My girlfriend asked me today why her 15" computer monitor looks about the same size as her 13" tv.

My understanding was that with tv's, the size of the tv is just the measurement in inches from one corner of the viewable area to the opposite corner. I think this also is true of lcd monitors. A 17" lcd monitor actually has a diagonal length of 17" as I understand it. So why is it that crt monitors dont follow this rule. Do they use a different measuring system for them and if so, what is it? Why do crt monitors measure about 2 inches shorter than they are actually quoted as? For example, why is my girlfriends 15" monitor the same size as her 13" tv?

Both CRT monitors and CRT TVs, in the UK at least, are measured from the corners of the tube, rather than from the corners of the viewable area.

It so her 15" monitor should really look the same size as a 15" TV.

Both CRT monitors and CRT TVs, in the UK at least, are measured from the corners of the tube, rather than from the corners of the viewable area.

IIRC that used to be the case, but after getting slapped around by the Advertising Standards people, monitor manufacturers agreed to quote visible area. So it may just be that the "15in" monitor is older than the "13in" television -- old enough to be before the rules changed.

Peter

As far I as I can see, from looking at a few monitor brochures they still quote the tube size as the headline figure. They do mention the visible size though, so a they say things like '17-inch (16.0" VIS)'. The model numbers still appear to include the tube size as well.

Other people referred to it, but to state it clearly, TVs are measured from the corners of the picture while monitors are measured from the corners of the tube. Monitor manufacturers now list both numbers.

Exactly why that's the case, I don't know. My guess is that the viewable area on a TV goes much further into the corners of the tube than on a monitor (as huge corner convergence issues on a TV are commonplace but would be significant flaws on monitors) and that the dimension difference between tubes and viewable areas on a TV is insignificant. In fact, it may be that the basic tube for a 15" TV and a 15" monitor is exactly the same, with more of the monitor's tube ``blacked out'' than the TV's. I haven't looked at a bare TV tube in a long time to verify that, though.

My guess is that the viewable area on a TV goes much further into the corners of the tube than on a monitor

That may be true, but I've seen several TV's that differ in viewable area. One Zenith in my house seems to have decidedly more plastic over the edges of the screen, enough so that station logos are right to the edge instead of spaced a bit.

Of course, it could just be that I have an older TV. I also don't know what size it proported to be.

One significant difference you'll notice between TVs and monitors that might play into this discrepancy is that TVs often (usually? always?) use plastic to cover areas of the tube that can display; monitors do not. That is, if you make the image bigger on a monitor, you acn see the picture getting cut off where there are no phosphors on the tube. If you make it bigger on a TV, the plastic casing will obscure the image first.

That may be true, but I've seen several TV's that differ in viewable area.

Yes. Set-top boxes and other gadgets whose user-interfaces are meant to be viewed on TV, have to restrict themselves to the "safe area" which means cutting off 10% of the full 4CIF (or QCIF, can never remember which is which) display resolution. IIRC the 10% is a very conservative figure, though, especially for modern TVs.

Peter

And a semi-related question. How are tv's with differen't aspect ratios measured? Do they still use the diagonal for those? a 42 inch diagonal on a wide screen tv would have a different screen area than a 42 inch diagonal on a normal aspect screen.

Other people referred to it, but to state it clearly, TVs are measured from the corners of the picture while monitors are measured from the corners of the tube.

They might be in the US. In the UK TVs are also measured from the corners of the tube, my 32" screen has a 29.5" viewable area.

For example if you take a look at:

http://products.sony.co.uk/productdetail.asp?id=2_1_2573

You will see Sony taking about their KD36NX200 36" TV which has a 34.6" visible area.

Correct.