Image contrast required at visual resolution thresholds for SO-1166 and Tri-X film
Niehaus, Joseph M
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The objective of the study was to determine how image contrast required in photographic emulsions to resolve tricolumnar bar targets visually varied as the spatial frequency of the targets increased. Threshold contrast for homogeneous targets, arranged in decreasing order of size and viewed under magnification with perfect optics, remains constant if the magnification is chosen so as to keep the apparent image size of the targets the same. However, when the targets are impressed upon photographic emulsions, threshold contrasts increase because of the disturbing effect of the grain clumps in the emulsion. Two films, Kodak SO 1166, now called Plus-~X Airecon, and Kodak Tri-X were analyzed to determine how the threshold contrasts were affected by the graininess of the films when the background densities of the films were .75 ± .1. Contrast in the photographic images was varied by introducing controlled amounts of stray light into a camera system with a beam splitter. Resolution values of approximately 2.5, 5, 10, 20, 30, and 50 lines/mm were obtained. Ten frames for each of the above values were analyzed by two trained observers, free to select magnification and viewing table illumination, and the three-bar groups within each frame called just-resolved by each observer were scanned with a recording microdensitometer to determine the minimum density difference ΔD required to resolve each of the groups. Average values of threshold resolution and the corresponding average minimum density differences thus obtained were then computed for each of the films and for target length to width ratios of five to one and essentially infinity. The data was plotted with ΔD as a function of resolution R, and third order expressions derived from the curves. It was found that the second and third order terms were of such small magnitude (10^-5R^2 and 10^-7R^3) that they could be disregarded for practical purpose and the resulting linear expressions were of the form ΔD = b+GR where b is a constant, .005, assumed equal to the nominal value of ΔD for visual resolution thresholds of infinitely large targets, and G is a variable dependent primarily upon the grain characteristics of the film. The expressions for the two films, where short (5/1) and long (∞) line targets are employed are: Short Line targets SO 1166 -- ΔD= .005 +.003R Tri-X -- ΔD= .005 + .004R Long Line targets SO 1166 -- ΔD= .005 + .0008R Tri-X -- ΔD= .005+ .0015R It was found that the increase in resolution obtained by using long line targets was about one and one-half times, but that the long line targets were much more difficult to evaluate visually because of the length of the lines.
Thesis (M.A.)--Boston University
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