CRT - Cathode Ray Tube
The "Cathode Ray Tube" is an earlier technology used in Computer monitors and Televisions. It works on the principle of 3 lights Red, green and blue which combine to give different pattern of colours. The CRT technology has now been superseded by TFT and LED screen.
How does CRT work
A CRT monitor may consists of a Cathode Ray Tube which is a vacuum tube containing one or more electron guns and a fluorescent screen. The electron gun generates a narrow beam of electrons, which strikes on a fluorescent screen to produce a tiny, bright visible spot. A typical monitor may contain 3 electron guns: one for each colour red, green and blue.