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I noticed that the Da-lite high contrast screen I have is only .80 and gray. I also have noticed that many say that the blacks on the brighter Sanyo PLV 60 are not as black as the Sony vw10. It seems that gray screens are never high gain. Does the brightness that a high gain screen add diminish blacks? There seems to be an attempt to reduce brightness to increase black level. For example the Sony has a Cinema mode which further reduces brigthness.Am I to assume that a very bright projector, say 2000 lumens will never have dark blacks? Any help explaining this would be appreciated.


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This is confusing to me, too.


My confusion is about how merely changing the reflectivity of the surface is going to improve the contrast ratio.


What is wrong with the following table?

Code:
Code:
Flat White     Gray
Gain                      1.0            0.6
Contrast Ratio            500:1          500:1
Peak White (footcandles)  20             12
Min Black (footcandles)   0.04           0.024
Does the Grayhawk screen and/or the DaLite screen have a polarizing filter and a 1/4-plate (or equivalent) somehow magically affixed in front of the gray substrate? If so, how is this polarizing filter more effective on low light levels than on high?


Is it really a perceptual phenomenon, and light meters don't register the same improvement that people report?


Is the light scatter that over-brightens the dark areas due to the inability of polarizing filters to be 100% efficient? Or is it due to light that scatters around the polarization filter? (If it's the latter, I think this explains how a polarizer at the screen *could* measurably improve the contrast ratio.)


It seems like the optical equivalent of Maxwell's thermodynamic demons must be at work. How does this magic work?


-yogaman

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Jaron:


Glass beads should reflect *any* incident photons regardless of how many there are.


Remember that Contrast Ratio describes the ratio of peak white to minimum black. There is still some light in the black areas.


Maybe this posing of the issue will help spread my cloud of confusion:


How could a glass bead "know" if an incident photon flux is coming from a very weak projector's whitest output or if it's coming from a very bright projector's blackest output?


-yogaman
 

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There's no polarizing magic going on. The grey screens *do* improve contrast, but only because they reject ambient light better than a white screen does.


In a theoretical perfectly light-controlled room with completely absorbent walls, this would be no no consequence. As the level of ambient light in your room increases, the contrast level will be helped by the grey screen.


They do not increase the contrast ratio of the light coming out of the projector, sorry to say. (Although I can see how better blacks can fool your eye into thinking the contrast ratio is better)


Andy K.
 

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Gray screens also provide a darker visual reference for an almost all pixels off screen in ambient light. White screens in ambient with all light off look white (big surprise). Gray screens provide a darker visual reference.


When CRTs were all that was available, getting more light was the technical problem. Now that we have light cannons, providing a darker background visual reference may be more important.


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Having had some time to think about it, and to fool around with a mirror, a flashlight (that's a "torch" for you mother-tonguers - no offence! http://www.avsforum.com/ubb/wink.gif ), and a crystal ball (well, a clear plastic ball), I think I begin to see how the glass beads that Jaron mentioned could do the job that Andy called "rejecting" ambient light.


If I hold the flashlight behind the sphere, I see a very bright image of the bulb in the center of the sphere. Similarly, if I hold the ball against a mirror, and hold the flashlight near my eye, pointed at the ball, I again see a bright image of the flashlight bulb. If I move the flashlight to one side of the sphere, the image of its bulb almost completely disappears.


A quick dusting of my freshman physics text reveals that this is due to total internal reflection which arises from the differing indices of refraction of air and glass (or plastic).


So the magic of the axially preferential gain (or "ambient light rejection") is in the spheres. This is a glass bead game.


And the reason the substrate needs to be gray instead of white is probably because the spheres aren't completely efficient. Their surfaces reflect some ambient light toward the viewer, and scatter some of the desired, axial light off axis. Also, in part due to packing limitations, some substrate areas will not have spheres covering them.


I suspect that the best gray level arises from an empirically determined tradeoff between the desire for bright whites, implying a highly reflective surface, and the desire to eliminate scattered light, implying a darker substrate. I have been told that Da-Lite and Stewart have respectively chosen gains of 0.8 and 0.9. It appears they chose different "sweet spots".


Has anyone had the chance to judge Da-Lite's material side-by-side with Stewart's? Did your preference vary with projector and ambient light brightness?


-yogaman
 

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Ken Elliott:


Sorry I missed your post while I was typing. I think you have voted for the perceptual process giving rise to the non-linear improvement in apparent contrast.


I don't deny that that could well be part of the effect here. The way the human visual processing system arrives at what you think you are seeing is truly amazing.


For instance, did you know you only have a relatively high concentration of color-sensing cones in the center of your field of view? The colors that you think you see around the edges of your vision are something your brain has largely "made up" or remembered.


Another indication of the great amount of visual processing that we do can be seen in the way we tend to see the color of an object in a consistent way, regardless of whether we see it in daylight or incandescent light. Take pictures an object under these two types of sources, and see how the color temperature affects the two recorded images.


But I'm comforted that the contrast-enhancing screen question seem to be based on some physics, too.


(To resolve if there are both a perceptual component and a physical one, one might use a spot photometer. I don't have one, unfortunately.)


Some interesting reading for the inquiring mind:

background
distribution of rods and cones
cone spectral sensitivities
visual dynamic range
contrast vs. acuity
brightness and contrast
seeing colour
retinal network

 

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Oh yes.. I forgot to mention, that.. not only could the corporations in ANY country file against your claim any time they wanted, and hold it up INDEFINITELY, without recourse by you.. they have the right at any time to look at the confidential aspects of your claim at any time, without your knowledge or consent.. in total breach of the client/patent-claim-process privelege. What the hell is that??!?!? Still think it's a democracy?


THESE are the kinds of things that have been happening lately... while you where asleep at the wheel.


Anyway.. to stay on subject a bit, maxwell is useful in these screen designs. hell, even color filtering in CRT lenses brings about non-linearities. Think about it.


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Ken Hotte

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[This message has been edited by KBK (edited 06-22-2001).]
 

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Quote:
Originally posted by yogaman:
This is confusing to me, too...


It seems like the optical equivalent of Maxwell's thermodynamic demons must be at work. How does this magic work?
You're starting to get warm.. it gets pretty ugly, to say the least.


Interestingly enough, the full set of maxwell's equations is EXTREMELY difficult to come by. Text's of his work, that are COMPLETE.. sell for a minimum of $4-5k. This contains the small bit that..when properly attached to Einstien's theories, actually accounts for the inconsistencies that make the planets 'stay' where they are.. suddenly realtivity works! The niggling little errors are gone!Heaveside's mauling of Maxwell's work is all that generally survives anywhere, and is used in motor design..which it was designed for. You see, Heaveside 'shortened' the equations, to remove the parts taht are not generally accepted or understood, and are considered to be unessesary.(for engineers, who did not need the more complex bits, he presumed) It's to bad that they are the small angular componets that are the manifestation of the strong force leaking into the weak interactive...throw them in, and suddenly everything works.


Of course, this allows for Over Unity engines, and the like, ie, perpetual motion machines. Gravitational vortices, etc. Reality... people!..instead of this false, incomplete, 'science'. No wonder you guys can't figure anything out. You are working with a flawed sense of reality.


To cut to the chase, and inspire a bit of thought..... there are AT LEAST 400 patents registered, and issued in Japan, on over-unity devices, and effects as patents. NOT A ONE has ever been issued in the USA. (recently patent law was changed, so that corporations could 'challenge' ANY patent application, and claim PRIOR 'art', simply by producing lab notes....what the hell is that?) All are turned down, as 'invalid' physics. What else is to be expected by a country that bases it's controllist behaviour and 'regime' on OIL? Kill the planet, for a buck, and all due to badly raised children. What a shame. Word to the wise, though..please understand the responsibility you bear if you go messing with this stuff. It ain't money, and soon after understanding it, you will agree. It's just too bloody dangerous in the hands of the average 'talking monkey'.


How much longer?

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Ken Hotte

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[This message has been edited by KBK (edited 06-22-2001).]
 

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yogaman,


For the record, I believe that both physics and perception are necessary considerations to appreciate the viewing quality of screens whether white, gray, black, pinkish-gray, purplish-gray, etc.


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Ken Elliott
 

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kelliot:


I apologize for misstating your position.


To everyone else:


Dean McManis owns a Grayhawk, and he has posted that the Grayhawk does NOT have glass beads. He continues, "the Grayhawk does have translucent optical coatings on top of the gray to reduce incedent light washout."


I don't know what to conclude about the physics of this coating material. If translucency were its only significant optical property, I don't understand how the coating could help the perception of contrast. Apparently the coating gain is about 1.6, though, because I've been told (or read?) that the Grayhawk base substrate has a gain of about 0.6; finished Grayhawk gain is published as 0.95.


But without more information about the surface coating material, I have to give up trying to understand how it works to preferentially reject ambient light. I apologize for dragging you all through my wild-goose-chasing attempts.


Regards,

-yogaman
 

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


" What brought on the idea of the silver backing was that I saw the silverish screen (#4) on ccrim's test and thought about it a bit... I had the ingredients around, so, try it out. About a month ago I was coming out of the local Home Depot and found a new can of rustolium brand, designer series silver metallic spray bomb can. There it was, new, on the ground. The cost involved appealed to my scottish side. I hopped over to it an picked it up.(scotts hop around on one shoe to save the other for later...)

Suddenly I have use for it!


After three coats of pearlecent, It is working quite well. Contrast had gone up a noticable notch. It tendsz to skew towards a bluish cast. I have a proper silver mix in the wings when I can afford to buy it. I am trying it out right now, in a semi-lit space. Daylight, overcast, with light coming into the room with all curtains closed. My room has far from the proper amount of ambient light control. The screen is too big for the projector.


The results, initially is that the depth preception of the picture has improved. slightly dimmer in the brights but much blacker in the blacks.

Proper sized screens, with modern projectors should have no problem. Mine is a used ECP 3000 from 1988."


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


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Ken Hotte

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[This message has been edited by KBK (edited 06-25-2001).]
 
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