In other threads I've talked about using an fs mirror instead of a second fresnel. While I was experimenting with a page fresnel held at it's focal point in front of my 19" lcd using the backlight for illumination (not a box) I noticed that it projected a brighter image than when I used the triplet. Even when I put the fresnel at the recommended 20mm distance I noticed no change in brightness using the triplet.

I thought it must be that the lens has too small an aperture for the lcd panel so I used a smaller video window using about a third of the screen but it made no difference. The best visible brightness was achieved by moving the fresnell to its focal which in my case is 16" and not using the triplet.

Now this is out of a box and not using lights for projecting just the lcd backlights. Maybe I would see flaws in the image if I used a fresnel as a lens in a real projector situation. Do we use a triplet because of the throw length or something else?

In a split setup has anyone tried moving the fresnel instead of using a triplet with a large lcd panel. If what I saw is true it looks much brighter.

I was just wondering if trying to squeeze that much light into the small triplet aperture isn't what causes so many people to seek higher wattage lamps.

Does someone want to try an experiment?

I'm not in a position to run a real test of this right now but I wonder if someone with a split design and a 15+ panel on a standard lens could try moving their front fresnel to where the triplet is and removing the triplet. With a bit of focus adjustment I think the brightness increase would be significant. I don't know about the quality of the image though. I can't tell that from what I've done so far.

The effects of a fresnel 20mm from the LCD and the effect of a fresnel further than it's focal length from the LCD are two completely different things.

There are two systems within the projector, one is the lamp or light system, which consists of the lamp, two fresnels and triplet, while the other system is the LCD, triplet and screen.

With the light system, the first fresnel is designed to collimate the light, with the second fresnel designed to focus the light at or near the position of the triplet into an area no larger than the diameter of the triplet lens. You can loosely refer to as "collecting light" and directing it into the triplet. Any light not directed into the triplet, will never reach the screen. The placement of the LCD can either be between the fresnels (split setup) or after the second fresnel (unsplit setup). You can read plenty of topics about which is better, but just keep in mind that the effect of the split setup is basically that of a page magnifier, so their is a "virtual LCD" formed with is larger than the LCD itself.

The second system is simply the LCD, triplet and screen. This is a basic principle of optics when you have an object (LCD), a lens (in this case a triplet acting toghther as a single lens, which in the case of the pro lens has a FL=500mm), and the image (screen). This is how your LCD size, throw, LCD-triplet distance and screen size are all related. (you can use the focal length calculator to quickly calculate these relationships)

Now... with the fresnel at 20mm from the LCD, all that is occuring is a magnifying of the LCD, so that the second system "sees" a larger (virtual) LCD size. However, if you remove the triplet, then move the second fresnel to a distance beyond it's focal length from the LCD, the fresnel will no longer be part of the first "light collecting system", but rather it becomes part of the second system, replacing the triplet. You can use a fresnel lens as a singlet projection lens, but it is of poor quality for this purpose. There will be all sorts of abberation caused by using only a single lens as a projection lens, as well as the fact it is a fresnel lens. One benefit of this system, it that the lens can be quite large, so there is no need for the "light gathering system" to focus light into a small region... hence moving the second fresnel far away from the LCD means it stops being a collector lens (or field fresnel) and becomes a projection singlet lens.

Hope this is understandable. SupraGuy wrote up a nice explaination of the two systems, and it's stickied someplace... i'll try and dig it up...


cheers,
gs


edit: you can check out Light Flow in the Pro Wiki Guide... look at fig. 1c... this is basically what you are doing, except you have replaced the triplet (A), with a fresnel lens (singlet)... the short comings of a singlet are discussed here in the Pro Wiki Guide as well...