Today I learned that you can "screenshot" an LCD with an LCD - Page 1
Forum section: electronics / general

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: January 12, 2018, 02:00:26 am
content: While troubleshooting a shitty 7in "hd" TFT display that got rained on, I came across this strange behaviour, something I've never seen before:https://youtu.be/PzSb4MgI0B4I did not know that LCDs are photosensitive, the mind boggles...Anyone else seen something like this before? Its so strange.

author: TerraHertz
author_rank: Super Contributor
timestamp: January 12, 2018, 10:22:39 am
content: Not photosensitive. They are electrostatic field sensitive. The top display (which I presume is turned off?) is sensing the electrostatic field from the pixel drives of the display underneath. Causing the liquid crystal fluid molecules in the top display to line up and polarize-rotate light passing through (or not), just as they do when their own pixel-drive elements are running.Prediction: If you put a fine metal mesh between the displays, it won't do that.Nor will shining a bright torch beam on part of the display do anything.And... you are the chosen artwork.

author: iamdarkyoshi
author_rank: Frequent Contributor
timestamp: January 12, 2018, 04:39:17 pm
content: Quote from: TerraHertz on January 12, 2018, 10:22:39 amSadly the display fell off and the cable ripped, so I can't do any further testing.But the OLED display on my phone had the same outcome of putting a negative image into the LCD panel.The top display seems to be malfunctioning, or the driver was malfunctioning, as I wasn't able to get an image from it, I'm not sure what state of "on" the top display was. But without powering the driver, the panel was just transparent with no strange effects.They came from one of those generic 7in HDMI TFT displays, and it was in the rain, so I took it apart to dry out the backlight layers, and was using my phone to backlight the LCD panel to see if it was working.Thats when I discovered this.So I got out my macbook so I could use my phone to record video.

author: Red Squirrel
author_rank: Super Contributor
timestamp: January 14, 2018, 11:27:39 am
content: For some reason this reminds me of when I first discovered what happens when you put magnets close to a CRT. That was always fun to do, but not very good.Those Dell CRTs we had at school had a crazy degausser though. BURRRRRRRRrrrrr when you hit the button, or sometimes when turning it on. Kids will be kids, and we always had fun with that.

author: Halcyon
author_rank: Unknown
timestamp: January 16, 2018, 01:10:31 am
content: Quote from: Red Squirrel on January 14, 2018, 11:27:39 amThose Dell monitors worked well and copped some abuse. The Sony Trinitron monitors also had a beefy degaussing function that would go off with a loud thud. It did the job though.

author: BrianHG
author_rank: Super Contributor
timestamp: January 16, 2018, 03:58:11 am
content: If your screen is active matrix and you are holding the row and column voltage strobes at the edge of a bias point, like in the case of freezing your LCD controller, of course it will be optically snapshot an image. Remember, the thin fets gates at each pixel are slightly photo-voltaic... Just like using a cut open c-mos dram chip with an added lense as a camera, the transistors sitting at the edge of their bias voltages will shift charge when exposed to light.To fight this unwanted effect in active matrix displays, the pixels need to be continuously refreshed.