A shadow mask is a thin metal screen filled with very small holes. Three electron beams pass through the holes to focus on a single point on a CRT displays' phosphor surface. The shadow mask helps to control the electron beams so that the beams strike the correct phosphor at just the right intensity to create the desired colors and image on the display.
An alternative way to accomplish the same task is taken by some CRTs. Instead of using a shadow mask they use what is called an aperture grill. The aperture grill technology was first patented by Sony in the late 1960s under the Trinitron brand name, which the company carried over to its line of CRT computer monitors. Instead of a metal mesh, this type of tube uses many hundreds of fine metal strips that run vertically from the top of the screen surface to the bottom. However aperture grill monitors have 1 or 2 damper wires that appear at 1/3rd and 2/3rds of the screens' height running horizontally through the screen to prevent vibration. Trinitron tubes below 17in or so get away with one wire, while the larger model require two. This is in fact normal for this type of display, but it does bother some people who may think there is something wrong with their monitor.
Compared to a shadow mask design, aperture grill CRTs has some advantages. First, they allow more of the electron beam to pass through to the phosphor; as a result the overall picture is brighter and sharper. One more advantage is that aperture grill monitor is vertically flat because the strips are run straight from the top of the monitor to the bottom; this type of tube curves only horizontally. It is evident that this reduces glare and let us see a more pleasant and less distorted image.
Although many think that aperture grille technology produce superior images, since the 1990s there have been many advances in shadow mask and hybrid technologies that gave an opportunity to consider personal choice or specific application. Once you are a gamer or graphic designer you must appreciate the geometry of aperture grill monitors. The geometry of aperture grill monitors is significantly better than geometry of shadow mask. Usually you even need not tweaking or control the monitor so perfect the geometry. And it is very hard to have perfect geometry on a flat shadow mask monitor, even after tweaking.
On the other hand, shadow mask monitors are able to have much crisper text at the same dot pitch as aperture grill monitors. The thing that made CAD professionals refuse to use aperture grill monitors. Primarily, it sure looks like the shadow mask would have much better clarity. But, surprisingly, today's aperture grill monitors are very good, and very sharp.
Shadow masks were going the way of the dinosaur, because they let much less light through - and this gives the aperture grill the advantage of brightness and brilliant colors. However, the arrival of inexpensive liquid crystal display (LCD) monitors and other flat-screen designs now challenges both aperture grille and shadow mask CRTs'.