Abstract: A photosensitive tube of the kind having a photoconductive target formed of a material such as lead monoxide exhibits an effect generally termed lag when the tube operates at very low light levels. Lag is undesirable and can be minimized by providing a low light level bias which illuminates the rear of the target. The present invention provides light bias in the form of small light emitting diodes which are placed externally of the evacuated envelope of the tube in localized recesses adjacent to a tubular anode electrode at a point where the anode diameter changes. In this way the light emitting diodes can be accommodated within the normal overall envelope of the tube and in a manner which conveniently permits an even diffused illumination of the target. This arrangement is particularly applicable to small diameter tubes to which conventional light bias cannot readily be applied.