Trigradient Tone Reproduction Method

changes. The light leak causes areas of the film to become fogged. Exposure A negative that has detail in both the shadow and highlight areas is exposed properly. However, when evaluating the negative image, it is necessary to consider the subject matter because less exposure is required for light sandy beaches and snowcovered terrain and more exposure is required for dark terrain, such as forests and industrial sites. When you are viewing a negative that has been exposed normally, patches of snow or light beach scenes appear overexposed. Inversely, patches of dark terrain or industrial sites appear underexposed. When the negative is completely underexposed or overexposed, the film sensitivity or filter factor (S/C) was set incorrectly or the automatic exposure control (ABC) in the camera system malfunctioned. Vacuum The lack of adequate vacuum in a serial-frame camera permits the film to sag away from the focal plane, causing the image to be blurred. The most common indication of insufficient vacuum is crooked data blocks. Miscellaneous Defects Reflections from the camera window of the aircraft, depending on the angle of the sun in relation to the window, can cause flare (nonimage-forming exposure) of the film. Condensation on a camera lens can result in a halo effect surrounding film image points. This is generally caused by rapid aircraft descent immediately before a photo run. AERIAL DUPLICATION 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 4-39