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LCD V CRT - part 1 of 1

by Mike McNamee Published 01/02/2005

BASICS

There are two basic technologies available for making a computer monitor. The older of the two is the Cathode Ray Tube (CRT) and the new kid on the block is the Thin Film Transistor (TFT). Within the TFT umbrella there are a number of sub technologies with fancier names than a women's shampoo advert. By comparison, CRT is simple, you get either Shadow Mask or Aperture Grill.

CRT has been around the longest and was used for the very first television sets which received a boost in popularity for the coronation of Queen Elizabeth II. CRT is bulkier, cheaper, heavier, hotter and less energy efficient. However it is stable, accurate and well understood and at the top end, such as the Sony Artisan or Barco Reference, unsurpassed by anything new.

Both CRT and TFT work in fundamentally the same way. A light "source" is used to light up red, green or blue pixels and the eye/brain fuses the dots into an image. The dots can take the form of circles, clusters of rectangles or lines.The CRT illuminates by scanning a beam of electrons over the inner surface of the tube, which is coated with phosphors. These glow under the beam and are made to behave themselves with the shadow mask or aperture grill. TFT is quite different. The light (which is a backlight consisting of a fluorescent tube) shines continuously onto a crossed pair of polarisers, which separate special bi-polar chemicals. The light passes through the first polariser but is trapped by the second because the light is by then polarised itself. The bipolar chemicals (also known as nematic twisted pairs) alter their optical properties if a small electrical current is applied to them. This causes them to "stand to attention" and allow the polarised light passage onto a red, green or blue filter, which then "lights up" that pixel. Under the magnifying glass, the TFT and CRT screens look quite similar.

Here the similarities start to end! The subtleties in the way the pixels are illuminated have profound knock on effects. Colour accuracy is of vital importance to our business and so we shall concentrate on the effects that TFT has on those parameters.

From the standpoint of colour precision the problems to date with TFT have been:

1. A dramatic loss of contrast and saturation when viewed off axis.


2. An inability to adjust the effective colour temperature of the back light itself.

3. An inherent response of output versus input, which is not the same as a CRT. A CRT can be adjusted on a simple gamma correction because this response is simple (a bit like a ski slope). By comparison the TFT has an s-shaped curve with an added complication of the blue response not quite matching the other two. This makes correcting the response with simple gamma tools (e.g. Adobe Gamma) more difficult.

4. A loss of light output over a period of time, particularly if the higher luminance of the TFT is employed - one solution is that taken by NEC in our review design, instead of grabbing the whole of the 270 cd/m2 available the tubes are set to run at 160 to 170 cd/m2.

The subject of this review, the NEC/Mitsubishi 1980SXi SpectraView claims to have solved the problems above - will they be able to change or minds about LCDs?

Other issues

Before you get carried away with the claimed solution of the colour issues you ought to consider some of the others as well.The TFT is a digital rather then analogue device and so it is optimised for discrete levels of screen resolution. In general if you move a TFT away from its designed resolution you pay a higher price in loss of clarity than you would with a CRT. This varies from model to model but by way of example, the reviewer's laptop is optimised at 1440x1050 pixel resolution. It is almost unusable at other resolutions as the numbers in the palettes become difficult to read. On the system upon which this magazine is set out, a 22" LaCie CRT is the main calibrated screen and is run at 1600x1200 pixels. An inexpensive TFT is used to hold the palettes and this will not run above 1024x768 resolution. This means that any mouse pointer movements across the main screen, lower than 768 pixels down are lost and cannot find the second screen until the mouse is pushed upwards. This is not too much of a problem as the bonus of the lower resolution on the palette screen is that the palettes are physically larger and so is their text. However, if you are going to use more than one screen look for a TFT with a thin surround - skip the built in side speakers! All of these considerations are irrelevant if you only use one screen.

If you are destined to use only a single screen, be sure you are happy with the resolution that your chosen LCD is capable of before you unsheathe the credit card! The notion of using two large CRT screen is an interesting one. However calculate your space and load bearing before you haul them onto your desk, a 23" CRT is a two-person job to move about for all but weightlifters. Space aside, it is an interesting notion because you could afford a couple of CRT's for the cost of a top end TFT at the time of writing.

The Up Side

In the interests of balance the upsides of TFTs should be highlighted. As well as looking cooler on your desk, they actually run cooler and have no emissions. In addition, their ability to run at higher screen luminance's means that you can comfortably work with brighter office lights should you choose to risk your bulb life. Many people claim that TFTs are crisper, which is not a view that the author subscribes to; at close working distances the pixel structure is visible on a TFT.


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1st Published 01/02/2005
last update 30/04/2014 16:20:53

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