ok, as mark over in Extreme mods has speculated, using a reflective polarizer , it may be possible to build a light engine that theoretically could double the light transmitted to the LCD(I think, anyway its a huge difference ;-)...
here is a link to an article describing a couple of designs:
http://www.nasatech.com/Briefs/June01/NPO20824.html

so how would we implement something like this? I have a couple of sheets of DRPF from 3m I can play with (11 inches square) ...ideas? suggestions? I have no clue what a "half-wave retarder " is by the way ;-)

interesting... by changing angle by mirroring of polarised light... so teoreticly doubles brightness... but how to change light direction?

well I think the key is that light not polarized in the right direction is reflected not "absorbed".... then the 1/2 wave retarder takes the light that is reflected back and changes its direction of polarization and it is then able to go thru the polarizer, it may take multple trips to do that, or maybe not...... I think

on second(well, ok, 20th ;-) viewing it looks like we need a 1/4 wave retarder.... hey.... isn't cellophane one? sigh..gotta go look at the AG thread..seems to me I saw something about that there...that would be too easy ;-)

edit: no its a half wave retarder according to the AG thread.....

heres a link paladin found describing a 3d system using lcds that has some cellophane info, it is a 1/2 wave plate....
http://individual.utoronto.ca/iizuka/resea...ellophane.htm#1

and just saw a post by mark that multiple layers of cellophane can be used to achieve 1/4 wave plate..plus he says you can eat whatever the cellophane was wrapping...off to fannie mays ;-)

The first layout that mikyd posted looks similar to the layout that I'm going to test with the parabolic reflector:

Parabolic Layout test

However, apparently Edmonds Scientific screwed up my order. They gave me an order number and everything, yet when I check my order history - nothing is there!

Additionally... the customer I bought my DVD Player (LCD Panel) has kept my cashier's check for 7 days (citing it must clear), so I haven't got that yet either! I WANT TO PLAY!!!

Mhh... i do not think you can change light photon direction (from X to Z), to get polarised 2x much light.... polar do not change direction, - it filters the light! ... polars produce ~40% filtering.. 10% are dropped by reflection and other stuff.. it is just my IMHO

well rereading another post it may be more like 40% increase..still worth doing though ;-)
.... from my understanding though, the 1/4 wave plate is what changes the polarization angle, follow the nasatech link in a previous post it explains the theory better than I can

Here is how I have proposed the elements be ordered. I like the idea of placing the quarter wave plate behind the bulb such that it will still serve its purpose, and yet not reduce the acceptable light exiting straight from the bulb. The problem is that there are no inexpensive quarter wave retarders capable of taking the heat with that kind of proximity to the bulb that are affordable.

1. Reflector.
2. Lamp.
3. Lexan.
4. Quarter Wave Plate (the Fresnel has been confirmed to not have retardation properties).
5. Collimator Fresnel.
6. Reflective Polarizer.

Vikuiti reflective polarizers do not have high enough extinction to replace a panels current polarizer and maintain high contrast. As such, this polarizer would work in addition to the panels stock polarizer.

The path that light will take is as follows:

Light straight from the lamp to the Fresnel will exit the lamp unpolarized, and thus pass through the wave plate unaltered. It will then impact the reflective polarizer. 84% of the energy that is polarizable will pass through and on for projection (50% of the energy is polarizable). The rest will be absorbed. The light that is not polarizable will reflect back, polarized. This light will enter the Quarter Wave Plate headed towards the Lamp. It will be circularly polarized by the Wave Plate. It will then impact the Lamp Arc Tube. Some of this light will be depolarized by the Arc Tube. This light will reflect off the Reflector and return having the same properties as the original lamp output. The circularly polarized light that is not depolarized by the Arc Tube will impact the Reflector as well, and have its circular polarization rotation direction reversed. Now it will reflect back and impact the Arc Tube where some will be depolarized by the arc tube. This diffused light will continue on, having the same properties as the original lamp output. The circularly polarized light that is not diffused will impact the Wave Plate and be converted from circular polarization, to linear polarization at 90 degrees to the direction that it entered. This is because the mirror flipped its rotation. 84% of this light can now pass the polarizer. The rest will be absorbed.

It's hard to imagine something being more optimal than this, unless the Arc Tube could be avoided. Of course, a prism can polarize light with near 100% efficiency (As opposed to 84% Vikuiti), but at our scale that is just not an option.

A similar design that intrigues me more, is the diffused light engine. I have posted another topic as it differs significantly:

http://www.lumenlab.com/forums/index.php?s...ndpost&p=102927

Mark.

I messed up the math in those posts .

Mark.

Why would you need a quarter wave plate. If my memory is correct, the lexan acts as a wave plate. Unless it is a perfect half wave plate then it should do the job adequately-just requiring a few extra cycles to reorient the light polarization completely. Maybe simple cellophane food wrap would also be a good substitute on the lcd side of the lexan. Some food wraps are designed to withstand high heat from microwaves.

What I am saying is that a quarter wave plate would be ideal, however not an absolute necessity for recycling the light. As an example, a 1/8 wave plate would recycle 50% of the rejected light into the proper polarization. The good thing is that the unrecycled light would just twist another 1/8 angle in its next pass. It's just a matter of finding transparent materials that act as wave playes.

Because eliptically polarized light will pass a linear polarizer unaffected. Thus it would pass the reflective polarizer, the panel polarizer and the panel analyzer unaffected and wash out the image. I could also take on a color shift.

The Lexan has not been shown to act as a wave plate, but the LL provided UV filter has. It seems to be a half wave plate, which definately would not work. These elements would need to be eliminated from the system or compensated for in phase shift choice.

edit I see that maybe you have read my post wrong. The plan is to use an alternative wave plate material. Like cellophane. The heat issue applies only to the NASA design.

Mark.

Yeah, I only skimmed through the posts. My bad.

how would one determine what acts as a wave plate? and what degree of a wave plate? when you say "cellophane" are you referring only to the crinkly stuff(clear candy wrappers like) would say plastic wrap(like microwave sealer) be the same?

I'd like to find that out too. So far I've found one "true" quarter wave plate 100mm in diameter for 1500.00 US.

holy...... yeah, alittle bit out of DIY range ;-) , well I know I read a link somewhere(scientific type paper) where they used cellophane as a cheap alternative to a half wave plate(they said it was actually 170 degrees but closer than their actual half wave plate) so would it take 9 layers of cellophane to make a 1/4 wave plate?

mark, you mentioned that elliptically polarized light will pass thru a linear polarizer unaffected.... how does elliptically polarized light come about? (in our context)

what an odd place the internet is:

"4.9 Construct your own 1/4-wave plate,Cellophane tape (NOT Scotch Magic tape) is an excellent plastic material for constructing yourown 1/4-wave plate.• Stretch some clear cellophane tape using your fingers, and then stick it down to themicroscope slide in your optics kit. This will take a little practice, but you should be ableto stretch it just enough to make a region with good 1/4-wave retardation. Use a narrowstrip so you can stretch it quire a bit. Clear polyethylene or food wrap also works, butyou may have to stretch it until it almost breaks. Now, combine this with a linearpolarizer to make a polarizer for right-handed light. Test your ¼-wave plate using reflection as described in Sect. 4.7."

from this link:instructor.physics.lsa.umich.edu/341/Chapter4.pdf

Build a quality home projector $400.00
Make your own wave plate $10.00
LL membership and a forum chuck full of researching, experimenting, can do DIYer's....PRICELESS!
sim

thats what I tell people about this place ;-) anyway heres another link to an ibm research paper that 2 weeks ago would have meant nothing to me..now I think I understand about 10% od it.... about increasing backlight/lcd efficiency using prismatic filters glued together, reflective polarizers,1/4 wave retarders...maybe someone else might see some ideas here? of course they also talk about using reflective color filters for the pixels and reflective black material for the matrix in the lcd...damn...we could use one of those lcds!

http://www.research.ibm.com/journal/rd/423/tanase.html

Oops. From what I know, elliptically polarized light as well as circularly polarized light will be linearly polarized by a linear polarizer. But not all of it (50% max): so not very effectively. That is why you need the near exact 1/4 wave plate.

I worded my first post here a bit funny. I made it sound like we are faced with the problem of finding cheap wave plates. Specifically, I said that there are no 1/4 wave plates capable of taking the heat in the NASA design. But that is why I posted the different design. It places the 1/4 wave plate after the Lexan so cellophane or stretched plastics can be used instead. Those that have followed the Lumens Theory thread know this was meant to be in there. The down side to the design is it takes a transmittance hit because all the light (even depolarized light from the arc tube) must pass through the plate, but cost and heat are not a problem.

This idea is tricky because not all the light that can be recycled will be recycled. The trajectories on return can throw the light way off hitting the reflector. So, like the diffuse light engine, this is going to be another balancing act: having the benefits make up for the transmittance of the additional films and inherent limitations.

I didn't know there was such a thing as a reflective color filter. Neat.

Mark.

I just had a read through, very intersting. I noted the date on the paper being 1997, surely if this works the technology must be in use somewhere today. I wonder if there are any current LCD's using this?

good catch. never noticed the date...makes you wonder....maybe it was just too expensive(the reflective color and mask)....

ok, I've been busy over in the diffuse light engine thread, mostly because finding a 1/4 wave retarder has been interesting, difficult but interesting.... finally I found some info on how to tell if something is a 1/4 wave plate and some intimations that
1: cigarette wrappers(or they were 50 years ago anyway) and
B: cd packaging
may both be 1/4 wave plates... so its off to the corner store and best buy
I think its because they are so thin compared to food cellophane and tape...

Again, finding the wave plate is not supposed to be the hard part . If you have any questions: just ask. Any clear plastic that was extruded or stretched is going to be a wave plate. It is the thickness of the plastic that defines what phase shift it induces. The amount of extrusion or stretching defines how efficient it is. Fortunately you can layer until the proper phase shift has been attained. I suggested chocolate box wrapping.

You can test if you have created a quarter wave plate by using a non reflective polarizer and mirror. Hold the polarizer in front of you. Place a sheet of the wave plate in front of that. Place a mirror in front of that. Shine a light through the polarizer and look for it's reflection to return back through the polarizer. It will not return if it is a 1/4 wave plate. Layer until you achieve this. Too many layers will lower the light output by a lot. A drop of clear oil (mineral oil?) between each layer will improve the transmittance by a lot.

Mark.

yes, as a matter of fact I mentioned the chocolate part in an earlier post and no-one ever answered how to tell the degree of retardation before.... definitly did not know the mineral oil thing, thanks.... mostly I was trying to find something I wouldn't have to layer too much....saran wrap and most food cellophane is usually a good 1/2 wave plate from all the college optics lab stuff I've found...

used a variation on marks test to look at a number of household cellophane items, I found it on a photgraphy forum, its the "disappearing coin" trick.... you put a silver coin on a black piece of paper with a light shining on it , then look at it thru the polarizer as you turn it...(well, its fun anyway) .... so far everything will take multiple layers and haven't found anything big enough to cover the entire reflective polarizer/box without multiple pieces and multiple layers... so as soon as I feel better its off to the store with a quarter and a piece of polarizer.... I'd like to find something that would take only a couple of pieces and layers to make the wave plate.
a question..... using the same white box setup I'm using for the diffuse light engine thread... no? need to go back and reread marks first post here...can't remeber if the reflector should go back in or not....

edit: ok, just reread marks post, reflector goes back in...but another question...if the light enginge were lined with reflecting material(something non-diffusive, mirror or aluminum foil or?) would this help in this scenario? I know most of the light won't hit the collimator in a useful fashion but some should right? it won't lose its orientation bouncing off the mirrored surface and will be flipped to the correct orientation on its return thru the wave plate right? comments?

How about reflective vinyl that they use on things such as stop signs, police cars etc? I am going to try some in my light engine, I may even cover the entire inside including the parabola with it.

As I understand it, the only light that will impact the Projection lens is light that can be traced through a circle the same size as the triplet at the focal point of the collimating fresnel. So if your reflector is that large for instance, then none of the surface behind or surrounding it will impact at a useful angle. If this design works good, then the small reflector is basically all you would need anyway. From what I can see, we just have to accept the fact that a lot of the light will not return to the reflector and will be lost.

Mark.

And what happens when we do not use a collimating fresnel, like me?

Hard to say. But my guess: it will work as well as it ever could. Your setup really is the ideal for this as I see it. But it still has the problem of returning light falling off acceptable incidence due to the naturally diffusiveness of the Vikuiti polarizer and the ideal of perfect world precision.

hmmm... I need to find a supplier for the polarizer I want to try it.

One thing to consider is that this stuff is very bright because it's not just reflective, it's RETROreflective. Which means that the light striking it will be reflected back in the same direction. This would be totally counterproductive for this application.

Someone on here was doing an experiment using it (actually a paint that created a retroreflective surface) with a spherical reflector. There it at least doesn't ruin the geometry of the light path, but to me it seems pointless.

If you created a flat retroreflective surface behind the bulb, then you'd have something functionally equivalent (in principle) to a spherical reflector.


Hope this helps,

Thanks, I wasted $10.. but... no biggie... that's the fun of it all. Now if I could just find the film I want... either the reflective polarizer or a reflective film that is mirror like, but transparent when looking at it dead on.

Apparently a short bandpass interference filter (like a dichroic mirror) Functions this way. Apparently they only selectively allow their designed wavelengths through if they are at head on incidence. Not sure if you could use a wide bandpass interference filter and get the same result. I don't see why not.

If that is the case, if it reflects any light that hits it on an angle, but it allows light that is dead on (0 degree incidence) then we just found ourselves a collimating film/sheet. See where I am going with this?

EDIT: Apparently a short bandpass interference filter (like a dichroic mirror)... These are big bucks are they not? especially in a size say 16x12"?

Of course I do . That is what we have been working on with the diffused system. Just our requirement was not a sheet that passes only head on light. We needed a sheet that passes any light that will wind up at normal incidence from the film. Refraction was allowed. In your case, refraction is not allowed so the requirement is a bit trickier.

I agree, getting one for cheap is not going to be easy.

ok.... so I've been thinking (watch out! ) and it seems to me that if I use the unsplit LL setup(Ikea/bulb/220 fres/300fres/LCD/triplet) that really the size of retarder that I need is only the size of the reflector at most? I know I've seen that someone tested a collimaitng fresnel with a laser pointer and the only light that made it thru the lcd to the triplet was a fairly small circle around the center of the fresnel(can't find it now...) and I think thats the same thing as mark is saying above also.... so the only light (or the majority anyway) that we need to be concerned about recycling is the cylinder defined by the reflector(or a triplet sized circle ) to the collimating fresnel right? or am I wrong? Mostly because its a lot easier to make(or buy) a 1/4 wave retarder that size, than the size of the entire lcd or fresnel...

you know, the more I think about it, I know I've seen light boxes on LL where the gate (light output ) was pretty small.... inwhich case we could achieve the majority of light recycling with a fairly small (relatively) piece of retarder and reflecto-polar

The light pretty much needs to return straight back in the direction that it came. That is why the reflective polarizer should be in the collimated area.

Ideally the wave plate would go between the lamp and the reflector, but I can't see us finding a wave plate that can take that heat. That is why I designed it all in front (and we take a performance hit for that reason).

I've seen filters rated for 400 degrees celcius, I'm sure you could find a 1/4 wave component made of some type of glass, 2 or 3" in diameter and you are set to go. Or am I right of base here?

You know something I don't understand is why the guys with collimating fresnels don't use a glass condenser lense that is diffused on the backside. Maybe it's triplet size, but If I drop the parabola and use a fresnel instead I am going to try using a condenser lens and throw a diffuser behind it to try and even out the light the fresnel sees.

Truth is I haven't looked too hard. I figured cellophane will at least get us to a proof of concept.Ground glass is apparently not very transmissive. And I look at it as though you have a fixed amount of light heading towards the Fresnel. If you add a diffusive layer anywhere in there you could throw a lot of that light off course such that it will miss the Fresnel, or be of the wrong incident angle to be projected.

Yeah, I knew someone would bring that up.. You can get it in different grades. I'd have to use something very lightly diffused.

well, I'm not quite sure what you're saying...let me see if I can explain myself a little better...in a standard LL setup, the only light that matters is the light coming from the arc right?.... if you draw a pyramid from the arc to the cardinal points of the collimating fresnel and go to about the halfway point between the fresnel and the arc, you should need only a "window" of at most 4.5 x 6 ".... where the lexan/cellophane/reflecto-polar would be....any light not transmitted by the polar will be reflected back to its point of origin, right? which will be about the same point for putting the hot mirrors that brain is sourcing now also....

well, most of the "real" wave plates are quartz or mica which probably could take the heat I think.... as for the 1/4 wave plate, I found a "optics eperiment kit" which includes 2 - 2"x2" 1/4 wave plate films.... its like 20$(good old edmunds..my "dream store" when I was young.... of course that many years ago, they mostly sold "how to make a wheel" and "make your own fire" kits

Follow the path of light going to the upper right corner of the small reflective polarizer (the edge of the pyramid). It hits the reflective polarizer, and continues off in an adjacent direction and probably hits somewhere around the upper right rear corner of the box.

ok, so how do you get that light back? a mirrored box?
edit: no... rereading marks first description, the only paths mentioned are the path incuding the arc and the reflector ...so... I'm lost again.... are you saying the polar needs to be the same size as the fresnel?

. Yes. The design puts a full sized reflective polarizer into the collimated area so that any light bounces straight back to where it came (but not exactly so some light will be lost). The wave plate can pretty much go wherever you want, but they don't work quite right as the angle of incidence exceeds 0 degrees. That is the other reason why I am suggesting using a full width plate in the collimated area (the other reasons being cost and heat).

no, I'm still confused...if you think about that first ray(upper right corner of the RP) why would moving the RP to the collimator make any difference? what I am talking about is a totally self contained light engine...the only exit from it will be a 4x6(or whatever the minumum for its distance from the arc should be), the arc itself will be at the focal length of the collimating fresnel , the light engine itself can be mirrored inside although I'm not sure that wil be a huge improvement.... think of it as a pre-polarizing light source.... all the light coming from it will be polarized in the correct direction..it will also eliminate the heat at the LCD that is produced by the 50% of the energy that the LCD polarizer normally can't use(there will still be like 15% I think absorbed just due to the inherent inefficiency of the LCD polar)

Basically you can't go any bigger than the same size area as the triplet. So no wider than 10cm with the pro lens.Exactly. But the reason why placing the reflective polarizer in the collimating area is because of heat and the fact that light will bounce straight back to where it came (but not really). Even with the little box you are suggesting the light still needs to exit the box eventually within the same area that the triplet samples, and even more importantly in a trajectory such that it will impact the fresnel. What you are suggesting would have a lot of reflected light missing the fresnel entirely. But the reality is that both systems have similar symptoms and so I can't say which would be more efficient.

If you want to test a reflective polarizer right by the bulb, then you will want a wire polarizer. They are good to somewhere around 200 Celsius. VersaLight is one brand. One thing that comes to mind is that if you do keep the system compact, and roughly within the active imaged area of the light engine, then you may be able to do a hybrid diffused/recycling engine on a smaller but more controlled scale than what we are toying with in the other thread.

That is, you could use a small flat white diffusive reflector and eliminate the expensive wave plate. The cool thing with that you could pretty much count on getting no less light than without a reflector, so everything you do could be a benefit. Has anyone tried a small flat white reflector (still a spherical)?