On 6/4/2013 7:26 AM, Arfa Daily wrote:


"Smarty" wrote in message
...
On 6/3/2013 9:54 PM, wrote:
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.
Your chosen example that the 3 guns are mis-registered uniformly, and
that the phosphors are all illuminated to 100% is not realistic. The
differences in phosphor efficiencies and the necessary beam currents
to achieve comparable light outputs are, as you acknowledged, quite
different. More important is the fact that mis-registered beams are
not, in general, spilling their mis-directed energy to reach 100%
phosphor saturation during a black and white program (versus the full
white example you have chosen). In general they will be generating
beam currents on the average well below peak white and perhaps closer
to black. The instantaneous beam current for, let's say, the least
efficient phosphor's gun, will be mistakenly exciting the most
efficient neighboring phosphors at the same time as the exact
opposite is occurring for nearby areas of the screen owing to the
fact that the most efficient gun is simultaneously exciting the wrong
phosphor area with too little energy. If the proposed mechanism /
concept made sense theoretically, then a black and white picture
should not show colored regions due to magnetization at all.

The ability to discern color differences has as much to do with human
vision as it does with the pure physical radiation of visible light
from the phosphor surfaces, and my partial explanation is that humans
see color variations which are much more subtle at some frequencies
compared to others. White and shades of gray reveal less whereas some
color fields reveal more.

For purposes of our discussion, the esential ingredient of impurity
of color is the non-homogeniety of the electromagnetic field due to
uncontrolled magnetic influences arising from nearby magnetized areas
including the shadow mask. To the extent that we are talking about
pretty drastic purity issues causing large blotches of color, some
areas of the CRT will have gross errors due to landings which are at
the extremes of the convergence system (and thus more likely to be
visible) or beyond the intended raster limits. Recall that the
purity control of the CRT and degausser is intended to deal with the
extremely small and subtle effects of the Earth's magnetic field,
whereas abrupt failure of the degausser is likely to impart a much
bigger residual effect unless corrected. Also, the magnetic
distortion is, unlike the Earth's field, very localized and highly
non-linear across the CRT surface, making the assumption that
spoiling of the beams occurs in any uniform way highly unlikely.



All of which is jolly interesting. Pity it doesn't match the actual
facts, as have been repeatedly put to you by a number of people very
well qualified to comment, by virtue of the fact that collectively,
they spent a very long time time working on this stuff at nuts and
bolts level, and have probably seen every possibility of purity error
on every type of CRT and under every set of circumstances possible ...

No disagreement that the well qualified folks here have seen it all and
done it all in terms of tackling all sorts of purity problems. My
intention has been to understand what causes the characteristics which
have been described, and I think a frank answer is that none of us fully
understand it but can offer explanations which are way beyond adequate
for repair purposes.