Author Topic: Transflective LCD polarizer orientation  (Read 1346 times)

0 Members and 1 Guest are viewing this topic.

Offline Lomax

  • Frequent Contributor
  • **
  • Posts: 499
  • Country: eu
  • Minimalist
Transflective LCD polarizer orientation
« 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. 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!

[attach=1]

:-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)?
« Last Edit: January 19, 2020, 03:56:12 pm by Lomax »
 

Offline Lomax

  • Frequent Contributor
  • **
  • Posts: 499
  • Country: eu
  • Minimalist
Re: Transflective LCD polarizer orientation
« Reply #1 on: January 19, 2020, 10:49:59 pm »
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.

[attach=1]

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.
 
 

Offline shakalnokturn

  • Super Contributor
  • ***
  • Posts: 1474
  • Country: fr
Re: Transflective LCD polarizer orientation
« Reply #2 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?
 
The following users thanked this post: Lomax

Offline wraper

  • Supporter
  • ****
  • Posts: 13192
  • Country: lv
Re: Transflective LCD polarizer orientation
« Reply #3 on: January 20, 2020, 02:02:15 am »
I have this old marine instrument with a large-ish LCD (~11x9 cm) which had been damaged by the sun.
And what the initial damage looked like? IMHO LCD itself might be damaged by sun to begin with.
 

Offline Lomax

  • Frequent Contributor
  • **
  • Posts: 499
  • Country: eu
  • Minimalist
Re: Transflective LCD polarizer orientation
« Reply #4 on: January 20, 2020, 02:21:32 am »
And what the initial damage looked like? IMHO LCD itself might be damaged by sun to begin with.

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.

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?

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?

 

Offline wraper

  • Supporter
  • ****
  • Posts: 13192
  • Country: lv
Re: Transflective LCD polarizer orientation
« Reply #5 on: January 20, 2020, 02:55:51 am »
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.
Ah, now I get it. IMHO you ordered wrong type of polarizer films. Something like for white on blue background LCD with backlight.

 

Offline wraper

  • Supporter
  • ****
  • Posts: 13192
  • Country: lv
Re: Transflective LCD polarizer orientation
« Reply #6 on: January 20, 2020, 03:06:14 am »
Did you buy reflective polarizer? Or used transflective on both sides? In which case what is used as reflector?
 

Offline wraper

  • Supporter
  • ****
  • Posts: 13192
  • Country: lv
Re: Transflective LCD polarizer orientation
« Reply #7 on: January 20, 2020, 03:16:42 am »
Guess I found why this happens. Probably display is STN but you are using film for TN.

[attachimg=1]
« Last Edit: January 20, 2020, 03:32:40 am by wraper »
 
The following users thanked this post: Lomax, shakalnokturn

Offline Lomax

  • Frequent Contributor
  • **
  • Posts: 499
  • Country: eu
  • Minimalist
Re: Transflective LCD polarizer orientation
« Reply #8 on: January 20, 2020, 11:05:48 am »
Guess I found why this happens. Probably display is STN but you are using film for TN.

That does indeed sound like a very likely explanation, thank you! Anyone have any idea where I might find polarizer films suitable for an STN LCD panel? The supplier I bought from only has one type of film.
 

Offline shakalnokturn

  • Super Contributor
  • ***
  • Posts: 1474
  • Country: fr
Re: Transflective LCD polarizer orientation
« Reply #9 on: January 20, 2020, 08:42:02 pm »
@ wrapper: What paper is that an extract from please?
 

Offline wraper

  • Supporter
  • ****
  • Posts: 13192
  • Country: lv
 
The following users thanked this post: Lomax, shakalnokturn

Offline wraper

  • Supporter
  • ****
  • Posts: 13192
  • Country: lv
Re: Transflective LCD polarizer orientation
« Reply #11 on: January 20, 2020, 09:01:09 pm »
Guess I found why this happens. Probably display is STN but you are using film for TN.

That does indeed sound like a very likely explanation, thank you! Anyone have any idea where I might find polarizer films suitable for an STN LCD panel? The supplier I bought from only has one type of film.
After giving some more thought, I think that polarizer films are the same. But when removing polarizer, you likely removed retarder film(s) as well which likely come as separate film layers. In description of some polarizer films at 3DLens.com it's mentioned they are suitable for STN.  IMHO try placing new film on the back of LCD and old original front film in front and see what happens.
« Last Edit: January 20, 2020, 09:03:58 pm by wraper »
 
The following users thanked this post: Lomax

Offline Lomax

  • Frequent Contributor
  • **
  • Posts: 499
  • Country: eu
  • Minimalist
Re: Transflective LCD polarizer orientation
« Reply #12 on: January 20, 2020, 11:54:28 pm »
After giving some more thought, I think that polarizer films are the same. But when removing polarizer, you likely removed retarder film(s) as well which likely come as separate film layers. In description of some polarizer films at 3DLens.com it's mentioned they are suitable for STN.


Thank you, that sounds reasonable. A quick search for "retarder film stn lcd" indicates that this is known as an FSTN LCD (Film compensated Super Twisted Nematic). Reading a bit more about how this stuff works it sounds like I wasn't far off suspecting some "dichroic magic" might be involved. Also explains the identical appearance of the naked polarizer films when viewed without the LCD panel.

IMHO try placing new film on the back of LCD and old original front film in front and see what happens.

With apologies for the damaged and dirty pieces used (the removal was messy), here's a photo of what the different combinations look like (at 45 degrees and in negative orientation):

[attach=1]

Note the difference in the dark portions indicated, where you can see the old front polarizer on top of the old and the new rear polarizer. The old rear film gives a darker and more purple "black" compared to the new film's lighter and bluer "black". So the rear films are not quite the same either :(
« Last Edit: January 21, 2020, 12:06:15 am by Lomax »
 

Offline Lomax

  • Frequent Contributor
  • **
  • Posts: 499
  • Country: eu
  • Minimalist
Re: Transflective LCD polarizer orientation
« Reply #13 on: January 21, 2020, 07:04:57 pm »
Confirmed: FSTN LCD displays need a retardation film on the front and on the back. 

[attachimg=1]

What chance do I have of finding these films?
 

Offline shakalnokturn

  • Super Contributor
  • ***
  • Posts: 1474
  • Country: fr
Re: Transflective LCD polarizer orientation
« Reply #14 on: January 30, 2020, 11:50:14 pm »
Did you get any further with this?

I'm looking around as I also have a front polarizer to replace on a C-STN display now.
I'm thinking the "A" suffix references at 3dlens.com could be worth trying.
https://3dlens.com/transflective-polarizer-film-200x250mm-with-adhesive.php
https://3dlens.com/linear-polarizer-film-200x250mm-with-adhesive.php

I tested their P200 reference on a Tektronix TDS220 some years ago, it wasn't adequate for the job.
« Last Edit: January 31, 2020, 01:49:44 am by shakalnokturn »
 

Offline pete1zed

  • Contributor
  • Posts: 10
  • Country: nz
Re: Transflective LCD polarizer orientation
« Reply #15 on: May 07, 2021, 09:17:33 pm »
Hi,

The original post said that when the new polarizer film was applied, the background was LCD appeared green/yellow, compared to a neutral light grey with the original polarizer.

Did anyone find a source for the polarizer film to eliminate the green/yellow background?

I have exactly the same problem (please see the images attached).

I tried linear and circular polarising film (left and right, and including a left and right with a retardation layer) -- all with the same green/yellow background issue.

Any ideas anyone?

There is a comment above saying a retardation layer is required on both the front and back, so maybe I'll try that next...

 

Offline pete1zed

  • Contributor
  • Posts: 10
  • Country: nz
Re: Transflective LCD polarizer orientation
« Reply #16 on: May 16, 2021, 08:40:41 pm »
Hi again: here is an update on my attempts to find a front FSTN polarizing film

I have ordered this FSTN polarizer film. 

The issue is....

From polarization.com:
Yes, an FSTN display needs a clear retarder between the top polarizer
and the LCD to covert the elliptical polarization coming out of the
display back to linear polarization.
Unfortunately, there is not just one kind of FSTN: there are many
variations needing different retardation and orientations
. That is one
reason we do not offer them, because the market for a specific one would
be too small.
There are some that use circular polarizers, what means that they have a
quarter-waver retarder at 45 degrees to the polarizer (for example we
have seen BMW navigation systems with a right-circular polarizer). But
the most common types need some type of elliptical polarizer.

To solve that, I also found this site that sells different retardation films,
http://www.polarizerfilm.com/fstn-polarizer.html

...but I'll wait for the above polarizer to turn up before making further orders.

« Last Edit: May 21, 2021, 11:48:41 pm by pete1zed »
 
The following users thanked this post: Lomax, shakalnokturn, YetAnotherTechie

Offline pete1zed

  • Contributor
  • Posts: 10
  • Country: nz
Re: Transflective LCD polarizer orientation
« Reply #17 on: May 21, 2021, 11:57:26 am »
Problem solved:  No more yellow background from my FSTN display using this polarizer

Note: this is the transparent polarizer for the front of the LCD, but they also have films for the rear

There is  a  join in the layers which runs diagonally. So max size of a screen you could fit in without having the line is 150x180 mm.  You could cut enough for two screens from the 350x300 mm sheet

Attached is a picture showing what happens if you fail to avoid the join

« Last Edit: May 21, 2021, 11:47:43 pm by pete1zed »
 
The following users thanked this post: shakalnokturn

Offline wraper

  • Supporter
  • ****
  • Posts: 13192
  • Country: lv
Re: Transflective LCD polarizer orientation
« Reply #18 on: May 21, 2021, 02:07:45 pm »
There is  a  join in the layers which runs diagonally. So max size of a screen you could fit in without having the line is 150x180 mm.  You could cut enough for two screens from the 350x300 mm sheet

Attached is a picture showing what happens if you fail to avoid the join
I guess you received an unlucky one. Most likely it was cut from a large roll, where sometimes joint may happen. The same thing happens with cables, sometimes copper strands from two pieces are twisted together and later covered with insulation. Normally place this defect should be marked but sometimes it's not.
 

Offline pete1zed

  • Contributor
  • Posts: 10
  • Country: nz
Re: Transflective LCD polarizer orientation
« Reply #19 on: May 21, 2021, 08:02:20 pm »
Or maybe these pieces with the stripe are correctly binned at the factory. 

A factory worker then gets these out of the bin and sells them. 

The piece I received has the diagonal stripe directly from corner to corner, which is how you would cut it to remove the un-usable section in the most efficient way from the legitimate product.

I do not know...
 
The following users thanked this post: wraper


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf