=Sensitometry.=—The most generally used system of sensitometry is that of Hurter and Driffield, commonly referred to as the “H & D.” By this system, in order to determine the characteristics of a given photographic plate, it is necessary to take a series of graduated exposures, a standard illumination of the plate being varied in known amount by a rapidly rotating disc cut to a series of different openings, or by some other suitable means. The negative thus obtained is developed in a standard developer for a definite time, at a fixed temperature, and is then measured for transmission on a photometer. The following terms are defined and used in plotting the results: intensity of light transmitted _I_ Transparency = _T_ = —————————————————————————————— = ——————— intensity of incident light _I__{O} intensity of incident light _I__{O} 1 Opacity = _O_ = —————————————————————————————— = ——————— = ——— intensity of transmitted light _I_ _T_ Density = _D_ = -log_{10}_T_ = log_{10}_O_ Hurter and Driffield pointed out that a negative would give a true representation of the differences in the light and shade of the object if it reproduced these differences by equivalent differences in opacity. This is equivalent to stating that if the densities are plotted against the logarithms of the corresponding exposures, a straight line should be obtained at 45 degrees to the axis of exposure times. If the line is at another angle the opacities of the negative will be _proportional_ to the brightness of the object photographed, but the _contrast_ will be different. A typical H & D plot is shown in Fig. 103. It will be noted that two curves are shown. These are obtained with different developments, and illustrate the fact that the contrast or proportionality between exposure differences and opacity differences is a matter of time of development. Each of these curves exhibits certain characteristics which are common to all made in this way. There is primarily a _straight line portion_, where opacities are proportional to illumination. This is commonly called the region of _correct exposure_. The slope of this straight line portion—the ratio of density/(log exposure)—is the _development factor_, commonly denoted by “γ,” a gamma of unity denoting exact tone rendering. Below the region of correct exposure is a “toe,” or region of smaller contrast, called the region of _under exposure_. Above the correct exposure region is another where the opacity approaches constancy (afterwards decreasing or “reversing”), called the region of _over exposure_.