PHOTOGRAPHIC MATERIALS 129 



exposure of equal total energy and at the same intensity. This effect, likewise, is of 

 no concern in normal photographic practice. 



The precautions which must be observed in photographic photometry to avoid 

 errors from these and various other sources have been discussed by Harrison ' and more 

 recently by Jones. ^ 



Graininess. — The photographic image is composed of discrete particles of silver, 

 and thus under sufficiently high magnification the apparently smooth image presents 

 a granular or mottled appearance. Since even the largest grains are only a few microns 

 in diameter, very high magnification would be necessary to make the individual grains 

 visible. Nevertheless, with negative emulsions particularly, the inhomogeneous 

 appearance becomes noticeable at low magnification, in some cases at less than 5 X 

 linear enlargement. This inhomogeneous appearance, termed graininess, seems to be 

 due to a clumping or grouping of the individual grains into more or less dense clusters, 

 with areas of lower density between them. This graininess is of no concern when the 

 image is to be viewed or printed in its original size but becomes very important when 

 the image is to be enlarged greatly. Thus it is important in miniature-camera work, 

 in motion pictures, and in certain scientific work, such as spectrography. 



Graininess is determined chiefly by the nature of the emulsion, but with any 

 particular emulsion it is affected by such things as the composition of the developer, 

 the degree of development, the density, the nature of the enlarging optics, the contrast 

 of the printing medium, and the sharpness and definition in the negative image. In 

 general, graininess increases with gamma, increases with density, and is more notice- 

 able if the negative image is not sharp. Graininess is also more apparent when there 

 are any considerable areas of uniform density in the print. Thus, in any attempt to 

 compare the graininess of two emulsions, all these factors must be held constant. 



Since the nature of the emulsion is itself controlled by so many factors, it is impos- 

 sible to make definite statements which hold without many exceptions. In general, 

 however, it can be said that graininess tends to increase as the emulsion speed increases. 

 Thus, when the image is to be enlarged to any extent, it is usually undesirable to 

 employ a very high-speed emulsion unless the conditions of exposure demand it. 

 However, since this relation holds in only a general manner, the emulsion speed should 

 never be taken as more than a rough indication of the probable graininess. 



The method of measuring graininess depends on the use to which the emulsion 

 will be put. Thus graininess of miniature negatives should be judged from carefully 

 prepared enlargements, ^ while graininess of motion-picture film should be judged under 

 actual projection conditions.^ For scientific work, such as spectroscopy, the relative 

 graininess can be judged from microphotometer tracings of regions uniformly blackened 

 to a density of 0.3. * Curves of this type for some typical commercial plates and for a 

 group of special emulsions supplied for use in spectroscopy are shown in Fig. 8. 



Methods of using microphotometer tracings to derive quantitative values for 

 graininess have also been proposed.^ 



1 Harkison, G. R., Instruments and Methods Used for Measuring Spectral Light Intensities, 

 J. Optical Soc. Am., 19, 267 (1929). 



2 FoRSYTHE, W. E., editor, "Measurement of Radiant Energy." Reference is made especially to 

 Chap. VIII, Measurements of Radiant Energy with Photographic Materials, by L. A. Jones. 



3 Crabtree, J. I., and Vittum, A New Ultra Fine Grain Developer, Am. Phot., 30, 188 (1936). 



* Crabtree, J. I., and C. H. Schwingel, Duplication of Motion Picture Negatives, J. Soc. Motion 

 Picture Engrs., 19, 891 (1932). 



° Mbes, C. E. K., "Photographic Plates for Use in Spectroscopy and Astronomy," Eastman Kodak 

 Co. 



6 Van Kreveld, A., Objective Measurements of Graininess of Photographic Materials, J. Opt. Soc. 

 Amer., 26, 170 (1936). 



GoETZ, A., and W. O. Gould, The Objective Quantitative Determination of the Graininess of 

 Photographic Emulsions, J. Soc. Motion Picture Engrs., 29, 510 (1937). 



