Exactly.

This would garuntee that the reflected light would pass back through the lamp and reflector. If these don't rotate the polarization enough then add the lexan in there. the lexan would double as heat shield/polarization rotator.

You should run into some problems with color if you don't get the offsets pretty close. As I understand it, wave plates are optimized for specific groups of wavelengths. For example, I believe in the case of a neutral white wave plate this is done by using a thickness that makes all the waves centered on Red Green and Blue exit lined up with the exact same offset between the electric and magnetic components. It seems to me there will always be some thickness that will allow arbitrary wavelengths to exit with the exact same compensation, despite that each wavelength entering may seem unrelated to the speed of the fast axis VS the slow and the thickness of the material. As a result, some colors will have to be offset by more than one full wavelength. But it gets the job done. Using that method you could build a wave plate optimized for any number of random wavelengths assuming practical thickness is unlimited. But if you get the thickness wrong, it could get really messy. People's Lexan won't likely have an optimal thickness.

How much recycled light will miss the arc area on the way back is a relative unknown. We can guess that quite a bit will miss, and therefore not be projected.

What we are doing here is getting all the options out there. Everyone has their own preference for order of experimentation, but each idea looks roughly equal in terms of potential pros and cons to me. The thing to remember is DBEF is capable of nearly a 100% (2X) improvement in brightness. It has been done. It is great that several ideas are in the works so there can be a shakeout.

The Recycler Element approach has some known cons. It is unfortunately seems like a pretty tricky build, and it requires that all light pass through 45 degree elements which promotes TIR losses, and is not the angle these polarizers work best at. But it has been done with a 50% improvement, so how can you go wrong?

Mark

Sorry Phutton, you’re correct. I was describing something completely different than what I wanted to. It was late and I had too many pain killers, that’s my excuse and I’m sticking to it.
This is what I was meant to say. We don’t use a point source so the rays from each end of the arc aren’t going to be collimated. This will cause the returning rays angles to be allot different than the primary rays.

DJ

I see what you're talking about. while theoretically the rays should be parallel after the collimating fresnel many of us (including me) move the lamp in farther to get more brightness. This would throw off the reflections since the rays after the collimating fresnel would be either converging slightly or diverging slightly.

One way to address this is to put the DBEF after the collimating fresnel and then slowly move the lamp in and out to obtain the position with the highest brightness. simply allow the system to align itself for maximum brightness.

pm me your addy DAZZ

That is part of the problem, but the fundamental is that the arc itself is not a point. What DAZZZLA is demonstrating is that light from the edge of the arc will not return to its origin. No moving of the lamp will help this.

Mark

Something just wasn’t sitting right with the polarization changer. Here’s the problem, I could see a strange glow in the test rig I made. Every second DBEF wouldn’t quite extinct, except for the last one on the end.
Click to view attachment


This is with all the DBEF orientated the same direction. I initially thought that changing the direction of every second DBEF would fix this. But it just moved the problem to another section.



I managed to find the drawings I made of a polarization changer from a commercial projector a year or so back.
I had one of those “DOH!” moments.
There was another element involved with it that I had completely forgotten about. There was a segmented mirror placed just before the polarization changer. Its purpose is to reflect the light from every second section. The reflected light would return to the source and be recycled. It also fixes the problem of crossed polarization.
Click to view attachment

So I’m going back to the first version .

Pic

Every time I see "1/2 wave retarder", I think of Prince Charles for some reason.

You are correct that the light will not return to its exact same origin upon reflection. what I'm saying is it don't have to. Just like the fact that our fresnels don't focus at a point. They focus on an image. that means there is quite a bit of wiggle room. Moving the lamp back and forth (towards fresnel and away from fresnel) is the method we currently use. It essentially garuntees we find the position with the brightest output. The same would occur here, except in the case of the DBEF the brightest output would also include the largest amount of reflected light making it through the triplet.

So it may not be a 100% increase in brightness. But for $5 any increase would be well worth it. I'ld be happy with a 10%, 20% 50% whatever increase.

And yes, there may be slight color shifts due to wavelength dependencies in the rotation of the light polarization. But again, so what. I doubt very much that any waveplate would only rotate one wavelength and not the others. ANY rotation would eventually make it through the DBEF upon multiple reflections. The only difference is that the wavelengths with greater rotations will require less reflections, and thus suffer less losses . This should change color slightly. But I don't think it will do more than shift the color temp of the light up or down a few thousand degrees K. If the shift is too much for taste then just use the lcd color filters to compensate.

By the way, change subject just slightly.

I'm considering putting these on the high power led setup I'm making. It uses 60 leds arranged in a honeycomb (hexagon) pattern, touching each other. It uses individual lenses (5 degree half angle) on each led. I'm thinking that each lens would act as a reflector for the reflected light from the DBEF. Essentially, the reflected light would enter each lens at the flat top and be reflected back out through total internal reflection (OK, they may not be TIR, but a large portion should reflect out)

I'm hoping this will actually be more efficienct than the standard fresnel/point source combo when paired with the DBEF. What thinks you?

Click to view attachment

We are definitely talking about 2 different things. That is all I was trying to clear up. Even still, the factor that you, and the other factor that DAZZZLA points out are both one of entropy. Moving the reflector isn't going to help that as I see it.My understanding is that the thinnest easiest wave plate to build is one that only cares about 1 wavelength and accidentally supports a handful of others. It takes careful specific effort to build for any more.All I know for sure is, the proportions of R, G, and B will be altered. I can't model in my head how much. I'm only suggesting that it could very likely be nasty. Maybe it will be acceptable. I don't really want to debate one way or another since my own assessment is based on approximation. Just giving a heads up. I think the point I am really making at this point is that precision is going to be better than none. I am willing on debating that.

Mark

Is this the first version? http://www.lumenlab.com/forums/index.php?s...st&p=253520

If so, it has the problem of doubling the width of the light on output.

I see what you mean about this: http://www.3lcd.com/structure_close_up.htm...changer_element

It has a 1.5 fold improvement (Versus 2) because only every second segment receives usable recycled light from the neighboring segment. Bummer. You'd think there would be a way around that limitation with all this stuff. Think. Think. Think .

Mark

Well, hot-diggety-dog and other exclamations of enthusiasm!

I found some white retro-reflective sheeting on Ebay that I think might work for me.

Here's the specs.....3M Diamond Retroreflective Sheet.

I'll let you know how it goes.

Tgreenwood

Got my DBEF. It's a little clearer than I expected. It acts as a regular polarizer on one side of the sheet. But the other side does not act like a polarizer. What I mean is on one side I can rotate the sheet in front of another polarizer and it will give me dark and light orientations. But flip the sheet over and it doesn't do that. As a matter of fact, it simply gives off slight color sheens like velvet.

anyhow, here it is in front of my 8.9" lcd.

Here it is aligned with the lcd polarizers.
Click to view attachment

Here it is perpendicular with the lcd polarizers.
Click to view attachment

If there is a protective film on one side that has birefringent properties, it will circularly or elliptically polarize the polarized light that passes from your monitor. Circularly or elliptically polarized light will pass through the reflective polarizer. Sounds to me like you effectively have is a circular or elliptical reflective polarizer. Which is exactly what we have been looking for. So if you have a film on there, keep it on, it could be making a great 1/4 wave plate.

If there is no film, then maybe you have an intentional circular or elliptical reflective polarizer. Sweet. Or they have changed the tech and the stuff only works one way now.

Mark

When I get home I'll check to see if there are 2 films stuck together. I'm pretty sure it's one film, though. I can assure you it is very uni-directional. If you flip it over then you can rotate the polarizer 360 degrees in front of the lcd and you get very little difference in transmittance. Although, like I said, there may be a shimmer of light. Very soft like velvet, but noticable nonetheless.

Edit: You know, now that I think of it maybe there are two films stuck together. If you look at the images you will see a brown and green line. I thought it odd that they would put those smack dab in the middle of the polarizer to interfere with the image. But maybe they can simply be peeled off.

Well, I couldn't find my camera but Mark's right on the protective film which appears to be on both sides. After peeling a corner of one protective side there was a mirrored finish which seem to also peel but not as easily. The other side was dark.

Interesting enough I put the film up against a desktop lcd, mirror side facing panel, and the corner without the protective films only polarized to dark while the rest of the protected film allowed light to come through but with a hint of wierd colors showing. The unprotected corner then lit up with the rest of the film as I twisted it. I then reverse the sheet having the mirror side facing me and the entire sheet including the unprotected corner responded accordingly when twisting.

Oh btw, if you haven't guessed I also bought 3 sheets of this.

Lol!!. If this stuff works like I think/hope it will, better stock up. ebay supplier won't know what hit him. He only has 200 or so of these in stock.

Well I'm convinced that is what you are seeing. Even if the films were on both sides, the thing with birefringent films is the axis matters. The fast axis of the film may be oriented perpendicular to the polarization axis on one side, while the film on the other may be parallel. What you should see is in one direction efficient polarization, but in the other, a slight hue shift as you rotate (Sepia to Blue).

Again, I would hesitate to peel either of the films off until you can investigate how good a circular polarizer you have. It must be a pretty good one, based on what you have said. This is after all, exactly what we are trying to build by adding the 1/4 wave film. Just yours is so perfectly integrated, you didn't even realize it. Which is perfect .

So (if I have thought this out right) if the polarizer is facing your monitor, and rotating it does nothing to the brightness, you can then peel off any film (if there is any) on the side facing you. It is not doing anything necessary. That said, I would just leave them both on, even though I can bet the film facing you (if there is one) could be throwing the polarization into a slight ellipse. But since we know it works pretty well as a polarizer from that side, it can't be off linear by much.

Edit of course, we may find the films are diffusing the light too much compared to what SIM calls a mirror surface. But for now, I would leave that action on until it becomes a problem.

Mark

Yep. that's about a good description of it.

Hey! we got a two-for-one on this. A reflective polarizer AND circular/elliptical retarder.

Sim, when you put your DBEF to the lcd at its brightest orientation is the peeled corner any brighter than the rest of the DBEF. If not, I say don't mess with a good thing and leave it on.

. Um, that's what I was... oh well. .

Mark

Just seconding what ya said. Ya got my support all the way!

When having the polarizer side facing the lcd and twisted the corner without the protected films got extremely darker compared to the rest of the DBEF itself, meaning the protective films seem to be allowing little light to come through (circular, eliptical?). When twisting (I think) at 90 degrees the entire film lit up.

Also, when having the mirrored side facing the lcd, when I twisted the entire film @ 90 there was that Sepia to Blue hue you described and no change to brightness but when @ 45 the hue practically disappeared. The corner, however, reacted like normal polarizer with no hue at all.

Umm, with either side facing the lcd - the corner, when oriented for brighter was clearer and sharper. It seems the protective film is slightly dulling clarity but, yes, without the protective film it is slightly brighter.

Another btw, I'm forwarding another 3 sheets to DAZZ once I get his 1/2 wave retarder film that he ordered from polarizer.com (saving on shipping to OZ, eh, CHEAPSTER ) Guess, he didn't need the retarder afterall.....right?

Sounds expected. If you get the axis of the wave plate to line up with the polarization axis of light passing through, and the axis of the next polarizer (the reflective) there shouldn't be a hue shift at all.

It all sounds good. What you have is normal bi-directional reflective polarizer with protective films on both sides. On the one side the protective film's axis is aligned with the polarizer's polarization axis. The other side has the film at 90 degrees to the polarization axis. That is why you see birefringence effects in one direction and not the other.He does, because these protective films are 1/4 wave retarders, and he needs a full 1/2 wavelength for the standalone Polarization Changer Element he is working with. He could stack the 2 films together, but nothing is going to beat a 1/2 wave compensation optimized for Red Green and Blue as in the polarization.com product.

The films on these polarizers are very unlikely to be optimized. It would be a shot in the dark if they are. But they seem to be pretty close, so it's worth trying before binning them.

Mark

ok.....I'm totally lost already on this with all this twisting and retarding . Wish there was a visual (3 dimensional preferably) of all this just so's the likes of me can keep up with you brainiacs .

Guess there's only one option left for me to do in order to handle this....

Mark,

How do you know these are 1/4 wave retarders and not 1/2 wave retarders. also, isn't that different from circular or elliptical polarization. My understanding (may be wrong) is that 1/4 or 1/2 wave retarders would still be linear polarization. If the plastic cover only rotated polarization by several degrees then we would still see the dark/light occuring as we rotate the polarizer around the lcd.

A half wave retarder rotates the polarization axis 90 degrees. But the polarization remains linear. This will allow you to still have a crossed state between the 2 polarizers. A 1/4 wave gives you perfect circular polarization. As I understand it, 50% of circularly polarized light will pass through a linear polarizer regardless of axis orientation and will exit linearly polarized. Anything between 1/4 and 1/2 will give you elliptically polarized light, which will have a preference for a specific axis, but will also pass some but definitely less light on an axis 90 degrees to that.

I know this is a pretty good 1/4 wave plate because you aren't noticing any bias for any specific axis as you rotate the polarizer. If it were elliptical or linearly polarized (and thus being 1/2 wave or not 1/4 wave film) you would see a bias for one specific axis as you rotate. That is, you would see the brightness (and not just the hue) change.

And I bet you would find around exactly half the light from your monitor is all that gets through regardless of orientation. You would get far more than half if it were a linear polarizer on axis with your monitor.

Wave plates aren't intended to rotate linear polarization. You need a helix or some other trickery to do that. These are meant to throw the electric and magnetic components of linearly polarized light out of phase with each other. The side effect of doing that is the impression of a rotation. It so happens that if you throw the phase out by 180 degrees (with a 1/2), you get a 90 degree rotation while preserving linear polarization and phase.

Mark

Thanks for getting that for me
I want to play too so just ship it across and I’ll get the retarder shipped to me.

Did I say I was not only a scrooge but also I’m impatient?

DJ

thanks Mark. Great explanation. I think I'm starting to get a clue.

OK, so here's my plan. Need some advice to see if I missed anything.

I plan on setting the DBEF on acrylic or lexan and place that on top of the led lenses. Well, it won't be resting on top, but it will be as close as possible. Here is the plan.

Click to view attachment

I guess a couple of questions. Which would be better as the base, acrylic or lexan. I know the lexan acts as a makeshift retarder. I was planning on using that before I found out that the DBEF comes with its own. Is there anything here that does not look like it will work.

I’d try the acrylic. If the protective layer is working then adding the polycarb may stuff it up. I’m not 100% sure that the Acrylic won’t effect things either.

DJ

Another thing to note is that the light reflected back from the DBEF may not travel the way you have shown. I think after it is reflected off the side of the reflector it will travel back down to the LED chip. Maybe it will be reflected from there or maybe it will just turn into heat

Yep, I thought of that also. If the output from the led lens was perfectly collimated then that would be the case. But the output from the led lens is slightly diverging around 5 degrees. That, combined with the travel distance to and from the DBEF (and also the reflection) may allow the reflected light to not be reflected back to the source. Or at least, that's what I am hoping for. then I rely on internal reflections to hopefully get a portion of that light back to the DBEF.

Maybe I'll try some type of experiment first.

In this case then wouldn't tgreenwood's corner cube reflector idea work? Or even a box lined with retroreflectors?

At first thought, since it looks like you are always relying on 2 reflections before the light gets another chance at the polarizer, you may actually need to use a 1/2 wave plate. I think much of the polarization will just be flipped back into the reflected axis otherwise.

And I wouldn't use Acrylic or Lexan. Just try to lay the wave plate and polarizer over the LED's.I'm thinking the retro reflector would need to go inside the LED (or be the LED), and the emitter would pretty much need to be transparent.

Mark

Has anyone tried the basic 1/4 wave plate idea with the spherical reflector yet?

Mark

Anybody have any updates?

Tgreenwood, I understand you received and are now working with retroreflectors. Any news?

DAZZZ, you should be getting your sheets of DBEF in the mail soon.

phutton, how's your 'DBEF on acrylic or lexan' idea coming along?

Mark, you still there?

I got the retroreflective material, it came in 3 inch by 8 inch strips with adhesive on one side.
At first glance, I wasn't very impressed. After 'testing' it with a flashlight, my estimation of the stuff went up considerably. The retroreflective property is quite impressive, but I don't know about the polarization rotation property yet.

I don't know if this is the genuine 3M retroreflective stuff that I wanted, but I am hopeful that it will work.

I need to dig out a regular polarizer sheet and use it for a real test because the repeated reflections using the reflective polarizer I have is screwing up the test. And I found out that my green laser pointer doesn't appear to be linearly polarized, but the red one is. I'll see if I can get the test set up properly tomorrow, and build a corner cube retroreflector out of three mirrors to hopefully test the rotation property to see if it really works before I spend more money.

Tgreenwood

I’m still here.
I’ve had some time away from thinking about the recycler, only last night started again. The break was helpful. I re-tested the diffuse DBEF to make sure I had it correct in my head. I must have looked at it the wrong way last time because the DBEF now appears to polarize to our advantage. This is my latest idea.
Click to view attachment
It could be made relatively thin, 5-10mm, so the simplicity of just adding it into the collimated section of the path is back on

Actually, I'm working on it and should get some results within a week. Putting the finishing touches on the high power led light engine. Got it glued to the heatsink. Tested all strings before glueing. Still gotta test the whole light engine to make sure I didn't fubar any strings while glueing to the heatsink.

I have the test setup ready. I have a mask ready to mount the DBEF. Once the light engine is up and running, I'll put that bad boy up against the leds and see if it will give me greater output than just a polarizer alone.

I must still be looking at it wrong. Basically what this is saying is that the red dot polarization passes straight through the polarizers, and green line polarization reflects off. It would be the same on both sides of the polarizer. In the drawing you have red dot polarization passing through one side of the polarizer, but also red dot polarization reflecting off the other side of the same polarizer. I don't think that is possible.

It seems like there has to be a way.

Mark

I found my plain polarizer sheet, and, okay, the corner cube retroreflector thing appears to work.

Here's what I did:

I set up a mini-maglite flashlight to shine through the polarizer and hit the retroreflector. I angled the polarizer in the beam so that any reflection off of the polarizer would be at an angle and away from where I was measuring the light throughput. The retroreflected light then went back through the polarizer and hit a white target I set up behind the flashlight.

If this worked perfectly, the retroreflected light would have its polarization rotated so that none of the light would make it back through the polarizer. Well it wasn't perfect, and here are the numbers I got.

I set it up with a plain mirror, a home made corner-cube retroreflector, and with the 3M retro material. With the plain mirror (no polarization rotation), I got between 225 and 234 lux. With my home made corner cube, the amount of light that made it back through the polarizer was 71 lux. With the 3M material, the light getting through measured 79 lux. So there is definitely some polarization rotation going on.

Now I'm sorry that I have cheap polarizer material that I got from a science toy website many many months ago. If the polarizer was better, I think I would have gotten better numbers.

In any case, it looks like if I use this retroreflective material with a reflective polarizer, it ought to recycle quite a bit of light for me.

Tgreenwood

I agree. It’s just that when I test it with my monitor I see something different . Maybe I’m missing something still. I'll see if I can confuse myself some more

I put a polarizer into my collimated light and re-tested the DBEF. Yep, the last diagram was a brain fart . Back to the drawing boards yet again .

DJ

It's a remarkably tricky problem. The more I think about it, the less confident I get. We just can't reflect and pass the resultant light off the same polarizer. I don't see how we can avoid that wall. If we could get different behavior depending on the direction the light passes through the reflective polarizer it would be an easy design, but that stands as the fundamental flaw. I think we need that kind of asymmetry. Without doubling the output light area.

One thing that I played around with is that the wave plate will not have an effect on unpolarized light, but will effect polarized light. That could be used somehow to give the asymmetry I think we need. Those are the kinds of clues we need.

Mark

Or this could be the clue.

I set up a mirror on top of a polarized light. Then I used another polarizer (analyzer) in front of my eye to view both reflected and direct light.


Here’s the interesting bit. When the polarized plane was parallel to the mirror or perpendicular to the mirror, correct polarization extinction was achieved. But when the polarization plane was at any other angle extinction didn’t happen in the reflected half. So either the mirror is creating elliptical polarization or it is de-polarizing the light.

OK I’ve done a few more experiments and come to the conclusion that what I was viewing was a result of Brewster’s angle. I was originally viewing the polarized light from a steep angle, when I lowered my viewing angle, extinction returned to what it should be.

(Off onanother tangent )
I wonder if we could use angled glass to create a form of reflective polarizer that is more suited to our light engines, Hmmm.

DJ

Both the DBEF and ½ wave retarder arrived today.
The retarder works perfectly. That is, it rotates the plane without any color variations, much better than the polycarb or other plastics I was testing.

The DBEF that SIMUL8R (Thanks Sim) sent me operates differently than the diffuse DBEF I have. It is asymmetrical in its reflective polarizing properties. It seems to be a normal polarizer with a reflective layer on one side only. What this means is that one side will reflect p-light and pass s-light. If the light enters from the opposite side of the film then p-light will pass but s-light will be absorbed. So not to good for the recycler.

I’ll investigate if the polarizer can be separated.

DJ

I just confirmed that it is a plain polarizer underneath. I dry peeled a small section. What I have now is a normal polarizer and a reflective polarizer. The new reflective polarizer extinct to a grey hue while the diffuse DBEF I already had has a purple hue so it’s more neutral. The question now is can I peel a whole sheet with out start-stop creases.

DJ