Originally Posted by curttard
Also, OP said he would only have to deal with ambient light "maybe 10% of the time" and even then only "slight" ambient light.
Reflected light can be as much of as a problem as mild ambient light unless the room is properly treated. That said, I myself have decided I prefer a white screen and a treated room. If I had 5000 lumens to play with, I'd then go with a gray screen and a treated room.
Originally Posted by pb_maxxx
true Kirnak. but what most people fail to understand is that projectors do NOT project black... therefore as long as there is ANY visible light (which means projected light of any source... then the following is true) the color of the screen will play an important role in not only the perceived black levels but also the visible (i'll call it 'actual') black levels. and as long as there is enough projected light to maintain white levels... then yes, both perceived and actual contrast can be achieved...through matter. and the greater the ambient/projected light is present... the more the screen color matters.
which is why you will never see a LCD/plasma/LED screen that is white.
Agreed on all counts except kind of one. Actual contrast. Depending on the room, perceived contrast can certainly be preserved over a lighter shade screen. Actual contrast can not be improved with any shade of gray alone. (Alone being the key word.) Let's simplify the math. Lets say we have a PJ that has a black level of one unit of light. It also has a max output of 10 units of white light. OK, we have a contrast ratio of 10:1. Now, add 2 units of ambient light. Our black level is 3 and our 100 ire is now 12. We have reduced our contrast to 4:1. We'll just assume a 1.0 gain screen. OK, let's go with a dark gray that only has 0.5 gain. 100 ire is now 6, black level is now 1.5. Still a 4:1 contrast ratio. So, what can be done to preserve more of our 10:1 contrast ratio? (It's quite impossible to exceed it.) That ties in with curttard's question.
Originally Posted by curttard
Sure. But any gain that pumps up the whites is also going to pull up the blacks, right? Will one grey screen have special properties that enable it to have, simultaneously, visibly darker blacks AND brighter whites than another grey screen?
It's theoretically possible, but I won't step into the argument over whether or not any specific paint has achieved it. How? Enhance the gain. Let's talk about the mechanics of how that would work in theory, then talk about the problems.
An ideal white screen with a gain of 1.0 would theoretically have a lambertian surface or reflection. Light hitting the screen would be reflected with equal intensity in all directions. An N8 gray screen with lambertian reflectance would also reflect light equally in all directions and would have a gain of about 0.8. Both would have the same measured contrast from the same PJ with the same settings, even with some ambient light present. The gray screen would have a better perceived contrast ratio in the presence of ambient light however, assuming the PJ had enough power to project a sufficiently bright picture. Why? because the ambient light level reflected off the screen to our eyes would be lowered, and as light levels get sufficiently low we perceive far less color in that light. That is precisely why colors look less washed out on a gray screen with ambient light present. Our eye will "see" improved contrast even though a sufficiently accurate probe will confirm no change in contrast from our white screen.
Dynamic range of the eye must also be considered. If you have a bright enough image you may be able to lower the luminance and not detect a difference. In that case a darker gray will appear to have darker blacks while maintaining whites. Everyone has different vision, so two people standing side by side may come to entirely different conclusions about the same test. The person with lower dynamic range in their vision may be absolutely convinced of an improvement in contrast, while the other subject with better dynamic range will just see both black levels and white levels decreasing. Both will give completely opposite opinions, both will be right, and both will think the other is lying!
Once we enhance the gain, we can save some of our actual contrast. How does this work? First off, what is enhanced gain? A passive screen can not amplify the light it reflects, but it can direct more of it towards the viewer. Let's imagine a perfect enhanced gain screen. It will reflect light only towards the viewer, and not off to the side. That way, more light is available for actual use. The perfect screen can't just reflect light like a mirror, it must reflect light in a wide enough cone that light is reflected towards the viewer from both sides of the screen. A perfect screen would reflect light evenly and equally throughout that cone. If the light is too strong in the center of the cone and too weak at the edges, we see a hotspot. A side benefit of this increased gain is the reduction of ambient light that originates to the side of the screen. Since light is reflected off the screen in a cone, the reflections from side generated light are directed away from the viewer. Since the viewer sees less reflected/ambient light, more of our original 10:1 contrast ratio is preserved. Both perceived and actual contrast is enhanced over our first two screens. The effect is additive so we see a much improved contrast. Ambient light from behind is not affected and our enhanced gain screen offers no benefit in that instance.
Of course, there is no such thing as a perfect screen. All of life involves trade offs, and so do screens. Some screens hotspot, some have grain or shimmer in the picture. Some are worse, some are better. The list goes on and has been debated endlessly; I have nothing to add to that debate. My white screen is wonderfully uniform, clear and sharp. I also have devoted and continue to devote a large amount of time and money to create a room with minimum reflected light and zero ambient light. I have traded time, money and space for a clearer picture. That's a trade off a lot of people can't accept. People with lower visual acuity will be bothered less by grain in the picture, as will people sitting further from the screen. They may not even notice the grain, therefore it's an intelligent trade off to ignore it in return for a benefit elsewhere.
It's all about making a decision about what's important to you. I highly recommend people paint a two square foot sample of what ever paint they are considering before painting an entire screen. Tape it up and watch different scenes from movies. Include dark scenes, colorful scenes and scenes with large white/light (Snow/clouds) expanses while panning. Walk back and forth to all viewing positions. look for both the weaknesses and strengths of the sample. every sample will have some weakness. Every decent sample will have some strengths. Project a uniform white field from a test disc. If your sample looks darker than the surrounding area, screen or other sample, then it has a lower gain at that angle. If it looks brighter, it has higher gain. If you notice a sharp drop off in brightness as you move, that's a hotspot. The sharper the drop off, the worse the hotspot. (There will ALWAYS be very gradual drop off as you move to the side unless you have a $10,000+ PJ. PJ lenses are not perfect.) It really doesn't matter what someone else accepts as a tradeoff, it only matters what is acceptable to you. Seriously, do not expect to find perfection in life, much less in a screen. That's why people keep experimenting and innovating everywhere and in every aspect of life and technology. There is always room for improvement.
Some type of louver technology could also be used to combat ambient light from the side. I have no idea of how feasible that would be for DIY and I applaud efforts to experiment along those lines.
Lastly, there has been talk of "selectively reflective" screens for quite some time. First off, it is categorically impossible to reflect only red, green and blue and not the secondary colors. (Cyan, magenta and yellow.) That's just plain physics and is as impossible as a perpetual motion machine. (Sony marketing notwithstanding.) That said, is it possible to reflect light only within the Rec. 709 standard and absorb light outside of that triangle? If possible, that would be an effective means of reducing ambient light. How effective? Is it possible? Hell, I have no idea on both counts. Also, I sincerely doubt anyone on this forum is actually qualified to debate the issue, myself included. What the hey though, it's a hobby and nothing wrong with experimenting. Experiments don't have to succeed to be a success. Most don't. Knowledge is more often expanded by failure than success. Success is great too though! If you come up with something that works for you, then that's all that counts.
Just my humble opinion.