AUTO-IRIS AND AGC
The range of light levels over which a colour TV camera is required to work is indicated in Table 6.2, and can range beyond 1:10 000 between candlelight and a bright sunlit outdoor scene. Since the practical contrast range overall of a TV system is seldom more than 50:1, and since we wish to see reasonably normal brightness and contrast levels in all circumstances of lighting, a very wide-ranging automatic video level compensator must be fitted to the camera. It takes two forms – auto-iris control; and a.g.c. by electrical feedback circuits. The multi-bladed iris is driven by a moving-coil armature rotating against a spiral hair-spring – similar to the arrangement of a moving-coil meter. Iris opening is proportional to coil current, a useful feature in that when the camera is off the iris is fully closed to prevent accidental damage to the pick-up sensor in the event of its being pointed towards a strong light. To supplement the video a.g.c. circuit, then, a current proportional to light level (hence luminance signal voltage) is fed into the iris coil, forming a control loop which maintains video output level at 1 V pk-pk. There is a trade-off in auto-iris operation: for best video signal-to-noise ratio maximum lens opening is required to get as much light as possible to the image- sensor; for optical reasons within the lens system best depth of focus field is achieved at small iris openings. For these and other reasons the more light in the scene the better, and a level of 2000 lux should be aimed at for good results. Such a level can be achieved indoors easily with the aid of one or two 1 kW quartz-halogen lamps. Cameras fitted with sensitive pick-up sensors can produce accept- able pictures from a scene whose illuminance is as low as 4 lux. For very low-light situations most cameras have a ‘sensitivity-up’ switch which overrides a.g.c. operation to increase gain (at the expense of definition and video noise level) under these circumstances.
Since the auto-iris and a.g.c. systems work on the average level of the luminance signal a problem arises where the mean level of the light in the whole scene is such as to compress wanted detail down to or below black-level, e.g. a person standing indoors between the camera and a window. To overcome this the auto-iris system can be inhibited by a BLC (Black-Light Control) switch, whereby detail in foreground subjects can be maintained, even though the surround- ing highlight areas are washed-out by light and colour saturation of the sensor surface.