Contrast enhancement in photographic copies of underexposed radiographs.

Abstract

A particularly difficult problem which ii frequently faces the medical photographer is the production of technically acceptable slides from improperly exposed radiographs. Dark originals offer no special difficulty as adequate correction can be obtained simply by increasing the camera exposure. Light originals cannot be salvaged so easily since attempts to control density by decreasing camera exposure usually result in copies that are flat and lacking in contrast. A striking gain in contrast in the copy results from photographing the original, utilizing reflected light (Fig. 1). A sheet of glossy white paper is placed under the film as a reflecting surface, and diffused lights at 45° provide the illumination. It is essential that intimate contact between the reflecting surface and the roentgenogram be assured, to preserve detail. A simple copying stand holds the camera, and the exposure time is best determined by trial and error (Fig. 2). Figure 3 is a schematic illustration of the mechanism of contrast improvement. A completely clear area of film has an optical density of zero and transmits 100 per cent of the light incident to it. Assuming a perfect reflecting surface, all the light again passes through the film with no loss. Any region with a measurable optical density such as B or C effectively attenuates the light twice, once before and once after reflection. Since B has a density of 0.3 it transmits 50 per cent of the incident light; and C, with a density of 0.6, transmits 25 per cent. Therefore, single transmission (viewbox conditions) establishes a ratio of two to one in light absorption. A second passage, by virtue of reflection, reimposes the same transmission rates, reducing B to a remnant of 25 per cent while C is still further compromised to 6 per cent of the original intensity. The effective opacity ratio of B to C is now four to one, a significant alteration of the contrast scale. A graphic rendition of readings of optical density made from transparencies of a stepwedge photographed on Kodak Direct Positive Panchromatic Film (35 mm) and processed simultaneously in Kodak Direct Positive Developer is shown in Figure 4. The original is represented along the abscissa and the duplicate along the ordinate. Reflected light conditions are seen as a broken line, the continuous line demonstrating the findings with transmitted light. The accuracy of tonal reproduction under viewbox conditions is observed in the straightness of the line, an original density difference of 0.3 duplicating as 0.35. It is pertinent that this relationship persists in both the light and dark portions of the original. The reflected light copy has a much steeper gradient in the zones of low opacity of the original, the same density difference of 0.3 now recording as 0.62. Note that the more opaque areas are not similarly affected and these tend to lose density differentiation.