 |
| Teravolt at English Wikipedia, CC BY 3.0 via Wikimedia Commons. |
These were used in TVs until cathode ray tubes (CRTs) were replaced by plasma and then LED displays. CRTs use an electron beam that traces out an image line by line on a glass screen, starting at the top left of the screen and working downwards. The path followed by the beam is just like how we read text on a page. A flyback transformer is used to produce a high voltage, in the order of tens of kilovolts, to accelerate the beam and make it strike a phosphor coating, the latter emitting light on impact. Only one point on the screen is illuminated at any one time, hence the bright spot at the centre of the screen when old TVs were powered down and the scanning coils were turned off. Persistence of vision and the 25 Hz scanning rate (50 Hz in total because of interlacing) give the impression that a complete image is displayed all at once.