The Subjective Sharpness of Simulated Television Images 



By MILLARD W. BALDWIN, JR. 



1. Introduction and Summary 



OF 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 sharp- 

 ness 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. The 

 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 that with images of present 

 television grade the tolerance for unequal horizontal and vertical 

 resolutions is already remarkably wide. These conclusions are sup- 

 ported by our experiments with small-sized motion pictures viewed at 

 a distance of 30 inches, about 4 times the picture height. It would 

 not be safe to extrapolate the results of these experiments to the large- 

 screen conditions of motion-picture theaters, because the visual acuity 

 of the eye may be expected to increase with distance in the range in 

 question,^ and for other reasons. 



2. Exposition of Method 



Image sharpness is to be measured by subjective test, employing 

 psychometric methods ^ which have been widely used in the measure- 

 ment of other subjective values. Test images are to be projected onto 

 a screen from 35 mm. motion picture film in such a way that the reso- 



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