How is a Television Screen Measured?
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Basics
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All television screens are measured by their diagonal length: the distance from the upper left corner to the lower right corner and vice versa. That's the big number you see on the package or in the catalog.
It's an advertising ploy: the diagonal length is always bigger than either the height or the horizontal length, which makes the TV sound is big as it can possibly be. It worked perfectly well in the past because all TVs used to be the same aspect ratio (4:3) no matter how large or small the TV was. With HD flat screens, however, it's caused a bit of a problem. Many HD TVs have screens with a much wider ratio--typically 16:9--in order to accommodate widescreen movies and similar programming. That can cause confusion when measuring for a television screen.
When Big Screen Isn't Big Screen
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A TV screen with a more rectangular shape will naturally have a wider diagonal length than one with a more square shape. Unfortunately, it doesn't mean that the screen itself will be any bigger. In fact, an older 4:3 ratio TV has a larger screen size than an HD TV of the same size. If you have an old 25-inch tube TV and you replace it with a 25-inch HD TV, you're actually getting less screen space than you did before. This becomes particularly apparent whenever you view an older TV show or movie shot in the 4:3 ratio. The widescreen TV places a pair of black bars on either side of the image, to keep it in its original shape so that it doesn't look distorted. The difference in overall size is thus very noticeable and can be extremely frustrating if you just paid a lot of money for a brand new widescreen TV.
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Measure Height, Not Diagonal Length
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The easiest solution to properly measuring the size of a TV is to go by vertical height, not diagonal length. Every TV lists its screen height along with other pertinent data in its owner's manual. If you're swapping out an old 4:3 ratio TV for a new HD TV, take a tape measure and gauge its height, then ask the salesman at the store to let you know what the vertical height is for any HD TV you want to purchase. Alternately, simply multiply the diagonal length of the older 4:3 TV by 1.22. The result is roughly the diagonal length of the new 16:9 TV needs to be in order to provide the same size image every time. Shows filmed in 4:3 will look exactly the same and shows in 16:9 will look a little bigger.
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