Abstract: In a forward-looking infared system of the type in which a cryogenically led infared detector array drives a light-emitting array whose video output is focused onto the target of an electro-optical multiplexer, such as a vidicon tube, which drives a CRT display, the video chain may be tested without cool-down of the infared detector. A point-source of light, such as a tungsten lamp mounted forward of and at the side of the vidicon target, is energized to project light onto the target surface at an acute angle, setting up a gradient of light intensity across each of the horizontally scanned lines.