The most commonly suggestion solution I find for short throw situations is the use of mirrors. I have certainly seen mirrors used successfully many times, and managed to use them myself a couple of times, but my stories are more painful than one might expect.

Mirror Considerations:
Size:
You may be surprised at the size of the mirror you need to gain significant throw distance. Here is how I calculate my mirror needs.
Td = Total distance from lens to screen needed for desired throw (as found in your projector manual or using a projection distance calculator) see my recommendations for projection calculator.
Ad = Available distance from your screen to an unmovable obstacle like a wall.

Td - Ad = Md Mirror distance. This is the distance your projector will need to be away from your mirror, so that the total optical distance from the projector to the screen = Td.
Now go back to your projection distance calculator and set the Md as your throw distance, and see how big the image will be at that point. Now multiply that by %115 for safety. That is the size of mirror you will need.

Lets say we have a 10ft screen, and our Td to fill that is 18ft. but all we have available is 10ft to the wall. Md = 8, image size at 8 ft is 4.4ft, so I would look for a 5ft mirror. Wow you might say, that sucks that’s a huge mirror, and yes it is, cutting down throw distances significantly is very hard. Cutting down the throw by a foot or two is much more unachievable, and even a small mirror is very useful.

Lets look at that problem again, I still have a 10ft screen lets see if I can convince people to let me move the screen away from the wall, I’ll need 18ft plus the 1.5ft depth of the projector 19.5 ft. That’s pretty far, they will never go for that, lets see if I just get a 4 inch mirror I can turn the projector sideways and eliminate the depth of the projector, there’s a good start, and the bonus for using a mirror. Now maybe I’m luck enough to have a 2ft mirror, I’ll look at the projection distance calculator, and I see that that will gain me 3ft of throw distance, now I’m down to 15ft of throw, I only need to push the screen 5 ft farther from the wall, not 9.5 ft farther from the wall, half as much, great.

Other notes on Using a mirror, it’s important to note that the mirror takes up space itself, I’m going to put it at something like a 45degree angle to the screen and projector, which means the image from the projector will be very keystoned; that’s fine, we don’t need to worry about that, but when I’m calculating distance the Td is going to be the distance from the lens to the near edge of the mirror + the distance from the near edge to the screen. If we are using a 45degree angle the distance from the lens to the far edge of the mirror + the distance of that edge to the screen should be the same, but that extra distance is in the Md part of the equations for the far edge, making our Md farther than the “optimal’ Md we used when we calculated things up above. That’s why I added the 15% to the mirror size. If you want to get fancy you could try and calucalate the side of a isosceles right triangle with a hypotenuse = the nominal mirror width: devide by 2 and add that to the Md and go back to the projection distance calculator to re-calculate the mirror size, but I’d stick with +15% if I wanted to keep sane.

Material:

it is recommended that you use a ‘first surface’ projection mirror for projection. There are two types of mirrors commonly used,

the first is a silvered glass mirror, however most home mirrors will have a layer of reflective silver on the back with the glass over top to protect it. This is the type of mirror to avoid, when your projector hits this the front surface of the mirror itself will reflect a bit of the image, and the silver on the back will reflect the rest, the result is a ghost or blur on the projection screen. A projection or first surface mirror has the silver applied to the top of the glass and therefor the full image is reflected. This makes a first surface mirror a bit harder to manufacture since the layer of silver needs to be perfectly flat, and it makes it more delicate since it can be much more easily scratched than a glass surface. This type of mirror will generally have the highest optical quality and lowest loss of light, but is likely to be very heavy, and therefor need a significant mounting structure.

The second type of projection mirror is a sheet of reflective mylar. Mylar is a plastic with some metallic properties, one of which is that you can manufacture a thin reflective “mirror” sheet of mylar film. Although mylar can be somewhat or almost totally transparent but the type used for this is an opaque mirror finish. This makes for a first surface mirror with no worry about a double image. This film is very lightweight and cheap (compared to large glass mirrors) and therefore makes a good projection mirror, however there will be more imperfections in the plastics and so you will loose more brightness, and a bit of sharpness with this mirror. This type of mirror is also fairly delicate, it can be punctured or stretched, and some mylar will go cloudy just when it it touched if it is stretched tight. The mounting hardware does not have to support as much weight as glass, but in order to get a perfectly smooth and flat surface of the plastic film it will need to be stretched flat over a ridged frame, the process of which is an art in itself.

Mounting:

As mentioned above you will have to very precisely position the mirror between the projector and screen so that you are able to align the image on the screen as with as best a fit as possible. Both the mirror and the projector would benefit from a very sturdy and finely adjustable mounting system. Many companies which sell mirrors also sell a mounting system which can cost as much as the mirror and projectors in some cases.
And remember due to the added complexity of the mirror aligning the image is much more time consuming, and minor or major adjustments to the keystone correction of the image are likely to be needed.

Tips:
If you can get a mirror a little bigger than you think it will make your life easier.
You might think a 3×4 mirror for a 3×4 screen makes sense, but You may fine a square mirror is more usefully due to to the keystoning of the image on the mirror surface.
If you have a software that controls keystoning for your image it will save a lot of time, if you can just get it big enough and in the right area and do the details with software it will save a lot of time and pain on your part focusing.

Here are some price lists for ready make mirrors, I have only used the American Science & Surplus mirrors myself.
cheap small mirrors
Low cost and Small - Medium Size Mirrors
Medium cost and Medium Size Mirrors with mount
Large Mirrors
Large Mirrors

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