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PostPosted: Fri Jan 05, 2018 1:13 pm 
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Is there a reason why CRTs typically used white beams of light and had to rely on a shadow mask to separate the colors, and the phosphors to do the coloring?


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PostPosted: Fri Jan 05, 2018 1:14 pm 
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I thought the CRT uses beams of electrons, not photons. The phosphors aren't a filter they're what produces light when excited by the electrons. (The light is coming from the phosphor not the gun.)


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PostPosted: Fri Jan 05, 2018 1:21 pm 
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What rainwarrior said.

You can't bend beams of light easily, but electrons (being charged) can easily be deflected magnetically (as in a TV) or electrostatically (as in an oscilloscope)

The very earliest of televisions did use "Nipkow" disks to ... uh, well, sure, let's say "bend" light ... but they were monochrome.

(CRT is "cathode ray" tube. "cathode ray" being an older term for "electron beam" because we didn't know what was coming off the cathode at first)


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PostPosted: Fri Jan 05, 2018 1:31 pm 
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It should be noted that electron beams have no color. Color+light is the last thing in the reproduction chain of the tube.

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PostPosted: Fri Jan 05, 2018 1:59 pm 
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Maybe you could say the Virtual Boy used scanning light beams? It had a vertical column of LEDs, and used a spinning mirror to send that light across the picture horizontally. (Too bad white LED tech wasn't cost effective by then, they had to go with red.)

lidnariq wrote:
The very earliest of televisions did use "Nipkow" disks to ... uh, well, sure, let's say "bend" light ... but they were monochrome.

Wow, that's interesting:
https://www.youtube.com/watch?v=8GYGxEk0btA
https://www.youtube.com/watch?v=D-yO07MQPMA

I like how it makes scanlines with a circular curve.


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PostPosted: Fri Jan 05, 2018 2:39 pm 
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They used three electron guns, one for the red phosphors, one for the green phosphors, and one for the blue phosphors. Each phosphor reacts to the one electron beam alone, not the others.

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PostPosted: Fri Jan 05, 2018 2:50 pm 
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The three phosphors each emit a different colour of light. The electrons coming in are all the same.

The way I understand the separation via the shadow mask: if the mask covers 1/3 of the space you can see a different 1/3 of the screen through it from any given angle. By placing the 3 electron guns (and the mask holes) at the right angles, you can line those spaces up with the intended phosphors.


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PostPosted: Fri Jan 05, 2018 3:27 pm 
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Distance between guns themselves and the focal point determines how far the mask is from the inner screen surface. Color purity is perfect (beams only land on their intended color) as long as the mask distance doesn't change, and it will when you have a white image for a long time, leading to small discoloration near sides of the screen. Newer aswell as lager tubes use invar mask which can be "driven" much harder before it deforms enough to cause color purity problems. Not to be confused with convergence which is the alignment of the beams to produce a single spot. On TVs in particular convergence is not very good near the edges, with blue and red breaking up quite visibly causing to small discoloration in edge details. Convergence is primarly controlled by the deflection system. The narrower the tube and its deflection angle the harder it will be to maintain good convergence aswell as focus, both mechanical (shadow mask distance and alignment from screen) and electrical. Flat tubes make it even harder to achieve than curved tubes, plus flat tubes weight a whole lot more due to needing whole lot more glass to withstand pressure from outside world.

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PostPosted: Fri Jan 05, 2018 3:38 pm 
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Are there any TV sets that use lazer beams and spinning mirrors to draw images onscreen?


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PostPosted: Fri Jan 05, 2018 3:52 pm 
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Sort of. Not in the way you're thinking, where the mirrors completely recapitulate the deflection coils of a conventional CRT.

Modern "laser" projectors often use the LASER diode (and/or diodes) to get a very narrow-band light source for higher energy efficiency and better gamut, and then reflect that light off a DLP array (and all of the pixels in that are, in fact, mirrors)


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PostPosted: Fri Jan 05, 2018 3:54 pm 
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There is one design that uses UV laser to scan across phoshpor stripes to produce image : https://www.prysm.com/solutions/laser-phosphor-display/

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PostPosted: Fri Jan 05, 2018 5:00 pm 
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psycopathicteen wrote:
Are there any TV sets that use lazer beams and spinning mirrors to draw images onscreen?

See "Laser video display" on Wikipedia.


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PostPosted: Fri Jan 05, 2018 9:38 pm 
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So people did try developing a laser based TV set. Any information on what type of screen it projected onto, or is there even a screen at all and it's all light flying straight out of a box?


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