The Relative Sensitivities of Television Pickup Tubes, Photographic Film, and the Human Eye

The threshold scene brightness which a picture-reproducing device can record, a measure of its "operating sensitivity," depends not only upon the lens speed and the exposure time, but also upon the amount of detail in the recorded image. A general expression for the "operating sensitivity" of a picture-reproducing device is obtained which includes these factors together with the threshold number of quanta per picture element. This parameter characterizes the "true sensitivity" of the given device. The "true" and "operating" sensitivities of four types of television pickup tubes, photographic film, the human eye, and an ideal picture-reproducing device are obtained. Eye and film have of the order of one one-hundredth the "true sensitivity" of an ideal picture-reproducing device. Some recent television pickup tubes have of the order of one one-hundred-thousandth the "true sensitivity" of an ideal device. To compare "operating sensitivities," the same exposure time and equivalent lens systems are taken for the three devices. A television pickup tube which has a photoelectric response of 10 microamperes per lumen and makes full use of the storage principle can record scenes with no more illumination than that required by some of the "faster" photographic films. The relatively low "operating sensitivity" of film results from the large amount of intrinsic picture detail (a picture element is taken to be a single grain). The human eye has a range of "operating sensitivities" extending from that of film to a value several thousand times higher.