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Old 01-07-2007, 10:58 AM - Thread Starter
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I'm a hobbyist, not a pro like a lot of you. I was trying to explain a CRT projector to a buddy at work who did teach science at some point in his life. I was trying to tell him that the projector works the same way as a CRT TV only three times. He told me that there had to be a "lamp" in order to project an image. I said it was just an electron beam scanning back and forth in a tube for each color. Honestly, and it should be fairly obvious by how, I don't know exactly how it works. Is the tube lamp-like? I rejected this notion based on my knowledge of how black is produced.

How is the image projected? Politely now, what do I tell my buddy?

Layman's terms for me please.
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Old 01-07-2007, 11:11 AM
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It's three tv screens, one red, one green and one blue.
the screens are really bright almost too bright to look directly at.

You can also use a regular 19" tv and use a fresnel lense to project a picture on a wall.

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Old 01-07-2007, 11:17 AM
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CRT projectors work much like any Vacuum tube. The Vacuum tubes in an old radio work on the same basic principles. There is a cathode which goves off electrons, a high voltage anode that draws the electrons to it, and a gate between them that varies voltage and controls how many electrons get through. With CRT there is an additional Phosphor face right next to the anode which lights up as electrons carsh into it.
Think of projection tubes as audio tubes on Steroids. There is a good explanation here
http://www.etechvideo.com/techarticle1.htm

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Old 01-07-2007, 06:19 PM - Thread Starter
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Thanks guys.
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Old 01-07-2007, 07:43 PM
 
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Quote:
Originally Posted by Crescent View Post

I'm a hobbyist, not a pro like a lot of you. I was trying to explain a CRT projector to a buddy at work who did teach science at some point in his life. I was trying to tell him that the projector works the same way as a CRT TV only three times. He told me that there had to be a "lamp" in order to project an image. I said it was just an electron beam scanning back and forth in a tube for each color. Honestly, and it should be fairly obvious by how, I don't know exactly how it works. Is the tube lamp-like? I rejected this notion based on my knowledge of how black is produced.

How is the image projected? Politely now, what do I tell my buddy?

Layman's terms for me please.

There is no lamp. The best way to explain is with the silly fresnel scam things you see on ebay. There, you can purchase plans to "Trurn your TV into a giant screen!!!111eleventy!" Okay, now stay with me here.

All that scam is, is basically how to take a large lens (say from an overhead projector one of those things your teachers/professors used when you were falling asleep in lecture) and put it in front of your television. If you remember way back to high school physics and playing around with lenses, if you have a lens like this you can focus a plane onto another plane. Your television screen is just such a plane, and it's lit up. Put a giant lens in front of it, and you can focus it onto a wall. Now, the thing with a regular television is that it isn't very bright. If you actually do this, you are basically making your own DIY CRT projector: you have a CRT that makes an image, a lens, and you're projecting it onto something. Thing is, the image is going to suck, because the TV is way too dim, the lens is terrible, etc.

A CRT works basically exactly the same way. You have a CRT tube, and you have a lens, and it gets focused onto the screen. Except that you have three tubes instead of one, and they're quite bright. But there are no lightbulbs involved. There are just three very tiny monochrome televisions in there, basically. That is probably the best way to explain it to someone. If they don't understand, then you could explain about the magic gnomes...
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Old 01-07-2007, 10:56 PM
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Hi,
The basic idea is simple. You have three very bright Cathode ray tubes A.K.A. little TVs (the "lamp" part). These are sent thru three lenses which once converged project a nice picture on the screen. Your friend is correct in needing a bright light. Just not that it needs to be a lamp. You may also want to ask him how that old rear projection TV that he saw at best buy worked.

If I am wrong on any of this please correct me.
John

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Old 01-08-2007, 12:09 AM
 
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Originally Posted by raven8474 View Post

Hi,
The basic idea is simple. You have three very bright Cathode ray tubes A.K.A. little TVs (the "lamp" part). These are sent thru three lenses which once converged project a nice picture on the screen. Your friend is correct in needing a bright light. Just not that it needs to be a lamp. You may also want to ask him how that old rear projection TV that he saw at best buy worked.

If I am wrong on any of this please correct me.
John

Yeah, I would just avoid the term "lamp" or "bulb" or anything like that. The light source is a CRT plain and simple. That's what it is.
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Old 01-08-2007, 12:19 AM
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Its more like 3 B/W TV's turned away from you, actually, with blue,green and red lenses on them.

Its not a lamp in the sense that there is nothing to incandesce like in an ordinary filament lamp (or a Coleman lantern or gas light with a mantle) that creates the light as there is in a modern DLP or LCD PJ.

Its closest to a simple florescent tube where the invisible electrons crossing the length of tube from end to end, excite a phosphor coating on the inside of the glass envelope that DOES emit light when excited (which,btw, is dissimilar to a 'neon' lamp that has a gas that emits the light.)

There is a similar phosphor coating on the inside of the CRT face that is excited, sending out photons created by the electrons from the tube neck hitting it.
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Old 01-08-2007, 12:29 AM
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Chris,
I'm sure you know that this was an over simplified description of the process. My reasons for using that 4 letter word starting with L were to explain how the system works. And that there are other methods to light up a screen.
John

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Old 01-08-2007, 12:37 AM
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They are definitely NOT black and white CRT's, with colored lenses on them. They are monochrome(single-color) CRT's. 1 each R,G,and B. Each CRT has its own specific type phosphor, which when exited by the cathode ray, emits the proper color. Some projectors have color filtering, either in the lenses or glycol, but this is only to isolate the intended color further, therefore refining color output. Even in the color filtered models, each CRT still produces the appropriate color.

If each CRT was grayscale(black and white), with color filters in front, our lives would be MUCH easier. Unfortunately though, the strength of color filter needed would greatly reduce light output, and probably not display color correctly across the entire luminance range. Also running gray light through a colored filter would not result in the correct color. Another point would be that the tubes would have to be driven so hard to produce decent light output through the filters that they would NOT last long before wearing beyond use. I am sure there are other reasons but these are the top ones I can think of.

1272 stack coming soon!:)
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