Optical qualities of prism sheet and glass substrate (LCD screen)

[Tom.G]

Summary:: Need help with defining optical qualities of two LCD screen layers that can severely hurt eyes when the screen is disassembled (what do they do to light to make it dangerous)

- a prismatic sheet and a lower glass substrate. Moreover, my eyes hurt even from the reflected light from the wall! The symptoms are exactly the same as when working with the monitor. Does anyone know what these two layers do to light and why it can hurt eyes?

Here are a couple Youtube videos on LCD displays. The first is a short quick look.



The second is about 34 minutes and the guy likes to talk but there is much information there. The prism layer is explained starting around 12 minutes. It is used for "light recycling," that is it redirects the backlight to go perpendicular thru the screen. Didn't you say that removing that layer reduced the discomfort?



Cheers,
Tom

 

[Vitina]

Maybe this?
https://www.physicsforums.com/threads/ophthalmic-migraine.71816/

Maybe flicker?
https://www.physicsforums.com/threa...amp-result-in-flickering.933119/#post-5893420

That thread discusses the variability of people's sensitivity to flicker. Some people can perceive flicker at frequencies that most other people don't.

What about when you use an old fashioned CRT screen? (if you can find one.)

What about when you watch TV at home?

What about when you go to a movie theater? (If you can find one still open.)

I agree with most of these studies...

As an old TV repairman, I recall that there was an on going balancing act between scan rate (29.5 fps US NTSC, 50 fps PAL Europe) and image retension of the phosphor.
Short retension ment flicker and retnal fatigue, long retension ment cometing of a moving object. Most opted for longer retention time.

Early computer monitors were monochrome and notorious for this.
LCD screens update on a pixle by pixle basis but used differing methods of maintaining and updating the image with florescent backlighting that operated from 120hz to 20khz somtimes creating a nausiating strobe effect.

Newer LCD screens have variable updates from 30fps to 240fps and a steady state led backlight. So the retension time can be very short and depending on subjet matter can affect people with "persistance of vision" problems.

I have this issue with some of these screens, early Apple CRT monitors were my biggest problem.
I have no problems with oled screens, no apearent or real flicker, no image shift due to thick LCD layers or shift in polarization, and no backlight.

 

[informerkh]

I have this issue with some of these screens, early Apple CRT monitors were my biggest problem.
I have no problems with oled screens, no apearent or real flicker, no image shift due to thick LCD layers or shift in polarization, and no backlight.

Hello, thank you for suggestions and happy holidays!

I doubt that this particular problem has any connection to this. For me, typical CRT frequencies of 75-85 Hz were difficult for perception, I saw how image 'pulses'. But my symptoms were completely different, my eyes were red and the head ached but I personally react to bad LCD in another way and clearly can differentiate my reaction. I was happy to purchase my first LCD and had zero problems with LCD screens for a long time.

CRT had only one drawback, flickering, although the picture felt more 'honest', like there was a 'surface' of the image, and LCD is the opposite - letters hang in the air (in the light) and there is no focus point for the eyes.

I didn’t think anyone’s eyes would hurt from the LCD, although now I found a bunch of similar complaints to mine, that started since LCD appeared and became widespread (early 2000s).

The problem I am fighting with right now is also present widely in modern oled screens.

I found a post (https://ledstrain.org/d/513-removing-the-metalic-semi-transparent-lcd-layer-helps) where the author removed a prism sheet from 5 different monitors and had a significant symptom decrease with every monitor. He said in the comments that nothing else helped. But he also said in another thread that LCD matrix itself is somehow problematic. It seems we came to the same conclusions.

 

[informerkh]

Here are a couple Youtube videos on LCD displays. The first is a short quick look.

The second is about 34 minutes and the guy likes to talk but there is much information there. The prism layer is explained starting around 12 minutes. It is used for "light recycling," that is it redirects the backlight to go perpendicular thru the screen. Didn't you say that removing that layer reduced the discomfort?

Cheers,
Tom

Hello Tom, happy holidays to you and have a great 2022!

I've seen a plethora of videos, including the first one, but the second one is new to me, it is way more detailed and interesting to watch. I found exact same 3M presentation that is featured in this video and yes, I told that removing this layer reduces the discomfort. Moreover, as I mentioned in another reply, I just found another person (https://ledstrain.org/d/513-removing-the-metalic-semi-transparent-lcd-layer-helps) who did exactly the same and reduced discomfort! He removed the prism layer from 5 monitors and got a significant relief, but he still could not find a way to use screen with no symptoms. He got stuck with the LCD matrix itself, he found it to be another cause of this problem. That corresponds with my experiences.
But LCD matrix is not a lens or at least should not be a lens, so it's hard to define what exactly happens.

I also wonder why prism layer that makes light go perpendicular trough the screen is problematic but any increase of brightness on the safe screen is completely OK for me. That is a big mystery to me.
I am not sure if all the light comes though it perpendicular, I've seen in some article that it reduces light angle but not completely.
I found a couple of articles about prism film and similar films (there can be few of them in one device):
http://archive.informationdisplay.o...play-marketplace-getting-the-light-through-tf
https://displaybly.com/brightness-enhancement-film-for-led-backlkight
And a big detailed part of a book (Applied Prismatic and Reflective Optics) that describes them (PDF file):
https://www.spiedigitallibrary.org/...sschapter&isFullBook=False&isResultClick=True
But I still can't figure out what's going on and how exactly this light is different from any other light. The most mysterious is LCD matrix (glass substrate with transistors) that should not be a lens.

 

[Tom.G]

Now if you could just find someone looking for a Masters Thesis subject, maybe you could talk them into researching the problem!

 

[berkeman]

Now if you could just find someone looking for a Masters Thesis subject, maybe you could talk them into researching the problem!

But the OP has still refused to conduct my single-blind experiment. That does not bode well for the future of this discucssion thread...

Yes, that is a great idea and I want to do something similar!
There are three problems with the execution:

1. People are scattered around the world and even within one country. As I need people who are willing to participate, have free time, have these layers... It is super hard to organize. And only two people are ready to disassemble screens right now, other simply do not have skill or waiting curiously for the results. But I hope things would improve.

2. Screen layers have to be found, so it should be exactly a bad screen, preferably the worst one for clear conclusions. Because of the next point in this list, the best thing to do is find some expensive modern smartphones with IPS screens and that takes some time and money (or less money and more time if service center can give some).

3. People have different sensitivity levels. I have high sensitivity but others have less. This problem is the most prominent on modern smartphones and most of them are OLEDs, so there's nothing to disassemble.

The lady I mentioned took the slightly bad IPS screen, disassembled it and found out that:
- she has way less problems with the exact same prism layer
- she has problems with a layer that comes above it, it is sort of another prism but probably multifunctional, we couldn't figure out its name. It looks very similar to prism film and may be some sort of reflective polarizer combined with prism film... I am not sure.
- she has pressing sensation that resembles phone when she takes lover glass substrate with transistors, that is glued with rear polarizer, and that undefined multifunctional layer together. She feels pressure when she applies them above the safe screen (with no image, just white background).

As I said, she is at the very beginning and she doesn't want to jump to conclusions.
This is a tricky problem and it affects people randomly, so there is no critical number of EEs to do all the tests yet. I hope it will change over time. Maybe if I have some clear findings or simple tests that everyone can do, there will be more data.

 

[russ_watters]

I replied to you in PM.
My problem has no connection to flickers and there were plenty of Ophthalmologists and flicker tests.

I just joined the thread (and we are discussing in the mods forum). A quick note for the overall problem (on PF and otherwise): if you want people to treat this/you like this is a real issue you need to take/treat it seriously. These flippant answers don't help your case. Surely with your education you must see that at face value what you are suggesting isn't possible, so if you need to be extra rigorous to show that it is real.

FYI, 3 or 4 years ago I had an issue that felt like someone stabbing me in the back of my eyes. It went on for months and an ophthalmologist found no issues. It was frustrating. I know the pain was real. And I was primed to reach for environmental issues. But reaching for answers in the minutiae of nothing is not useful.

 

[jrmichler]

But the OP has still refused to conduct my single-blind experiment.

When I see something like this, I start to wonder if the OP has a real problem, or is playing games with us. People with real problems are willing to check everything, even those things that appear to be obvious. People who are serious about solving a difficult problem will follow the advice given earlier about documentation, then show the document.

I suggest one more test. Modify a monitor so that the problem is minimized. Then have somebody else randomly switch between that monitor and a second monitor. Both monitors must be identical in appearance, the switching must be random, and the switching must be done out of your sight. The switching MUST be done such that you have no idea which monitor you are looking at. The test needs to be repeated several dozen times, and the result of each switch, or fake switch, recorded. And you must go into the test with an open mind.

 

[informerkh]

I clearly see I've come to a wrong place with the question. The majority have no actual experience with tech (or complicated optics) to make any advices, aside from very basic advices about how to organize data collection.

The blind test was proposed solely to prove to the contributors of this discussion that the problem is real and there are no tricks and no placebo. This very idea seems very odd to me, as on tech forums people usually are aware about eyestrains of different kinds and do not start from 'Prove it is real' or 'It is placebo'. Does a blind test make a slightest contribution to my research? No, as the problem is most definitely real for the everyone experiencing it (I provided a link to another person who tried the same experiment few years ago, but it was ignored, as I see). The only use of a blind test for me is to prove that the problem exists to hardware manufacturers, and that could be done when we know what to look for, what to test in every case.

The document in Russian (which as I said, was created long ago): https://4pda.to/forum/index.php?showtopic=943228&view=findpost&p=110571086

Now if you could just find someone looking for a Masters Thesis subject, maybe you could talk them into researching the problem!

Tom, if such a chance will appear, I will use it, but it seems that it requires more skills and time than a Master Thesis, we spend a lot of time already and there's still a long road ahead.
Your suggestions were interesting and valuable, thank you for your inputs!

Everyone else, sorry for the bother. I'll ask somewhere else.

 

[berkeman]

Whelp, we tried our best. Thanks everybody for trying to help the OP to alleviate his issues.

Thread is now closed.