Cathode Ray Tubes (CRTs)

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Cathode Ray Tubes (CRTs)

Post  admin on Fri 23 Jul 2010, 2:33 am

Three electron guns are placed inside a vacuum in side the body of the CRT screen. The electron guns fire a series of electrons at a screen coated with phosphorus. The actual screen is coated in dots of phosphorous, also known as phosphor dots. As the electrons fired from the electron gun hit the screen, the phosphor dots will emit light according to which electron gun fired the electron. In a colour CRT display, there are three electron guns, which will each fire a different electron stream, red, green and blue. As the electrons travel through the vacuum of the CRT screen, the path of the electron is moved through the use of an electric field or through the use of magnets, which will direct the electrons to the appropriate part of the screen. The combination of the various colours and the position of the phosphor dots will create the final image which is displayed. An issue with CRT screens is the use of magnetic forces to direct the electrons to their position on the screen. This created problems with multimedia setups such as speakers, which contain magnets. Placing them too close to a CRT screen caused the CRT screen to display images incorrectly as the magnetic interference from the speaker would push electrons out of their path.

As phosphor dots are used to display images on a CRT screen, they will only illuminate for a short period of time before dimming. This requires that the electron beam fired from the electron guns to constantly move across the screen many times a second to maintain the image. Different techniques can be used to maintain an image on a screen, such as raster scans and refreshing. Both utilise the high speed scanning frequency from the electron guns to maintain the image. The ‘refresh rate’ is the number of times the screen is scanned to maintain an image expressed in Hz. A refresh rate of 120Hz means that the electron gun is refreshing the screen 120 times a second.


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