Lies, Damned Lies, and CRT Monitor Statistics
by Blake Linton Wilfong -- The Wondersmith!

I've got a conspiracy theory. CRT monitor manufacturers, who have already faced a class-action lawsuit for advertising picture tube sizes instead of viewable image sizes, are still exaggerating their claims. Worse, these exaggerations are detrimental to consumers' purchasing decisions. I'm about to spill the beans on a dirty little secret, so pay close attention.

I'm interested in all this because I'm starting to feel the need for a new monitor. The "14-inch" freebie that I got with my 4-year-old computer still works, but it's already annoyingly blurry at 800 x 600, and I'd like to bump up the resolution another notch to better handle basic Web browsing and critical applications like KaZaA. :-)

I'm sorely tempted by flat panel displays. Not only are they space-efficient, but they have crystal-clear--should I say liquid-crystal-clear?--displays, consume much less power, and generate less heat than conventional CRT monitors. People used to complain that flat panels had slow response times, creating blur or afterimages when playing games or watching video, but their speed has improved so much in recent years that this problem is now generally regarded as solved. Also, flat panels only operate well in their native resolution, which is usually 1024 x 768 for 15" models, and 1280 x 1024 for 17" and 18" models. I don't see much problem with that, as modern 3D graphics cards are quite capable of handling these resolutions without bogging down. There just isn't much reason to drop to lower resolutions nowadays. Finally, the prices of flat panel displays have plummeted lately, to the point that they're getting fairly competitive with CRTs, especially when you factor in the electricity savings.

No, the only major problem with today's flat panel displays would seem to be overall image size. Do you really want to view 1024 x 768 graphics on a wimpy little 15" screen? Sounds awful!

Well, take heart! I'm about to prove to you that 15" flat panel monitors have almost the same usable screen area as 17" CRT monitors. (And I've seen with my own eyes that a high-quality 17" CRT monitor can display 1024 x 768 beautifully.) CRT monitor manufacturers grossly exaggerate the size of their displays, whereas flat panel manufacturers don't.

If you've taken a close look at the fine print of any CRT monitor advertisements in recent years, you'll notice that they all specify a smaller "viewable image size". An ad for a so-called 17-inch CRT monitor typically includes the following parenthetical note: "(16 inch viewable image size)". The truth is that, unlike a flat panel display, a picture tube can't draw anything close to its edges, so the usable image size is smaller than the claimed monitor size. This discrepancy became the subject of a class-action suit several years ago, with the result that ads for CRT monitors now include the "viewable image size" specification.

But 16" is still significantly better than 15", right? Wrong, because even the smaller viewable image size is still exaggerated! We can prove this with some very simple calculations.

According to the Pythagorean Theorem, the square of the length of the longest side of a right triangle is equal to the squares of the lengths of its two shorter sides. The simplest case of this is the 3-4-5 right triangle, shown below:

If the shorter sides of a right triangle are lengths 3 and 4, then the longest side is length 5. As Pythagoras observed, 3 squared (9) plus 4 squared (16) equals 5 squared (25).

Computer displays happen to have the same shape as a 3-4-5 right triangle. That is, the ratio of a computer display's width to its height is 4 to 3. If you consider a computer display's width and height to be the short sides of a 3-4-5 triangle, then the computer display's diagonally-measured image size is the long side of that 3-4-5 triangle. We can use this fact to calculate width and height from a diagonal measure, or vice-versa. The ratio of a computer display's diagonal measure to its width is 5 to 4 (and to its height is 5 to 3).

So to get the diagonal measure of a computer display image, simply multiply its width by 5/4, or 1.25. (Or multiply its height by 5/3, or 1.66666...) To get the width from the diagonal measure, divide by 5/4, or 1.25. (Or to get the height, divide by 5/3 or 1.66666...) It's that simple.

Let's try this out on a flat panel display. The image width of most 15" flat panel displays is exactly 12 inches. Multiply that by 1.25 and you get 15, its diagonal measure! The image height is 9 inches. Multiply that by 1.66666... and you get 15 again! It works!

Now we should be able to figure out the width and height of a 17" CRT monitor's actual displayed image. Let's use a specific real CRT this time, the Viewsonic E70f. This is a very nice monitor; you can view its full specifications here. Bear in mind that I'm not singling out Viewsonic; their monitor specs just happened to be easy to find. OK, the E70f's viewable image size is stated to be 16.0 inches. Plug that into our formulas, and you'll find that the image width should be 12.8 inches, and its image height should be 9.6 inches. Let's check that against Viewsonic's claimed factory-set display area. They say the width is 310 millimeters and the height is 230 millimeters. (Why do they give diagonal measure in inches, but width and height in millimeters? This question is left as an exercise for the reader.) Multiply by 0.03937 to convert to inches, and you find that the width is approximately 12.2 inches and the height is about 9.1 inches. Huh? That's substantially smaller than we calculated from the 16.0 inch diagonal measure! Indeed, the diagonally measured viewable image size calculated from these figures is only about 15.2 inches. Something is rotten in the state of Denmark. (To be fair to Danes, I must point out that Viewsonic is actually headquartered in the state of California.)

Let's give Viewsonic the benefit of the doubt. They also state that the monitor's "Maximum Scan" is 320 millimeters by 240 millimeters "(dependent upon timeing [sic] used)". I assume this means that the factory default configuration for the E70f leaves a bit of room for error around the outer fringes of the display, and it may sometimes be possible for the user to manually squeeze a little more image size out of the CRT. Converting (and rounding liberally in Viewsonic's favor), we get 12.6 inches wide by 9.5 inches high. These figures predict a viewable image size of about 15.8 inches. So if you manually adjust your E70f monitor's settings, you might be able to get a viewable image size that, when rounded up to the nearest inch, matches Viewsonic's claims--though Viewsonic specifically states that you might not succeed. Though Viewsonic does have a rationale for its claims, in my opinion all CRT monitor manufacturers should specify viewable image size based on reliable factory defaults, not user-tweaked settings which might or might not actually work. Why should monitor makers' standards be less strict than those of Intel or AMD, who advertise the speed at which their processors are designed to function reliably and consistently, not the maximum speed to which they might be overclocked?

It seems likely that most users will leave the E70f at its factory settings, with a viewable image size of 15.2 inches. That's very close to the 15" viewable image size of a flat panel display. To think of this another way, the 310 millimeter width and 230 millimeter height mean that the E70f's image area is about 110.5 square inches. Meanwhile, the 12 inch width and 9 inch height of a 15" flat panel display provide an image area of exactly 108 square inches. The 17" CRT monitor's display area is thus only about 2.3% larger than the 15" flat panel's--an insignificant difference.

Moving up a notch, the 19" Viewsonic E90f has a claimed viewable image size of 18.0". But its factory default display area of 352 mm by 264 mm implies an actual viewable image size of about 17.3"--barely more than that of a 17" flat panel, and smaller than an 18.1" flat panel's.

Yup, there's definitely a flat panel display in my future.

 
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