From: brett@jericho.uucp (Brett Cheng)
Subject: Re: Pixel line on 14" Color Monitor on Purpose? Help!
Summary: correction & explanation of shadow mask/aperature grille
Keywords: shadow mask aperature grille Trinitron
Organization: S-MOS Systems, Inc. (Vancouver Design Center)
Distribution: comp.sys.mac.hardware
Date: Fri, 8 Jan 1993 18:10:32 GMT

In article <C0FvH4.795@usenet.ucs.indiana.edu> jgraham@bronze.ucs.indiana.edu (the End) writes:
>
>This faint line is the price you pay for the excellent resolution. The 
>grid of wires that creates the "mask" for .26 mm resolution run vertically
>and this is the supporting "crossbar" that keeps them from a-janglin.

This is basically correct...

>A few companies have eliminated this minor annoyance by incorporating a 
>new material called "Invar" into the traditional shadow mask apperature
>grill used on all the lower resolution monitors (NEC, ect.). Although
>this new material achieves a finer grid, and thereby a comparable dot-pitch

This statement above is not really correct...

Some corrections and further info:

Firstly, "shadow mask" and "aperature grille" actually refer to 
two different technologies used for color CRTs.

A shadow mask refers to a thin sheet of metal with tiny holes or slots
whereas an aperature grille consists of vertical wires held in tension.
The purpose of either type of mask/grille is to block the electron beams
so that the correct color phosphor dot (R, G or B) is illuminated.
Something like 80% of the electrons are blocked by the mask.

Because of this, one problem in shadow mask CRTs is the heating of the
mask by the electron beam, causing expansion which shifts the whole mask, 
thereby shifting the position of the holes so the beam does not strike
the correct phosphor. Various mechanical/thermal compensation 
schemes are used to compensate this effect of expansion of the whole mask.

Another expansion phenomenon is called "doming" - this refers to 
small areas of the mask expanding due to heating from the concentrated
beam, also resulting in shifting of the holes. To compensate for this
some CRTs use a material called INVAR which has a small coefficient
of thermal expansion. This minimizes the doming problem. I think
that invar is relatively expensive, so its not used in all CRTs.

A Trinitron tube uses an aperature grille, which because of the thin
vertical wires held in tension, avoids the doming problem.
A side benefit of this is that the screen face is vertically flat.
The thin horizontal is indeed used to stabilize the wires in the horizontal
direction. As noted in previous posts, large Trinitron tubes have
another horizontal wire in the top half of the screen.
Another advantage of the aperature grille is higher transmissivity
of the beam to the phosphor.

So, the statement from the previous post that:
 >A few companies have eliminated this minor annoyance by incorporating a 
 >new material called "Invar" into the traditional shadow mask apperature
 >grill used on all the lower resolution monitors (NEC, ect.). Although
 >this new material achieves a finer grid, and thereby a comparable dot-pitch
is incorrect. i.e. Invar does not eliminate this horizontal wire
because it never existed in shadow mask tubes - only in Trinitron ones! 
I don't think that Invar in itself provides a finer grid...
Rather, the manufacturers using shadow mask technology
are now using new materials to improve an inherent problem with that 
technology (ie doming). Perhaps by reducing the doming problem it is
possible to use finer dot pitches in a shadow mask.

Hope this clears things up.

BC

-- 
Brett Cheng
uunet!jericho!brett    brett%jericho@uunet.uu.net
Newsgroups: comp.sys.mac.hardware