To Fold or Not To Fold...

That is the question...

When thinking strictly in terms of projected image brightness, I understand that you can actually get a brighter image if your light source shines through the LCD from the rear.


This also means that you will have to bounce the image off of a first surface mirror to flip it right way around before the projection lens.

Since you have to use a mirror in the folded design you must lose some light there.

So is it worth it?
inline projection

Any time you add a mirror to the light path, you will lose some of the light. It does not matter if the mirror is before the LCD or after the LCD. The very best silver mirrors are 98.5% reflective, so they lose 1.5%. (Not practical to use because they tarnish quickly.) Aluminum first surface mirrors (which you would need if it is after the LCD) are at most 95% reflective, so they lose 5%. It goes down from there as you go to second surface, etc.

So if you want to get the absolute most light possible from your projector, don't use any mirrors. Flipped or reverse images are not a problem, as long as you plan ahead: You can mount the LCD any way you need to, to make the image come out right on the screen! With no mirrors, the correct orientation would be upside down, with the light going through from front to back. (Where "front" means the surface that you would see before stripping it.)

Digital image flipping is useful when you mess up and mount the LCD the wrong way, but it only works if you always run the LCD from your computer. If you have other sources, like a cable or satellite decoder, then it is a lot smarter to mount the LCD the right way to begin with.
Thanks for the replies... but Im still confused.

Lets say I dont have the choice of digitally flipping the image.

So either I shine the light source from the back of the LCD to the front... and use a mirror to flip the image around... or I shine the light from the front to the back of the LCD and not use a mirror.

From what I understand it is better to shine the light from the back of the LCD to the front because thats the way LCDS were designed to pass light.

The question is: Is the light loss suffered by shining the light from the FRONT of the LCD to the back greater, or less than the light lost from doing it the other way and using a mirror to flip the image around.

>From what I understand it is better to shine the light from the back of the LCD to the front because thats the way LCDS were designed to pass light.

I have never read that anywhere. I doubt if you could measure the difference, if there is any at all. There are polarization filters on both sides of the liquid crystal glass layers. The only difference would be that the light goes through the liquid crystal, then the color filter in one direction. Or vice versa in the other direction. Why should it matter?

You can measure the 10% or so you lose even with a good first-surface mirror.

OTOH, this is a DIY forum, so I suggest you try an experiment to see if you can measure any difference.
So... I unfolded my lightpath, flipped the LCD around.... and it does seem a wee bit darker.... Id need to see a side by side comparison to be sure though...

Oh well.... its much easier to get corner to corner perfect focus now... and when I get that larger front fresnel from I should have enough room in there to tilt the fresnel enough to be able to sit the projector on the floor, tilt it up, and still correct the keystone to get a square image... so Im happy.


2005-03-14 12:47 am
Digital flipping doesn't always work; even on faster computers, the rotate and flip functions can slow the display down a lot. And if you're trying to use a dual-monitor setup (so you can get the computer running while the MH bulb warms up) then digital flipping is a big pain. Turning the LCD is OK, but you've committed to a horizontal setup. My projector uses a mirror after the lens. The mirror can be swiveled around the lens, effectively rotating the projected image. I can display a landscape or portrait projection regardless of whether the projector is on its side or standing vertically.

If you don't need to do any of that, then turning the LCD is simplest. I accept the mirror losses, which aren't noticeable as long as you have a decent mirror.