Some Factors Affecting the Choice of Lenses for Television Cameras

The design of a television camera for a particular use involves the selection of a lens which, under the poorest conditions of illumination which are expected, will form a sufficiently bright image to meet the requirements of the pickup tube used. When the size and sensitivity characteristics of the pickup tube and video-frequency amplifier, the required signal-to-noise ratio, and the angle of view of the camera are known, the specifications of the lens can be computed. If a tube having a sensitive surface of width W inches and an operating sensitivity of s microamperes per lumen is to be used in picking up a scene having a surface brightness B candles per square foot, with a horizontal angle of view α, the following equations can be used to determine the lens to be used: T, focal length: F=-cot -inches 2 2 0.064 W VITBs numerical aperture: tf= I,/ 0 IL, N in which T is the light-transmission factor of the lens, I,, is the equivalent root-mean-square noise current at the input of the amplifier used with the tube, and N is the required signal-to-noise ratio. The paper discusses the derivation of these equations and includes charts to facilitate computations. Comparison of predicted results with the observed performance of the apparatus have shown good correlation.