3
$\begingroup$

In my office today, there was strong sunlight shining onto my LCD screen. I could see that in patches, the image became quite a bit darker than the rest. I could return the image to normal my pressing my hand on the area for a minute. This seemed to be a heatsinking effect.

The LCD screen was getting very hot. It was on the threshold of being painful to press my fingertips against it. With a thermometer with a blackened tip, I could measure around 45 degrees C, but the screen could have been higher.

My question is what happens to the screen to cause the dark patches when the screen gets hot?

$\endgroup$
  • $\begingroup$ The temperature allows the liquid crystal to randomize, destroying (overcoming) the alignment the applied electric field is trying to create. So, you go back to an 'off' pixel that is black (or mostly black with some residual alignment). $\endgroup$ – Jon Custer Sep 1 '16 at 16:45
  • $\begingroup$ @JonCuster Answer in answers, not comments - it wouldn't hurt to post one now. $\endgroup$ – wizzwizz4 Jun 28 '17 at 16:02
  • 1
    $\begingroup$ @wizzwizz4 I found a source backing up his explanation and posted an answer (since he seemed content to leave that as a comment for nine months) $\endgroup$ – Doktor J Jun 29 '17 at 5:10
3
$\begingroup$

Quoting from Vartech Systems, a supplier of rugged displays:

LC displays (LCD) have a well-defined isotropic or operating temperature limit, above which the actual liquid crystal molecules will lose their orientation and will assume a random orientation instead of ‘twisting’ through the light valve.

Basically, the crystal orientation in the liquid crystal gets randomized, and the effect is that light can't pass through, so the image is darkened. I suspect that as the temperature straddles that operating temperature limit you may get a dim image where some crystals are still more or less in alignment while others are randomized.

Reading a bit further on,

Isotropic conditions will cause positive image displays to become dark (see image below), while negative image LCD's become transparent. This is the Nematic-to-Isotropic Transition Temperature or NI Transition.

Most computer LCDs are positive image displays, and thus you wind up with your dark display.


The site goes on to note that temperatures above 100°C (212°F) can permanently damage the coating on LCD displays, though Samsung claims that storing your display at temperatures above 45°C (113°F) can damage it, so it's possible that Vartech's 100°C threshold is specifically a property of their ruggedized displays.

Vartech also mentions that as temperatures drop the viscosity of the LC increases, resulting in slower response times (which will first manifest as "ghosting", and further as very slow image updates/transitions, like a bad PowerPoint slideshow in slow-mo). The site doesn't make mention of whether excessively low temperatures can permanently damage a display, but the line

Low temp effects are usually reversible

suggests that it is at least theoretically possible for extremely low temperatures to cause irreversible damage.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.