Hi, excuse me if this has been asked before, I couldn't find anything -probably didn't search the right words.

I haven't built my own PJ yet so I don't have experience about fresnel lenses and what I've read so far in the forums haven't given me any more insight. Just thought I'd mention it before I go on.

Fresnel lenses, the basic design suggests using a 320 mm collector lens. Now the first question: can fresnel lenses have much shorter focuses than 320 mm, like 80 mm, 120 mm etc.? If not, why?

Second question: I'm assuming it's possible to have collector fresnel lenses with a much shorter focus now. Following the basic design ( light > heat glass > fresnel > lcd > fresnel > triplet lens ), if I used a collector fresnel with a very short focus instead of the 320 mm one, and placed a second collector fresnel in front of the other, would it cause too much distortion or project the lense lines of the fresnel? I'm asking this because I think if it can be done without compromising the quality, it would greatly decrease the overall size of the design. Look at the diagram.

The setup on the top is the default design; (excluding the light and the heat glass on the diagram) fresnel > lcd > fresnel > triplet lenses. Below the collector fresnel has a shorter focus and a second collector fresnel in front of it, with the triplet lenses shown in 2 different zoom settings.

Click to view attachment

Has anyone tried something like this? If so, how were the results? I think the design in the diagram also allows a much better and easier adjustment for the projection size. Or, even with shorter focused fresnels, is this impossible? What do you think?

What you have to think about is the FL of the triplet as well as the FL of the fresnel.

DJ

edit: Then, in the case of my diagram, I'd have to use a triplet suited to the 2nd collector fresnel only, right? Regardless of the 1st collector fresnel? Then I'd suspect that, a smaller triplet suited to the 2nd fresnel, will not produce as good a projection as the 80mm, making the whole idea useless, am I right?

what about room for air and heat? unless you mounted the bulb on the end of the main case in its own box HHHMMMM........ thinking


I will get back to you on this thought

Brian kc0kfg

Interesting Idea, but I am not SURE if it would work... But, GREAT idea...

I'm no expert, but I'll share my understanding and someone who knows something can step in and correct me where I'm wrong.

The triplet is to be focused on the surface of the LCD panel. The field fresnel is there to herd the light from the corners of the LCD onto the triplet so that the corners aren't totally dark.

In principle, if the triplet had the right focal length, it seems to me that this idea would work (except the zooming part that you show, unless you're using a varifocal lens).

However, because of the ridges formed by the rings on the fresnel, the light entering or leaving the side with the grooves should be perpendicular to the smooth surface. So, any design that has light non-perpendicular to the fresnel on both sides is going to result in the appearance of rings.

Does that make any sense?


Take it easy,

You would need a triplet suited to the combined FL of both fresnels and you will have a hard time, if not imposable, finding one with a field of view large enough to see the entire LCD. Another consideration is the magnification the fresnels and triplet being so close to the LCD will have. Your projector will end up needing to be placed to close to the screen to be practical.

DJ

For the triplet it works like this a 320mm fl triplet and a 320mm fresnel if placed near the triplet would add to about a total fl of 160mm so the throw will be real wide now this does work. A regulat glass lens works too.