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Creating Colour in LCD Displays

In order to create the shades required for a full-colour display, there have to be some intermediate levels of brightness between all-light and no-light passing through. The varying levels of brightness required to create a full-colour display is achieved by changing the strength of the voltage applied to the crystals. The liquid crystals in fact untwist at a speed directly proportional to the strength of the voltage, thereby allowing the amount of light passing through to be controlled.

In practice, though, the voltage variation of LCDs can only be achieved at a slow speed, perhaps as slow as 25ms. This may be adequate for mainly static screen applications such as office or web browsing, but leads to motion blurring on videos and games. To combat this, manufacturers reduce the screen colours from 8 bits to 6 bits per colour per pixel – from 24 bit true colour to 18 bit colour. Although this might not seem like much, it’s actually a massive drop in the number of colours that can be accurately represented on a screen.

In a true-colour monitor there are 24 bits of colour data per pixel, divided into three groups of 8 bits each giving 256 possible colour shades for the colours red, green and blue. This is known as RGB colouring, the basis of a large part of computer colour theory and practice. When blended together this system gives an enormous colour range, with 256 x 256 x 256 = 16,777,216 possible colours per pixel. If, on the other hand, the colour depth is reduced to 18 bits, with only 6-bits for the red, green and blue hues, there are now only 64 different possible shades per element. This results in 6 bit colour LCDs delivering a maximum of 64 x 64 x 64 = 262,144 colours, a massive loss of around sixteen and a half million colours!

However, there are two major advantages to losing all these colours is a much increased response time, considerably reducing motion blur. The colours that are lost are imitated through a form of dithering, and also a technique called Frame Rate Control (FRC) which displays alternate shades on successive frame refreshes to average to the desired colour in the human eye. This can produce adequate colour information for perception to be fooled, except where the difference is too great as flicker may then be seen. The second advantage is simply cost: 6 bit LCD monitors are far cheaper to produce.

As multimedia applications have become more widespread the lack of true 24-bit colour on LCD monitors remains an issue. More businesses and home users are using more demanding colour applications involving photographic or video work, CAD or DTP, so they have demanded more from their monitors.

The screen response times in LCD monitors has come down considerably, to a great extent relieving many of the ghosting and motion trail issues. However, true 8 bit colour in flat panel monitors remains hugely expensive, and only available in very high end LCD monitors. On the next page, we’ll see how TFT LCD monitors made a considerable difference to colour representation in affordable LCD monitors.

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