International projectionist (Jan-Dec 1947)

Television, Films and the Human Eye By DR. ALBERT ROSE RCA Laboratories, Princeton, New Jersey Research reveals that the television image Orthicon tube and the eye are equally sensitive, far surpassing the fastest photographic film in its response to light. A MOVIE patron steps from the bright outdoors into the dim intenor of a theater. Some moments elapse before his eyes become sufficiently adapted to the dark to guide him to a seat. The seat he chooses is circumscribed — within limits. He cannot, for example, approach within arm's length of the screen with the intent of determining whether the heroine wears 60 or 80-mesh sheer hose, for by design the seats stop thirty or more feet short of the screen. The viewer of a home television receiver is not subject to the same limitations. The aim, at least, is that he may view his picture with enough light in the room to see comfortably. There is, moreover, no barrier to prevent him from taking his seat where he can exhaust the last particle of picture detail. Both these considerations, i.e., picture brightness and viewing distance, determine the technical standards that a picture must meet in order to be judged satisfactory by the eye. Both are more severe for a television picture than for a motion picture. Because the judgment of picture quality is made by the eye, it is highly desirable to be able to specify the performance of the eye quantitatively and in such terms or units as> will allow ready comparison with the performance of motion picture film or a television system. The problem of choosing a performance scale that can be applied with equal validity to motion picture film, the eye and television pickup tubes is considerably simplified by the fact that all three devices are subject ultimately to the same limitations. These limitations are set by the statistical fluctuations in the absorption of light. The fluctuations can give rise to graininess in films, to "noise" in a television picture, and to limited halftone discrimination in the eye. They also permit a unified approach to performance evaluations — an approach the technical economy of which can readily be appreciated since only a single number, corresponding to the light efficiency, is needed 14 to specify the performance range of a well designed picture pickup device. And this statement holds equally for the eye, photographic film and pickup tubes. Here, in brief, are a few of the conclusions to which the aforementioned unified approach leads. More Sensitive Than Film The sensitivity of the image orthicon and the human eye are approximately equal and each is about ten times as sensitive as photographic film. The comparison pictures in Fig. 1, when properly interpreted, support the factor of ten between image orthicon and film sensitivity. At two foot-lamberts, both pickup tube and film can record pictures. At lower scene brightnesses, only the pictures transmitted by the image orthicon are present. The fact that motion pictures are viewed at less than one-tenth the brightness at which the original camera recorded them is in support of the increased sensitivity of the eye over film. The discrimination of the eye (for halftones and resolution) is enhanced when looking at brighter pictures. This FIGURE 1 These photographs show the increased efficiency xjf the image orthicon over the( most sensitive film under normal and low lighting conditions. Picture on left side of each film square shows the image as picked up by orthicon and photographed direct from kinescope. Picture on right of each square is the image as recorded by camera. Only when light was maximum was film able to record an image, although orthicon tube was successful when illumination was reduced V9 percent. Figures under each square indicate relative intensity -of light. means that both television pictures and motion pictures must improve their quality hand-in-hand with their brightness. Because television pictures, in order to be viewed in moderately lighted surroundings, are likely to be brighter than motion pictures, the quality of the television picture should exceed that of the motion picture. This burden of improved quality is passed on mostly to the television pickup tube, in that improved quality needs more illumination in the original scene or, preferably, more sensitivity in the pickup tube. A particularly interesting problem arises when the original scene to be picked up is not as bright as the picture reproduced on the kinescope. For thisrjicture to be judged satisfactory, pickup tube performance must exceed the performance of the eye in the same proportion as the kinescope brightness exceeds that of the original scene. Much confusion has been generated by comparing the "limiting resolution" of motion pictures with the number of lines of a television picture. The conclusion has usually been that a motion picture is two or three times, as the case may be, better than a television picture because the limiting resolution of film is 1,000 to 1,500 lines and the number of lines in a television picture is only 500. This conclusion is at least misleading. The limiting resolution of a picture is not as important in the eye's judgment as is the response, or signal-to(Continued on page 16) INTERNATIONAL PROJECTIONIST