Originally Posted by Rinehart
Ah very good, thank you for that. As I said, I suspected that I had made a fundamental mistake, and there it was: the electrons don't pass through the insulator. I had always envisaged it as something like what you get with a spark plug or spark-gap transmitter, when the voltage rises high enough the electrons jump from one electrode to the other. Of course this produces a lot of heat, but through the miracle of magic thinking, I assumed that didn't matter.
So let me see if I've understood:
1) in targets with photoemissive materials, light falling on them creates free electrons which escape from the material altogether, the number of which depends on the intensity of the light.
<YES>
2) these electrons eventually do settle back on the material, almost all of them at some other point.
<Yes, in the iconoscope. In the image orthicon they are propelled to the target and do not resettle on the photocathode.>
3) this process is constantly occurring all over the plate, with the result that on average the distribution of electric charge over the plate as a whole corresponds to the image. (Perhaps the expression "electron image" comes from this?)
4) the purpose of the electron beam is to deliver a constant electric current to the mosaic, one that can move across it without needing a physical conduit, like an electric wire.
<The action is to deliver a constant current. It was probably the original intention to deliver a variable current depending on the charge on each element, but it didn't work that way.>
5) at the moment that the beam strikes the mosaic, emitted electrons from other points on the mosaic can't settle on the point the beam strikes. This causes a discontinuity; the amount of charge that can't settle there corresponds to the number of photoelectrons released at that point.
< At the moment the beam strikes an element of the mosaic, the element emits a number of secondary electrons that is influenced by the charge (voltage) on that element. The variation in this secondary emission is unfortunately much less than the total charge that an element could accumulate if there were no electrons raining back on it the rest of the time.>
6) this causes current flow to the signal plate, and because the beam moves over the mosaic in a very precise way, the signal is a representation of the electron image.
<The instantaneous change in charge causes current to flow to the signal plate...>
7) unfortunately the scanning beam itself can cause electrons to be liberated from the mosaic, and these are called secondary electrons. These also are sent to the signal plate, resulting in a lot of signal noise.
< It's not just that the secondary "rain" causes noise, but it also cancels out most of the signal since it discharges the mosaic elements to a great extent.>
8) to reduce the number of secondaries sent to the signal plate, there is a secondary electron collector which will rout them to the anode to complete the circuit instead of being sent to the amplifier and then to the transmitter.
<yes>
Can I ask you a couple more questions? The first concerns an unimportant problem in an oscilloscope and doesn't pertain to a camera tube, but which is a vital problem for a picture tube: how did they get around the problem of varying the intensity of the electron beam without changing the cross-sectional area of it?
<Careful design. Sorry to be flippant, but gun design is very complex and I would have to study it further to give an answer of HOW the desired spot distribution is obtained. No design is perfect, and spot size does change with current, but the effect should be minimized. The other aspect that should be minimized is any extended halo, which would produce a fog around highlights. Oscilloscope guns typically have a good central spot, but also have a halo that makes them unsuitable for pictorial applications.>
And secondly, what kind of oscillator produces the sawtooth wave that drives the deflection circuits?
< The power stages are different for electrostatic vs. magnetic deflection. Horizontal magnetic deflection in particular depends on using the yoke inductance to turn a voltage pulse into a current sawtooth. Vertical and and horizontal electrostatic deflection typically use a linear amplifier. Vertical magnetic deflection is basically linear, but will include some pulse voltage to overcome the yoke inductance. Several different types of oscillators are possible to drive the power stages.>
To answer your question about the RCA amateur iconoscope tube, the insulator was in fact transparent, and I should have mentioned that it was. The article I read didn't give any details as to what it was made from. (It also mentions that the signal electrode was transparent, but doesn't give any details about that, either.) This wasn't the only difference, however: for one, it only produced a 120-line raster without interlacing, and the secondary anode voltage was only about 600V, so perhaps that has something to do with it?
Could you send me the publishing details of the textbook you have quoted? I'll try to see if I can get a copy from Bookfinder.com.
As always, thanks for your help.
PS: Oh hang on, there's one more I have thought of: it seems that in the early days of TV, to create the effect of a dark space like an office in the middle of the night, they used the normal blinding light but simply reduced the gain on the camera signal. I suppose that it's the equivalent of Day For Night shooting in film. Does anyone know how convincing this was? If it's anything like its cinema counterpart it would have been dreadful, but you never can tell.
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