Transparent LCD
Reuse old LCD screens
Almost a decade ago I made my first own transparent LCD. The Images are lost to time but the process is straightforward. Remove the LCD from its casing, remove the front polarizer, install it into a box with plenty of lights inside, and that’s it. You have a transparent LCD. The color accuracy is not good but as always that’s the compromise.
The idea was to use it as an advertising box but a wrench fell (literally, a wrench) on it and broke the screen. Since then I have not returned to this project as even then it was overcrowded with such boxes. At that time I just wanted to see just how difficult was to repurpose an old LCD as a transparent LCD. The answer is that it’s not that difficult.
A 32’’ LCD can be found for free at a scrapyard or for about $100 tested working. It is also important to note that most LCDs end up at landfills due to broken backlight, the same thing we are removing to make LCD transparent.
Recently I found a video on Youtube that shows the process perfectly, and its use of it is also interesting.
All liquid crystal displays (LCD) operate on the principle of being able to 'twist' polarized light as it passes through a 'nematic' liquid crystal. The orientation of each liquid crystal in a display is governed by an electric field applied to a transparent electrode, through an array of thin-film transistors (TFT). The liquid crystal is normally 'sandwiched' between two polarizing filters at 90 degrees to each other. Polarized light enters the back of the liquid crystal from the back-lit LED. When the nematic crystal is not energized, it 'twists' the polarized light by 90 degrees so that it passes through the second polarizing filter. When an electric field is applied to the liquid crystal, the light does not get twisted so gets blocked by the second polarizing filter.1
So, by removing the first polarizing layer we get light passing through the screen regardless of in which state the crystals are. This still leaves one polarizer. This is the backlight polarizer. The issue with polarizers is that they let only light of certain orientation which in practice means that to push light trough so you can see the other side clearly you need at least 50% more light in the box then normal.
What surprised me is that such transparent LCDs never really caught on for promotions. The reason is that such boxes are sold at a premium, and they should be but don’t need to be. Old LCD screens are more than suitable for repurposing as digital signage instead of them filling landfills. There is no need to buy a new screen and have them end up in today’s delivery nightmare. If multitouch is needed, that adds additional cost but even that could be removed with something better but less costly like LeapMotion. In today’s time, a way to interact with content on the screen at public venus contactless is much more preferred than touching a surface that countless others have touched.
There are “novel” methods of having a transparent screen but as with all things it comes at a compromise. One such technology is Electroluminescence where the phosphor is excited into glowing by electricity. This technology is mostly used in EL wires or flexible panels. Electroluminescence has been known for a long time, half a century. It is an interesting technology that will be covered in-depth in an additional post.
https://physics.stackexchange.com/questions/155391/what-happens-if-you-remove-the-polarization-filter-from-a-computer-monitor