Sensitometric-densitometric testing requires a method of providing the exact same exposure to different emulsions, or the same emulsion type that is processed differently, and then comparing the resulting densities.

Sensitometers make controlled exposures that are suitable for sensitometric-densitometric testing procedures. Densitometers measure density.

For sensitometric testing purposes, you must provide a way of exposing sensitized material with a known quantity and quality of light.

One of the first requirements for sensitometric control is to have a sample of the light-sensitive material that has been exposed properly under measurable and reproducible conditions. For sensitometric test purposes, it is common practice to expose a strip of film, so a number of varying exposures are made on the same strip. This series of controlled exposures is made with a sensitometer through a series of neutral-density filters.

Ideally, a sensitometer should be designed so you can do the following:

Predetermine the total amount of exposure.

*Determine the difference in exposures given to various areas.

*Control the color quality of the light.

*Reproduce or duplicate the exposure consistently.

*Obtain a wide enough range of exposures to produce densities ranging from very light to dark.

Scenes that might be photographed include a wide range of brightness values that are represented on a negative as areas of varying amounts of density; however, these different densities are scattered throughout the picture area and are difficult to measure. For the sake of simplicity, uniformity, and reproduceability in the application of sensitometry, the exposures produced by a sensitometer are arranged in a series of gradually increasing steps. These steps correspond to the relative brightness values of a normal scene. The exposures are made on a length of film or paper of the same type that you are processing. This sample is called a sensitometric strip.

"Photographic exposure" is defined as the product of illumination and time. The two important parts of a sensitometer are the light source and the device for controlling the amount of light transmitted to the emulsion. Since total exposure is the result of the intensity of illumination and the length of time it is allowed to act, the exposure may be varied by changing either the intensity or the time.

Light sources for a sensitometer must be carefully chosen, and their characteristics must be precisely known. The intensity of the light must also be known. The time of exposure should correspond closely with actual photographic practice and must remain constant over long periods of time. In addition to this, the color temperature must be known, must remain constant, and must correspond closely to the quality of light that is likely to be used in practical photography. These qualities allow the test strip to be exposed under conditions as close as possible to those that occur in practical use. When you base sensitivity measurement on a consistent factor that is known, the response of light-sensitive emulsions, under practical conditions, is predictable. Thus various emulsions may be compared to each other. You know that emulsions often change in apparent sensitivity (speed) with a change in the spectral composition (color) of the exposing light. Processing also has an effect on the speed of emulsions.

When incandescent lamps are used in sensitometers, they operate at a color temperature of about 3200 K. The color temperature of sunlight is about 5400 K. Therefore, you may need a filter to alter the color of the lamp, so it is equivalent to the spectral energy of sunlight.

Differences in time can also lead to errors due to reciprocity law failure. The EG&G sensitometer, used commonly in Navy imaging facilities, permits you to change the exposure times as well. The exposure can be changed from 1/100 second to 1/10,000 second. This allows you to test the reactions of emulsions to various exposure times.

A sensitometer is used to produce a logarithmically graded series of exposures with values that are already known. These are generally arranged in steps from low to high. A part of the strip is left unexposed, so the gross density of the material itself may be determined.

The device or method used to vary the amount of exposure must also meet certain requirements. The device should be able to produce an exposure range that conforms closely to that found in actual practice, to be accurate, to be consistent, and to have no significant effect on the color quality of light. A step tablet is used in a sensitometer for this purpose.

Step Tablet

The 11- and 21-step step tablets are most commonly used in sensitometers. These step tablets are comprised of a series of neutral-density filters with densities that range approximately from 0.05 to 3.05. On an 11-step step tablet, one f/stop exposure difference (or 0.30 density difference) exists between each step of the tablet. On a 21 -step step tablet, a 1/2 f/stop exposure difference (or 0.15 density difference) exists between each step of the tablet. Selection of a step tablet should be based on the emulsion latitudes and contrast differences in pictorial and copy films and papers. A 21-step step tablet is used normally for long-scale films, and a 11-step step tablet is used for short-scale films and printing papers (fig 2-2).

Processing Sensitometric Strips

Sensitometric strips (sensi-strips) are made by processing under controlled conditions of time, temperature, agitation, and chemical activity. This holds true whether the material is black and white or color. Black-and-white control strips are normally made in the lab, while color control strips are obtained from the manufacturer of the material being processed.

When you are machine processing control strips or sensi-strips, they can be attached to either a roll of film or a leader. To reduce the likelihood of bromide drag, ensure the leading edge of the sensi-strip has received the least amount of exposure in the sensitometer. Following this procedure, the strip goes through the processing machine tanks with the rest of the roll and receives the same processing. Remember that all steps in the processing of control strips and film are important if the process is to maintain high standards and uniformity of results. Each step in the entire process must be carried out as carefully and as systematically as possible with uniform times and handling techniques in each step from exposure to drying.

Refer to figure 2-3 for the relationship between the sensitometer, step tablet, and sensi-strips. CHARACTERISTIC CURVES

The characteristic curve shows the relationship between the exposure and the density, resulting from

Figure 2-2-Step tablets.

the development processes. It is also called an H and D, D log-H, D log-E, or sensitometric curve. Characteristic curves are plotted on graph paper. The vertical axis represents the amount of density, created by each step of exposure. These logarithmic values are read directly from a densitometer. The density values increase from the bottom to the top of the graph.

The horizontal axis of the graph is used to indicate log exposure (log H). The horizontal axis represents the amount of exposure the material received. Exposure must be converted to a logarithmic value, so the scale of density and exposure are uniform. Exposure increases from left to right on the horizontal axis.

When the sensitometric strip is made with the Eastman Kodak No. 2 Step Wedge (21 step), there is an increase of about 0.15 in density between each step or an increase of 50-percent density. The individual steps should be calibrated by reading the step on a densitometer for the greatest accuracy. The densities of steps range closely from 0.05 to 3.05, but the density of the individual steps may vary slightly. For illustration purposes, the step tablet used in the following example has exactly 0.15 density differences between each step.