Friday, February 04, 2005

How to reveal you are an idiot - part one of a series

The other day I was in a computer shop in Tottenham Court Road, and I overheard a customer explaining things about laptop screens to a person who he was shopping with.

"This one has an LCD screen, and this one has a TFT screen. See the difference?"

It is actually true that the terms "TFT" and "LCD" mean different things.

LCD means "Liquid Crystal Display", meaning that the each pixel works by having a voltage applied across a liquid crystal element, the opacity of the element varies in accordance with the voltage, and when a light is shone through it it appears light or dark depending on the voltage. (Colour displays work by having three crystal elements per pixel, each with a different coloured filter - one red, one green, one blue).

TFT means "Thin Film Transistor", meaning that there is a transistor attached to the back of each crystal element, which controls the voltage applied to that element. TFT is synonymous with "active matrix" when applied to LCD displays. TFT screens are sharper than old kinds of passive matrix LCD screen, and as a consequence passive matrix screens are no longer available on laptops.

Now it is also possible for Thin Film Transistors to be used to control other kinds of display besides LCDs. Some people are quite optimistic about a new display technology called Optical Electroluminescence (OEL), and prototype OEL displays generally do have the pixels controlled by Thin Film Transistors.

So, it is therefore technically true that not all LCD displays are TFTs, and also that not all TFTs are LCDs, but for practical purposes the two terms are synonymous, particularly when referring to laptop screens. So the person in the computer shop was doing a fine effort at demonstrating that he did not know anything.

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