Last fall I bought a used InFocus LP290 projector, and in January I opened it up and found that the polarizing filter for the blue LCD was melted. Since then I’ve been keeping an eye out for a replacement filter, and Monday night I parted out a dead Optoma projector from Jeremy for the polarizers. Tonight I fitted one to the InFocus, with surprising success.
LCDs and Polarizing Filters
A long time ago, I think I knew how LCDs work, but I’d forgotten until a great conversation I had with Dave, who posts comments here from time to time and is immensely knowledgeable about LCDs. To paraphrase:
In high school physics, we learn that light behaves as both particles and waves (both of which can be demonstrated experimentally). In the wave model, light is a transverse wave: it “wiggles” from side to side in a plane containing its line of travel. (In contrast, sound is a longitudinal wave — it makes compressions and rarefactions back and forth only within the line of travel.)
Normal light has waves wiggling in all directions around the line of travel — up and down, side to side, and all angles in between. A polarizing filter only lets through light waves wiggling in one particular orientation, say up and down. It also lets through the up-and-down component of waves wiggling at an angle other than up and down; but waves wiggle side to side have zero up-and-down component and get completely block.
Digression: polarizing filters are used in photography and in sunglasses to block glare. It turns out that the light waves in certain kinds of glare have been polarized by their reflection off the surface of whatever object is glaring, and polarizing filters can completely block those types of glare. It’s a really cool effect to rotate a polarizing filter in front of something that’s backlit at a fairly steep angle and watch the glare appear and disappear.
I have this description backwards. See Dave’s comment for an excellent description of how it really works.
Back to LCDs: the crystals in LCDs pass light waves untouched when they’re inactive, and twist the polar orientation of light waves when active. This is the key to how LCDs work.
An LCD screen like on a cheap calculator has a polarizing filter in front, the LCD panel, and a backing mirror. Light from the room hits the filter and only the portion in the proper orientation (say up and down) passes through. That light then passes through the LCD, and where the LCD is inactive the light maintains the same orientation. It reflects off the back, passes through the LCD untouched again, and is already in the correct orientation to pass through the polarizing filter on the way out. Thus where the LCD is inactive, you see light areas.
Where the LCD is active, it twists the polarization of the light waves by 45°. (There are other possibilities, but I’m talking about simple and cheap.) It reflects off the mirrored back and passes through the LCD again, getting another 45° of twist for a total of 90° with respect to the polarizing filter. That’s exactly the angle of polarization that’s completely blocked; so where the LCD is active, you see dark areas.
In projectors, with light passing through the LCD only once, it has to work a little differently. I’m guessing the LCD does 90° of twist, with polarizing filters both before and after.
So without the “before” polarizing filter in the blue light path of my projector, the darkest the blue beam could ever be was about medium-bright, because it only got half as filtered as it was supposed to. Instead of true blacks, I got medium blues. White was still white, of course, because that’s where the light was supposed to pass through (and still did).
Colored Polarizing Filters
Looking at the optics inside projectors, there are lots of things reflecting, refracting, and filtering light, and I often see color tints and have trouble determining which thing I see is the color filter. So after taking the first polarizing filter off the Optoma and finding it to be a dark blue-grey, I thought it was a neutral grey and didn’t think any further about color.
Tonight I found that each polarizing filter is a different color, complemenrary to the color of the light path it’s installed in. I was having trouble figuring out why you’d want to have a color filter and a polarizer. Since the yellow-orange filter blocks blue light in the blue beam, it seemed it would simply dim the blue beam all around. Okay, that leads to blacker blacks; but (apparently) in the same way as just using a dimmer bulb. That didn’t make any sense.
Then I looked through the colored polarizing filter more carefully.
The two pictures are taken with the same camera settings, the first with the orangey filter held in front of the lens horizontally, the second vertically.
Both pictures have the orangey cast to them, but notice in particular the color of the glare on the edge of my desk. In the second picture, it’s nearly white, not orangey like everything else. That’s where the glare is.
I believe this filter passes all colors of light in general, and specifically blocks blue light that has opposing polarization. That’s why the screwdriver looks about the same in both pictures — everything except blue gets through, and the light from the screwdriver isn’t polarized so the blue is blocked. The glare on the edge of the desk is polarized, so even the blue gets through and gives a more even white.
That’s kinda cool. And that means the filter should do an extra good job of darkening blues where the LCD is active. With a complementary-colored filter in each of the three color beams, it means it should do a really good job of making black blacks.
Fitting the Filter to the Projector
The InFocus and Optoma obviously don’t use the same size filters, as the Optoma filter clearly doesn’t fit the InFocus carrier. Fortunately, the Optoma filter is larger, so at least I can hack it to work.
(The camera doesn’t represent the filter color very well, by the way. In real life, it looks very similar to the deep orangey-yellow of yellow liquid food coloring, not as pink as it looks here.)
I confirmed the proper orientation for the filter by very carefully slipping it into position in the projector and holding it there with a needlenose pliers with the projector running, and the proper orientation is horizontal. Too bad, since the Optoma filter was as tall (short dimension) as the InFocus carrier is wide (long dimension), and it actually fit really well in the carrier the wrong way.
But with a little bending of the carrier edges, I got the too-large filter secured in (and hanging out of) the carrier well enough to reassemble the projector. The glass isn’t parallel to the carrier any more, but I can’t think of any reason that’ll make a difference. You can see the glass slide sticking too far to the left, and slanting forward, in this picture, between the silver carrier mounting tab above and the LCD housing below.
The projector works now, and the blacks are black again! I’m not ready to close it up and call it done until I sit next to the projector and test it for an evening to make sure it’s not going to melt down and light my house on fire, but I’m optimistic. And if it works, I’ll think about cutting down the glass the filter is mounted to, so it fits properly.
You have the “active” and “inactive” states backwards. Maybe it’d be better to state “electric field present” and “no electric field present”. When no electric field is present, the LCD rotates the plane of polarization. When the electric field is present, the LCD does not rotate the plane of polarization.
The reason for this is that the liquid crystal molecules are long, thin (cigar shaped) molecules. When an electric field is present, these molecules stand up on end (perpendicular to the plane of the glass sheets). This allows the light to pass with no change in polarization. When no electric field is present, the liquid crystal molecules lay down parallel to the glass plates.
However, there is a twist in the orientation of the molecules as you go from the front glass plate to the back glass plate. This is obtained by coating the insides of the glass plates with a material which causes the liquid crystal molecules to want to align parallel to these alignment materials. One alignment material is polyvinyl alcohol (PVA) which has been oriented by rubbing (PVA is a long, thin molecule, too.).
The twist from the front glass sheet to the rear glass sheet is 90 degrees in a typical TNFELCD (Twisted Nematic, Field Effect, LCD), and this rotates the plane of polarization of light by 90 degrees in the case where no electric field is present (and results in no rotation in the plane of polarization of light when the electric field is present, and the liquid crystal molecules are standing on end). An improved version uses a 270 degree twist in the liquid crystal (known by the jargon “super-twist”).
Usually, there’s a polarizer and reflector on the back of a LCD. In normal operation, in the state where no electric field is present, then the light goes through the front polarizer, through the liquid crystal and has its plane of polarization rotated by 90 degrees, whereupon it goes through the rear polarizer (Note that the orientation of the front and rear polarizers are normally crossed (e.g., rotated by 90 degrees with respect to each other).). It is then (specularly) reflected (which maintains it’s plane of polarization), and goes through the rear polarizer, through the LCD whereupon its plane of polarization is again rotated by 90 degrees, and then out through the front polarizer.
In the state where an electric field is present, the liquid crystal molecules are standing up perpendicularly to the glass sheets, and do not rotate the plane of polarization of light as it passes through them. Thus, the light passes through the front polarizer (and is polarized), then passes through the liquid crystal where the plane of polarization is not affected, and when it hits the rear polarizer is then blocked. Any light that leaks through (since polarizers are never 100 percent efficient) is reflected, goes back through the rear polarizer, the liquid crystal, and is blocked by the front polarizer.
There may be some variations on this (I don’t claim to be an expert at all aspects to liquid crystals, since it’s been 20 years since I’ve been out of the field!). So, anyone who is contemplating working with these should do a little research.
Dave
Very good read, full of useful info. Thanks!
Hi,
Thanks for the great articles you’ve posted on projectors!
I’m having problems with my Philips LCD projector, it’s quite old now but still works fine except for the dark greenish shine on dark/black scenes in a movie.
I understand that this might be a polarizer problem, which one should I look for? Or could it be something else?
Thank you.
Aykut, if you’re comfortable taking the projector apart (and confident you can put it back together — mine have lots of delicate flex-pcb cables that would be easy to tear, so make sure you find and disconnect all of them before pulling PC boards out), look for a missing or damaged polarizer in the green light path. It sounds like your green LCD may not be showing full black because a polarizer is missing.
It can be hard to tell which color is which, so look around inside a bit.
Whatever you do, don’t separate the flex-pcb cables from the LCDs themselves. My experience is that you’ll never get them back on properly, which leads to solid bright lines across your display.
Thanks for the hints! I’ll try to take it apart some day, and hopefully put it back together as well
nice and useful blog btw!
Hi Keith,
Thanks for your blog, it’s really useful. I have the same problem as you, please see in my picture here: http://i28.tinypic.com/291doag.jpg
My projector is a Toshiba TLP560.
What are my choices, please help. I don’t think I have any chance of getting hold of a broken projector in order to steal one of its filters. Do you know of anyone who successfully used something else, such as a photographic filter?
Thank you for any help, I am as fried up as my filter these days,
D.
Dragos, I’ve been in contact with someone who tried different types of filters not made for projectors and I believe they all melted. I think you really want to find a filter out of a dead projector, and I know that’s not easy.
Hi Dragos,
I found the photography filter melted down immediately in the light path of the projector. I guess that because it’s mounted between two pieces of glass, it has very little capacity to dissipate heat.
I did make a new polarizer out of cutting up an LCD display (liquid inside is said to be poisonous). This surprisingly did hold up to the heat, however, it was less efficient at polarizing the blue wavelengths and threw off the color balance noticeably. You could try replacing all three color polarizers with this material to keep the color balance, if you can’t find any replacement blue-blocking polarizing material.
- Marsh
That’s sad news, guys…
Marsh Ray, could you give me a few examples of LCD display that I could ruin and would work? What would be, you know, dispensable?
You know if is possible to use any normal polarized? the ones that i need to fix my nex projector cost 120USD each one
and the three are burned
Jose, Marsh’s experience is that everything other than a projector polarizer melts. Your least expensive way to fix it may be to find another identical or similar projector that’s broken in a different way and take its filters.
Hi everyone,
My LCD rear pro TV has a yellowing picture thanks to a burning out blue polarizer.
Are these parts interchangeable? Sony still make them for some of their projectors, as do some other manufacturers.
Unfortunately Samsung don’t make the one for my TV anymore.
I did manage to pick up some new polarizers for another TV, and tried to retrofit them into my TV but it was if they weren’t even there when i powered the TV on again.
Perhaps i need to retry this and rotate the filters by 90 degrees instead of only flipping them around?
John, definitely try rotating them. It? I hope it’s just one filter that you’re changing, or one at a time? If you are working with two, rotate only one of them.
I bet that’ll do it!
Keith only working on one faulty blue polariser. Unfortunately the replacement one is smaller, thicker, and made from plastic.
The original one is obviously bigger (in length and width), alot thinner, and made from glass.
I came across this thread http://www.avsforum.com/avs-vb/showthread.php?t=657027 that has pictures of the polarisers for a Sony unit.
I would be willing to invest in one of these, but am hoping someone on that site can get me the dimensions first.
Hi, to all
I’ve seen this post and here my situation
I’ve clean all the optic engine (with optics special liquid) of my e LCD Philips projetor by unfortunatly when cleaning the polarizers the cleaning fluid was to strong and messed this up. I’ve damaged to of the pols less one that stayed ok (the ones that became clean, and I mean really clean, pol surface was dressed up) had a kind of mirror surface.
Now, Marsh’s tried a pol from cutting a LCD (that worked, according to him), does anyone knows what type ? (LCD from calculator type display ???, or what ??)
Please comment, thks
PS. The projector works without these front pols (if you take them all to avoid unballancing the colors), but you’ll get grays insted of blacks.
How can I reload the infocus software after taking the projector apart and putting it back together? (I have a LP290)
Matt, I have no idea what this software is that people keep referring to. I’ve had my projector apart and back together and it still projects, has menu controls, etc.
Thanks for the quick reply. I only get a blank white screen.
Matt, whoa, that’s not good!
Have you tried all the different inputs to make sure it’s not a problem processing a particular one? Except you should still get the on-screen display. Huh, that’s really weird.
Yes. I tried the different inputs, but you are right about the on-screen display. I am going to take it apart again and put it back together. Maybe I missed something.
If you have any suggestions, I am certainly open because I am not an expert by any means.
And thank you for your help thus far.
Knowing that it worked for you was helpful.
Matt, how ’bout you forgot to plug the LCD flex-PC connectors back in?
Keith,
Had to step out for a bit, but I wanted to let you know that you were right on. I had plugged them back in, but one or more of them were not in far enough. Thanks again for all your help.
Also, if I have a green yellow blur in the center of the screen, what do you think it could be?
Matt, look for melt in the polarizing filter for your blue light path, maybe also your red one, like I had. Note that the filter for the blue path doesn’t look blue.
Hi Keith,
I was able to get my hands on an LG blue polariser, but it’s marked as a ‘mirror’ on the packaging. To the naked eye it’s the same colour as the polariser when you’re looking at it (orange) but when you look through it, it doesn’t have the same properties when rotating it.
In fact, everything comes through with a blue tint, and as you rotate the polariser left and right the blue tine changes to purple.
I was told that a polariser is a polariser, but this to me seems totally different. You think there’s *any* chance that this will work?
Thanks,
John
John, I think there’s a fair chance it’s actually what you’re looking for.
I don’t know, try hold it in front of the projector lens and rotating it and see what happens?
Hi Keith,
I’m the same john that posted earlier in the thread, i’m trying to locate a BP for a samsung LCD RPTV. THe one that’s already in there has the same properties as the ones you’re describing on this page, the LG one which i’m thinking of retrofitting is like i just described in my last post.
You think it may work?
THanks!
John
Hi,
I’ve posted some time ago a question about polarizer replacement (using a LCD polarizer). Well, laking on replies, I’ve tried an ordinary LCD from a monocrome (black) multimeter. I’ve removed the POL and placed it on the POL holder. SUCESS, the LCD POL worked so well that my wife deleted a new projector from my birthday presents list (should I be glad with it ?, yes, because I was going to pay for it anyway).
So, POL replacement can be done for just a few dollars (euros in my case), but it’s not a straight forward process (that can be explained in another post if someone is interrested).
Regards
John, I think there’s a fair chance it’ll work.
But even more, I believe you can preview what effect it will (or won’t) have by rotating the filter in front of the projector lens. Now, as I think about it, I believe the filter that was melted in mine was the filter before the LCD, so you’d really need to test your filter where the original was to get the correct effect.
But you should be able to tell whether it will polarize blue light correctly by rotating it in front of the lens. If it has a chance of working, you should see the amount of blue in your image vary dramatically as you rotate the filter. If it does, then I think it’s worth opening the projector to try to put the filter inside.
Hi Keith,
I might give it a go but i don’t like my chances. the original one that’s burnt behaves the same way as yours (orange glare turns white when rotated), the LG replacement doesn’t do anything when rotated. The blue tint changes to purple when moved left and right.
I need to get the glass cut down for it to work also so there’s a bit of effort before i can even trial it.
I have a feeling that however it behaves with the naked eye will be the same once installed.
John
I have a Hitachi TX200 with a bad in (before panel) BP, causing the center screen to be yellow and the edges blue. I managed to get some polarizing material sent as a sample and verified it was the right way up, relative to the white glare reduction relative to the original part, but all I get is an even blue flooded picture. Turn it through 90 deg as a test and I get no blue at all, as expected.
I can’t work out what’s going on as from what I have read it should have worked initially – any ideas?
Abe, you’re going to think this is silly, but double- (triple-) check the flex-PC cable to the blue LCD. It is so easy to get wrapped up in what you’re doing with the polarizer that you forget you unplugged the LCD cable when you were dismantling and missed replugging it when reassembling.
Abe: Try flipping the material over. The polariser material only works one way. When i took the burnt polariser out of my projector to inspect it, i put it in backwards and it was as if it had no polariser in there at all..
Can you tell me where you got the polariser material from?
Keith:- Had this apart many times now so I know it almost off by heart! Unfortunately the main board sits over the panels and must be disconnected and removed before I can get to them. It’s annoying that I can’t run it whilst experimenting, and it takes so long enough to dismantle, mod, reassemble & mount that it makes multiple tests a real pain.
John:- Spoke to a schools science supply place I found somewhere & they sent me a small sample. Specs say that it’s good for 70 deg C (measured the surface temp of the existing polarizer with a thermocouple whilst running – 62 deg C) and verified that it cuts surface white glare (as per original part) either way around. Bit stumped as to why it’s not working.
No, the polarizer material works both ways (e.g., either direction of light going through it). However, most devices seem to like to mount the polarizers at a 45 degree angle. Thus, if you put one in backwards, the alignment is such that it doesn’t cross with the other polarizer, and doesn’t perform the light blocking function (e.g., it acts like it’s not in there).
As for where to get polarizers, back when I was doing commercial optical work, we got ours from Edmund Scientific. Of course, they pretty much deal in surplus materials, so sometimes the things they have vary a bit. On the other hand, their prices are sometimes much better than retail prices.
Dave
Abe: If you could spare a small piece or put me in touch with the plac you got yours from it would be greatly appreciated.
Maybe not all polarisers are created equal? I noticed that some projectors have an IN and OUT polariser, but from what i can see on my projector and the InFocus one on this page, there is only one?
Dave:- I did try to match the orientation of the original filter, but I agree that some experimentation is in order. I’m going to try to rig up a way of testing while it’s apart using a mains light source, then I can try twisting it about.
Dave:- Got the sample from here “http://www.sciencestore.co.uk/StoreOpticalComponents.asp#PolarisingFilm”
only thing i can think of (unfortunately) is that it’s either not a blue polariser (just general polariser like on sunglasses?) or the specs are different.
All blue polarisers that i’ve seen so far are orange to the naked eye (like the ones in this page).
Keith,
This link is for the LP280 firmware download v1.26 that everyone keeps referring to. Follow the link then scroll all the way down past the useless pdf files of the user manual, reference guide, and ceiling mount instructions to the download link for the firmware.
https://portal.infocus.com/support/Product%20Pages/downloads.aspx?mic=LP280&on=downloads
I know in an earlier post one reader could not get the computer to recognize the the projector via the ps/2 cord or the USB connection, so hopefully this download comes with a ReadMe file explaining how to get it into the projector. Along those lines…do you know if it’s possible to Flash the update via the IR port? Some computers have the IR transmitter and software/stack pre-installed and have the ability to transfer different types of files via IR…would you happen to know if the projector has the capability of receiving such files via the installed IR receivers?
Hope this link is helpful : )
Teri
Hi there,
Need some assistance. I realised that my Panasonic PT-L759 LCD Projector polarized filter melted too after reading some posts here. There happen to have 3, each with different colours when looking into through the flourescent. Can anyone suggest where i could get 1 that is coloured red and another seems to be coloured green. Really need help and appreciate anyone who could recommend where i could get the polarized filter. Another thing, is it possible to use some alcohol or thinner to get rid of the melted filter film and just replace with a new polarised film instead of replacing the entire glass panel as well. Thanks a lot for your assistance
Forgive me if I get this a bit wrong, I’m new to optics.
If I’m understanding this correctly, the polarizing filter for the blue channel doesn’t actually need to filter blues because that is done upstream via mirrors. The main function of filtering the blue is to help create darker blacks(by filtering whatever the mirrors missed).
I’m asking because I have a burned out blue polarizer and finding another working blue polarizer is a bit difficult/expensive. Regular polarizers are quite cheap however.
Eric, I would say: The blue polarizer works in conjunction with the blue LCD to create black in the blue channel.
Yes, it’s possible that a plain polarizer (rather than a blue polarizer) would work. Just find one that can withstand the heat, and come back and tell us of your results!
Keith,
I found your website a year or so ago, after purchasing a LP290 and realizing the picture was always REALLY blue. I took my LP290 apart and discovered a melted filter that looked nearly the same as the one in your picture. Since then, I was able to procure a broken LP290, and I replaced my filter with one from it, and it worked!
However, here is my problem: now the projected image has one color (green-ish looking) that is misaligned from the others. I guess I must have bumped something or…? Did you ever get your LP290 back together? Did it work fine when you did? Do you (or anyone else that happens to read this) have any idea what I might be able to do in order to fix my picture?
Thanks very much for posting all this, and I would be very appreciative if you were able to get back to me!
-Allen
Hi, I have 2 faulty projectors. One is Toshiba TLP 260 and Canon LV-X6.
Toshiba: Image has a blurr spot which is growing. might be faulty polarizer but change is costing me SGD$1000+.
Canon: The left and right side of projected image has different colour. The left is yellow/orange while the right is blue. might be prism faulty but change is also costing me SGD$1000+.
Can anybody help me regarding my problem with a less cost? I have an important presentation this Friday. Would truely appreciate any advice
Thankyouthankyou