Colour receiver tubes
CRTs used for colour display have three separate guns, one for each pri- mary colour arranged to bombard a screen which is coated with three different types of phosphors, one for each primary colour. The three phosphors are either grouped in triangular delta (3-colour triad) or in-line formation. When a group of triads is caused to glow at proportioned colour intensities, the eye is deceived into perceiving that the primary colours occur at the same point and so it discerns a spot or a dot corre- sponding to the mix of the three colour intensities.
Typical voltages for colour tubes are considerably higher than their mono- chrome counterparts with the final anode voltage in the region of 30 kV.
Purity
For correct colour reproduction, the red, green and blue beams must strike only their own particular phosphors and no other. This is known as purity. Purity adjustment involves changing the magnetic field formed by the
scan coils both in terms of strength and direction to move the beams so that they strike the correct phosphor coating of the screen. This is achieved by the use of a pair of two-pole ring magnets placed along the neck of the tube and a rotating disk magnet as shown in Figure 9.8.