Sensitometry is the science of determining the photographic characteristics of light-sensitive materials. In sensitometry, special test or control strips are prepared by accurately exposing the material with varying amounts of light. These test strips are then processed.
A sensitometer is an instrument used to produce the special test strips called sensitometric strips. A sensitometer is used to produce these sensitometric strips because it provides consistent and repeatable exposures of a known quantity and quality of light. The sensitometer is used to expose a strip of film with varying amounts of known exposure on the same strip of film. Since the sensitometer provides repeatable exposures each time, any changes in density indicates a change in processing. In Navy photography, the sensitometer is used to expose black-and-white materials only. There are several uses for sensitometric
strips; but in this training manual, we are only concerned with monitoring a process. Here the sensitometric strips are used as control strips. Control strips are made and processed under the controlled conditions of time, temperature, and agitation. This is true for both black-and-white and color materials. Black-and-white control strips are usually made in the photo lab, while
color control strips are obtained by the manufacturer of each material.
Ideally, a sensitometer should be designed so you can accomplish the following objectives:
1. Predetermine the total amount of exposure.
2. Determine the difference in exposures given to various areas.
3. Control the color quality of the light.
4. Consistently reproduce or duplicate the same lighting conditions.
5. Provide a wide range of exposures.
The sensitometer used most commonly in the Navy today is the Egerton, Germeshausen, and Grier (EG&G) sensitometer (fig. 10-24). This sensitometer uses a
Figure 10-25. Step tablets.
daylight balanced xenon flashtube. It can also produce exposure times from 1/100 to 1/10,000 second.
A sensitometer is set up to make only one exposure. In order to provide a range of exposures, a step tablet is placed between the light source and the light-sensitive material. The step tablet is a strip of neutral-density filters in equal increments, ranging from 0.05 to 3.05. This range provides a 10 f/stop range. The most common step tablets are as follows (fig. 10-25):
1. 21-step tablet-1/2 f/stop between each increment.
2. 11-step tablet-1 full f/stop between each increment.
Courtesy of X-Rite, Inc.
Figure 10-26. Densitometer with both transmission and reflection capabilities.