scanned with 

 a physical microphotometer having a comparatively narrow slit. 



The determinants of edge gradient have been less studied than have the determinants 

 of resolving power, but it is known that the maximum gradient has a maximum with 

 respect to exposure. It would be expected that the maximum gradient would increase in 

 gamma, but present knowledge indicates that it increases less rapidly. The dependence 

 on wavelength has not been studied with modern techniques, but older studies indicate that 

 gradient increases with decreasing wavelength. The values in Table 608 are for >»/2 

 and tungsten light at the optimum exposure. 



Both resolving power and edge gradient are inherent properties of the emulsion and are 

 relatively inflexible. It is possible to improve them by bathing the material in dye that 

 absorbs the light to which the emulsion is sensitive, but this is rarely practical because of 

 the concomitant reduction in speed. 



178 Mees, C. E. K., The theory of the photographic process, chap. 21, Macmillan, 1942. 



179 Mees, C. E. K., Proc. Roy. Soc. London, vol. 83, p. 10, 1909. 



(continued) 

 SMITHSONIAN PHYSICAL TABUS 



