Displays for Computers and PDAs

Rant on displays for computers, and replacements for LCDs.

LCD displays are basically a failure. You can't see them in the dark, and you can't see them in sunlight. They work just fine in office lighting (where you probably have a cheaper desktop machine anyhow), but not on the move. To get around this problem, they have to use power hungry backlighting. Notebook computers in particular don't really work on batteries. They just hold their breath until the next power point. This hasn't changed since the first commercial colour VGA LCDs in 1986.

It has taken a decade and a half for LCD construction costs to come down from out of sight, to now taking aim at an antique technology like the CRT monitor. I've been waiting to replace my CRT display with an LCD for the past six years, and unless the old monitor breaks, I'll probably be waiting a few years more. Why? Because LCDs are just about impossible to see in a sunlit room!

The most obvious display characteristics are the physical size of the display, and the next most obvious is the number of pixels. Dividing the pixel count by the size gives you the pixels per inch resolution. This ranges from about 70 to 150, with most being about 90 to 125 dpi. An old fashioned laser printer gives 300 dpi, or about 9 times the resolution.

There are two basic type of colour LCDs.

Passive matrix
There are lots of name variations, mostly including super twist nematic, or STN. Basically twisting polarised light when the charge on a column and a row of pixels is activated. Prone to slow refresh (so you lose the cursor). A dead technology for anyone who can afford the cost and power consumption of the active matrix display, it is basically only for situations where cost means everything.
Active matrix
Thin film transistor, or TFT. Each pixel is driven directly. Solves the problem of slow refresh, has a higher contrast, and colours are more saturated. It looks much better. It also costs more. It used to be much more, but now the difference is less.

There are three ways to organise the construction of the layers of an LCD display.

The backlight provides all the light seen by the user. You generally have a compact fluorescent tube at the end of a light guide behind the display. Typically used by notebook computers. Good indoors, utterly impossible to view outdoors.
Same as transmissive, except you put a partially reflective layer between the lightguide and the display. Backlight isn't as bright, but outdoors, some light is reflected back after passing through the display once. The maker adjusts the reflectivity to get the results they want.
Mirror layer behind the display. All light enters the display, and bounces back. For lighting indoors and at night, you put the lightguide on the front of the display, and sidelight from the front rather than the back. Not as efficient for indoor use (light goes through display twice, and frontlighting isn't as efficient as backlighting in the first place), and the geometry needs to be somewhat more precise.

The construction also determines how you organise to artificially light the display. Displays are normally backlit. Everyone recognises this isn't the brightest method. The light from a compact fluorescent tube (usually) comes from the side via a light guide, bounces off the back.

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