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Phil Allison[_2_]

There are two things present that have to be understood. There is
convergence, which is making sure the 3 electron beams hit the same
spot/area at the same time. There is purity, which is making sure the
3 beams hit their respective phosphors.

Let us assume that for some reason, all three electron guns come in at
an incorrect angle thru the shadow mask or grid or screen. The blue
electron gun hits 50% on the blue phosphor and 25% on the red and 25%
on the green phosphor. The red gun hits 50% on the red phosphor and
25% on the blue and 25% on the green phosphor. The green gun hits 50%
on the green phosphor and 25% on the red and blue phosphors. All
three phosphors are illuminated at 100%, so only differing electron
beam strengths due to compensating for differing phosphor efficiencies
will be noticeable in any color shading of white and gray areas of the
picture.

Assuing that the convergence of the three images is correct, and only
purity is bad, it will take a trained eye to tell that there is a
purity problem on a B+W picture. But, as soon as a color picture is
presented, colors will be noticeably "off".

** Sounds correct.

When adjusting the " purity magnets " on a conventional CRT, one sets the
test pattern to red and tries to get a even red colour all over the tube
face.

When switched to give a white raster - slight colorations may still appear
in odd spots. These will vary with the orientation of the set too, due to
the earth's mag field, meaning that final adjustment may need to be done in
situ.

.... Phil