certain threshold. At point B, sufficient numbers of photons are

available to convert the larger silver halide crystals to metallic

silver. This statistical event accounts for the "toe" of the H & D

curve. When the number of photons arriving are larger than the

threshold, the conversion of the silver halides to metallic silver

depends on the projected area of each crystal. The crystal size is

carefully controlled during the production of the film. The

distribution of grain sizes provides a linear portion of the H & D

curve denoted part C in Figure 6.1. Because the crystal grain sizes

are limited in range, the film "saturates" at point D on the curve.

When all of the silver is converted, additional exposure can not

further reduce the transmission of the developed film. In fact, some

films will lose density with addition exposure (part E on the curve).

This effect, known as "solarization" is rarely used except by

photographic artists for special effects. This curve indicates that

the film response over a wide range of exposure is quite non-linear.

 Measurement of Film Characteristics

 In order to model the film accurately, it is important to collect

an adequate sample of data points. These samples must be gathered in

an experiment in which the conditions are representative. The

exposing energy and wavelength must be in a range consistent with the

eventual use of the film. Typically, a sensitometer and densitometer

are used to obtain the data points for the response of the film. The

densitometer exposes the film to a known irradiance for a fixed time

period. A Kodak step wedge is placed between the sensitometer light

source and the film to provide a wide range of exposures on the film

simultaneously.