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Discussion Starter · #1 ·
I assume this must have been done.

What I mean is, measure the on/off (white/black) contrast from the projector itself with a lightmeter, and then compare it to the measured contrast from the screen with various paints/materials, so we can see how much of the contrast the projector is capable of that the screens maintain.

I'd be curious just how much better the contrast is on a grey screen (assuming a light-controlled room) versus a white one.
 

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It would take a very expensive light meter in order to get the values from the dark portions.

I don't have one. Mine was only in the $700 range. You would need to spend thousands in order to do this properly. :spend: We've thought about this in the past.
 

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Discussion Starter · #4 ·
In a light controlled room there is no difference in contrast numbers. Contrast is the same on both screens.
I was thinking maybe it would be better on grey screens (as some often claim) simply because there is less light reflected and bouncing around, plus less effect of any ambient light.

It would take a very expensive light meter in order to get the values from the dark portions.

I don't have one. Mine was only in the $700 range. You would need to spend thousands in order to do this properly. :spend: We've thought about this in the past.
Bummer. I'm surprised something as basic as ftL would not be able to be accurately measured with the average meter.

Thanks for the answer.
 

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I was thinking maybe it would be better on grey screens (as some often claim) simply because there is less light reflected and bouncing around, plus less effect of any ambient light.
If there was any ambient light in the room, or the room surfaces were a light color, you would be correct. I believe what Yiannis means by a "light controlled room" is what we colloquially call a "bat cave" and is a room with flat and very dark, or black, surfaces so that pretty much every surface except the screen itself absorbs a LOT of light. In such rooms is where a pure white screen really shines (pun intended ;)).

Bummer. I'm surprised something as basic as ftL would not be able to be accurately measured with the average meter.

Thanks for the answer.
You have to realize that a PJ that has even just a 4000:1 contrast ratio (that is really that figure) means that if the white areas are reflecting 20 fL (which is bright for a normal screen) the black areas would only be reflecting 1/4000th of that which if my math is correct would only be 0.005 fL. If the true CR was say 50,000:1 the blacks would only be reflecting 0.0004 fL. Most light meters aren't designed to read such low reflectance values because about their only use would be to measure CR which is a VERY niche usage.
 

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If there was any ambient light in the room, or the room surfaces were a light color, you would be correct. I believe what Yiannis means by a "light controlled room" is what we colloquially call a "bat cave" and is a room with flat and very dark, or black, surfaces so that pretty much every surface except the screen itself absorbs a LOT of light. In such rooms is where a pure white screen really shines (pun intended ;)).
Bingo!!

Exactly my point!!:T:T


My friend Cuttard, the grey screen could help the contrast ratio on secondary reflections (the light that gets back on the screen after bouncing on the wall, ceiling e.t.c.) because the gray color absorbs better the light than the white one. So your image will appear less ''washed out''. In an ideal room with black walls and black velvet curtains, the contrast ratio on both screens will be the same. Bottom line is that the amount of light that's leaving from both screens is the same, so the light that is being reflected and bouncing around in the room is exactly the same but the gray screen has the advantage on secondary reflection.


As far as measuring the contrast ratio, i fully agree with what Mech and Don have said.
 

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Discussion Starter · #7 ·
What I meant was, I have navy fabric on the screen wall and coming out on the ceiling and side walls maybe 8 feet in a basement with no windows, and there is still light bouncing around from the pj and screen, enough that I can clearly see everything in the basement.

Anyway, thanks for the responses.
 

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What I meant was, I have navy fabric on the screen wall and coming out on the ceiling and side walls maybe 8 feet in a basement with no windows, and there is still light bouncing around from the pj and screen, enough that I can clearly see everything in the basement.

Anyway, thanks for the responses.
This is normal for the average home theater. Most screens (DIY and commercial) are relatively diffuse reflectors which means they reflect back the light that hits them in all directions equally (or close to it). Bright scenes will effectively light up your room unless the room surfaces are dark colors. The higher gain the screen is the more light will be reflected back toward it's source (the PJ) so the less side walls will be illuminated, but too much gain and you will get hot spotting. One of the best solutions for this situation is a retroreflective screen since it is designed to reflect most of the light back toward the source without hot spotting, but there are no good DIY solutions that are retroreflective, the few I have seen use glass beads that are too large.
 

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I'm not sure that the numbers would be massively helpful even if there was a readily available method of measuring at home. There are a number of factors that impact the overall image quality and perceived quality. Some people will correctly state that the screen can't change the projector's contrast ratio but I would add, that a better screen can negate the importance of the projectors contrast capability. As can the lighting conditions in the room.

The general principle is that a projector can't produce black from its red, green and blue light sources. Black is the absence of light. In a pitch black room, the absence of light looks close to black. In a bright room, the darkest color the projector can produce is limited by the screen color. That is why blacks look light grey on a white screen with the lights on. Using a darker color like a neutral grey closes some of the gap.

Coming back to the numbers, they start to get a bit meaningless when you see claims of 150,000:1 contrast ratio. With even a little light, the image on such a device will look washed out on a white screen like any other projector. If you notice, the higher contrast ratio's are always on the dimmer projectors which are only really effective with the lights off anyway. For those, I would bet the only difference between grey and white is perceived rather than measurable.

In my opinion, it makes a far greater difference when using brighter professional devices in the 7000 lumen to 20,000 lumen range with high ambient light (or even sun). These devices tend to have 2000:1 contrast ratio's but are bright enough to look great on darker screens. I can even get a watchable image on a matt black surface with an 8000 lumen device. When projecting on black, blacks really are black but again, the projector's capability has not changed. My perception has though and to me, that's all that counts.

Trying a surface in your room with your equipment will be more informative than technical readings which are normally only good for marketing the product. due to the other factors, two projectors with identical contrast ratio's will not always offer identical performance, just like two screens with identical gains can differ.

BTW, I purchased some glass beads that had a powder-like consistency (ie small enough) for $5 and they worked fine for a diy screen. I have made better screens using other methods but the glass beads did work without difficulty.
 

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In my opinion, it makes a far greater difference when using brighter professional devices in the 7000 lumen to 20,000 lumen range with high ambient light (or even sun). These devices tend to have 2000:1 contrast ratio's but are bright enough to look great on darker screens. I can even get a watchable image on a matt black surface with an 8000 lumen device. When projecting on black, blacks really are black but again, the projector's capability has not changed. My perception has though and to me, that's all that counts.
Why should be making any difference since the light that has to leave the screen is exactly the same? Let's assume that we have the perfect room (zero reflections) and two screens, one white and the other black. In both cases the light that has to leave from both is 12 FL. As i am thinking, the only way for that black screen to do a better job with the image in terms of contrast is to handle better the probable blooming. If the projector doesn't suffer this, i can't find any other reason why they won't look the same.

Welcome to forum!!
 

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Check out the attached pics which illustrate my point. They show a comparison between high contrast grey screen goo and my own home made black-screen mix. The room is brightly lit from above, the side and the blinds are open letting mid afternoon sun shine directly in the projectors path.

Low brightness projectors are only watchable with the lights off anyway so there is a limit to how much you will notice the difference a high contrast screen will make.

A brighter projector is better able to cope with a darker (less reflective) screen surface which can cover some of the gap in the projectors ability to display a black image. As brighter projectors have lower contrast ratios and require less gain, the benefits of a high contrast screen are really noticeable. reflectivity from the white walls is a very minor problem in comparison to the sunlight shinning on the screen in these images.

If you turn off all the lights, my white screen looks better than all of them. Make sense?
 

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I can see your point but your experiment is a whole different story from what we discussed!! First of, there's a projector light that hits both screens simultaneously. Therefore, the conditions are not the same. You can't have the same amount of light reflected from both screens with only one light source at the same time.

High contrast screens is just a marketing denomination since screens are non made by increasing something that's not there by the projector. As all the screens cannot create light, same principal goes to contrast ratio also. The perceived contrast is different because of the surrounding light that wash out the image much more on a brighter screen than the darker one.
 

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My point was that it makes a bigger difference with bright projectors. On a very dark screen, a dull projector would struggle to give you any image, even with the lights off. Brighter projectors let you experiment with darker surfaces which offer more noticeable results. I agree, you can't change the projectors contrast ratio, but you can help a projector with a lower contrast ratio with a darker screen.

Reflectivity is one aspect in defeating ambient light. As black is the absence of light, white screens do not look good, even with a really bright projector when there is ambient light. Making them more reflective or giving a higher gain will not close the gap either. The contrast ratio is what the device can achieve between it's darkest and lightest output. However, a device with 50,000:1 will have washed out blacks with the lights on when a white screen is used. If the screen is black, then an absence of light will look black, even with the lights on, even if the device only has 2000:1. Essentially, you are making up for the projectors lack of ability to produce black light by supplementing it with black pigment. You then rely on the projectors brightness to give you a bright white for contrast with the black.

Not sure what you are saying on the comparison. The screen goo looked the same with or without the blackscreen material over it. The projector was calibrated for the screen goo (to the extent it was possible with the bright conditions).

I have no vested interest in showing one option in a worse light than the reality (so to speak). I am not selling my blackscreen mix (or any other specific screen technology). I know some of the companies who are trying to sell will use some tricks like shining a light on screen they want to look worse for extra effect. That is not what is happening here.

My point was just that, in my opinion, low brightness projectors only look good in the dark and in the dark, screen color is less important than when there is ambient light. With ambient light, the darker the screen, the better the blacks look but only really bright projectors can take advantage of the darkest surfaces.
 

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You have to realize that a PJ that has even just a 4000:1 contrast ratio (that is really that figure) means that if the white areas are reflecting 20 fL (which is bright for a normal screen) the black areas would only be reflecting 1/4000th of that which if my math is correct would only be 0.005 fL. If the true CR was say 50,000:1 the blacks would only be reflecting 0.0004 fL. Most light meters aren't designed to read such low reflectance values because about their only use would be to measure CR which is a VERY niche usage.
A little OT, but how low (and high) can such a light-meter still accurately measure fL? As long as a projector's CR stays basically the same despite throw distance (which I admit curiosity about) and the meter could safely take a very high fL rating, couldn't you just move the projector closer to the screen until it provided a readable black-level?

I agree that CR shouldn't really change with different screens in a "cave", I'm just curious about the possibility of you guys adding Native-CR results from properly calibrated projectors. ..and I'd love to know how much affect throw distance has on contrast.
 

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You have to get the meter to read in it's specified range though. If the reading is below the spec, the reading can't be considered accurate. For instance, the i1Pro 2 can measure light in this range: 0.2 - 1200 cd/m2. So the most contrast you could ever possibly measure would be 6,000:1. You could test contrast, but you'd have to use a projector which has a contrast ratio lower than that number. Also, I couldn't find what the accuracy of the i1Pro 2 is within that range. Without knowing the accuracy, you'd have know way of knowing the accuracy of the result. If the accuracy at the low end is +/- .1--which seems reasonable for such an instrument--then a measured contrast of 6,000:1 would only have an accuracy of between 4,000:1 and 12,000:1. What that means is that you could take one measurement and get 4,000:1, take another and get 12,000:1, and you'd still have no idea which measurement was higher or lower than the other. Basically, any contrast measurements taken with commonly used meters are completely worthless.

I think if you play with the math enough, you can show that a passive screen can't affect contrast ratio. It can however, affect perceived contrast. You can prove that by just visually comparing a gray screen and a white screen in a bright room. Controlling ambient and reflected light will actually allow you to maintain the PJs rated contrast, screen color can not.
 

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Nak, i wouldn't agry that the the measurments with light meters are so worthless. First of, 1200 nits are 3768 lux, that's pretty high as a value even for bright H.C. projectors. I agry that we shouldn't push our instruments to the limit, but, there are ways for making the necessary compensation. For instance, i use the distance of 61'' screen to measure on/off and have at the same time the real lumens of the projector. At that distance, in most cases, no instrument will read 3700 lux. For dark readings, if a reading is not possible from that distance, i would get closer the instrument to the lens to take a reasonable safe reading. I know that is not the most fair thing to do but as you know, few lux over or under change the contrast ratio drastically. Therefore, doing that, i only have a more or less approach about contrast level. Needless to say that off the screen, during a film, we see entirely differente figures of contrast ratio......so for me, knowing exactly the CR up to exact 0,000001 lux, is quite meaningless.

If you want the extreme accuracy of C.R. you get a Minolta and the story ends.:)
 

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My point is though that a very small margin of error on the dark side will result in an enormous margin of error in the calculated contrast. For a contrast measurement to have any meaning at all, you have to find out what the accuracy of the meter is at the low end of the scale. Errors in max lux will have small effects, but errors in min lux will render the comparisons absolutely invalid. Since these meters are intended to measure things other than min lux, it's safe to assume there are inaccuracies in the meters at the low end. Before you can get any useful contrast information at all, you have to know what the accuracy of the meter is. That way you'll know your margin of error in your contrast calculation. Even comparisons are invalid, because the results of two comparisons will almost certainly fall within the margin of error.

I have not been able to find any specifications regarding the accuracy of the i1 meters. I'm sure they don't publish the information since it's irrelevant to their intended use. It's very possible that x-rite has not even bothered to determine the accuracy at the low end of the scale. Testing costs money... Regardless, you'd have to call x-rite and speak to an engineer. You'd have to be sure you were speaking to an engineer too. Others probably wouldn't have a clue. Unless of course they have a spec sheet they haven't published. One thing you know with complete surety; there is some level of inaccuracy at the low end. No meter is perfect. Even meters costing $20,000 dollars and designed to take contrast measurements will have some inaccuracy. The difference being those meters will have smaller inaccuracies, and the margin of error will be included in the specifications--since that margin is important to their intended use.

It's not about extreme accuracy of Contrast Ratio measurements, it's about any accuracy whatsoever. As in my example a couple of posts above, a very tiny margin of error at the dark side results in a margin of error from a 4,000:1 CR to a 12,000:1 CR. Since you don't know the margin of error in your dark measurements, you have absolutely zero idea of your margin of error in your CR comparisons. It's possible--even very likely--that your margin of error is even worse than my example. Probably a lot worse.
 

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The pros (spectros) are well known for their inaccuracies in dark readings, so i agree with you that you can't take valid measurements with these as far as it concerns the CR. On the other hand, colorimeters like i1 display pro or lightmeters such as CA813 can take these readings pretty safely even in dark end.
 

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You get better readings down to 10 and 20 ire for sure with the i1 disply pro. You're still missing my point. You have absolutely no idea what the accuracy of either of those gauges are at the levels we're talking about, because the accuracy is not published. In the case of the CA813, you have some published specs, but not what level these are accurate down to. Just because you get a reading, doesn't mean it's accurate. Worse, you don't even know what the tolerance is. Without those numbers, your contrast readings are meaningless. The first task would be to ascertain from the manufacturer what light level a given meter is accurate down to, and what specifically is that accuracy. Small errors won't affect your gamma adjustment or visibly affect your color balance. Small errors in black levels can render CR calculations utterly worthless.

Now, if you can get the tolerance figures for the level of light you want to measure, then you can possibly make some meaningful calculations. Without those tolerances, you can not. It's really just that simple. You have no idea if your margin of error is +/- 500:1 or +/-50,000:1. And there is a margin of error.
 
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