Dual Fresnel

[Dwiel]

Hey guys!

Just thought I'd let you know what I'm doin cause it's pretty cool (IMO)

I recently bought a new LCD cmv1512 to replace my NEC 1545v, and I am really hapy with it... besides the point... Anyway, I have 2 split fresnel lenses. I have been trying to get the best combination of placement and fresnels to get the best image... With one method, I didn't even need a projection lens! the edges had really bad chromatic aberation though :-(

Anway, I was thinking that if I was able to move all of the parts (glass, fresnel, LCD, etc) closer to the bulb, they would catch a lot more light. I also knew that by putting 2 lenses in series, the focal length becomes 1/(1/f1+1/f2). This meant that I could put 2 fresnel halfs right next to each other, one a 9" fl and the other something like 11", and get a combination effect of a fl of 5". I know have my fresnels much closer to my 250w HQI and gathering much much more light! My setup looks like this:

Bulb - 5" - glass - 1/8" - fresnel 9"fl - 1/2" - fresnel 11"fl - 1/2" - LCD (not in yet... using a transparency) - 1 1/2" - fresnel 11"fl - forget" - prj lens

This is a very bright and high quality picture that is being projected. I was afraid that many of the negitive effects of some fresnels like hotspots, circles, etc might appear due to the increase of their use, but the picture is still very nice!

I suggest to those of you who happen to have an extra split fresnel, or really, I only used an extra half of a fresnel, give it a try. the formula 1/ft=1/f1+1/f2 where ft is focal total, f1 is focal length of the first lens and f2 is focal length of the second holds pretty true.

I'll be sure and posts results when I get a hold of a friend's digicam, but I am sure that the picture is 2x brighter with the current setup than how it was when I simply used the split 11" fl fresnel by itself.

comments/suggestions?

[freeupyermind]

im very interested in this, it coud result in much smaller designs in the future as well as a lot less light loss, something im wondering about, is it possible to do this with projection lenses to cut down on size nedded between projector and screen?

[Dwiel]

at one point, I was using only 4 split fresnel halfs. There was no projection lens used, and I had a very large image at a very short distance from my screen. The last fresnel was about 6' away from the screen. The picture didn't even fit on my wall(screen) which is 12' across! There was about 2' on each of the walls next to the one I was projecting on, so I'd say that if I had a 16' wall, the picture would have taken up all of it... maybe 17' Also, the picture was still much brighter than I would expect when trying to project an image so large. I think that this is due to how close together everything was. I dont remember if this was before or after I moved the front fresnel to around 5" from the bulb. I'd say that if you are interested in rear-projection, this might be a very good method for shortening the throw distance. The only problem, is that even if your fresnels are warped a little bit, they all compound each other, and the result is not as pretty as you might want it. Luckily, only one of my fresnel halvs is very warped, and I have had it under about 50lbs of books for around 18 hours, so hopefully it wont be too bad. Luckily, that lens is one that comes before the LCD, so it isn't as imperative that it is perfectly flat.

Are you trying to do rear-projection or front projection? With front projection, it seems that the PJ would need to be so close, it would really be in the way of the projector... My projector has shrunk in size though, so now from the bulb to the proj lens is only around 26" it used to be closer to 32"

just wondering why you wanted a large image witha short throw distance.

[ace3000_1]

dwiel have u seen my light engine in my thread? lol its the same thing, ive managed to get 4x the brightness compared with just a single frensel, the multi frensels also way out doo's the split frensel design, 3x the brightness in mine now compared to the split frensel method, also u dont have to use a frensel, u can use another condenser, the first frensel is just acting as a secondary focal condenser and enhancer with can be replaced with a normal pcx lens, but the shorter your focal is the bigger the lens will be as u are concentrating that light from the first condenser into a more concentrated narrow beam with the second, hence giving u a brighter image, there are also a few other tricks but ill let u work those out for yourself lol.

Trev

[freeupyermind]

im working on a rear projection system and i want about 160 inch screen but i only have about 33 inches of space for the image to project, can you use multiple projection lenses to make the image bigger, and how do you do it without a projection lens, does anybody have any diagrams, or could make one up in paint real quick?

[Hezz]

Freeup,

what you are proposing sounds a little unrealistic without a very wide angle lens with a short focal length. How big of a LCD are you planning to use. A lens that could do this without much distortion would be very costly because of it's high end and critical engineering and multiple lens with aspheric surfaces.

Hezz

[Dwiel]

Yeah, the extra fres is acting more like a large condensor. It allows you to put everything much much closer thus increasing the angle which the light will be accepted and giving you more light. I didnt realize it was 3x and 4x brighter though. What did you do to get 4x brighter? I only just now saw your posts in the 'show your pics thread', but thats it.

free: I'm not sure what the deal is for your rear-projection, but I believe that all you need to do is stack 2 pj lenses right after eachother. the only problem is that if the last lens isnt big enough, you will cut your image down If you want to know what size the lenses need to be, draw out on paper the focal lengths etc, and get it so that you have where the light is going from the next to last pj lens to the screen. Now if you want to put in another lens, simply make sure that when you put it in that former picture, it can 'see' all of the light comming from the previous lens. It gets tough when the projected image size gets large because the size of lens you need to see the whole picture needs to be large very quickly after the lens...

maybe a pic will help me:

[ace3000_1]

dweil the first frensel doesnt need to be as big as the rest if u use the right condenser, u must use the right condenser to get the best performance from this system, and that my son is a secret lol, i managed to get 4x the image brightness with my setup, if i change a few things wich may be in this model but definatley in the next it should be 7x brighter, ill be fooling with my light engine this week, keep an eye on my thread to see how i go with it, in mine ill also be using a pyramid that makes it even brighter, though at the moment im deciding weather to use white or mylar for the inside, there's been a few updates on the pyramid too between cruser and myself, but in mine the light is finley tuned and guided to the lcd and to the projection lens with maximum intensity and brightness from the bulb and its spherical reflector, if i change my reflector to a parabolic reflector i should have an increse of 40% of light wich is somthing im looking into now.

Trev

[ace3000_1]

what you are proposing sounds a little unrealistic without a very wide angle lens with a short focal length. How big of a LCD are you planning to use. A lens that could do this without much distortion would be very costly because of it's high end and critical engineering and multiple lens with aspheric surfaces.

i totally agree with hezz, to get an image so big with such a small throw it would be nearly impossible to have the whole image in perfect focus and an even brightness, and its why rear projection tv arent 100inches.

Trev

[ace3000_1]

Have a look here dweil this is what ive done, also i dont use 4 frensels i use 3, 4 will loose u light.

Ace_3000....pictures of Projector?