This viewer used the diode image converter employed in the active systems but with a higher sensitivity photoemissive photocathode and a further degree of intensification provided by arranging for the electron-optical geometry to give a demagnified image at the phosphor. With careful selection of the objective and eyepiece lens parameters a significant improvement in perception was obtained. Variable magnification by adjusting the voltage applied to special electrodes provided the optimum overall magnification according to the level of night brightness in the range from starlight to full moonlight. The variable magnification ensured that under different night-sky illumination levels the relatively low level output image did not impair the dark-adapted conditions for the operator’s eye, thereby optimising its photon detection efficiency and leaving the operator with his visual perception away from the phosphor screen largely unimpaired.

When dark-adaption of the eye was not mandatory, brighter output images were provided by using the diode converter in a cascade image intensifier. The EHT power supply circuitry of these tubes was also designed to meet conflicting requirements of adequate protection when the scene was flooded wih light by flares or gunflashes, rapid response and fast recovery.