Shadow Mask Definition

Monitors work by aiming a beam of electrons at a blob of phosphor, which in turn glows. This glow is what we perceive as a pixel on the screen. Your standard colour monitor has three dots (dot triad) at each location on the screen; red, green and blue. There is a corresponding electron gun for each colour which emits an electron beam of varying intensity - this corresponds to colour brightness. To ensure that the electrons from each gun strike the corresponding phosphor, a 'shadow mask' is used. Because the three electron beams arrive at slightly different angles (from the three separate electron guns), it is possible to construct and align the shadow mask such that the electron beam from one gun will strike the correct phosphor dot, but the other two phosphors will be in shadow. This way, the intensity of red, green and blue can be separately controlled at each dot triad location. The shadow mask is usually an invar mask (64% iron & 36% nickel) which is a thin plate with small holes punched in it. Only about 20-30% of the electron beam actually passes through the holes in the mask and hits the screen phosphor, so the rest of the energy is dissipated as heat from the mask. As a result, shadow mask monitors are prone to colour purity problems as they heat up due to slight shifts in the position of the holes relative to the phosphor dots. Shadow masks - or their equivalent - have made mass production of CRT's possible.