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Discussion Starter · #1 ·
I posted this in the screens forum, but this forum gets more traffic...


I use a 600 lumen XGA LCD projector, the Mits X100.

What I want is a gray screen (improved perceived blacks), but with some additional *postive* gain, like 2.0-2.5-ish to improve the overall brightness on a 7-8 foot wide screen.


The gray screens discussed here, either home brew or commercial materials like the Grayhawk, seem to all be lower than 1.0 gain.


Does physics allow a > 2.0 gain gray screen that raises the foot lamberts overall, but still improves perceived black level through the gray color?


What if Da lite sprayed the glass beads used in their glass-beaded white screens (~2.5 gain) onto a gray colored backing material rather than the stock white material? Wouldn't this give a proper "gamma-corrected" surface for the reflected image? i.e. blacks should be perceived blacker, but mid and white tones should appear brighter due to the increased gain of the glass beads (?).


Essentially, what digital projector owners need in a screen material is a material that reacts to the projected image light according to an optimal gamma-curve- i.e. for a light level at or below the black level light output of the projector, the screen reflects nothing. At higher and peak light levels, the screen material reflects an optimum, calibrated ft-lambert light level, calibrated to ISF standards.


Of course, a material that could react in this fashion probably would cost more than the projector shooting images onto it http://www.avsforum.com/ubb/wink.gif ...

 

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The Grayhawk is a grey screen with as far as I know a base grey at about .7 which is enhanced to about .85-.9 with a higher gain coating.


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Ken Elliott
 

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I don't understand why you would want a screen material to do gamma correction. This would mean some kind of nonlinear screen response. Projectors have the controls available to do this and it seems like it would be much more difficult to precisely adjust the spectral response via screen material. I'm confused on this one.


John Moschella
 

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You are correct in your suspicion that physics and technology at this point won't allow it (as far as I'm aware). Either a screen amplifies light reflection, or it diminishes light reflection. I have seen the Greyhawk and the Dalite Day-mat grey screen (albeit in commercial applications rather than HT), and they work simply by reflecting less light. The difference between them and a grey wall is that the grey color used has been carefully selected NOT to change gamma mixtures of the RGB signal - something you or I could probably not do by going down to home depot and picking up some grey paint. Also, the money you pay for such a screen goes to quality control (surface uniformity and flatness) and customer service. Are they overpriced? Probably - but if you want the best regardless of how much actual quality gain you get and are willing to pay for it...go for it.
 

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Discussion Starter · #5 ·
re: "This would mean some kind of nonlinear screen response. "


EXACTLY!


For digital projectors (especially LCD's), we want a screen that reflects nothing (or close to it) when the ft-lamberts at a given spot on the screen are equal to or less than the black level ft-lamberts output from the projector. At ft-lamberts above the black level of the projector, the screen can respond linearly.

 

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Quote:
Originally posted by Rgb:


For digital projectors (especially LCD's), we want a screen that reflects nothing (or close to it) when the ft-lamberts at a given spot on the screen are equal to or less than the black level ft-lamberts output from the projector. At ft-lamberts above the black level of the projector, the screen can respond linearly.

[/b]
That statement makes sense to me and if it were possible would allow a LCD to have black instead of gray. Apparently I just misunderstood your reference to "gamma-corrected". The way I understand it though this is not gamma correction, I would describe it as a notch filter that cuts off low intensity irradiance.



John Moschella
 

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Discussion Starter · #7 ·
re: "I would describe it as a notch filter that cuts off low intensity irradiance."


YES!


Maybe a lens add-on could accomplish this. The add-on could be produced in many models, with a varying "notch filter black level" set point, depending on the black level output of your particular projector.


Sorry for the confusion on gamma terminology- my intent was to express the concept of the amount of light reflected from the screen relative to the instantaneous amount of light emitted by the projector, adjusted by a function ("gamma function") to correct for the above zero black-level light output from all digital projectors.


 

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"re: "I would describe it as a notch filter that cuts off low intensity irradiance."


Not to rain on your parade, but forget about it. The entire screen would have to be an active device that senses and responds differently to different intensities.


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Noah
 

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What it sounds like you want is to eliminate stray light and enhance the contrast of your LCD system. You could add polarizers that are aligned with the LCD's to your screen.
 

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Discussion Starter · #10 ·
What I'm aiming at might also be described as a "high pass photosensitive filter", designed into either a lens or screen material. The "photo-crossover" light level would be determined by the black level lumens output from the projector (and divided by the surface area of the screen or lens to obtain ft-lamberts).

 

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Discussion Starter · #11 ·
re: "The entire screen would have to be an active device that senses and responds differently to different intensities."


...or a screen material with a photosensitive chemical coating that acheives the same result, i.e. reacts to the impinging light level quickly enough to reduce the black level in dark scenes.


The more I think of this, the less far out it seems. The material/coating would *not* need to react as fast as 1/24 second (film frame speed) to adjust its gain based on the impinging light level- it could change its instantaneous gain (gain varies over time and over the surface area of the screen, depending on light level projecting onto that unit area).


If you think about how most real films are shot, dark scenes occur over several minutes or more. I have no issues with scenes that have bright areas with dark areas (shadows, etc) in the same frame. It's only low contrast, dimly lit scenes that last for many minutes (or an entire movie, like Dark City http://www.avsforum.com/ubb/wink.gif ) that I have issues with gray black level and low level detail. The screen could take several seconds to change its gain. Minutes is probably too long, but a second or two would probably be OK to really improve the apparent black level from digital projectors.


Maybe its time to concoct a "mad scientist" paint mixture with light-reactive elements for a home brew screen...
 

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RGB,


Shawn at Cygnus has been researching such a lens or screen material.


I don't know of any material that has the non-linear properties that you are refering to, but if a material exists Shawn will find it (if he is still looking)


Now on the perception side of things, I think the best image comes when your peak whites are roughly in the 20 to 30 ft-L range. If you go too low, white will eventually start to look grey. If you go too high, low level detail might be overally exagerated and the blacklevel will be too objectionable on dark scenes. This is not to say that a super bright projector is a bad thing, just that it needs to have a grey screen to go along with it.


Also keep in mind that gamma adjustments in general can greatly enhance an image. The problem is finding a source and/or a projector that has it.


If someone comes out with a magic filter or screen surface, that will be cool, but unless you have a PhD in Chemistry or Physics, I wouldn't worry too much about trying to find it all by yourself.


My two cents.


-Mr. Wigggles


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******************

Take for example, the gesso backed pearlecent MIX. The pearlecent MIX is

translucent in nature. It has a natural tendacy to be reflective, or to have a

inherent 'gain characterstic'. This is why the low number was found when

the lamp/meter combination was placed up against the PURE pearlecent

(painted upon a piece of drywall that had previously been primed).

The high number comes from the nature of the two coatings as they work

together. In this case the highly diffuse gesso, and the slightly less diffuse

pearlesent MIX on top of it. Ambient light is a serious problem with this

combination.


*****************

http://www.avsforum.com/ubb/Forum9/HTML/000277.html


Non-linear -multiple coating- black level enhancing screen making systems. That was the whole point right from the beginning.


You could get the simple, one shot coating system, or, if you think that you are handy with spraying systems, you could go for your own *perfect* creation, that maximizes YOUR black level and available contrast range, for maximum fidelity and image 'believability'. Your choice. Make it yourself, or have it built.


CRT screen systems inherently -if the product is done right- only need a single pass paint system, but falsifying that black for a digital projector immediately gets you into a headache... that if properly looked at, actually is your salvation, instead.


The black level perception will shift depending on:


the room ambient light

the size of the screen

how the light hitting it causes the eye to operate

the settings on the projector

the inherent contrast range in the projector

the age of the bulb

the gain of the sreen

the layering of the screen

screen/room/projector dynamic response curve

etc, etc, etc.


"Where do you want to go today?"


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goosystems.com


Ken Hotte

[email protected]


[This message has been edited by KBK (edited 05-13-2001).]
 
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