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Discussion Starter · #1 ·
OK you HT nut/photographers out there...

I've been reading posts after posts about using red filters to improve picture contrast and I was wondering what a circular polarizer or a linear polarizer would do to the picture?


In photography, the polarizers cut out a lot of the glare to produce a nice, crisp image on 35mm film simply by turning the filter slightly clockwise (or counterclockwise). I was wondering if anyone in here have tried to attach one in front of a projector lens and see if it improves the picture quality any.


If someone has a polarizer, please do this simple experiment to see what happens and report results in this thread.


THnX.
 

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Knowing how polarizing filters increase color saturation and contrast with 35mm film, I stuck one on front of my pj but the image was much to dim. Much darker than with a ND20 filter. Maybe someone with a superbright pj could try it.
 

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I just tried my circular polarizing filter on my XP21. When the rotation was just right it darkened the picture with some increase in color saturation. Overall the pq was less pleasant than the unpolarized image. When it was rotated from the optimal position it gave a weird color cast that varied with the rotation.

-R
 

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I am no physics expert, but in my photography experience polarizers work best when there is a large amount of reflected light from the sun creating a haze.



.02 cents



...if that.
 

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I tried a glass polarizing filter on my Hitachi a little while ago with similar result. As I rotated the filter clockwise or counterclockwise, it would gradually allow more and more red or yellow light through, until the image completely turn red or yellow. At optimum position it actually cuts out the slight purple tint on the famous Hitachi grey top and bottom bars, enriched the colours and the blacks. However it cut out so much light that it dim the picture signifiantly, and killed most highlights.
 

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Red filters work because the light sources used in these projectors (typically a UHP bulb) are short on red light if you want a D65 colour temp. So by using a red filter you can increase the blue and green gain on your projectors panel / chip to improve contrast.


A polarizing filter will not help your picture, especially for an LCD based pj, as the LCD panes polarise the light in a single direction anyway.


Cheers


Brett D
 

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In photography, a linear polarizer works by reducing light reflected from water, windows etc (making it polarized) more than unpolarized light. This is in theory not expected to help a projector since all the light is already polarized.


Nevertheless, I tried a polarizer on the Hitachi 5500 and by rotating it, the color shifted between green and purple. At the optimum angle, the whites were white and the blacks were black (purple cast was gone!). Unfortunately the best polarizers out there, still only transmits 42% of unpolarized light (50% theoretical maximum for coated glass).


Thus, a polarizer can be used to eliminate the purple, but it converts a 1500 lumens projector to a 630 lumens one (worst case scenario)... Increasing projector white level (contrast) caused washout of the highlights :-(.


I believe that the reason this worked on the Hitachi LCOS is that the green light is polarized in a different plane compared to the red and blue light. At the optimum angle, equal amounts of red, green and red are absorbed, and the color of white light is therefore unchanged.


Incomplete black level, on the other hand, is caused by light leakage (mainly red and blue to cause the Hitachi purple blacks) and may be unpolarized or, more likely, in a perpendicular polarization plane . Thus, the polarizer helps in this case by reducing the red/blue light leakage.


For increased contrast ratio, the absorbance of red, blue and green must be less than the absorbance of the light leakage. This may be the case for the Hitachi, because the red/blue is only ~45 degree from the green. Thus, maybe only 45% of the polarized light is absorbed, while 58% (1-0.42) of the light leakage is absorbed.


This would have been a great remedy for the Hitachi, had it not been for the large light loss!!
 

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In early LCD projector designs, a linear polarizer filter called an "analyzer" was employed. It was installed beyond the LCD panels (near the combining block) to boost contrast. As rbrande mentioned it cut the output by at least half and as manufacturers started pushing lumens this configuration fell to the wayside.


Thumper
 

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Quote:
Originally posted by BSN
OK you HT nut/photographers out there...

I've been reading posts after posts about using red filters to improve picture contrast and I was wondering what a circular polarizer or a linear polarizer would do to the picture?
BSN,


Can't help - and most certainly can hurt - if you are using projectors like D-ILAs where the digital chip works by

altering the polarization state of the light that is supposed to go on screen vis-a-vis the light to be dumped.


A polarizing filter is used in photography to cut glare due to reflected ambient light. The situation in HT isn't

analogous to photography in the slightest.


The key to good HT and good photography is to understand what the various filters do - rather than taking an

empirical approach.


Dr. Gregory Greenman

Physicist
 

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Discussion Starter · #10 ·
Quote:
Originally posted by Morbius



The key to good HT and good photography is to understand what the various filters do - rather than taking an

empirical approach.


Dr. Gregory Greenman

Physicist



O...kay Dr. Greenman, sir.

Relax. It was just a simple experiment. I'm perfectly aware of what filters do as I am into photography myself. It's just that I didn't have a polarizer on hand and thought that others who do, can do this simple test. Saves me from going out and buying one myself.

Don't need to go into the deep end with ostentatious pedantics. We're all educated in here. Some are just more showy than others. Don't get me wrong. There's something to be said about establishing your credibility (by placing "Dr." or "PhD" by your name every once in a while) so that you are perceived to be knowledgable in your field, especially in forums like these where you don't know anyone's background. But at least limit it to the topic on hand.



Anyway, thanks guys. It was just a theory of mine in that I thought it would have a slight improvement in contrast, but not at the cost of significant decline of overall brightness. I guess it only benefits the Hitachi's with the purple tinge, but even then it's not much of an improvement to the picture quality.


You saved me time and money. I love this forum.
 

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Very good question, BSN. Thanks for asking it. I think "Dr." Greenman was complimenting you.


You were seeking exactly the knowledge to which he referred. Instead of just buying the filter and slapping it on (empirical), you asked a group who may have more scientific knowledge of the effects of your proposed experiment.


Although I do have to say that simply applying a solution based on an "understanding" of the components is simply empirical as well. Educated, but empirical. Actually, if we are discussing the technical merits of BSN's inquiry, his method would actually be annecdotal, not empirical.
 

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Discussion Starter · #12 ·
If that was the case, then I stand corrected.

But I didn't take it as a compliment.
 

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Quote:
Originally posted by Jim Scarbrough
Very good question, BSN. Thanks for asking it. I think "Dr." Greenman was complimenting you.


You were seeking exactly the knowledge to which he referred. Instead of just buying the filter and slapping it on (empirical), you asked a group who may have more scientific knowledge of the effects of your proposed experiment.
Jim,


Correct you - thank you for explaining this to BSN.

Quote:


Although I do have to say that simply applying a solution based on an "understanding" of the components is simply empirical as well. Educated, but empirical. Actually, if we are discussing the technical merits of BSN's inquiry, his method would actually be annecdotal, not empirical.
Yes, perhaps "anecdotal" would have been a better word.


Now - where's that thesaurus....


Dr. Gregory Greenman

Physicist
 

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I have now carefully measured the effects of a polarizer on the Hitachi 5500 purple blacks.


Sad to say, but my initial impression was wrong.


The polarizer does indeed lower the black level, but not more than a ND filter. Thus, the polarizer reduces the light leakage and the direct light with the same relative amount.


This was determined using a calibrated photocell and measuring the intensities for RGB at different IRE levels.


See here for details of the measurement technique in general:
http://www.smartavtweaks.com/
 
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