How a CRT Works

A normal, old-fashioned TV like the kind that has been around since the 1920s is called a CRT television, or a Cathode Ray Tube TV. If you took it apart and looked inside, what you would find is a big glass tube shaped like a funnel. At the narrow end of the funnel there is a thing called an electron gun. At the wide end of the funnel, there is a flat piece of glass where the image appears. This big flat piece of glass is what you are looking at when you watch TV.

On the inside of the tube at the big end, there is a layer of thousands of tiny phosphor dots. A phosphor is any chemical that will absorb energy of one kind and turn it into light. In the case of a CRT TV, the phosphor absorbs energy from the electron beam that hits it and turns that energy into visible light that we see on the screen. The phosphor dots are grouped together in threes, with one phosphor dot that is red, one that is green and one that is blue. The three dots combine together to make one point of light, or pixel, that you see on the screen. Three dots of phosphor make one pixel. The camera tells the TV what color each pixel should be and how bright it should shine. The electron gun shoots out three electron beams. The beams aim at one pixel on the screen. One beam will hit the green phosphor dot, one will hit the red dot and one will hit the blue dot of the pixel.

Then, back toward the narrow end of the funnel, there are coils that can move the beams around on the screen. The coils can move the electron beams left and right, up and down, and hit any pixel on the screen. So the coils steer the beams across the screen dot by dot, and down the screen line by line. The electron beams exactly mimic the scanning in the camera, so they create the exact same image the camera sees. At the end of the image, when the TV gets the end-of-image signal from the camera, the beams start back at the top of the screen and paint the next image. This happens 30 times a second. You see a moving image.

Making any color from red, green and blue phosphor... If you have three dots of phosphor, and one is red, one is green and one is blue, you have a pixel. A pixel can make any color of light on the screen. If you mix red, green and blue light equally, they blend to make white light. If you turn off all three dots, you get black. If you light up only the green phosphor, you get green light, and so on. The strength of the electron beam controls how bright the phosphor shines.