Is the practical limit of projector contrast in a real room less 500:1?



Quote:
Originally Posted by orion456 /forum/post/18098905


As I understand it, a single candle in a room produces 1 Lux and a 1000 watt ANSI projector bulb on a 100" screen produces less than 400 Lux. If so, then the maximum contrast visible on the screen is 400:1. Even if the candle is far away and produced only 1/10th of a Lux, you could not see a real contrast greater than 4000:1; on/off or ANSI contrast.


For comparison in your own projector room - the full moon on a dark night produces 1 lux, the quarter moon = 1/10th lux, moonless night = 1/500th lux and the average family living room with the lights on is 50 lux.


Assuming that 1/1000th of the projector light reflected off the screen finds its way back to the screen, that would be 0.4 Lux, making anything better than 4000:1 contrast impossible?


How then can these projectors that claim 10,000:1 and more, actually show such contrast? Or is the real answer, they can't?

Click to expand...

It sounds like you may have been misled by some guy in Australia who didn't know what he was talking about for a lot of what he said, or read a site that I think likely basically plagiarized what he said without really understanding all of it. While both articles I'm thinking of have some good information (most articles do), they also have some stuff that I think has misled a lot of people.


First of all, do "real rooms" have that much other light at all times? Getting rid of other lighting tends to be the easiest thing, at least at night. It is getting rid of reflections that tends to be much more difficult, and even a white room can give basically infinite on/off CR if other lighting is taken care of.


As far as the candle situation, it depends largely on what the light is, where the light is shining, etc. In your case "far away" may only be 10' since light falls off as the square of the distance and so at 10' from a candle you only get about .1 lux. If your white is 100 lux then you would be limited to about 1000:1 on/off CR if you had that situation. But then a fair number of people have light that is blocked by something else, like light behind a couch, ceiling lights that they have point down without any going directly to the screen, etc. Light that doesn't go directly to the screen can be much brighter before the reflections from it affect the images the same.


When these same kinds of questions were asked before I did some actual measurements in an off-white walled setup I have where at the back of the area to the side there are lights shining down. My memory is that with a couple of these bulbs on I could measure something like 100 lux a couple of feet beneath them, but the amount of light that made it to my screen and washed things out was more like around a lux because of the way that much of the light gets blocked and has to bounce around.


Light can be a problem like you say, but as far as your 1/1000th of the projected light making it back to the screen, this is an example of where a room can kill ANSI CR, but high on/off CR can still be useful. Let's take a room where even more of the light makes it back to the screen so that in the ANSI CR test the ANSI CR off the screen couldn't be over 100:1 even if the projector had as much ANSI CR as on/off CR and basically infinite of both. If you don't have other lighting, then a projector with "only" 10000:1 on/off would be limited to around 70:1 in a checkerboard of 10% video on black even in a white room if it had about a 2.3 gamma and the room that limits things to 100:1 would take that down some for a final of around 40:1. Even in that room if the projector's on/off CR was raised to 1,000,000:1 it could do close to 100:1 in that 10%/0% checkerboard instead of 40:1, so there is a case where the on/off CR could improve those dark mixed images even in a light colored room.


And if instead of looking at a full screen 10%/0% checkerboard we considered images like a 1/4 screen checkerboard of the same with the rest black, in that fairly light colored room the projector with 10,000:1 on/off CR and 10,000:1 ANSI CR could do around 60:1, while the 1,000,000:1 on/off CR projector with 10,000:1 ANSI CR could do around 370:1 for the same image.


I used the contrast ratio estimator here:

http://home.roadrunner.com/~res18h39/contrast.htm


with a room reflection level of .02 to limit the off-the-screen ANSI CR to around 100:1 for the above numbers.


Try the above with a projector with less on/off CR like 2000:1 or 4000:1 and a person can see that if they get rid of other lighting, on/off CR can still benefit them a fair amount even if they have less than black walls.


The above isn't even counting things like black screens that can reject a lot of off-angle light.


All that said, if a person really does have a lot of other light that they can't get rid of (not just reflections from the projected light) then getting a high lumens projector can give them higher ending on/off CR and ANSI CR off the screen than a dimmer projector where the projector itself has higher on/off and/or ANSI CR. CR in the images always matters, but when there is a lot of other lighting it is the white level of the projector and the black floor of the room that mostly determines the total range, with the black floor of the projector playing less of a role.


--Darin

Thanks for the detailed explanation. As usual, things are never as simple as they first seem.


In summary:


Maximize contrast by minimizing external light sources or at least directing them away from the screen.


A brighter projector can offset some external light caused contast loss.


Contrast varies with frame brightness.

I've wondered about the effects of introducing tiny bits of ambient light on the on-screen black levels in my room.


On one hand my instinct is to think that the perceived/measured contrast on screen would have a sort of ratio-relationship with ambient light in the room. In other words:

a tiny ambient light, say a tiny equipment light, would have a greater impact on the black levels of a projector that had "infinite black" than, say, a 6 year old LCD projector with fairly high black levels. If the LCD projector's "black levels" on the screen were actually brighter than the light falling on the screen of the equipment light, then the equipment light could do nothing to "wash out" that blacks of that projector's black levels. The LCD black levels are already higher than the ambient light given off by the equipment light.


BUT...then I'm reminded that light is additive. That's why if you start with even a tiny light source, like a small candle, but keep adding more tiny light sources (more candles) you get more and more light in the room. So, presumably, even when you have the room fairly well lit by a lot of candles, adding one more candle does indeed raise the light level (whether by a perceptible amount to our eyes is another question).


Do I have that right?


If so, it makes me wonder about the teeny bit of light that still seeps into my room. I have black-out blinds, but the seal is not perfect. At night (I only watch at night) there is a little, mild glow of light getting in through the non-sealed portions at the top of the windows. I have presumed it is not enough to affect my black levels. Because with blinds down and the lights off, the screen is FAR blacker than when my JVC is simply projecting "black." But...if light is additive then isn't it possible that even the teeny bit of light getting through the windows "adding" to the projector's light? And hence actually raising the black levels?


I dunno. Anyone?


(BTW, to try to see the effect of the light getting into the room from the windows, I have sat in the pitch dark in the room for over 5 minutes waiting for my eyes to adjust.

Even after 5 minutes I can not see the screen in the room - the room appears utterly pitch black).


As for directional lights and the effect on the screen contrast: I have the Stewart ST-130 1.3 gain screen material in a dark, light controlled room. I'd been worried by reports that the ST-130 was particularly susceptible to ANY ambient light washing out the image and that if you wanted ANY ambient light the more directional, gray-based Firehawk screen material was to be preferred. However, I designed my lighting - a combination of pot lights and track lights - so that I could have a "zone" of track lights on directly over the viewing sofa, pointed down, for task lighting. Since the sofa is quite dark and non reflective, this has worked out great. I have been amazed to see that turning on those lights over the sofa (which is 10 feet from my screen) has barely any perceptible effect on the on-screen image! It remains looking about as bright, contrasty and gorgeous as with the lights off. So, obviously, the type of ambient light in the room really matters. If I had been using wall sconces that sent light out more omnidirectionally including toward the screen, no doubt I would not have enjoyed such happy results.