|NotTaR of Television Sets : Color CRTs - shadow masks and aperture g..
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All color CRTs utilize a shadow mask or aperture grill a fraction of an
inch (1/2" typical) behind the phosphor screen to direct the electron beams
for the red, green, and blue video signals to the proper phosphor dots.
Since the electron beams for the R, G, and B phosphors originate from
slightly different positions (individual electron guns for each)
and thus arrive at slightly different angles, only the proper phosphors
are excited when the purity is properly adjusted and the necessary
magnetic field free region is maintained inside the CRT. Note that
purity determines that the correct video signal excites the
proper color while convergence determines the geometric
alignment of the 3 colors. Both are affected by magnetic fields.
Bad purity results in mottled or incorrect colors. Bad convergence
results in color fringing at edges of characters or graphics.
The shadow mask consists of a thin steel or InVar (a ferrous alloy)
with a fine array of holes - one for each trio of phosphor
dots - positioned about 1/2 inch behind the surface of the phosphor
screen. With most CRTs, the phosphors are arranged in triangular
formations called triads with each of the color dots at the apex
of the triangle. With many TVs and some monitors, they are
arranged as vertical slots with the phosphors for the 3 colors
next to one another.
An aperture grille, used exclusively in Sony Trinitrons (and now
their clones as well), replaces the shadow mask with an array of finely
tensioned vertical wires. Along with other characteristics of the
aperture grille approach, this permits a somewhat higher possible
brightness to be achieved and is more immune to other problems like
line induced moire and purity changes due to local heating causing
distortion of the shadow mask.
However, there are some disadvantages of the aperture grille design:
- weight - a heavy support structure must be provided for the tensioned
wires (like a piano frame).
- price (proportional to weight).
- always a cylindrical screen (this may be considered an advantage
depending on your preference.
- visible stabilizing wires which may be objectionable or unacceptable
for certain applications.
Apparently, there is no known way around the need to keep the fine
wires from vibrating or changing position due to mechanical shock
in high resolution tubes and thus all Trinitron monitors require
1, 2, or 3 stabilizing wires (depending on tube size) across the
screen which can be see as very fine lines on bright images. Some
people find these wires to be objectionable and for some critical
applications, they may be unacceptable (e.g., medical diagnosis).
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