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Discussion Starter · #1 ·
I'd be curious to know what other forum members think about

using DLP projectors with 6-element color wheels (RGBRGB)on

gray surfice material. It seems to me that without the clear segment, the picture tends to be a bit dark on gray screens. Maybe a unity gain surfice would be more appropriate -- at least if the projector is outputting less

than 800 to 900 ANSI Lumens. Don't get me wrong. I think six-element is defenitely superior (no rainbow, better color saturation), I just think they need higher gain screens.


 

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There's no inherent reason why a 6-element (or 3-element) wheel would need a higher gain screen. If you get 12-16 ft-lamberts off a grey screen in a darkened room there's no reason why one would need to go to a higher gain. That being, said projectors without clear section generally are spec'ed at a lower brightness. I prefer to think of those brightness figures as being less inflated not actually lower. Besides, Stewart's Greyhawk is nearly unity gain (0.9+), less than a 10% different in brightness should not be noticeably darker.


Regards,


Kam Fung
 

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My take on the whole thing with DLP VS. grey screens:


The DLP is UNIQUE amongst digital projection systems. It is dependent on an entire different set of characteristics than the LCD,DILA, and..is actually leaning slightly more towards that of a CRT unit, believe it or not.


It is the only one that relatively speaking, relys on a stroboscopic-persistence effect for it's actual output parameters, very much like that of a CRT. The DLP 'flashes' at you, and so, in comparison, it's response curve linearity ,as far as relational aspect to screen grey levels, are not in any way directly and linearly comparable to that of a LCD ot DILA.


Each and every set of eyes has a different 'persistence' characteristic. When combined with the lamp type output spectrum, refresh rate, and most specifically screen grey level, ambient room lighting, etc... you get a projection system that has a differnt response curve linearity that the other projection types do not completely share. The extra variable of each pair of eyes, and the cognitive senses behind them, ends up creating a situation where there is much more sensitivity to grey levels.


Much more care must be taken witht DLP projectors when attmpting to get to the right screen size/ambient light/grey level/etc than any other PJ type... IF you wish to target the exact right contrast range for your tastes and perceptive conditionals.


Remember, I told you way back, that the situation would be changing so fast that it was going to turn out that there would NEVER be exactly one specific perfect grey level screen out there for all PJ types and situtations. The black level of digital PJ's combined with PERCEPTIVE situtions make the exact right grey a MOBILE enterprise, that shifts with each 'appliction'. There MUST be a RANGE of greys, otherwise, this AIN'T gonna work.


Throw in the constant evolvement of digital PJ's, this short time INTO their development, and you end up with a situtaion where screen sizes, PJ's, and grey levels almost have to stay together. You sell your PJ for a new one. You try your New PJ on the old grey screen.. and you find out that your perfect match.. is gone. The old screen should have stayed with the just sold PJ.


A different grey level has more than a passing chance to be more correct for your new PJ. Not as simple a evaluation task as you may imagine. You have a LIMITED contrast range in a Digital PJ. The best you can hope for is to maximize it as much as possible by playing with screen grey levels, each and every time you make a change. Sad, but true.


Which is why I am so HOT on painting systems. Very important. You see.. I like to think ahead, and this was one of my first considerations when looking at paint systems. I thought installers would love it. They get a chance to serve their customers BEST. With Digital PJ's.. the quality of the install comes right down to the SCREEN.


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Ken Hotte

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[This message has been edited by KBK (edited 07-06-2001).]
 

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Discussion Starter · #4 ·
I acquired one of the first GrayHawk screens (.88) and was enjoying the results with a Seleco HT200. When I went up to the new HT200DM, the picture was very dark and I couldn't get the color to balance out. I just snapped on my old StudioTek 130 material, and BANG! All is well again.

Hence my conclusion that gray screens are good for digital

imaging devices, provided they are not 6-element DLP's with 600 ANSI Lumens. I'd be curious to see what I'd get with a unity gain material.
 

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Ken:


I'm looking at a Sanyo XP21N on a 6' x 8' screen. Using the brightness analysis suggested by Mr. Wiggles in another thread, I come up with 30 lumens per square foot with a Grayhawk and 26 lumens with the Da-lite high contrast.


I had this thought that using the lower gain (more gray) screen might actually be the better approach with this very bright projector. What do you think?


Dan Houck
 

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Pete,


What was the screen size of the GrayHawk you tried out with your HT200DM? And what is your throw distance?


I think I might be getting that projector, and my initial plan was to use a 16:9 GrayHawk. I thought I might get enough screen brightness since I'm using a small screen (82" diagonal, 173" throw distance) The room will be almost completely dark. Do ya'll think a GrayHawk is a bad choice for this setup?


And while we're at it... isn't there some standardized way of figuring this out? I mean, we know the rated output of these projectors (800 lumens for the HT200DM), and we know the gain of the screen (.95 for the GrayHawk). There should be some formula for plugging in throw distance, screen size, and a modifier for ambient lighting conditions... and then we'd know what kind of minimum gain we need for the screen. Or is this just too subjective to reduce down to a formula?
 

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Discussion Starter · #7 ·
foldedpath,


I'm using a 72" wide image and the projector is about 17'

back. I only watch at night so the room is dark. There may well be a formula as you speculated, but for me it's a matter of empiracle evidence: DM + GrayHawk = overly dark and unnatural color; DM + 1.3 StudioTek = snappy picture and good color. HT200 (non DM) + StudioTek 1.3 = O.K. but not as good as with GrayHawk. Since from a reflectivity standpoint the difference in the two projectors is several hundred ANSI Lumins, I can only conclude that the light output of the DM is better served on a screen with unity gain or higher. Perhaps Don Stewart can illuminate (no pun intended) this issue.
 

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The worst thing you can do with the current crop of 3 or 4 segment wheels is to use a high gain screen.

The ultimate test and worst case scenario to bring out this artifact is to use a silver screen

Ive tried this even using dlp projectors that I do not see any rainbow on including the Dwin and the Seleco. Using a silver screen with these units brings out this artifact so bad you cant even watch a movie.


Just today I tried the new Seleco 200dm with the rgbrgb color wheel with a silver curved screen and the rainbow atifact is completly gone. You can dart yur eyes left to right, sideways and no matter how close you sit the artifact is completly gone. These things really work.


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Sorry I took So long to get back to your question, Dan.


Here's what I would do.


I would first pay attention to the single item in the equation that will be the greatest arbiter of video quality, and the one with the most compromises and limitations built into it. That is, believe it or not, the PROJECTOR. It is the single most limited item in the equation. What you have to do, is feed the unit, on the input that you will use the most, a high contrast signal. That signal should be calibrated in it's brightness and contrast chartacteristics on a Direct view RGBHV (VGA) monitor. The blacks should be fully delinitated, and the peak whites should not be 'clipping', or be crushed in their dynamic range. You adjust the monitor itself when under multiple high contrast signals, so that this effect is involving the monitors characterisic problems as little as possible. Then, you adjust the source you are using so it is as linear as possible via this 'corrected' VGA image. Back and fourth unitl the two are as correct as possible, but the real reason is to get the source settings exactly right.


So, the signal is then fed into the projector you are using, and this is viewed on a standard 'white' sort of screen, preferrably one with a middle gain figure, about 1.3 or more. This is used, as it has less in the way of interfernce of the near blacks by ambient lighting conditionals getting in the way. On this screen, you calibrate the PJ ALONE, never touching the source sigmnal AT ALL. PJ only. Remember, you just calibrated that imaage of the source. Don't mess with it.



What you want to do, is get the PJ to scale the blacks off it's 'black level' or 'noise floor' as linearly and deliniated as possible, without peaking the whites too hard, and smushing them together. Fight with this until the best compromise is achieved. If the system in use is digital in question, settings should be written down.


Of course, all these things are done with MULTIPLE source images, otherwise you will fall into the trick of calibrating your PJ for one particular movie. Once the PJ is set up EXACTLY right in these regards, then you might try messing with the source again, but it is in all probablility, not nessesary... you are already at it's point of maximum linearity.


Write all the setings of the two if at all possible, the PJ and the source settings. This is incase you have doubts, and wish to mess with things,a nd probably will once you ahve a grey screen on the wall...


Then you can FINALLY make an attempt to mess with a greyscreen. You have maximized the parameters of the system that give you as MUCH contrast range AS POSSIBLE in your system. Anything else is perceptive, and delivered by the greyscreen effect. So, what it comes out to, is that maximum contrast ratio and lumen output figures turn out to be relatively USELESS in figuring out whether one greyscreen is more suitable thatn the other.


First, the most linear and expanded contrast range calibration possible on the front end, and then the attempt at a greyscreen choice. No numbers will help you here, just time, thought, and experience. Any other way will stand a very great chance of shortening the contrast ranage of your PJ system.. even though it will seem to be enhanced....inititally. After understanding dawns, and the excitement wears off, the reality sets in, and the intitial choice might not turn out to be the right one.


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Ken Hotte

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[This message has been edited by KBK (edited 07-07-2001).]
 

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Discussion Starter · #10 ·
Alan,


When you speak of a curved silver screen are you refering

to something like an old Kloss Novabeam screen? I have one of these in my basement, but I figured it was destined for

the town dump.
 

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Pete,


I'm surprised that the DM looks so dim on a screen that size, I know that it's real-world performance does not appear to match the spec, but I didn't think it was that far off. Of course using the first generation Greyhawk may have something to do with it, the newer one have almost 10% more gain. In any case, it appears that for the 200DM you are sacrificing too much brightness to improve the already excellent black level of the DM. That does not necessarily extend to other 6-section wheel projectors (Plus Piano, for example), it really depends on what their real-world light output is. There's no way to tell that except by measuring it.


Regards,


Kam Fung
 
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