The Subjective Sharpness of Simulated Television Images

F the many factors which influence the quality of a television image, the one which is generally indicative of the value of the image and the cost of its transmission is the resolution, or sharpness. This resolution factor has always been reckoned in purely objective terms, such as the number of scanning lines, or the number of elemental areas in the image, or the width of the frequency band required for electrical transmission at a given rate. The subjective value of sharpness has not previously been considered. Some recent tests with a small group of observers, using out-of-focus motion pictures in a basic study of the visual requirements on images of limited resolution, have thrown new light on the evaluation of resolution and sharpness. T h e results appear of sufficient interest, particularly when interpreted in terms of television images, to warrant this presentation. We shall use the word sharpness in the sense of a subjective or psychological variable, with a strict technical significance in keeping with our experimental method, and we shall use the word resolution in the sense of an objective or physical variable. We find that as images become sharper, their sharpness increases more and more slowly with respect to the objective factors. We find also that as images become sharper the need for equal resolution in all directions becomes less and less, and t h a t with images of present television grade the tolerance for unequal horizontal and vertical resolutions is already remarkably wide.