Electronics > Repair

Transflective LCD polarizer orientation

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Lomax:
I have this old marine instrument with a large-ish LCD (~11x9 cm) which had been damaged by the sun. Since the instrument was basically free I figured it would be worth spending some time and $$$ to try and fix it, so I ordered new films from 3DLens.com. These arrived promptly and after struggling to remove the old films and clean up the glass I fitted them at a 90 degree angle to each other. But the result wasn't quite what I had hoped for!



:-DD

What did I do wrong? Neither of the new films have any yellow tint whatsoever, both protective films have been removed, and I made sure the glass was spotless before I fitted them...

Edit: Could it be that the polarizers should have been mounted at a 45 degree angle to the LCD panel (while still at right angle to each other)?

Lomax:
After comparing with the original polarizers I have found that they were indeed oriented at +/- ~45 degrees - but trying this with the new films does not get rid of the colour cast, I am only able to change it to an orangey-red. So orientation is not the (only) answer here.

I found something interesting when comparing old vs new though; the old front polarizer appears to be made up of two layers, one of which had partially peeled during the (somewhat violent) removal. When combined with a piece of the old rear polarizer on the back of the glass, the part of the front polarizer which still has this second layer looks normal - but the part where it's missing exhibits the same colour cast I got with the new films. Conversely, in negative mode the dual layer part shows as black while the single layer portion is blue.



What might this second layer be? Do I need a different type of front polarizer film to get rid of the colour cast?

It is worth noting that the new films show no sign of any colour cast when looked at without the LCD glass panel in between them. In fact, when compared side by side with the original front polarizer it is impossible to tell new from old - the transmitted light looks 100% identical and neutral.
 

shakalnokturn:
I have had the same sort of problems replacing polarisers on Tektronix TDS200 scopes, hard to get optimum contrast and viewing angle with new polarisers.

I'd be interested in some documentation comparing TN display variations, how to determine which type is being used without datasheet and how to find the appropriate polariser for a known type.

I think the basics of the problem is that the polarisers need to be adapted to the LCD's twist angle on top of being at 90° with one another when aligned with the LCD. Maybe also wavelength considerations in newer polarisers?

wraper:

--- Quote from: Lomax on January 19, 2020, 03:09:17 pm ---I have this old marine instrument with a large-ish LCD (~11x9 cm) which had been damaged by the sun.

--- End quote ---
And what the initial damage looked like? IMHO LCD itself might be damaged by sun to begin with.

Lomax:

--- Quote from: wraper on January 20, 2020, 02:02:15 am ---And what the initial damage looked like? IMHO LCD itself might be damaged by sun to begin with.
--- End quote ---

The LCD is fine - the damage can be seen in the "before" picture above. There was no trace of it after removing the damaged films.


--- Quote from: shakalnokturn on January 20, 2020, 01:48:00 am ---I have had the same sort of problems replacing polarisers on Tektronix TDS200 scopes, hard to get optimum contrast and viewing angle with new polarisers.

I'd be interested in some documentation comparing TN display variations, how to determine which type is being used without datasheet and how to find the appropriate polariser for a known type.

I think the basics of the problem is that the polarisers need to be adapted to the LCD's twist angle on top of being at 90° with one another when aligned with the LCD. Maybe also wavelength considerations in newer polarisers?

--- End quote ---

There's definitely a second layer of some sort on the back of the front polarizer, which when missing gives rise to the colour cast and poor contrast. But what is this layer? I doubt very much it is a polarizer, and it's not a simple colour filter either. There is no difference in appearance between the areas which have both layers and those where it has peeled off, when the film is viewed on its own - yet it has a dramatic effect on the colour and contrast of the LCD when placed on top of it. Perhaps some kind of dichromatic magic?

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