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| Typical Number in Hospital: 3 | Cost Bands: 3 | References: 3 |
A graph, or a complete picture can be built up on a special CRT screen which retains the image as electrostatic charge on a fine insulating mesh behind the screen. When the whole screen is illuminated by low-velocity electrons from a 'flood gun' the electrostatic mesh forms a mask so that the original image can be seen on the screen.
These CRTs have been used extensively in electronic test instruments and in ultrasonic B-scanners because of the ease with which the image can be stored and erased. The main drawback (for B-scanning at least) is that the display is bi-stable, so that each point on the screen is either blank or at maximum brightness. This is only suitable for line drawing since no 'grey scale' is possible. Many bi-stable CRTs can also be driven in 'variable persistence' mode which does allow some modulation of the brightness, but the image is continually fading. The variable persistence facility is useful during the setting-up procedure for some tests.
It is expected that high-speed computer graphics memories, in conjunction with conventional short-persistence CRTs or TV- type monitors, will supplant storage CRTs.
Content and Design Copyright 2000 Dr. Malcolm C Brown. See Title Page for more details