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Klaas Robers wrote:Harry, you need quite some space in front of the flying spot CRT. If you look to the first photo in your posting, you see left of the CRT a Printed Circuit Board with a radio tube, which is a photo multiplier. I guess that you need at least a distance equal to the diameter of the screen. And place the slide as close as possible to the surface of the screen. This is because the system uses so called "proximity focus", that will say no focus at all.
For a good flying spot scanner you should use a CRT with short persistence, mostly blue phosphor.
Klaas Robers wrote:If you use a larger solar cell, then it doesn't work any way.
Because seen from the detector, the light spot on the tube should be able to hide behind a small spot on the slide.
I think also that for the set-up of your first photo, there should have been an opaque plate with a small hole just before the PMT.
That then is the "eye" of the light sensor. If the light sensitive area is larger, the light of the CRT-spot always can find a route around a small dot on the slide. And then the resulting picture is unsharp.
I hope that this makes clear to you how the flying spot system works, to get a sharp picture.
In fact I don't believe any way in proximity focus. I fear that the second photo that you sent is fake. With proximity focus you cannot have a sharp picture for standard definition TV.
Of course the space between the CRT and the PMT should be absolutely dark. I don't know how they obtained that in photo 1, may be that the cabinet was absolutely closed when it was operated.
In our 1970 TV-studio at Philips Research Lab (Nat.Lab.) we had a flying spot scanner for slides. But we had a lens that made a sharp image of the CRT-screen onto the slide.
So when you could look at the surface of the slide, you saw a sharp spot of light that scanned the slide. Then there was a colour splitting prism and 3 photo multipliers for Red, Green and Blue. There was no lens between the slide and the PMT's. But we obtained razor sharp pictures in 625 lines PAL.
Klaas Robers wrote:Yes Harry, those pictures were really unsharp.
If you place a black opaque hood over the PMT, and drill a small hole that "looks" to your slide, then you might get a sharper picture. And yes of course, the pinhole will make the output signal lower. But the PMT is sensitive enough if you increase the supply voltage.
Any way you should increase the space between the PMT and the slide, and make the space between the slide and the flying spot tube as small as possible.
And make a black tunnel between the pinhole and the transparent part of the slide, such that the pinhole sees only light from the slide. It is not impossible that such a tunnel was also part of your first photo, but that it was removed for making the photo.
Realize yourself that when you make the raster on the CRT larger and smaller, that you zoom the picture, and if you shift the scanned raster that you can do pan and tilt.
Steve Anderson wrote:I haven't read through all of the above postings thoroughly but there's an element where the persistence of the CRT phosphor can play a part. What is needed for a slide scanner is a very short persistence CRT - the opposite of a P7. A P1 (a general purpose green CRT phosphor) won't cut it. The decay in the light output is too slow, even at NBTV rates. The result is smearing and generally poor resolution.
Either a shorter persistence CRT is required or the image is scanned at very low rates and then sped up to generate the final image - not very practical. Our eyes may not see it, but the PMT (or whatever) certainly does. The reason P1 phosphors were used is 1) The sensitivity of our eyes to the green part of the spectrum, 2) The afterglow of a few milliseconds assists in the same.
PMT's are generally most sensitive in the blue end of the spectrum, a perfect match for a short-persistence blue CRT.
Steve A.
Klaas Robers wrote:Yes Harry, in fact you make a pinhole camera for the PMT. A pinhole is a lense with an undefined focus distance. So the pinhole makes a sharp image of the slide AND of the CRT-screen at the same time. A pinhole photo camera makes snapshots that are always sharp.
The idea of the mirror is not bad, however also for the mirror you need space in front of the CRT-screen.
Your original idea of a solar cell against the slide, drawing 2 and 3, does not work. Consider a white slide with a small black spot on it. When the light spot on the CRT is positioned just below the black spot, light of the CRT will shine around the black slide spot and illuminate the solar cell on other places. That is the reason that you will get an unsharp picture. Only large black areas will hide the CRT-spot almost completely.
The size of the pinhole close before the PMT defines the sharpness of your resulting picture. Something to experiment with.
The fourth drawing that you made is how we used the Flying spot in our Colour TV studio at the Nat.Lab. We had a disc with 20 slides. The avatar that I have, the lady with the roses, was one of those slides. The EBU, European Broadcast Union, had made about 15 of those slides, that were all well known by the experts, so they could judge the colour reproduction of the new TV-sets. On a certain moment, when the studio manager was cleaning up, I got a set of those slides.
The persistence of the CRT is also a point, very good Steve. However, if it is not too long, you can compensate for it in the video amplifier. And indeed blue phosphors have a much shorter afterglow. I have seen flying spot scanners for SSTV that used a P7 CRT and placed a blue filter somewhere in the light path. Then they used only the short persistence blue phosphor and discarded the yellow long afterglow.
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