Two methods of producing high-quality reproductions of black-and-white aerial film are in use today. One method is the specific tone reproduction method. The second method is simpler and more feasible for shipboard use, so it is discussed in more detail in this chapter. This method is called the trigradient tone reproduction method or the 1.00 print gamma method.

The following criteria is recommended as a guide to optimum photographic quality and product uniformity in producing duplicate positives or negatives from original aerial negatives. The overall objective of these recommendations is to ensure that a maximum amount of intelligence information is retained in an optimum form.

Only the straight-line portion of the characteristic curve of the duplicating or printing material must be used. For most duplicating film, the straight line lies between densities of 0.40 to 1.80. Thus the D-min should be close to 0.40 and the D-max should be no more than 1.80 in the duplicate.

* Normally, the contrast of the duplicate is correct when the density range between the D-max and D-min falls between 0.80 and 1.20, preferably near 1.00.

The requirement for using the straight line is met when the exposure level of the printer is correct. The contrast requirement is met when the processing is correct. Specifically, the contrast of the duplicate can be increased or decreased relative to the original by increasing or decreasing gamma, respectively. To achieve these goals, you must use some form of tone control to guide the printing and processing operations. The duplication of aerial reconnaissance imagery requires that exacting standards and controls be stressed. This helps to ensure that the imagery is of the highest quality.

SPECIFIC TONE REPRODUCTION METHOD

The purpose is to match the characteristic curve and the density range of the original negative to the characteristic curve of the duplicating material being used.

TRIGRADIENT TONE REPRODUCTION METHOD

The trigradient tone reproduction method of duplicating is an objective method for determining printing and processing requirements. This method allows you to select one of three standardized processes. Each process produces a different contrast or gradient. The processing requirement is selected by determining whether the density range of the duplicate should be increased, maintained, or decreased. By doing so, you can alter the density range of the imagery, if necessary, in each generation. Thus the density range of the final product conforms closely to the desired tonal values (based on a D-min of 0.40).

Trigradient Control Curves

As stated previously, the trigradient tone reproduction method is based on three tone-control curves (or printer curves): high contrast, medium contrast, and low contrast.

Basically, the printer curves are produced the same way that sensi-strips are made for process monitoring. Instead of using a sensitometer to expose the film, the contact printer is used to expose the film through a step tablet. Since the amount of exposure is unknown, the horizontal axis of the curve indicates the density of the step tablet. The vertical axis represents the density produced after the duplicating film is exposed through the step tablet and processed.

Each set of printer curves consists of a family of response curves that graphically display the various tones produced in the duplicating material when it is printed under various exposure and processing conditions. Each curve is labeled with the exposure setting used on the printer to create the curve (figs. 4-30 4-31, and 4-32). These response curves enable the density range of the imagery to be altered, so the density range of the final product conforms closely to the desired density range (1.00). The exposure of the printer is important since it determines the placement of the tonal values of the imagery being reproduced on the sensitometric response curve of the duplicating material. Ideally, all tones should fall on the straight-line portion of the response curve to ensure that the images are reproduced uniformly. The D-max of the imagery being duplicated is used to determine the required exposure since it reproduces as the D-min in the reproduction.

Trigradient Tone Reproduction Procedures

The following procedures are used in the trigradient tone reproduction system:

1. Obtain the three sets of tone control curves (high, medium, and low contrasts) for the particular duplicating materials being used.

2. Determine the D-max and D-min densities on the roll of imagery to be duplicated. Remember, the D-max and D-min are the areas in which you want to retain detail, not necessarily the areas of highest and lowest density. Do not use specular highlights or completely black areas to represent the D-max and D-min.

3. Determine the density range in the original, and determine whether the tonal range should be increased, retained, or decreased. Do this to obtain the desired density range (usually 1.0) on the duplicate.

4. Based upon results of step 3, select the set of response curves closest to the density range (high, medium, or low).

5. On the horizontal axis, locate the density of the step tablet that corresponds to the D-min density that you selected. Go up from this point until you intersect the 1.40 line and draw a tick mark.

6. On the horizontal axis, locate the density of the step tablet that corresponds to the D-max density that you selected. Go up from this point until you intersect the 0.40 line and draw a tick mark.

7. Using a straightedge, select the characteristic curve closest to your straight line. If the curves cross, choose the curve closest to your D-max because this density controls the exposure of the D-min on the duplicate.

8. Expose and process the duplicate film according to your determination in step 7.

9. Read the D-max and D-min of the duplicate film. If these values are not within established tolerances (0.05, for example), another duplicate must be made. Minor adjustments in exposure or processing may have to be made. For example, if the D-min is too high, then less exposure is required. If the D-min is within tolerance but the D-max is too low, then more development is required (to raise the contrast).

The entire tone control system is based upon data produced when the system is established. For the system to be accurate and reliable, all of the major variables must be controlled so they can be carried out on a repetitive basis.