An automatic dark current control for a television (TV) camera or the like comprises the combination of a dark current level sample-hold circuit and a unique low level clipper in parallel with the video output line from the camera vidicon. An electronic switch in the sample-hold circuit is closed once during each field for the duration of one horizontal line sweep of the vidicon electron beam behind the mask on the photosensitive vidicon surface to charge a capacitor to a level corresponding to the dark current present during that line interval. A signal proportional to the charge on the capacitor is applied to a dark current level adjusting circuit which measures the difference in actual dark current over a predetermined value during the one field. The difference or control signal is applied to a clipper circuit comprising a high gain differential amplifier and a diode switch connected across the input and output of the differential amplifier as well as the video output line. The clipper operates effectively on millivolt differences between control and video signals to automatically limit the dark current in the output signal to the preset level.
Output signal samples provided from a solid-state imager, responsive to dark current accumulated in its CCD registers, are supplied to circuitry for measuring dark current noise level. The measured level is compared to a reference value for generating an error signal controlling the application of power to a thermoelectric cooler cooling the solid-state imager. The imager is cooled sufficiently to reduce dark current generation. A servomechanism for keeping dark current level acceptably low with minimal power consumption by the thermoelectric cooler is thus provided.
A vertical deflection circuit is used in a camera tube in a color television camera in which the camera tube is provided with an optical filter having an optically black part extending in parallel with a beam scanning direction of the camera tube. A vertical deflection circuit comprises a circuit for generating a saw-tooth voltage for vertical deflection, a vertical deflection coil for the camera tube, and a circuit for supplying the saw-tooth voltage thus generated to one end of the vertical deflection coil in DC coupling. Due to temperature fluctuation, a vertical deflection width fluctuates with respect to a position corresponding to a black part of the filter, as a reference.
Output signal samples from a solid-state imager, responsive to dark currents accumulated therein, are applied to slope responsive circuitry for generating a signal which can be used, for example, for field shading correction or imager temperature control. Dark current slope responsive measuring circuitry is particularly advantageous over prior dark current measuring circuitry since it is uneffected by DC offset of the dark current signal which results from imager mask misregistration or carrier diffusion when receiving radiant energy.
A straylight compensation circuit which corrects for excessive dark current features a blanking circuit to ensure a stable black level without spikes and blanking noise. The video is integrated to provide a measure of the average picture level and is added to a reference level to compare with the blanking level in the original signal. The blanking circuit has a differential amplifier with diodes to ensure a correct and stable black level without noise or spikes.
An image processing apparatus constructed in a manner such that an image signal is inputted, the inputted image signal is clamped to a predetermined reference voltage in a clamp circuit, an optical black level of the clamped image signal is detected by a detection circuit, the detected optical black level and a predetermined black level are compared and a correction amount is calculated in an operation circuit, and the optical black level is corrected by operating the correction amount obtained by the operation circuit for the clamped image signal in a correction circuit.
