Transflective LCD polarizer orientation

[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!





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]

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.

[Lomax]

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?

[wraper]

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.

[wraper]

Did you buy reflective polarizer? Or used transflective on both sides? In which case what is used as reflector?

[wraper]

Guess I found why this happens. Probably display is STN but you are using film for TN.

[Lomax]

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.

[shakalnokturn]

@ wrapper: What paper is that an extract from please?

[wraper]

@ wrapper: What paper is that an extract from please?

https://books.google.lv/books?id=zmlQDwAAQBAJ&printsec=frontcover#v=onepage&q&f=false

[wraper]

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.

[Lomax]

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):



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

[Lomax]

Confirmed: FSTN LCD displays need a retardation film on the front and on the back. 



What chance do I have of finding these films?

[shakalnokturn]

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.

[pete1zed]

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...

[pete1zed]

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.

[pete1zed]

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

[wraper]

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.

[pete1zed]

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...

[CharlieD]

This topic is very interesting. I change the polarizer film of a Tek TDS2002B scope with bad polarizer fml (a typical fault) and although it improved compared to how it was, it was left with a purple hue where it should have been black, see pics below for comparison.

Confirmed: FSTN LCD displays need a retardation film on the front and on the back. 

(Attachment Link)

What chance do I have of finding these films?

Any suggestions to improve this and what could be the causes?

How looks like


How should look like

[pete1zed]

Hi,

It is possible that the direction of polarization is not correct.  You could check this roughly by rotating the film until you see the correct display colours.

It is possible that there a retardation layer missing in the replacement film, or present but not wanted.  See
https://tinyurl.com/retardation-layer-1

Let me know if I can send you a free sample with the retardation layer.

One issue is that you may have inadvertently peeled off the old film, but left the original retardation layer in place, which is almost impossible to detect.
I had a case where I spent a lot time trying to find the correct polarizer, and then discovered I'd not removed the old clear plastic retardation layer. 
Therefore I had two retardations layers by accident.
After I removed the original retardation layer, the regular LCD polarizer worked fine.  Just check that you removed all the film by checking the exposed layer is glass and not plastic....

.... except, if you can't get replacement film with the polarizer, you'd want to ensure the old retardation layer is in place so it can be re-used.

Of course, there may be no retardation layer at all.

This is  a confusing answer I know, so if you do find the solution, please reply here so we can all benefit.

Also,
(1) Can you just buy a whole replacement display from Aliexpress etc? 
(2) Have you tried to contact Tektronix for the information?
(3) Good luck...


When running into a similar problem, I got the following answer from www.mecanusa.com
I think it is also possible to send them a small sample of the original and they will try and figure it out. 


-Pete


----------------
Thank you for contacting Mecan.

Here below link is for our lineups of retarder film.
https://www.mecanusa.com/polarizer/phasedifferencefilm.htm

Unfortunately, we have no knowledge of the type of polarizer used in calculator
and we don't know which kind is appropriate for you.

If you would like a sample, we can provide small pieces for free.
Size is about 8 x 8 cm, and we will send you with regular air mail from our US office.

You may also want to try a circular polarizer.
A circular polarizer is made by laminating a linear polarizer and a quarter waveplate together,
so it might be that the original polarizer may have been a circular polarizer.

Here below link is for our lineups of circular polarizer film.
https://www.mecanusa.com/polarizer/circularpolarizer.htm

Unfortunately, there is no self-adhesive layer in either product

Click here for our adhesive sheets for optical films
https://www.mecanusa.com/polarizer/adhesivefilm_MPD62.htm

If there is any other request or inquiry, please contact us anytime.
We look forward to your reply.

Best Regards
Ryo

[CharlieD]

Hello Pete1zed:

First of all, thanks for your answer.

It is possible that the direction of polarization is not correct.  You could check this roughly by rotating the film until you see the correct display colours.

I did, it was either this colour, or a kind of white colour in the other extreme case of polarization direction, unfortunately I don't have any pictures of that. I decided this was the best direction of polarization in terms of contrast/colour.

One issue is that you may have inadvertently peeled off the old film, but left the original retardation layer in place, which is almost impossible to detect.
I had a case where I spent a lot time trying to find the correct polarizer, and then discovered I'd not removed the old clear plastic retardation layer.
Therefore I had two retardations layers by accident.
After I removed the original retardation layer, the regular LCD polarizer worked fine.  Just check that you removed all the film by checking the exposed layer is glass and not plastic....

I'm not really sure, I'm gonna open the scope and check this, and I will reply.

(1) Can you just buy a whole replacement display from Aliexpress etc? 

Too expensive for me  unfortunatelly, It would be more expensive than the scope itself (cost around 50 bucks with a bad channel, already repaired). Also, I'm from Cuba, get spares and buy online is almost imposible here.

I think it is also possible to send them a small sample of the original and they will try and figure it out.

Same problem, I'm from Cuba, send something to another country is difficult.

I asked here hoping that someone had a similar issue, but I'll find out everything I can, I really wish that the scope can get the original display colour. I'll be posting updates in the future.

BTW, the polarizer filter I'm using is this:
https://3dlens.com/linear-polarizer-film-200x250mm-with-adhesive.php

A friend send it to me from USA.

Many Thanks.

[Lomax]

Nice scope! And an absolute bargain at $50 

I'm afraid what @pete1zed is saying WRT the missing retardation layer is most likely correct. I'm sorry to hear it's so difficult to place international orders from Cuba - maybe your friend in the US can help get you at least a sample piece of film with a colour correction (retardation) layer? Should be possible to send as a letter... Please let us know how you get on!