Anatomy of an LCD screen

Have you always wondered what’s in an LCD screen? Reading on the web, there are a lot of different ideas on what it is made up of. As part of what I do I replace LCD screens in laptops, when it is worthwhile. So around the office I have a few “cracked” screens. A few customers have said “the screen isn’t cracked, but there are big lines and black parts radiating out like cracks”, and right enough the actual surface of the screen isn’t cracked, but the appearance is definitely that of something cracked!

So, I took one apart! And I found no less than 11 layers in the screen I took apart, and a touch sensitive screen will have at least one more layer over that probably.

1. Starting at the back there is a plain white plastic panel which is fitted in the aluminium frame of the screen, pretty securely. It serves no purpose other than protecting the rear elements of the screen, and is somewhere to stick all the part number labels, and the ribbon connectors. It is also a sort of reflector for the LED lighting. The frame is what all the various layers sit in, and what is screwed into the lid of the laptop.

2. In front of that is a clear plastic panel. One edge of this panel sits on a row of 18 light emitting diodes embedded in one edge of the screens frame. The light from the LEDs will travel through clear plastic, and because of internal refraction, will come out of the front and rear of the plastic sheet, reflecting forward off the backing sheet.

3. Next is a translucent piece of film, which may some purpose, but looks like a diffuser to me, just to spread the light from the LEDs more evenly.

4 & 5. Two layers of plastic film with a prismatic lens effect, probably just more diffusion (layer 3 may have between between 4 & 5, all three layers may be just to spread the light better).

6. Another slightly more translucent film with a black edge, probably to stop stray lights getting through to the business layer.

Next comes the business layer that displays the screen content, that the lighting system behind shines through. It consists of 5 layers.

7 & 11 are thick adhesive plastic films, which are very tough and transparent, the main purpose of which seems to be to protect the user from shards of broken glass should the screen break (think laminated windscreen!).

8 & 10 – two thin sheets of glass, to protect and stabilize layer 9, so that it does not get damaged by bending or the cat sitting on it. These are the bits that crack, but because they are under the film, are rarely seen. These glass sheets may be polarised or contain colour filters. Cracked glass will damage the electronic components sandwiched between the glass sheets, hence the black areas on a cracked screen, presumably.

9 – the bit that does the work, a thin sheet of transistors and liquid crystal devices and other electronic wizardry, if you look closely when a piece of supporting glass is removed you can just see the matrix of pixels with the naked eye. Attached to this thin film are ribbon cables that connect it to the circuit board that controls the screen.

All 11 layers are located in the aluminium frame, and held together by silver sticky tape! A few self adhesive strips help to locate some layers internally, and there are locating pegs and holes to line up some of the various pieces of film, which suggests that at least some of them may be polarising screens or colour filters, but that isn’t apparent to the naked eye.

If I were a bit more artistic I would draw a picture for clarity, but I hope this written description satisfies someones curiosity!

It’s worth pointing out that this was a 10″ netbook screen, so may be a little simpler. Looking at a 15.6″ widescreen (without breaking it down) I see that the LEDs are housed in the two sides of the screen, and that the aluminium frame is on the front not the back of the screen. Also some screens have a different form of lighting (not LED) so may be different.

Posted in Computer Stuff

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