Evaluation of Image Intensifier Tubes Using Detective Quantum Efficiency

Abstract

In recent years, many new image detectors have been developed that are sensitive to extremely low light levels. These devices have given new importance to a performance criterion called detective quantum efficiency, or DQE. The subject of this investigation is to develop the basic experimental technology necessary to measure DQE of image detectors, and to apply these techniques specifically to photographically recorded image intensifiers and to unaided photographic emulsions. DQE is defined as the square of the ratio of output S/N to input S /N. Input S/N is determined by the input photon statistics. Output S/N is determined by measuring the recorded image. Two ways of measuring DQE are the single-level gradient method and the two-level AD method. The AD method gives a more appropriate measure of the performance of image intensifiers that have light- induced background. A special two -channel projector was built to measure DQE by the AD method. Its function is to superimpose the image of a signal target upon a uniform background irradiance. The same instrument can be used to obtain DQE measurements using the gradient method. DQE was measured for two image intensifiers (a high -gain TSE tube, model P829D, made by EEV, and a two -stage cascade intensifier, model C3301 1, a Carnegie tube made by RCA) and two unaided photographic emulsions (Kodak IIa -O and Kodak Medium Contrast Projector Slide). The results show that different image detectors may have considerably different DQE performance. For example, at 425 nm, DQE values for the Carnegie tube were more than 20% (the quantum efficiency of the input photocathode is 28 %) whereas both of the unaided photo- graphic emulsions gave peak DQE values no greater than 0.8 %. In addition, the peak DQE of the Carnegie tube occurred at roughly 10' photons /cm2 whereas the peak DQE of unaided IIa-O occurred at 109 photons /cm2 (both at 425 nm). Furthermore, single photon event detectors (such as high -gain image intensifiers) have a peak DQE near zero exposure with DQE decreasing as exposure increases whereas multiple photon event detectors (such as unaided photographic emulsions) have zero DQE near zero exposure, a peak DQE at an exposure which yields a corresponding output density of about 0.2 above fog, and a decreasing DQE for further exposure increases. Also measured were granularity, light- induced background, system modulation transfer functions, and relative system speeds.