Dark current compensating lens iris control

Abstract

The iris of the lens of a closed circuit television camera is controlled to maintain an approximately constant magnitude video signal irrespective of dark current generated by the camera tube. The television camera includes an optically black filter covering the initial horizontal scans of the camera. A dark current sample is taken for each video field during a horizontal scan behind the optically black filter. A compensated video signal is generated by substituting the dark current sample for blanking portions of the video signal. The compensated video signal and the dark current sample are passed to a differential integrator the output of which is used to drive a motor which automatically controls the iris of the camera lens to maintain a selected magnitude video signal. Manual control of the iris can be selected by the camera operator; however, a protection circuit couples the automatic iris control to partially close the lens iris if lighting conditions could potentially damage the camera tube while the iris control is in the manual mode.

Dependencies are inferred from the claim text.