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WESTERN ELECTRIC SYSTEM 377
The film technician is called on to provide suitable negative exposure and positive timing and appropriate development of the negative and the positive so that the negative exposure as it varies from moment to moment shall appear as a positive transmission similarly varying. In other words, the contrasts of the negative exposure must be faithfully reproduced as contrasts of positive transmission. To accomplish this, we go back to the work of Hurter and Driffield, who forty years ago established the requirements.
Hurter and Driffield showed that every photographic emulsion may be described by a characteristic curve, known since their work as the H and D curve. It is convenient to plot the data in logarithmic terms to show the relation between the exposure and the resulting photographic effect. We choose the logarithm of the exposure, measured in meter-candle-seconds or in any other convenient units of light energy, and plot the logarithms of successive exposures against the resulting densities. Photographic density is defined as the logarithm of the opacity. Opacity itself is the reciprocal of the transmission, which is the ratio of the amount of light transmitted by a piece of developed exposure to that which falls upon it. We shall for the present avoid the troublesome technicalities of specular and diffuse densities, and consider that satisfactory measurements have been made of the exposure and of the density resulting.
If a series of graded exposures are made on a series of areas of a photographic film, and a curve plotted between the logarithms of exposures as abscissas and the developed densities as ordinates, we find the underexposure region represented by a portion of the curve concave upward, followed by a straight portion corresponding to the region of correct exposure, and finally the overexposure which appears as a curve concave downward. The slope of the straight line portion is determined, for any particular type of emulsion, by the development — this slope is called gamma and defines contrast. Fig. 5 exhibits an H and D curve obtained from Eastman positive film.
Curves of this kind are obtained for the emulsions for the negative sound record and for the positive prints, for various developments. It is thus possible to determine what development to give for any desired contrast.
Hurter and Driffield showed that perfect reproduction in