Why did projection color tvs lag?
 

Just wondering, why is it that color rear projection sets came in to being so late in the game (the 70's)? What were the main limitations keeping even small 3-crt projection sets from coming into being, especially since they could have used 3 simple and small black and white picture tubes and shoot for much smaller screen size than what they eventually reached (65 inches)? Was it that good lens systems couldn't be built yet? The tubes still were just not bright enough and so would have been too big? Convergence would have been too complex without solid state electronics? I mean how hard would it have been to make something a lot smaller than that, like 25 or 30 inches, but still bigger than 15" and 21" complicated and expensive color pic tubes? And rectangular to boot. Even some of the the first BW sets in the 40's were projection and that was with only one pic tube.

I worked for Sony many years ago and knew several design engineers at the Trinitron display division. As I recall, Sony projection sets were manufactured in PA. They had several challenges but the three big ones were, first, achieving an acceptable field of view, the second was screen brightness, and the third, convergence.

Most people expect a bright screen image comparable to direct view CRTs. Achieving this in a rear projection set is a real engineering challenge. Driving a bright CRT is part of the solution; the real engineering marvel was the development of the high efficiency screen. It's basically the same problem encountered with a film projector screens - brightness fields are generally confined to narrow viewing angles. Solving this problem was essential to creating a screen with uniform high brightness over a wide viewing angle. Projection sets today use a specially constructed bipanel. It's a combination of a large fresnel lens matted to a lenticular light diffusing panel. The combination is designed to create a wide horizontal angle of view within a limited vertical aperture. This helps to concentrate the light to the most desirable viewing angles. This is a diffusion screen because light diffuses through it. Recall, that some of the earliest projection sets used reflected screens. The CRT assembly mechanically sat in front of the TV set and projected light onto a screen which then reflected light back to the viewer. Reflected screens do take up a lot of room. The use of diffusion screens made it possible for engineers to make projection TV sets more compact.

The second achievement was engineering a liquid cooled high intensity display tube that would resist phosphor burn. Operating a tube at high intensity does risk burning tube phosphor if sweep circuits suddenly stop working. Many early projection sets were prone to accidental phosphor burn with this type of circuit failure. Today's sets are better at sensing and preventing these problems - although they still happen. However, even with an efficient screen, to compete with direct view CRTs, projection set require high intensity projection tubes - things get expensive because 3 are required to create a colored image. I can recall once looking down into the CRT's of a projection chassis operating on a design test bench. The CRTs were so bright they can blind you! You are basically accelerating electrons onto a very small area with typically more 30KV accerating voltage. I saw a demonstration of a tube operated with liquid cooling removed and boy did it heat up quick. The faceplate got so hot that the phosphor was totally cooked. Projection tubes do tend to push CRT brightness technology to the limit.

The third problem was convergence. Early projection sets, without all the fancy computer alignment stuff found in todays sets, typically made the task of convergence overwhelming for most technicians - especially corner convergence problems.

Of course, there is trend to make modern projection sets light weight and portable - somewhat hard to do with traditional projection systems; however, DLP technology really changes all that now. Digital light projector use optical semiconductors instead of CRTs. This is a totally different technology that's revolutionized home theater and business applications.

Tom

Regarding the brightness issue, a simple calculation shows:

Assuming
1) 100% collection of all the CRT light output (of course you can't),
2) a 5-inch CRT and a 50-inch picture;
that is an area ratio of 100 to 1.
In this overly optimistic case, to make a bright (50 ft-lambert) picture requires the image on the CRT to have a 5000 ft-lambert brightness, roughly equivalent to a sheet of paper in full sunlight. Any practical inefficiencies due to f-number of the optics, etc, either lowers the viewed brightness or requires a brighter CRT (The vertical directivity of the screen compensates this to some degree.)

early projection not suitable for everyone
 

I agree, screen brightness was not that of crt from rear projection, owned a Kloss model one from 1980? Watched it using a wall --- original curved screen not vailable-these took up a lot of space. Corner convergence was bad due to this. Produced a beautiful picture and worked for many years. :yes:

Then I owned a 3 CRT zenith similar projector from late eighties. Was designed to shoot on flat wall, had much more brightness--can be watched with room lights on-- showing 10 foot screen image. The convergence to tweak right was a bear , proper convergence was only acheived right before I sold it. Still a great set-- had to buy special cabinet for unit to be at right height and parallel to projection wall. These units being expensive usually people only sold them if they had major issues. These were generally hard to find in good used shape.

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A single-CRT projo with color wheel will flicker badly unless it has video storage so it can run at 3 times the frame rate. At that point it will still have noticeable color fringing on motion on a large screen. Single-chip DLP projos with color wheels run at up to 360 fps or so to reduce the problem.

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There was at least one plan that was printed in an electronics magazine in the 50s that had a color wheel in a Norelco projection set. The wheel only needed three filters instead of the usual six. I believe that one of the early CBS prototypes used a projection tube as well.

Steve

Here is a link to a huge color projector that I hope to have restored in the next couple months: http://www.myvintagetv.com/amphion_600.htm

It will be interesting to see what kind of picture quality can be had from 1960's technology and money was no object.


Chuck

Here is one made in 1955:

http://www.earlytelevision.org/color_projection.html

I know I saw a build-it-yourself article using a color wheel and a 1-tube projector - don't know if I have a copy.

I think the same article that gave details for building the Colordaptor had instructions on using a Norelco projector and small wheel.

http://www.earlytelevision.org/colordaptor.html

The color wheel adaptation appeared in

"Color Television Selection - Operation - Servicing, 1957 Edition, a Compilation from Radio and Television News, i.e., that mag with all the color models on the front showing a bootlegged picture of Lucille Ball.

Here's the first page of the article

Very cool. How much juice is that sucker supposed to draw?

Specs say 650 watts.

Chuck

Guess you'll be wearing your lead apron Chuck when working on this monster live! :eek: Golly 40KV and all those X-rays shooting around. Makes you wonder how many Techs/Engineers got nuked before they discontinued the model. :smilemad:

Talk about realism, they were probably watching nuclear tests on that TV, and getting radiation and heat from the 650 watts of heat pumping out of it at the same time!
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