Gated Image Intensifiers And Applications

ABSTRACT This paper describes the gating properties of some first and second generation image intensifier tubes and the application of these tubes as preamplifiers for solid state image sensors.Results are given of the performance as a function of the gate pulse width of a single stage first generation inverter tube, a second generation wafer tube and a hybrid tube coupled to a solid state image sensor. 1. INTRODUCTION A gated image intensifier tube offers the possibility of cutting- off the photocathode current by applying a relatively low voltage change to the gate electrode, while maintaining the normal operating high voltages. The tube combines the function of image intensification with that of a fast electro-optic shutter.In most cases the gate voltage is applied to the photocathode it­ self. The speed of gating, in terms of minimal pulse width, rise and fall times, is determined by the required pulse height (gating vol­ tage), the electrical capacity and conductivity of the photocathode.Another important parameter is the cut-off ratio: the output brightness ratio of the tube in gated-on and gated-off mode. When the tube is coupled to an integrating device, the cut-off ratio determines the minimum duty cycle or ratio between gate-on and gate-off time intervals per integration period.Gating second generation wafer tubes is relatively straightforward since a gate voltage of only 200 V is required. With standard tubes a minimum pulse width of about 20 ns can be applied. For shorter gating times down to 1-5 ns, the conductivity of the photocathode substrate has to be improved in order to prevent the so-called Iris effect, which is caused by the limited charge propagation velocity in the thin photocathode film.1First and second generation inverter tubes can be gated with the focusing electrode or a special gating electrode. The required gating voltage is determined by the electron optics. In order to keep the gating voltage low, the gating electrode should be as close to the photocathode as possible. Most standard inverter type tubes can not be gated, since the electron optical design is that of a simple diode in which there is no separate focusing electrode. Inverter types that do have a separate focusing electrode can be gated, but show a limited cut-off ratio* This is caused by photo-emission from the focusing