Which home theater projectors can be shelf mounted
That's a pretty easy question to answer. The first rule of thumb is this, if you want to shelf mount your projector, you'll almost certainly need a projector with adjustable vertical lens shift. This assumes you want to put the projector on a shelf in the back of your room - and I mean set it on a shelf - not mount it inverted, or anything else unusual.
To be more specific, short of listing dozens of projectors, let's just say, that virtually all but three or four 3LCD type home theater projectors can be shelf mounted.
LCoS projectors designed for home theater (JVC, Sony), also all have vertical lens shift (not not necessarily "business LCoS" projectors like some Canon's which sometimes find their way into home theaters).
And that brings us to DLP projectors. Few DLP projectors
offer lens shift, and therefore few can be shelf mounted.
For example, although Optoma has an extensive line of home theater projectors (more models than anyone else), all are DLP, yet you would be hard pressed to find one projector sporting adjustable vertical shift.
BenQ, on the other hand, offers lens shift in most of its home theater projectors, with it only absent in a couple of entry level models.
Why is Lens Shift critical to shelf mounting a projector
First, let me advise of an exception. You can shelf mount any projector that lacks lens shift. However, those projectors all have "fixed" lens shift, that determines exactly where the projector must be placed vertically, relative to the screen. Since projectors lacking adjustable lens shift, tend to be designed to be even with the bottom of the screen, or even lower, then to shelf mount one of these projectors, you are normally going to have to have that shelf between about eighteen and forty inches above the floor, depending on the projector model, and how far down your screen comes.
Mounting that low, doesn't work for most people, as seating and other things normally get between the projector and the screen. Still, it is possible!
With variable lens shift, a projector typically defaults to having its lens even with the center of the screen vertically, and the "variable" aspect allows the projector to be placed higher or lower, as the adjusting the lens shift moves the image, up or down on your screen.
Most projectors with variable lens shift, have enough to allow the projector to be anywhere from even with the top of your screen, to even with the bottom. Those with more range, may even have enough to allow the projector to be a full half screen height above (the top) or below (the bottom) of the screen. For an example, if you have a 100" screen, that would mean that the projector could be as high as about 25" above the top, or 25" below the bottom of the screen - or, of course, anywhere in the middle.
My own theater has my projector mounted about even with the top of the screen, a very typical setup.
Why Shelf Mount your Home Theater Projector
Assuming you want a fairly permanent installation, your choices are basically - shelf mount or ceiling mount. Each has its advantages, however, essentially all projectors can be ceiling mounted, whereas slightly more than half of all home theater projectors (my best guess) can be shelf mounted.
The advantages of shelf mounting are as follows:
1. Usually lower installation costs. This is due to several things, first, many ceilings don't have any power, so power has to be brought up to a ceiling mount. By comparison, most walls do have power readily accessable, so it's just a matter of moving the power up the wall a few feet.
The other thing is that you will probably need shorter cable runs if shelf mounting. That's not always the case, but typical.
2. Not all ceilings have access above them to allow easy running of cabling. (I know this nightmare - I previously ceiling mounted projectors until a couple of years ago.) Running through walls is - well, running through walls - Standard Operating Procedure.
3. With ceiling mounting, the projector, the ceiling height and the size and height of the top of your screen all determine where the projector can be mounted. With high ceilings, typically your projector will hang down a bunch of feet on a pole. In my room, for example I had about a 6 foot pole with the projector at the bottom. It wasn't pretty, and my wife never liked that setup. By comparison she strongly favors the shelf.
. Shelf mounting often is easier for doing maintenance, such as cleaning filters, or replacing the lamp. This is especially true for lamp replacement, as perhaps half of all home theater projectors (although few of the more expensive ones), do require unmounting from a ceiling mount, to change the lamp. And, not only is that a hassle, but then you have to remount and carefully reallign the projector - basically a pain in the butt!
Downside to Shelf Mounting Your Projector
1. Audible Noise! Where are you sitting? If your room is pretty squarish, you are likely sitting not very much forward of the back wall. This would mean that you are sitting close to directly under the projector. And that means projector fan noise may be an issue. (A good reason for placing the shelf up very high. On the other hand, if you sit well forward, fan noise should not be an issue. (If you are ceiling mounting, in such situations, in the first case, the projector is probably well in front of you, and therefore quieter, and in the second case, you might be right under the projector.
Note: If you are pretty much sitting under the projector, you might want to make the shelf larger than necessary to block out any direct path for noise coming directly from projector to your ears. (Putting some sound absorbing material on the shelf, can further reduce fan noise.
2. Simply said, a back shelf is as far away as you can get a projector from the screen, in your room. That normally means having the zoom lens somewhere between fully telephoto, and in the middle of its range.
The bottom line, is that projectors, when shelf mounted, aren't as bright as when ceiling mounted closer to the screen.
With today's home theater projectors with 2:1 zoom lenses or more (that includes all Panasonic, Epson, and Sanyo HT projectors, to name a few, that means you are giving up lots of lumens. With a 2:1 zoom lens, in full telephoto (typically about 20 feet from a 100" diagonal screen), most home theater projectors deliver anywhere between 55%, and 65% of the brightness to the screen, that they would if in the closest position (typically about 10 feet from a 100" screen).
The bottom line, is that when ceiling mounting, you are normally closer to the screen, and therefore get more lumens, and it can be a rather significant difference.
Is it practical to shelf mount any projector with vertical lens shift
No, it's not. Your room may not be the right shape - the room too long for the size screen, so you can't mount the projector as far back as your wall. Fortunately most 3LCD and LCoS projectors have zoom ranges around 2:1 or just a little less. Off the top of my head, I can't think of any lens shift equipped 3LCD or LCoS, with less than 1.5:1. (I'm strictly talking home theater projectors here, not business ones.)
Thanks to these zooms with such wide range between closest and furthest away positions most people can shelf mount.
DLP projectors again, tend to be the exception. BenQ, who offers variable lens shift on their PE8720, W5000, W20000, and other projector's has lens shift that will allow vertical placement anywhere between top and bottom of the screen, but their zoom lenses range varies between only 1.15:1 and 1.35:1, basically just a few feet of placement flexibility at most. Fortunately, the zoom lenses on the BenQ's are fairly long throw, to accomodate the typical room size/distances for shelf mounting, but a much higher percent of owners of these, will not be able to shelf mount. (I for one had the BenQ PE-8720, and was able to shelf mount it, thanks to having a very large screen, which allowed the projector to be pretty far away.)
With a BenQ, or other DLP with limited zoom range, if shelf mounting is what you want, you will have a greater chance of being able to do so, if you are flexible in terms of the size of your screen - you just might have to go a size or two larger or smaller than what you might have picked, to allow that projector to be the necessary distance away.
What else do I need to know about shelf mounting my projector
First, remember as you figure out the placement, that specs provided by manufacturers (unless otherwise pointed out) are based on measuring from the front of the lens, to the screen, not from the middle or back. Since that can be as much as a 2 foot difference, pay attention.
Next, ventillation is important. Different projectors will have different recommendations, but generally, you'll want to have at least 5-6 inches of clearance in the back. All shelf mountable projectors vent hot air out the front (typically at an angle, or the sides), to help out.
Don't stick your projector on a bookshelf, etc., surrounded by other things, ventillation is very important to longetivity of your lamp and your projector. That said, stuffing it in a shelf surrounded by books won't work for long anyway, most likely the projector will just overheat quickly and shut down.
If you are doing a shelf, and are concerned about heat, drilling large holes or placing other cut-outs on the shelf to allow more ventillation is always a good idea, but probably not necessary. The shelf I use is about 30x24 inches (30 wide), and I've had no problems with either my old BenQ, or my current JVC RS1 (both rather large home theater projectors.) I've always managed to provide just barely 6 inches in the back for the cables, and for air to circulate, and I've never had a heat related shutdown, etc.
Since you are using lens shift, the projector should be perfectly level, both horizontally and vertically - essentially perpendicular to the screen. That will allow the lens shift to maintain a rectangular image. If the projector is tilted at all, you end up with more of a trapazoidal shape.
Calculating projector lens shift
Virtually every projector with lens shift will have a table in it's manual that will tell you the vertical (and if it has horizontal lens shift), and horizontal placement range. For example it might say for a 100" screen, that the offset is 16.5" maximum, which would mean the maximum height of the projector would be 16.5" above the top of the screen surface. Other manuals, however may just provide a formula - easier for odd sizes, but requires more thinking on your part to translate.
One thing of importance to remember. With most projectors vertical and horizontal lens shift severely affect each other. To get the maximum vertical lens shift, you can't use any horizontal shift, and vice versa. Fortunately few use horizontal lens shift at all, except for fine work - an inch or two if the projector's placement was not correctly calculated (such as forgetting that its lens is mounted off center, and centering the shelf to the screen).
Therefore, if you will be needing most of the vertical lens shift, try to place that shelf so that the lens is dead center - left to right - with the screen, so as not to waste your flexibility on horizontal lens shift.
If I'm shelf mounting and using lens shift, do I need to invert the projector
No, there is not normally any reason that makes you flip the projector, if it has vertical lens shift. The only possible exception that I can think of, is on rare occasion, you may find a projector with a lot more shift above, or below, the center point, than the other direction. In that case, you could, conceivably find an installation where you want to mount, say, very high, but the only way to get up that high is if the projector is inverted. More likely though, it would be the other way around, most manufacturers, I would think, if not offering the same amount of shift in both directions, would have more shift to the top.
Mind you, that doesn't mean you can't mount the projector inverted, under a shelf. I seriously considered that last year, when buying my JVC. the JVC outputs about 900 lumens maximum in brightest mode, and I decided when I bought it, that, if it wasn't as bright as I wanted for sports viewing, I would seriously consider mounting a Panasonic PT-AX100U (now the AX200u), inverted to the bottom of the shelf. JVC on top, Panasonic underneath. The Panasonic would give me twice the lumens. Fortunately, that wasn't necessary.
For most folks, especially when adapting an existing multi-purpose room (family room, living room) to do double duty as a home theater, shelf mounting is often the preferred solution. Typically it will save money, and effort in terms of the overall installation, and provide (brightness notwithstanding), performance at least as good as with ceiling mounting.
These are the basic issues. One smart move, is to figure out, up front, whether you want to/need to go with a shelf mount, as it will quickly help you narrow your projector choices. And, anything that quickly helps you choose between a whole bunch of good projectors in your price range, is generally, a very good thing!