Do fresnels need to be sandwiched between glass?

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Do fresnels need to be sandwiched between glass?

If I build a frame around each fresnels, would that be still necessary?

what about having 1 glass per fresnels?

I am trying to have least materal between the screen and the light source as possible while trying to keep the cost down .

You want to keep the path tranversed by the light as simple as possible.

Typical glass reflects about 4% of normally incident light.

Taking into account both boundaries you will get (1-0.04)^2 transmitted which is about 92% tranmission.

Add two panes of glass and you'll be down to about 85% transmission.

It gets even worse when you consider each fresnel lens transmits about 92% of the light incident upon it surface, the LCD panel wlil transmit less than 50% of the light shining on its surface since the light source is not polarised. Then you have 6 more boundaries in a triplet lens so you better hope they're coated for anti-reflection or you're going to end up with a nightlight.

This is where all your lumens go. I noticed Toms hardware were foolish enough to assume they have a 3500 ANSI lumen projector when more than half of the photons won't even go through the panel when its white/clear!

Bottom line is use one panel of glass as a thermal barrier and don't use any more!
That's the thing.

I understand that it will cut down the amount of light that will get to the screen.

But I am also worried that fresnels will bend or alter its shape if it does not have some support?

I guess I will have to actually test the results and see how much of a difference it makes.

Because I do see some results with dble sandwiched fresnels that are quite nice.

thank you!!!
Yeah definately worth testing it in real life, i suppose it depends on your application and how bright the final image ends up. For my projector i was thinking of making a stiff frame from metal or wood to hold the second fresnel lens in place and attaching the first fresnel lens to the thermal barrier glass.

I'm trying to find some UV reflective glass to stops the fresnels degrading as they are highly absorbent below 400nm. The only stuff i can find right now is designed to transmit IR frequencies which is not a good thing either!
Hmm.. how come using a non split design is better than splitting the fresnels?

I just did a simple calcuation assuming normal incidence on the percentage of photons stricking the triplet lens compared to the initial number striking the thermal barrier glass:

With split design and 1 sheet of glass transmittance is:

0.5*(0.96^(2*3)) = 0.39

With unsplit design its obviously the same answer:

0.5*(0.96^(2*2))*0.96^2 = 0.39

I presume you can use the thermal barrier glass as a support for both lenses giving less distortion?

I've also seen composite fresnel lenses bonded together which claim transmittance of 92%, this would boost transmission by about 3%.

Hmm.. i think i'll experiment with this as well..let me know how it turns out if thats ok?
Using a non-split fresnel design doesn't have anything to do with transmittance. It has to do with image quality. The thought is that instead of having the objective lens focus on the nearest fresnel it will instead focus on the LCD, thus giving a better image.
I purchased UV protectant from lumenlabs. I was thinking that it was a glass with a film.. but behold its just a thin film!!! :p blah...

so my plan right now is...

Light source (400w MH) --> IR glass -- > super thin glass: UV : fresnel---> LCD ----> fresnel -----> triplet -------------->screen.

I am planning to build a frame for both fresnels, so hopefully that should be enough to hold it in shape for long time :)

ill post some pics in few days with results!
support & UV filter

I use a piece of thin Lexan XL10 (from Home Depot) to support my unsplit fresnels, AND it filters out > 99.8% of the damaging UV.

It is very easy to cut with a metal cutting blade in a jigsaw, but you do have to be careful not to scratch it while cutting. (MUCH easier than cutting acrylic fresnels!) It is also pretty heat resistant, but I would not put it close to a MH lamp without some IR filtering first.

I only use one piece of Lexan, since my fresnels lay right on top of it. I have read posts from people complaining their unsupported fresnels warped after a while. I think that is the result of both gravity & heat combined.
unsplit long-throw

Yes, I have 220 & 770 mm fl fresnels together, about 15 mm before my 15" LCD. Then I have a 600 mm fl process lens, but I have also tried a 457 mm fl opaque projector lens.

The 770 mm fl fresnel was from lumenlab, but I plan to replace it with a 3dlens 550 mm fl fresnel as soon as they become available. Then I will be able to get rid of the large gap between my fresnels and my LCD.
lexan transmittance

I think it is more like about 90% transmittance for one 0.093" thickness of lexan XL10. I just measured a scrap piece that I have, which is somewhat scratched and dirty: 88%

Still, I think that is much better than the UV filter sheets I have seen. Around 90% seems to be the norm even for the clearest glass you can find. A good dichroic IR filter (hot mirror) will claim 90% transmission of visible light and 10% transmission of IR. That is much better than the plastic film version (Rosco Thermasheild) that only transmits about 81% of the visible light.

The real light sponge is the LCD: Every bit of light goes through a color filter, and through the initial polarizing filter. Every red filter absorbs the green and blue, every blue filter absorbs the red and green, every green filter absorbs the red and blue. So the maximum you get through the color filters is near 33% transmission. Then the initial polarizing filter absorbs all the light that is not in the right polar orientation: near 50% transmission. That is a maximum of 16.5% transmission.

Want some real numbers? I just measured the LUX before my lexan: 31700 LUX. After one piece of lexan, two fresnels, and the LCD with a 100% white image: 4900 LUX

That is 15% transmission. The lexan & fresnels get it down to about 73%, then the LCD takes 80% of the remaining light.

There are many different Lexan varieties. The polycarbonate resin itself will absorb most of the UV, but it then turns yellow over time as the polymer strands are damaged. To make it acceptable as a window glass replacement, GE added a UV blocking coating to one side. That is XL10. If you use it, then you need to have the UV blocker facing the MH lamp. On the sheet of XL10, there should be a sticker saying "This side facing out", or something to that effect. It may also say the same thing in the printing on the protective cover, (printed on one side, green on the other). "Out" means "facing the sun", or in our case "facing the MH lamp".

There are other varieties of Lexan: In Canada they sell one with the UV blocker on both sides. (Do they think Canadians can't read the instructions? :rolleyes: ) If you can't find XL10, then look for another variety that has a UV blocker on at least one side.

I love working with it: Much easier to drill than acrylic, and you can cut it with a metal-cutting blade in a jigsaw. Very tough material!
Yeah i've had a look at some lexan and it seems the best stuff i can get over here in the uk has transmittance (87+/- 5)% with the double UV coating you mentioned. Good thermal charateristics too. As for fresnel lens setups what do you guys think of this? I used a router to recess the lens in a sheet of mdf, then sandwhiched it with another. It's very rigid now and looks dead flat.

An externally hosted image should be here but it was not working when we last tested it.
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