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July, 1945 PRODUCTION OF VANDYKES 57
light getting through, T = — • Density, first studied in terms of silver
Jo concentration per unit volume, was established as the logarithm to the
base ten of the opacity = logio = logio •
The properties of the photographic emulsion as the result of controlled incident energy or exposure, depend upon the treatment of the silver salts during preparation, the gelatin protective colloid, the type of development, and many other factors. Fig. 3 illustrates a typical characteristic curve of a photographic emulsion showing the relation between density as defined above and standardized exposure to white light as obtained on an Eastman sensitometer, type lib. As this exposure is increased to give a perceptible density for given developing conditions, the density varies almost linearly with exposure in the section known as the toe of the curve. A print in this density region would be underexposed and thin with little difference between densities. Above this toe portion is the straight line portion wherein density varies linearly with log exposure. This is a most useful section of the characteristic curve because the opacities of the silver image are proportional to the exposures produced by the corresponding brightness of the subject. The shoulder section of the curve represents a region of almost constant density with increased exposure. A print in this section would be overexposed and dense with little difference between densities. A study of Fig. 3 will show that the straight line portion of the curve can be represented by the equation :
Density = y (logw Exposure + K}
The factor 7 is a measure of the slope of the straight line portion and is thus a measure of contrast. Contrast is a function of manufactured stock characteristics and color temperature of exposing light, but if the image be placed in the region of correct exposure, contrast can be altered by development. The length of the straight line portion is termed the latitude of the stock and again is a function of manufacture, principally emulsion thickness. The ratio of densities in the region of correct exposure, then, is determined by subject contrast; the differences between densities are determined by development. It is common in the photographic industry to utilize these principles of sensitometry in controlling the speeds, latitudes, and contrasts of photographic materials.