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The Joys of a Low Gain Screen  

post #1 of 10
Thread Starter 
No screen really has gain. All screens just reflect back the light from the projector. They don't add any. So-called high gain screens really are directionally reflective. Thus more light is reflected back to a specific viewing area, usually an area inside the boundaries of the screen itself, close to the projector. Thus by NOT uniformly reflecting the light we get more brightness to a specific area of the home theater. Sit outside that viewing area, much outside the perimeter of the screen, and you have much less brightness.

This nonuniform reflection exacerbates the problems inherent in CRT projectors and generally produces a lower quality viewing experience. No CRT projector has perfectly even brightness from screen center to the edges. Red, green, and blue are projected from three different locations and converged, as best we can, on the screen. High gain screens amplify these shortcomings for all to see.

To demonstrate this go to any video store and observe the rear projection systems. Notice the ugliness of the picture as you walk around. Watch that color shift. Hey, just move your head around and you will know its a rear projector and not a TV. Notice how the brightness drops off drastically as you move from the center to the side. You have just noticed the shortcomings of a high gain screen.

On the other hand, a low gain, matte screen like the Stewart Studiotek 130 has extremely uniform reflection characteristics. It has the same viewing characteristics as a giant TV screen, as opposed to the rear projection system.

Imagine every beam of light hitting the screen like a nuclear explosion, scattering reflected light equally in all directions from every point on the screen! Just like your computer monitor. Now the reflected light is perfectly mixed, it no longer matters that red, green, and blue are being projected from three different locations.

The "mixing" of the light evens out the nonuniform brightness level of our less than perfect projectors. Center to edge brightness uniformity is improved. We can move our head from side-to-side and there is no color shift. We can walk around the room and the picture looks exactly the same. Our guest sitting way over against the wall sees the same picture as the one sitting right next to the projector. Colors look better from the mixing of the light as well. The screen appears to us like a giant computer monitor or TV screen.

I had the good fortune of reading Joe Kane's explanations of the ideal home theater screen and he influenced me to buy the Stewart Studiotek 130. And I love it. Better to chose a smaller screen for adequate brightness and select a screen gain of 1.5 or less. Obviously if high gain screens provided more brightness without sacrificing picture quality, everyone would own one.

For any of you who think they wish to own a high gain screen, go look around and view one, then a low gain screen. All you need to do is walk around the room while you are watching the picture. You will instantly notice what I've been talking about. I hope this post helps many in carefully chosing the best screen.

By the way, a bright white painted wall has a "gain" of 0.9 to 1.0. Try projecting on the wall to see all the benefits of a low gain screen. If you are short on cash, paint the wall with several coats of the brightest FLAT white paint you can find. It will serve you well until you can afford your screen.

Don't compromise the picture quality from your expensive projector with a high gain screen.

[This message has been edited by David Levinson (edited October 30, 1999).]

[This message has been edited by David Levinson (edited October 30, 1999).]

[This message has been edited by David Levinson (edited October 31, 1999).]
post #2 of 10
I read about half of this but gave up because it was very difficult to read without paragraph breaks. Just FWIW.
post #3 of 10
Mr. Levinson,

Ditto. You just keep it up, man.

I do a fun demo when setting up projectors...I use safe-release 3M masking to mark my centers and define the 1.33 image...then play a full white field. The light drop-off is obvious...the center tape is nearly gone, but the ones at the edges jump out at you. Whenever I put the tape on a 1.3 screen, it disappears everywhere.

The difference in making a convincing image is considerable. Anything higher than 1.8 'gain' loses that sense of three dimensionality...of 'being there', or looking through a window...and 1.3 screens from Da-Lite or Stewart do the best job.

Sorry guys...screens make a difference. I've seen a lot of 'em, and while I enjoy them all...if you really want the most convincing image...do it right.

post #4 of 10
Which is why I am pushing at creating the best 1.5 gain mix that I can. I am using a 7 inch projector. Pushing the gain limits is about all I can reasonably do. Designing the lower gain screens is a bit too easy.

---Place Signature Here---
post #5 of 10
Guys, you have to admit that under ideal conditions, high gain makes a lot of sense. Glass bead screens have been poo-poohed by videophiles since they give the screen an annoying texture, but things have changed. Bead sizes are now in the sub 10-micron range giving the screen an extremely uniform and smooth surface. If you are going to sit in one place and watch a movie (I don't walk around while watching) and all the viewers are within the "half-angle" of the brightness range glass bead screens are incredible! There is incredible uniformity to the picture because most of the light just comes right back at you, minimizing projector edge brightness problems.

I, too have seen a lot of screens in my CRT setup from 1.0-2.8. I'll go with the high-gain every time. As always, it depends on your setup. Sure, the low-gain option is the safest way to go, you're sure to get a pure image from all viewing angles. But if the viewing situation is just right, high gain makes a lot of sense.

As always, YMMV. Make sure you can return a screen if you don't like it.

post #6 of 10
Sound good Mike! KBK, do you take returns on empty paint cans? http://www.avsforum.com/ubb/wink.gif

Kidding aside, what KBK is doing isn't pushing the laws of physics, but is stretching the psychovisual parameters of different ways of getting gain. After all, you can get identical gain figures from just about any color screen, but the viewing experience sure won't be identical.

It is not scientific instruments that watch movies in your home theater, but inevitably subjective individuals. So physics theory and measurments aside, it is conceivable to get a higher than reference level (1.3 gain) reflection from a screen without necessarily encountering equally proportional amplifications of the inherent problems of various projection devices such as David mentions above in his initial post.

KBK's analysis of the 1.3 gain recommendation for standard industrial products seems to be dead on (a safe and sure recommendation). However it stands to reason that what holds true to guide purchasers of standard industrial grade products should not necessarily be considered as a reference when considering highly customized experimental products such as KBK's screen coatings. And even if some of the constituants prove to be problematic in certain viewing situations, there is no reason why their side-effects couldn't be reduced to a negligeable level or why they couldn't be replaced with better all-around performance elements.

It all boils down to what projector you use, what size screen gives you a "Cinematic" experience, and what level of tolerance you have to specific projection devices' drawbacks... without forgetting the HT seating issue for those who prefer a narrow lower gain screen with many viewers widespread across an endless front row. http://www.avsforum.com/ubb/wink.gif


post #7 of 10

I would characterize the achievments of the different screen types in a different way. For projectors with a given black level, brightness and contrast ratio you get specific ranges of brightness for the different screen materials. The brightness levels I use below are only a hint and shouldn't be taken as exact.

CRT projectors
            gain             |0                  brightness          100%|
CRT, peak   1.0               +-------------------------------------+
CRT, ANSI   1.0               +------------+
CRT, peak   2.0                +-------------------------------------
CRT, ANSI   2.0                +------------------------+
As you know CRT has normally no problem with peak brightness in overall dark images. But in overall bright image some people wish to have mor juice, especially when the screen is large. For this a screen with gain will help. As the black level on a 1.0 gain screen is below 0.1 lumen it is no problem for the black level if a higher gain screen is used. Overall bright images look much better and overall dark images still look good. The limit on gain in this situation is given by the methods how the gain is achieved. Side effects like color-shift and hotspotting are the most annoying.

DLP/LCD projectors
            gain             |0             brightness          100%|
DLP, on/off 1.0                        +----------------------------+
DLP, ANSI   1.0                        +-----------------------+
DLP, on/off 0.5 gray              +---------------------------------+
DLP, ANSI   0.5 gray              +----------------------------+
For DLP/LCD projectors with more than 1000 lumens it is normally not necessary to use a gain larger than 1.0. They have plenty of brightness, but as the black level of most such projectors is rather high with values well above 2 lumens, overall dark images do not look very good. Here does the gray screen come into play. It moves the apparant lumen range down a bit and therefor makes the black better, but also has a bad effect on brightness by making it lower. It doesn't help to use a projector with more brightness because the on/off contrast ratio is rather constant. This results in higher black level with more lumens. So if you want to come to a black level well below 1 lumen, the lumen in overall dark images *and* overall bright images will be not be very high. To be able to really compete with CRT in this regard the DLP/LCD projectors must have a much higher contrast ratio. An on/off contrast ratio off 1000:1 would be able to compete with CRT on peak lumen. Together with a brightness of 1000 lumen this would be a real killer. Currently only the cinema black chip of TI can achieve this, but this chip is not available to the hometheater market.

What else can be done to improve the image from DLP/LCD/D-ILA?
The gray screens are certainly a step in the right direction to improve the overall appearence of images from DLP/LCD/D-ILA projectors with overall dark images, but the image will not look as bright as before.
Is there really no way to improve contrast after the light has left the projector lens until it reaches our eyes?
Is there no material which doesn't have a linear reflectivity or transparency over a specific intensity?
It would probably already help to have a material which has some threshold were below the threshold less light is reflected or transmitted than above the threshold. This could reduce the black level without hurting the white level to much.
What is with the non-linear response from phosphors or flourescensy (sp?)?
Is there really no physical effect which can help here and is not to expensive to use?

A lot of questions, but this probably makes this discussion interesting.


[This message has been edited by Emil Naepflein (edited 10-20-2000).]
post #8 of 10
While you may get the brightest image from a glass bead screen with your eyes at projector level you also get the most color shift and hotspotting. Viewing slightly off-axis, while staying well within the half gain angle will even out the image tremendously. You still get most of the hi gain advantage while also benefiting from the uniformity that results from being at nearly the same viewing angle respective to the projector at all points on the screen. Ray trace a low-ceiling mounted retro-reflective CRT setup and you'll see what I mean. I have a relatively narrow HT, so there no seats that fall outside the half angle. From the prime viewing location in my setup (below the projector) the angle formed between the projector/screen/viewer never varies more than 3 deg. to all points on the screen.

You can see that with certain setups where all viewers sit within the half angle, glass bead makes a lot of sense. By the way, the Hi Power material never reflects a gain less than 1.0 from any vantage point.

post #9 of 10
Originally posted by Pionek Wieczysty:
I've read that movie theatres replace their screens (and high gain ones at that) after a few years of use... Assuming that I'm not a chainsmoker living near a scrubberless blast-furnace, how long will it take for the gain to degrade significantly?


I am sorry... but I could not pass this post without a comment.

Most commercial theaters will only replace a projection screen under the following rigid policy.

1. The theater screen must have at least a 5 foot tear in the viewing area. No less than 10 customer complaints must brought to the attention of the theater manager before acknowledgement of screen tear is forward to regional management. Regional management must ignore torn screen memo for no less than 30 days prior to forwarding the memo to corporate purchasing. Purchasing must leave memo in "in basket" for 2 weeks. After the 2 weeks call screen manufacture, order new screen and demand replacement screen within 24 hours.
2. When screen surface has NO LESS than 50 percent of said viewing surface area covered with stuck Gummy Balls, then theater manager may request a replacement screen. The same procedures as torn screen stated above will strictly be adhered to. Note: If popcorn machine is also in need of replacement or repair, said popcorn machine will be given priority over screen and other projection equipment replacement.

The truth is we have a lot of of theater screens that have well over 15 years on the job.

Gee...I hope I don't get in trouble for this.
post #10 of 10
I wondered if they ever did anything about those gummy bears we gnawed on for a second and then threw at the screen... as kids. http://www.avsforum.com/ubb/smile.gif


Dan Henderson
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