Problem of Infrared Television-Camera Tubes vs Infrared Scanners

Infrared sensitive television-camera tubes have responsivities orders of magnitude higher than scanner cells, yet the latter give better images of terrestrial scenes by their own thermal radiation. It is shown that although the camera tube electron beam reading mechanism saturates to limit maximum signal-to-noise ratio (SNR) to about 100, the large ir photon flux permits SNR up to 10(5) for typical nonsaturating cells that respond from the visible to 12 microm, despite their lower responsivity. Since ir images have very low contrast, ir sensitive camera tubes would be preferred only for wavelengths shorter than about 2.5 microm where photon flux is small and high responsivity is required.