The Vidicon Camera Tube

Take the whole tube apart in your head into four parts, front to back. THE FACEPLATE is glass with a transparent conductive coating on its inner surface — tin oxide, the same material that is on the inside of a shop window heater and on the LCD glass later in this batch. It is called the signal plate and it is the tube's output terminal. It is transparent so light gets through it and conductive so charge can be collected from it. THE TARGET is a layer of photoconductor evaporated onto the signal plate, a few micrometres thick. In the original vidicon this was antimony trisulfide. It is a semiconductor with a bandgap around 1.7 eV, which puts its response across the visible spectrum, and in the dark it is a very good insulator. THE SCANNED FACE is the back of the target, and there is nothing on it. It is bare photoconductor facing into the vacuum. THE GUN is at the far end, with a deflection yoke and a focus coil round the tube, exactly as in the cathode ray tube blueprint this one links back to. NOW THE MECHANISM, and it is worth going slowly because it is not what people assume. The target is a capacitor. The signal plate is one plate; the free back surface is the other; the photoconductor is the dielectric between them, a few micrometres thick, which makes the capacitance per unit area large. START WITH THE BEAM having just scanned a spot. The beam is a source of electrons at essentially zero volts, so it charges that patch of the back surface down to cathode potential and then stops — it cannot push it more negative than its own source. The signal plate is held at maybe +30 V. So every patch of the target is now a small charged capacitor with about 30 V across its few micrometres. NOW LEAVE IT for one frame time, one fortieth of a second in a 25 Hz system. IN THE DARK, the photoconductor is an insulator and the charge stays. The patch is still at cathode potential when the beam comes round again, so the beam has nothing to do and no current flows. Dark equals no signal. IN THE LIGHT, the photoconductor conducts in proportion to the illumination, so charge leaks through it from the back surface to the signal plate. That patch of back surface drifts positive. How far positive depends on how much light fell there and for how long — it INTEGRATES. WHEN THE BEAM RETURNS it must put back exactly the charge that leaked away, to pull that patch back down to cathode potential. That replacement charge, flowing through the signal plate to the external circuit, is the video signal. Three things follow immediately and each one matters. IT INTEGRATES, so it is far more sensitive than a device that only samples the light at the instant the beam passes. The whole frame time is exposure time. This is the same reason a photographic plate beats an eye at astronomy. THE SIGNAL IS A CURRENT INTO A CAPACITOR, so the tube's output impedance is high and awkward and the preamplifier that follows it determines the noise floor of the entire camera. Camera tube preamplifiers were an art in themselves. AND THE BEAM RESETS AS IT READS. There is no separate erase. Reading and clearing are the same operation, which is elegant and also means you get exactly one look at each frame.