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.
