A Note Regarding Optoma HD73 Ceiling Mounting
If you are using a 3rd party universal ceiling mount (as most do), this may be helpful: The Optoma uses M3 type metric screws. The maximum length for the screws is 10mm, the minimum is 7.5mm.
Considering The Optoma HD73's Digital Image shift
This is a nice touch that has been on many Optoma home theater projectors and some others (the old NEC HT1100 come to mind). It allows you, if you are projecting less than the full 1280x768 image, to move the live portion of your image up and down.
The best example would be this. You are watching a DVD. You have the usual letter box at the top, and the bottom. With digital lens shift you could move the whole movie image down to the bottom, eliminating the letter box at the bottom (and doubling it at the top). This does not affect the quality of the image!
You can also use this in part, to deal with the 16:10 aspect of the projector, and the large lens offset. You could raise or lower your DVD or HD image (100" diagonal screen) by that 1.6 inches, thus removing 1.6 inches of offset, getting your projector a bit closer to the top or bottom of the screen. That's not much, but might help.
Rainbow Effect and Screen Door Effect
The Optoma HD73 projector is DLP, and as such has a pixel structure that is far less visible than found on most LCD projectors. As a result, for practical purposes, the Screen Door Effect (SDE) is not an issue. If you sit close enough to a large screen to actually pick up the SDE, you are probably sitting so close that the image is too soft to enjoy.
The Optoma is comfortable at about 1.1 times screen width. with a 100" screen, you should have no issue with SDE if you are sitting around 7-8 feet feet back. For those of you who never want to see a pixel structure, even in credits and large stationary white areas, will probably prefer to be more like 1.3x back. Most LCD projectors (which have much more visible projectors would require about 1.4x to 1.5x screen width), as their equivalent to 1.1x on the HD73. The one exception is Panasonic's PT-AX100u, which uses "Smooth Screen" technology, and has less visible pixels, but is also a touch softer than other projectors. (Personally I like the immersion of sitting close. I sit just about 11 feet from my 128" diagonal screen (10 feet 7 inches wide). That puts me between 1x and 1.1x. I can see the pixels in white areas like text credits of movies, but otherwise its not an issue.
In other words, if you like the big image feel, DLP projectors are for you!
DLP projectors rely on a single DLP "chip" and bounce white light against it. Color images are created by timing the light against a spinning color filter wheel that typically have red green and blue segments (and sometimes white). Without getting into the intricacies, some people, due to the color wheel, see flashes of red, green and blue, referred to as the Rainbow Effect. Early DLP projectors (about 10 years ago had 1x (one times) wheels. Today's projectors use faster wheels to reduce/eliminate the effect. Most business projectors use 2x wheels while almost all home theater projectors today use 4x or 5x wheels. (A few older entry level HT projectors still use 2x.) Having faster speed wheels on home theater projectors is due to the rainbow effect being more visible on motion video than still (business type) images.
The Optoma HD73 projector uses a 5X wheel, whereas the HD72 used a 4X color wheel. That basically means that in terms of the Rainbow Effect, the HD73 is as good as any. Few folks can pick up the rainbow effect at all even with a 4X wheel, and less can, with a 5X. Myself, I'm slightly sensitive to the rainbow effect, and do pick it up occasionally (very dark scenes with bright, fast moving objects), with a 4X wheel. I rarely spot it with a 5X wheel, even on those types of scenes. I consider the move to 5X to be an important improvement for some potential buyers.
Problem is, no one seems to know what percentage of the population has an issue with the effect on a 4x projector or a 5x projector. If I had to guess (a wild one), I would say 5x DLP projectors are only an issue for 1-4% of the population, and I don't know if that number increases by 25% or 100% for a 4x color wheel. Pure guesswork.
Let's say that dealers report very, very few returns of DLP home theater projectors due to the effect, but if it bothers you, bingo: Get an LCD projector.
This is the usual tradeoff. Some people see the rainbow effect, and therefore buy LCD, while many people who like to sit closer to their screens for more immersion (the really BIG screen feel), prefer DLP because pixels are less visible (screen door effect). The good news - most buyers are happy with either technology.
In summary, there's a small possibility that you might not notice the rainbow effect with an HD73, but will on some other DLP's like Optoma's HD72 or HD70. I suspect, however, that if you see it on a 4X machine,, you'll probably also see it on the higher speed, but less frequently.
I certainly wouldn't avoid the HD73 because of the rainbow effect unless you already know you are susceptible to it based on viewing other projectors, because very few will notice it. If the HD73 is your first DLP projector though, power it up, feed it a movie and check it out right away (projecting on your wall will be fine), and be aware of your dealers return policy.
Optoma HD73 Light Leakage
The HD73 home theater projector is well designed to prevent light leakage. There is no issue from the vents on the bottom or sides. There is some reflection coming out of the lens that points toward the bottom of the projector, however it is by no means bright. If you are ceiling mounting very close to the ceiling, and your ceiling is white or near white you will be able to look up and see it. It certainly won't be bright enough or evident enough to draw your attention from the screen!
HD73 Audible Noise levels
The projector as noted elsewhere, claims 27db noise levels (low power). Since most will run the projector with AI on, however, the fan will run louder. If I had guess, I would put the noise level at full lamp power (or AI on) around 32 to 33 db. As a result, in full power, the fan would have to be considered on the noisy side. Those demanding a really quiet projector, that they never notice (even on quiet movie scenes) will not find the HD73 acceptable in full power. But, I should point out, that few are that picky. For most the audible noise just "dissapears" much as your hot air heating system, or your refrigerator. You just tune it out.