Originally Posted by Domcorleone
Below is what my space looks like. I would like to be able to have the shades partially up occasionally or be able to turn on the ceiling light up to 50% (its on a dimmer) and still be able to get a decent picture.
The higher the fraction of light that is hitting your screen with an angle away from your eyes less than the material's "half light viewing cone", the less ambient rejection you get. The ideal screen would redirect all light falling on it from the projector back to your eyes directly (i.e. ultra high "gain", enabling ultra low power projectors to deliver very, very bright images!). Think about a whole bunch of tiny mirrors all angled perfectly to let you see the projector lens and only the projector lens. Only
light originating from the projector would enter your eye. Light originating from anywhere else in the room would bounce harmlessly elsewhere. But if you shifted a foot over on your couch, then you'd see no image (also, your friends and family see nothing). So in the real world, screens either wastefully diffuse light in all directions, only a tiny portion of which hits your eyes, or redirect it over a narrower "viewing cone". Anything outside that cone (ambient light OR
projector light) simply doesn't make it to your eye.
That's the basic trade-off of ALRs. You reject some ambient light (yay!), and at certain angles you get more projector light coming to your eyes (aka "gain"... yay!), but you also reject a lot of projector light if any part of the screen is outside the "viewing cone" (boo!), and you get more
reflected ambient light from inside the viewing cone than you would have (boo!). So in your case (with lots of ambient light coming from behind the projector) a high-gain ALR screen would look worse
than a simple white screen. With blinds, light will bounce around at all angles and this isn't as true anymore.
Half light viewing angles range from very small (15° or so) for the most highly light-rejecting materials, to 180° (perfect lambertian reflectors, like a white piece of paper) for non-ALR materials. My guess is that most materials do a mix of both of these: some components of the ALR surface act like tiny pieces of white paper (that do nothing for ambient light) and some act like tiny reflecting mirrors (at a variety of angles). Tiny mirrors and tiny pieces of paper. That's how I think of it.
But you might say: paper and mirrors can't be right: some screens have low gain and
small half light angles. How do they do that? Simple: make your "tiny pieces of paper" in the screen surface a dark color, to absorb both ambient and projector light. So all
the light gets much fainter, but then the tiny mirrors send a bunch more light to your eyes (if they're angled correctly) than paper would to make up for that (this by the way this is how a dark-looking screen like Black Diamond can have a gain of 1.4!).
And this model lets you think about "poor man's ALR". If you have a very
bright projector, you can just make your entire screen out of tiny pieces of dark
paper, no mirrors (aka "gray screen" or "black screen"). Then both ambient and projector light arriving from any angle get highly absorbed, and then diffused equally in all directions. Very wasteful of the light. How could that help? Since the projector hopefully outshines the ambient light in the room by a very large margin, you get a nice high contrast image that isn't too bright (just be careful no-one wearing a white shirt walks in front of the screen, you might go blind). Metric of note: direct sunlight on a white screen produces 10,000 foot lamberts. So to illuminate a 120" 16:9 screen with the same flux as uniform direct sunlight, you'd need a projector with a half million lumens (and a screen with a gain of 5 thousandths). Direct sunlight is insanely
bright, no projector can ever compete. During the day with blinds closed, my screen vicinity measures about 3 ftL. Much lower at night obviously.
PS: Okay you can tell I'm a scientist here, but I worked out the maximum theoretical gain of a screen composed only of "tiny mirrors" all ideally aligned towards your eye: 8 x (distance to screen/diameter of pupil)^2. I.e. for a 12' distance with a dilated 8mm pupil, gain of ~670,000. If you'd like light in both of your eyes (wasteful you), then you have to accept of gain of "only" about 30,000.