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[ The PC Guide | Systems and Components Reference Guide | Monitors | CRT Characteristics ]

The Shadow Mask and Aperture Grill

The monitor is made up of millions of tiny red, green and blue phosphor dots that glow when struck by the electron beam that travels across the screen to create the visible image. The dots are extremely small and the beam is traveling very quickly in order to cover the screen fast enough to allow for a smooth and stable image without flicker.

To create a precise and crisp picture, it is necessary to make sure that the electron beam for each color strikes only the correct dots intended for use for that color. The normal way that this is done is by using a fine metal mesh called a shadow mask. The shadow mask is designed to the same shape as the surface of the CRT, and the electron beams shine through the mask. By carefully positioning the mask, the beams only strike the correct dots. The idea is similar to one way a sign can be made--a piece of paper is cut out with the shape of the letters and then laid on top of another surface. Then the paint is applied through the holes in the paper; the paper itself prevents the unwanted areas from being colored.

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. 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. These strips perform the same function as the shadow mask--they force the electron beam to illuminate only the correct parts of the screen. The most common type of tube using this design is Sony's popular Trinitron, which is used in many brands of monitors.

Compared to a shadow mask design, aperture grill CRTs have some advantages and one significant disadvantage. One advantage is that they allow more of the electron beam to pass through to the phosphor; this results in what many consider to be a brighter overall picture. Some also say that the picture on this type of monitor is sharper. Finally, because the strips are run straight from the top of the monitor to the bottom, this type of tube is flat vertically; it curves outward as you go from left to middle to right, but not as you go from top to middle to bottom. Most people find that this reduces glare and results in a more pleasant and less distorted image.

The major disadvantage of using the aperture grill is that a bunch of thin metal strips don't have the same physical stability as a metal sheet with holes in it (the shadow mask). This means that the metal strips can tend to vibrate. To correct this problem, one, two or three thin stabilizing wires are run horizontally across the screen--more are used for larger screens. These eliminate any problems with the metal strips moving around, however they cause an unfortunate side-effect: the appearance of very faint lines where the stabilizing wires are.

These lines are extremely faint and not usually noticeable unless you are looking for them, but cause a lot of controversy because many people don't understand what they are. Invariably, they display a full screen of white pixels one day and notice the line, and then think there is something wrong with their monitor. This is in fact normal for this type of display; it does bother some people and these should not use a CRT that uses an aperture grill.

Next: Image Quality Factors

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