Originally Posted by odyssey
The sequential CR does increase with decreasing aperture size in the illumination system before the DMDs.
I definitely agree that it can when done right. From what I have heard about SIM2's system I'm not sure if it does or does much. It could, but I'm just not sure. It should depend on multiple factors, including what the openings are before they start closing the DI down.
Originally Posted by odyssey
The reason is the same as with lens apertures: stray light is blocked and has a much higher percentage effect on the off state versus white. The illumination system aperture is the current modification of the Barco DCI projectors. The CR increase is from about 2200:1 to 5000:1 and the former TI engineer who developed the aperture technique thinks that a matching smaller lens aperture will not increase the sequential CR significantly further.
With that much improvement it sounds to me like the opening in the lens and the opening before the DMDs were not matched up well before this iris was added. I wonder how much those DCI DMDs are capable of under ideal conditions and ignoring uniformity. I'm not sure what they have for those as far as black backing, dimple issues, etc. compared to consumer chips. I was able to get about 9k:1 on/off CR with an Optoma H79 over 3 years ago with a fixed dual iris system. It came at the expense of uniformity and light output, but I felt it was worth it with my High Power screen in my dark theater.
I think of some of this iris stuff (with DLPs at least) as going beyond what I think of as more of a traditional view of just blocking off axis light, or what might be thought of as bad light. I'll cover what I mean and you can add or correct things if you want.
Think of a cross section of the lens where an iris will go as a quarter shape and size. Then an iris in this same plane as dime shape and size. Then put an opening before the DMDs that perfectly lines up with that dime shape in the lens. If we look at the light going through that dime shape in the lens for white and black the amount of light is basically the area times the amount of light per area. Let's assume for a second that the white going through that dime shape is coming straight off the mirrors to that opening and is uniform at that point, while the black is bouncing around inside that chamber and is uniform as it hits the quarter sized shape in the lens. This basically means that the native CR of the chips is high to a spot, but they bounce some light fairly randomly (like off the dimple, mirror corners, etc.) Of course this is all looking at a pretty idealized case to get the point across and how much of this is relevant to real cases depends on how much the chips can actually do. Anyway, even though in this case the white is going through the dime shape uniformly and so could be though of as on-axis light, I think there is an opportunity to improve the on/off CR further by blocking some of it.
Considering the above case, block half the dime shape in the lens. What happens here is that the white level goes down to half and so does the black level, since both are uniform across that area and the area is half. So, no improvement to on/off CR. But now go into the opening before the DMDs and block half of it in a way that perfectly lines up with the new half-dime shape in the lens. In theory the white level wouldn't drop here because the 2 openings are lined up perfectly (in reality it likely drops a little). But the key here as that the amount of light that hits the DMD has gone to half and the black level across the quarter shape in the lens has dropped to half. So, white went to half by closing both irises down, but the black level went to 1/4th, meaning on/off CR doubled.
The further the on/off CR of the projector is from what the chips are capable of the more I think the above applies. As things approach the CR that the chips can do for light traveling basically straight to the dime opening in the lens (not bouncing off walls before getting there), the less the above applies. At least that is how it looks from my experience and understanding.
When I was doing something like you are considering with putting a smaller lens iris in, the way I approached it was to take all the front elements out of the H79 lens until I got back to the one with the iris. I put a piece of white paper on something so that I could put it back in the lens and see what the light looked like at that opening for black and for a full field of gray or white. If you did something like this (might want to use sunglasses or stronger
) then I think you could get a reasonable idea about whether a smaller opening in the lens would improve your on/off CR. If for something like a 20 IRE gray field you saw that most of the light coming through the lens was concentrated in one area and a video black image didn't have the same pattern then you could probably improve your on/off CR by changing the opening to block the parts that are dimmer for that full field gray. And by looking at the video black image you might be able to see more issue with black coming through one spot in the opening than another.
If you try this I'll be interested to hear what your findings are.