On 2-D displays, such as computer monitors and TVs, the display size (or viewable image size or VIS) is the physical size of the area where pictures and videos are displayed. The size of a screen is usually described by the length of its diagonal, which is the distance between opposite corners. It is also sometimes called the physical image size to distinguish it from the "logical image size," which describes a screen's display resolution and is measured in pixels.
Optimal screen size and viewing distance
The TV image is composed of many lines of pixels. Ideally, the TV watcher sits far enough away from the screen that the individual lines merge into one solid image. The watcher may sit even farther away and still see a good picture, but it will be a smaller portion of their visual field. The resolution of the human eye (with 20/20 vision) is about one minute of arc. For full HDTV resolution, this one minute of arc implies that the TV watcher should sit 4 times the height of the screen away. So the ideal set size can be determined from the chart below by measuring the distance from where the watcher would sit to the screen in centimeters (or inches), dividing that by 4, and comparing with the screen heights below. Do not be surprised if the observer can see the pixels at this viewing distance as most normal people see far better than 6/6 ([20/20 vision]). If the user is replacing a standard definition TV with an HDTV this implies that the best visual experience will be with a set that is twice as tall as the standard definition set. As the average size LCD TV being sold is now 38"[when?], which is only about 15% taller than a 27" standard definition TV, this means that most consumers buy HDTV sets that are smaller than what they could utilize. Cost and budget also limit screen size.
Display sizes of common TVs and computer monitors
Knowing the actual height and width of a screen is useful when deciding which one to purchase. Common dimensions are listed in the table below. If the display is not listed, then the following equations can be used. Note that D is the diagonal (in centimeters or inches), W is the width (in pixels), and H is the height (in pixels).
|(in)||(cm)||w (cm)||h (cm)||A (cm²)||w (cm)||h (cm)||A (cm²)||w (cm)||h (cm)||A (cm²)||w (cm)||h (cm)||A (cm²)||w (cm)||h (cm)||A (cm²)||w (cm)||h (cm)||A (cm²)|
The method of measuring screen size by its diagonal was inherited from the method used for the first generation of CRT television, when picture tubes with circular faces were in common use. Being circular, only their diameter was needed to describe their size. Since these circular tubes were used to display rectangular images, the diagonal measurement of the rectangle was equivalent to the diameter of the tube's face. This method continued even when cathode ray tubes were manufactured as rounded rectangles; it had the advantage of being a single number specifying the size, and was not confusing when the aspect ratio was universally 4:3. In the US, when virtually all TV tubes were 4:3, the size of the screen was given as the diagonal with a V following it. In Japan and other markets, the size of the outer diameter of the tube was given. What was a 27V in the US would be a 28" in Japan. However the V terminology was frequently dropped in US advertising referring to a 27V as a 27". This was not misleading for the consumer as the seller had to give the actual screen size by law. Liquid crystal displays use 100% of their surface to display, thus the VIS is the actual size presented to the viewer.
A problematic practice was the use of the size of a monitor's imaging element, rather than the size of its viewable image, when describing its size in publicity and advertising materials. On CRT displays a substantial portion of the CRT's screen is concealed behind the case's bezel or shroud in order to hide areas outside the monitor's "safe area" due to overscan. These practices were seen as deceptive, and widespread consumer objection and lawsuits eventually forced most manufacturers to instead measure viewable size. On cathode ray tube monitors, VIS differs noticeably from the full tube size, which is generally about 25 mm (1 in).
When the common aspect ratio went from 4:3 to 16:9, the new widescreens were labeled with a W. A screen that is approximately the same height as a 27V would be a 32W. Recently Vizio and other US TV manufacturers have introduced even wider screens with a 21:9 aspect ratio in order to match aspect ratios used in cinemas. In order to gauge the relative sizes of these new screens, the screen aspect must be considered. Indeed, in a commercial market where multiple aspect ratios are being sold, it will always take two numbers to describe the screen size, some combination of diagonal, aspect ratio, height or width.
Set sizes are frequently given as a "class" as screens from different manufacturers will have slight differences in size. However the "class" should be within 1/2" of the actual size. The reasons for the different sizes within a class stem from differences in the manufacturers' equipment. LCDs are cookie-cut from one large piece of glass. As manufacturers move from one size to another, newer larger sizes must fit on the same size glass, though with fewer displays being cut from it. Some sizes fit well and maximize glass utilization, other sizes fit more poorly and waste glass. As an example, in some cases, increasing the screen size by even 0.1" can cause an LCD manufacturer to go from 12 screens fitting on their glass sheet to 9. This would make them uncompetitive with other screen makers.