Contrast

T.O. 33B-1-1 6-93 radiation exposures. Special illuminators are required to view radiographs with a density of 3 to 4. Radiographs with a density over 4 are extremely difficult to "read." A density of 2 to 3 is recommended for all radiographs. 6.8.6 Contrast. Maximum contrast is achieved in radiography when the maximum X-ray image contrast is coupled with the maximum available film contrast. High density radiographs viewed with high intensity illuminators provide the best radiographic contrast. As one of the factors that affect sensitivity, contrast should be high. Some of the general rules regarding contrast are as follows: (1) Contrast increases as kVp decreases. (2) Contrast increases as film development increases. (3) Contrast increases as film speed decreases. (4) Contrast decreases as kVp increases. (5) Contrast decreases as film development decreases. (6) Contrast decreases as film speed increases. 6.8.7 Fog. Fog is the darkening of radiographic emulsion caused by humidity, heat, cosmic radiation, certain chemicals, out of control development chemicals, scatter radiation and bad development practices. It is defined as the darkening of the film emulsion by an inadvertent cause. The fog level of film brings no useful information to the film and merely creates a high background that reduces contrast and image visibility. The faster the speed of the film is, the more susceptible it will be to fogging. 6.8.8 Distortion and Magnification. Some of the factors that cause distortion and magnification are discussed in other areas of this manual. However, distortion can also be caused by improper alignment of the X-ray machine and/or film in relation to the object. If distortion is excessive so that areas are obscured, it may be necessary to radiograph the object at a different angle. The total distortion or magnification that can be tolerated on a radiograph will depend upon the desired sensitivity and the geometry of the object itself. 6.8.9 Kilovoltage and Processing. Any attempt to evaluate a radiograph must take into consideration the conditions under which the radiograph was made. The effects of different kilovoltages and processing techniques cause a variation in contrast and latitude. 6.8.10 Viewing and Reading. GENERAL. Viewing and reading of the radiograph is the final step in the radiographic inspection procedure. The radiographer must be aware of the various factors that can influence his decision. Some factors are density of the film, artifacts on films as a result of handling and processing, level of illumination for viewing radiographs, response of human eye to differences in light intensity and the acuity of vision. 6.8.11 Viewing Conditions. Reading large numbers of radiographs is a strain on the eyes and fatiguing to the film interpreter. The environment of a film reading area should be pleasant and SHALL be free of objectionable background light that may cause reflection on the radiographic film. Two and one-half (2.5) foot-candles of ambient light measured at the viewer is optimum for viewing. This light level will aid the film interpreter by accommodating the eye so they are more sensitive to light. When attempting film interpretation, the radiographer should wait at least three (3) minutes before reading film, when coming into the viewing room from ordinary artificial room light. When coming from full sunlight, the interpreter should allow five (5) minutes for dark adaptation before viewing. If the eyes are subject to the full brightness of the illuminator during changes of the radiographs, at least thirty (30) seconds re-adaptation is necessary (see Figure 6-43).