This is the tube we call “screen” on a non-LCD video monitor. At the opposite side of the screen there are three electron cannons, one for each video primary color: red, green and blue. As you know, electrons are invisible, so these cannons pump electrons towards a phosphorus mask dyed on the corresponding color. These masks are located behind the screen glass you normally see as “screen”. When the electron beam hit the mask, the impact point starts to glow, turning it “on” – i.e., making it visible. Depending on the intensity of the electron beam the point can glow more or less, enabling different shades of the corresponding color.
The CRT can make other colors besides red, green and blue by “mixing” them. Since these points are very tiny and very close to each other, your eyes will see each group of three points (called triad) as a single point. This single point is called pixel.
Monochromatic video monitors have only one electron cannon and one phosphorus mask, hence their inability to show more than one color.
