Originally Posted by SolRebel
Thanks! Couple questions about the need to have the projector vertically centered with the HP screen. 1. If the proj and screen are lined up vertically, does the viewer need to be also? Basically can I raise my screen up a bit to get it closer to center or will the uncreased viewing angle just negate any gain increase anyway? 2. How do you get a projector in line with a screen and the viewer without getting the viewers head in the way? Thanks
See the imaginary line from the center of the projector's lens to the center of the screen? The closer you get your eyes to that line, the higher the gain of an HP screen. Obviously, you can't put your head in the path of the light. A retro-reflective screen like the HP bounces most of the light back to where it came from. It "collects" the light instead of scattering it more evenly like "neutral" screens. That's how it achieves its ~2.8 gain for a person sitting in the "best" seat. That's the same reason the gain goes down as you move away from the sweet spot. More of the light going to the sweet spot means less light goes to the eyes of people sitting off to the sides. Neutral gain screens reflect it pretty much evenly to all viewers. Screens can't create light. They just reflect it differently. If you ceiling mount and sit a few feet below the projector's beam, the gain will be minimal for an HP. If the projector is high enough, the gain drops below 1.0, at which point another screen choice probably makes more sense.
An angular reflective screen is appropriate for people who ceiling mount. My Stewart Firehawk (which was angular reflective) worked best when the projector was ceiling mounted, since it achieved its 1.3 gain for people a few feet below the projection lens. When I got a small HP sample from DaLite, I put it in front of the Firehawk screen. From my normal seat, with the projector on a high shelf, the Firehawk was brighter. As I stood up and brought my eyes closer to the projection lens, the Firehawk dimmed and the HP got brighter. I also put up a sheet of printer paper. As I moved from the normal seating position up toward the projection lens, the piece of paper stayed the same (since it reflected light randomly and maintained an even brightness no matter the position). The Firehawk got dimmer. The HP got brighter. At one point the Firehawk and the HP were the same brightness (roughly the same as the piece of paper), as their unique properties brought the gain to about 1 for each.
When astronauts landed on the moon, they left retro-reflective mirrors at several spots. A laser projected onto the normal lunar surface disperses light like a neutral gain screen. It's so far away that virtually none of it returns for measurement. When that same laser hits one of the retro-reflective mirrors, enough light returns to be measured accurately. That has helped scientists conduct experiments ever since those early and sadly abandoned days of lunar exploration.
When a lamp on my projector dims enough that eco mode isn't bright enough, I switch to high lamp. Once I feel it's necessary to do that, the lamps typically haven't lasted too much longer and need to be replaced.