Picture Generation
Sensor system: Tube/CCD
The charge coupled device (CCD) is a solid-state device which has rapidly replaced the conventional camera pick-up tube (Plumbicon and Saticon). Both the CCD and the camera tube use a light-sensitive material which changes its resistance when light falls on it (photo-conductive). The optical image on the sensor therefore results in an electrical charge pattern being formed which is processed in the camera to produce the video signal. The CCD is a special integrated-circuit and is often referred to as a ‘chip’ camera.
Camera configuration
Video cameras may use either a single device to generate the video signal, or a three-path system.
Single-sensor cameras. These use a single device to convert the relative brightness of an optical image into three electrical signals, by putting a grill consisting of red, green and blue filter stripes over the face of the sensor. The image is thus split into the three required primary colours which are detected and processed by the camera to produce a colour video signal with no colour fringing problems. This type of camera does not work too well under low light levels and does not resolve fine detail very well. It costs much less than the three-sensor camera, and has considerable merit for closed-circuit television applications, or for news ‘stringers’.
Three-sensor cameras. Red, green and blue optical images are obtained by using a system of prisms and special filters (dichroic filters). The three separate images are converted in the electrical signals RGB which are used to produce the video signals for recording purposes (i.e. composite or component). Professional cameras all use three-sensor systems, the picture quality being far superior to the single-sensor system. Three-tube cameras have the disadvantage that unless the three electrical images are identical in size and shape the synthesised image on the colour monitor will have colour fringes. The registration of the three images is an important feature to check on any camera specification. Three-CCD cameras do not suffer from this problem as the chips are factory set to extreme accuracy, and are cemented into place on the surfaces of the light-splitting block.
Resolution
The ability to resolve fine detail in a televised scene is a measure of the ‘resolution’ of the camera. The resolution performance of a tube camera is determined by the tube characteristics and scanning system; the Saticon has slightly better resolution than the Plumbicon tube. The resolution of a chip camera is limited by the number of pixels on the chip. Advances have produced chips with over 700 pixels in the horizontal direction.
The CCD camera has rapidly replaced the tube camera for most applications, for the following reasons:
| Ease of operation |
●Smaller and lighter in weight. ●Uses less power. ●No warm-up needed. ●Minimum of technical alignment fewer controls. ●No registration needed. ●No lag or burn-in effects on highlights. ●Less light required, approx. one f-stop more sensitive. |
| Robustness |
●Very rugged construction. ●Not affected by vibration—no microphony. ●Not affected by external fields—electric or magnetic. ●CCD does not have limited life like tubes. ●Very reliable in operation. ●Cheaper to operate. ●Operates within temperature range -20°C to +45°C. |
| Performance |
●Good signal/noise ratio, better than 60 dB. ●Excellent dynamic range. ●Good colorimetry (with infra-red filter). ●Good dynamic resolution—less blurring on moving objects. ●Resolution determined by number of pixels (currently better than 700 pixels horizontally enabling 700 lines/picture to be resolved). ●Vertical smear, a CCD effect where extreme highlights produce a vertical line, usually red in colour; this is eliminated in the Frame Transfer CCD and reduced to an insignificant level in the Frame Interline Transfer CCD. Smear is a characteristic of the Interline Transfer CCD, but it must be remembered that it only occurs on an extreme overload, i.e. greater than five-stop overload (×32). ●Geometry of CCD sensors is inherently good, however any geometric distortion in the lens will be difficult to correct. CCD cameras therefore require good quality lenses. |