I see a few new designs starting using mirrors to fold the light path, and comments about using the mirror to adjust for keystoning while keeping the fresnels fixed. Let's say you are doing a vetical single mirror folded design with a 45degree mirror to fold the light path to the PJ lens, don't you have to make the lens adjustable up and down (lens needs in and out movement for focus or zoom)? Since you are moving the mirror, it will direct the light cone away from the lens, causeing a (large?)drop in brightness. I'm not seeing this, can someone explain in more detail?

Thanks
Ed

Hello ednigma,
It can't be done with flat mirrors. People keep saying that it will work but they wont say how. Simply saying it works, but not backing it with information, translates to me to meaning wishful thinking.

DJ

Here! Here!
Well said.

OK thanks, I thought I was missing something obvious.

Still would like to know how it could be done. Do you need curved mirrors or a prism?

Right or wrong, I'd also like to know what the supposed theory is to keystone correction with a single flat mirror.

Regards.. Ed

Works fine on OHPs with the flip out mirror after the projection lens.

Hello Brain,
Would you care to elaborate?

DJ

I believe the way they do it is by tilting the projection lens and then adjusting the mirror to throw the picture back at the screen.

Any chance of a simple diagram?

DJ

Hi dazzzla,

http://www.lumenlab.com/forums/index.php?showtopic=1780

the first 2 drawings show the mirror setup after the projection lens these are adjustable.so you would tilt the box and then adjust the mirror for keystone
and to bring the projection back to the center of the screen.
peter

Hello pjgibbs,
I still don't see keystone distortion being corrected with a flat mirror. The only way to counteract keystone distortion is by introducing distortion, equal but opposite, into the chain somewhere in the optical system. There are 4 ways that I can see keystone distortion being corrected:
Pre-distorting the image displayed by the LCD (digital keystone).
Pre-distorting the image by magnifying one or more sides (optical keystone).
Physically angling the LCD and object lens. (Still a form of optical keystone)
Physically angling the screen.
Any other that I don't know about yet.
A flat mirror will only reflect an image and make it appear as if it is coming from a point behind the mirror, it doesn’t distort the image. Don’t get me wrong, I would love this to work but you can’t change the laws of physics.

DJ

I am also thinking of building a folded design but for a small LCD panel. Can't you correct for keystoning the exact same way as with a non-folded design?

If you mean by using a field fresnel in front of the LCD, yes it will work just the same. If your LCD is small enough you might consider lens shifting.

DJ

as brain already pointed out... it will work with mirrors
think about it, on an OHP you adjust the mirror to project the image onto the wall.
If the OHP is on a flat surface and the mirror is in the center of the projection screen, then the ideal angle for the mirror is 45 degrees.

however if the mirror isn't in the center of the screen and you tilt it so the image is up higher... you've got keystoning happening, thus you are adding distortion using the mirror

ergo, if you need to correct for keystoning then you could use a mirror, however you would want the mirror in a fixed position (ie at 45 degrees with the floor) sitting above your lens (in a vertical projector) and tilt your lens until the image is free of keystoning

yes


yes


No. The distotion is caused by the top and bottom of the screen not being equal distance from the object plane.


yes as long as the mirror is at the centre of the screen.


???

DJ