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Why does my monitor have 1/2/3 faint horizontal lines on it?

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Your monitor likely uses a Sony Trinitron picture tube. Trinitron 1 tubes can be recognized because they are curved only in the horizontal plane, but are flat vertically. Typically, the number of lines seen depends on the monitor size:
< 17" 1 line
17-21" 2 lines
> 21" 3 lines

Because of the technical nature of how CRT's work, few people understand the details of how they operate. As such, many laymen have viewed Sony's Trinitron design as being a proprietary black box because they don't understand Sony's technical documents. A fairly well accepted description of the way these tubes work follows.

For a description of how a standard shadow mask CRT works, see "What is a shadow mask?".

Sony's Trinitron design uses a variation of the shadow mask called the aperture grill (or guard grill). Rows of very fine metal strips run vertically down the screen, separating columns of coloured phosphor which are arranged in alternating stripes of red, green and blue. This configuration allows the phosphor strips to be placed closer together than conventional dot triads, and the fine vertical wires block less of the electron beam than traditional shadow masks, resulting in a brighter image, and less thermal buildup and distortion. Coupled with changes in the way that the electron guns are arranged, this design results in a crisp, bright image. However, the vertical strips are so fine that they can be set into motion when contacted by the electron beam (thermal changes) . This would result in a shimmer on the screen which would be quite distracting. To remedy this, Sony puts horizontal stabilizing wires across the vertical ones. This reduces shimmer, but results in one or more fine horizontal lines being visible on the monitor. As mentioned above, the number of lines increases with monitor size. Usually, these lines are only visible to a discerning viewer when looking at a bright, solid background. If you're an experimentalist, try gently smacking the side of a Trinitron monitor, and look to see the wires shimmer. Basically, the horizontal lines are a minor trade-off when compared to the superior brightness and vertical flatness of the Trinitron screen.

What follows is a statement from Sony about their aperture grille design:

Since its introduction in 1968, Sony has produced more than 70 million CRT's. Award-winning Trinitron CRT's are used in a multitude of applications, including high resolution displays for the computer industry. Today, Trinitron CRT's are used by Sony and other leading manufacturers to meet the ever increasing demands and expectations of computer users throughout the world.

One of the unique features of the Trinitron CRT is what is called the Aperture Grille. An Aperture Grille consists of a series of long vertical slits fastened with strong vertical tension to a steel supporting frame. Electron beams pass through the Aperture Grill to illuminate phosphor on the faceplate. The vertical tension of the Aperture Grille absorbs any thermal expansion, thus eliminating the problem of doming or color spill and resulting in a superior picture quality.

Since the CRT requires a vacuum to function, a damper wire which is approximately 15 microns in diameter is strategically placed on the Aperture Grille to reduce susceptibility to resonance. The "line" that some customers see on the screen is not a fault but the damper wire which has always been an integral part of our Trinitron technology.

We hope that our customers will continue to consider the overall attributes and excellence of the Trinitron system when evaluating our products.
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