Picture signal generator with light level control

Claim 1

In picture signal generating apparatus including an image tube having a photocathode for receiving incident image-conveying radiation and a storage target for storing an image-representing charge pattern and an image intensifier section for accelerating photoelectrons from the photocathode to a focused image-representing charge pattern on the target, and means for deriving a video signal from the target, the improvement comprising fast-acting servo means including first error signal generating means for continuously deriving a first error signal from the difference between the average video output voltage and a first adjustable reference voltage, accelerating voltage modifying means responsive to said first error signal for modifying the photocathode-to-target photoelectron accelerating voltage in a direction to maintain the average video output voltage at a desired level relative to said first adjustable reference voltage, slow-acting servo means including second error signal generating means for continuously deriving a second error signal from the difference between said first error signal and a second adjustable reference voltage, and incident light intensity-modifying means continuously responsive to said second error signal for modifying the intensity of light incident on said photocathode in a direction such as to reduce the magnitude of said first error signal, whereby the operation of the slow-acting servo means minimizes the condition to which the fast-acting servo means respons.