CATHODE RAY TUBE

Typical Number in Hospital: 100

Cost Bands: 1,2

References: 6

The most common form of display device used for the presentation of high-speed or text data is the cathode ray tube (CRT). The device, first introduced widely in the 1930s, consists of a glass envelope containing a very high vacuum in which a narrow beam of electrons is directed towards a phosphorescent screen. The electron beam is steered by electrostatic or magnetic means to cause illumination of different parts of the screen. Its real value lies in the extremely high speed by which the bright spot can be steered to different parts of the screen.

There are also specialized types of CRT (storage CRTs) used to retain the track of the bright spot. These are used in some ultrasonic scanners to build up the picture. Other types of 'non-fade' displays utilize computer memories to store the image. Most CRTs for monitoring ECG or pressure waveforms used to have long-persistence phosphors so that the path of the spot could be seen the whole way across the screen as a 'comet's tail'. These have now been largely superseded by short- persistence tubes backed up by a relatively simple computer memory to retain the whole line. Thus several cycles of a waveform are viewed clearly and this can be 'frozen' for closer examination.