DIYbanter - View Single Post

Smarty

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.