PICK-UP TUBES
Of the two ‘live-image’ pick-up devices in current use the vacuum photoconductive tube was the first-comer; the more common solid- stage sensor will be described later. The generic term for photoconductive tubes is vidicon, the many other type names arising mainly from the use of different target materials.
A vidicon tube (Fig. 5.14) has an electron gun broadly similar to that described for display tubes, with the difference that the gun is designed for a very small beam current (< 1 μA) and very small spot size – around 20 micron diameter. It has electrostatic focusing, but this is supplemented by a magnetic focusing coil which is coaxial with the vidicon tube. The effect of the static magnetic focusing field is to impart to the individual electrons in the beam spiral paths, which at intervals along the beam length all cross through the same point. Adjustment of the strength of the axial magnetic field by varying the d.c. focus coil current enables one of these focal points to coincide with the target surface at the front of the tube. Another important effect of the magnetic focusing system is to alter the deflection characteristics of the scan yoke to those of orthogonal scanning, in which the scanning beam hits the target at right angles to its surface regardless of the deflection angle involved.
In a vidicon tube the final gun electrode is a wall anode, and a mesh is fitted over its outer end. Between the high-voltage mesh and the low-voltage target layer exists a steep potential gradient, and the electrons travelling down this gradient are greatly decelerated to impinge on the rear of the target at low velocity. The rear surface of the target is now effectively connected to the cathode via the electron beam and is thereby charge-stabilised to the cathode potential.
Vidicon target
The electron beam in the vidicon is not modulated from the gun end of the tube save for blanking during field and line flyback intervals. Even so the beam current does become intensity-modulated as a result of the target’s behaviour.
The image is focused on the front of the target which consists of two layers on the rear surface of the glass faceplate: first a transparent film of tin oxide on the glass, then a light-sensitive layer of semiconductor material as shown in the inset to Fig. 5.14. A low potential of, say, 30 V is applied to the tin oxide layer, setting up a potential difference across the thickness of the target. In effect we have a capacitor, and in total darkness its leakage resistance is very high. Where light is present on the faceplate, however, the resistance of the semiconductor layer becomes low, discharging the capacitor. Resistance is inversely proportional to light level so that a charge pattern is set up on the surface of the target to correspond with the pattern of light and shade focused on it.
As the electron beam scans out the interlaced pattern of Fig. 2.1(b) on the target, it restores each picture element in turn to cathode potential, see Fig. 5.15. This obviously involves recharging the ‘capacitor’ represented by each picture element to a ‘standard’ charge
– the potential difference between the target connection and the cathode. The charging current involved will depend on how much charge has been lost due to illumination of the semiconductor layer, i.e. the incident light level. In total darkness, as when the lens is capped, virtually no beam current will flow; on any brightly lit area of the target a strong charging current will flow. This charge current necessarily flows back to the source of target voltage, and its fluctua- tions represent the patterns of light and shade being scanned and analysed by the vidicon’s electron beam – in fact they form a video signal similar to that of Fig. 2.2. To convert this current (typically 200 nA) to a voltage we need only pass it through a resistor, represented by R1 in Fig. 5.15. Across R1, then, appears the video signal for amplification and processing.