HUMAN-COMPUTER INTERACTION
SECOND EDITION
HUMAN-COMPUTER INTERACTION
SECOND EDITION
The random scan display, also known as the directed beam refresh, or vector display, works differently from the raster scan. Instead of scanning the whole screen sequentially and horizontally, the random scan draws the lines to be displayed directly. By updating the screen at at least 30 Hz to reduce flicker, the direct drawing of lines at any angle means that jaggies are not created, and higher resolutions are possible, up to 4096 ¥ 4096 pixels. Colour on such displays is achieved using beam penetration technology, and is generally of a poorer quality. Eyestrain and fatigue are still a problem, and these displays are more expensive than raster scan ones, so they are now only used in niche applications.
The DVST is used extensively as the display for an analog storage oscilloscope, which is probably the only place that these displays are used in any great numbers. They are similar in operation to the random scan CRT but the image is maintained by flood guns which have the advantage of producing a stable display with no flicker. The screen image can be incrementally updated but not selectively erased; removing items has to be done by redrawing the new image on a completely erased screen. The screens have a high resolution, typically about 4096 ¥ 3120 pixels, but suffer from low contrast, low brightness and a difficulty in displaying colour.
Most people who habitually use computers are aware that screens can often cause eyestrain and fatigue; this is usually due to flicker, poor legibility or low contrast. However, there have also been many concerns relating to the emission of radiation from screens. These can be categorized as follows:
The CRT is not the only screen technology in existence. If you have used a personal organizer or notebook computer, you will have seen the light, flat plastic screens, often in shades of blue/grey, but increasingly in colour. These displays utilize liquid crystal technology and are smaller, lighter and consume far less power than traditional CRTs. These are also commonly referred to as flat-panel displays. They have no radiation problems associated with them, and are matrix addressable, which means that individual pixels can be accessed without the need for scanning.
Similar in principle to the digital watch, a thin layer of liquid crystal is sandwiched between two glass plates. The top plate is transparent and polarized, whilst the bottom plate is reflective. External light passes through the top plate and is polarized, which means that it only oscillates in one direction. This then passes through the crystal, reflects off the bottom plate and back to the eye, and so that cell looks white. When a voltage is applied to the crystal, via the conducting glass plates, the crystal twists. This causes it to turn the plane of polarization of the incoming light, rotating it so that it cannot return through the top plate, making the activated cell look black. The LCD requires refreshing at the usual rates, but the relatively slow response of the crystal means that flicker is not usually noticeable. The low intensity of the light emitted from the screen, coupled with the reduced flicker, means that the LCD is less tiring to use than standard CRT ones, with reduced eyestrain.
This different technology can be used to replace the standard screen on a desktop computer, and this is beginning to happen to a small extent. However, the particular characteristics of compactness, light weight and low power consumption have meant that these screens have created a large niche in the computer market by monopolizing the notebook and portable computer systems side. The advent of these screens allowed small, light computers to be built, and these have created a large market that did not previously exist. Such computers, riding on the back of the technological wave, have opened up a different way of working for a number of people, who now have access to computers when away from the office, whether out on business or at home. The different manner of working has given rise to different forms of software; one example is an integrated package comprising the basics of the systems used at work, coupled with a communications module. Working in a different location on a smaller machine with different software obviously represents a different style of interaction and so once again we can see that differences in devices may alter the human--computer interaction considerably. The continued interest in the notebook computer market has fed back an investment in developing LCD screen technology, with supertwisted crystals increasing the viewing angle dramatically. Response times are also improving, which is necessary for even simple animation success such as blur-free cursor tracking. Colour LCD screens are more expensive and require more power, but are increasingly popular as they make it possible to have a complete multimedia system whilst on the move. Just as it is now virtually impossible to buy a traditional black and white monitor, it will be interesting to see whether colour LCD screens supersede greyscale by the time the third edition of this book is prepared.
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