Publisher Summary This chapter discusses two basic types of image intensifier. First, uses five stages of transmitted secondary electron emission from thin films for the multiplicative process (TSEM), second and more recently, a cascade type. In cascade type three photoelectronic amplifiers of the single stage photocathode- phosphor type are cascaded with good optical coupling to provide the required amplification. In the case of the TSEM tube, a typical peak current gain for a dynode is between 5 and 7 and occurs at a primary accelerating potential of about 5 kV. As a result, five such stages of multiplication give a peak overall electron gain of between 3000 and 18,000 which, taken together with the light-gain from the combination of the input photocathode and output phosphor, gives an overall light gain greater than l05. Examination of the dark current rate in the cascade tube cannot easily be performed in the same way as for the TSEM tube that is by viewing the output with a photomultiplier feeding a discriminator and scalar.