Pixel density

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"Ppi" redirects here. For other uses, see PPI.
The square shown above is 200 pixels by 200 pixels. To determine a monitor's PPI, measure the width and height, in inches, of the square as displayed on a given monitor. Dividing 200 by the measured width or height gives the monitor's horizontal or vertical PPI, respectively, at the current screen resolution.
The square shown above is 200 pixels by 200 pixels. To determine a monitor's PPI, measure the width and height, in inches, of the square as displayed on a given monitor. Dividing 200 by the measured width or height gives the monitor's horizontal or vertical PPI, respectively, at the current screen resolution.

Pixels per inch (PPI) or pixel density is a measurement of the resolution of a computer display, related to the size of the display in inches and the total number of pixels in the horizontal and vertical directions. This measurement is often referred to as dots per inch, though that measurement more accurately refers to the resolution of a computer printer. PPI may also be used to describe the resolution of an image scanner or digital camera; in this context, it is synonymous with samples per inch.

For example, a display that is 11 inches wide by 8.5 inches high, capable of a maximum 1024 by 768 pixel resolution, can display about 93 PPI in both the horizontal and vertical directions. This figure is determined by dividing width (or height) of the display area in pixels, by width (or height) of the display area in inches. It is possible for a display's horizontal and vertical PPI measurements to be different. The apparent PPI of a monitor depends upon the screen resolution (that is, number of pixels) and the size of the screen in use; a monitor in 800 by 600 mode has a lower PPI than the same monitor at 1024 by 768 mode. The dot pitch of a computer display determines the absolute limit of possible pixel density.

Typical circa-2000 cathode ray tube or LCD computer displays range from 67 to 130 PPI. The IBM T220/T221 LCD monitors marketed from 2001 to 2005 reached 204 PPI. The mid-2007 launched Toshiba Portégé G900 Windows Mobile 6 Professional phone came with a 3" WVGA LCD having a "print-quality" pixel density of 313 PPI.[1] In January 2008, Kopin Corp. announced a 0.44" SVGA LCD with an astonishing pixel density of 2272 PPI.[2][3] According to the manufacturer, the LCD was designed to be optically magnified to yield a vivid image and therefore expected to find use in high-resolution eye-wear devices. It has been observed that the unaided human eye can generally not differentiate detail beyond 300 PPI, however this figure depends both on the distance between viewer and image, and their visual acuity. Modern LCDs having upwards of 300 PPI pixel densities, combined with their evenly lit and interactive display areas may have vastly more appeal to users than the best prints available on paper. Such high pixel density display technologies would enable true WYSIWYG graphics and further, pave the way towards the elusive "paperless office" era.[4] For perspective, such a device at 15" screen size would have to display more than four Full HD screens (or WQUXGA resolution).

The pixel density is useful for calibrating a monitor with a printer; software can use the PPI measurement to display a document at "actual size" on the screen.

PPI can also describe the resolution in pixels, of an image to be printed within a specified space. For instance, a 100x100-pixel image that is printed in a 1-inch square could be said to have 100 pixels per inch, regardless of the printer's DPI capability. Used in this way, the measurement is only meaningful when printing an image. Good quality photographs usually require 300 pixels per inch when printed.[citation needed]

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[edit] Calculation of PPI

Theoretically, PPI can be calculated from knowing diagonal size of screen in inches, aspect ratio and resolution. This can be done in two steps:

1. Use the Pythagorean theorem to determine physical width and height of screen from its diagonal and aspect:

w2 + h2 = d2
w:h = 4:3
w^2 + \frac{9}{16}w^2 = d^2
\frac{25}{16}w^2 = d^2
w = d\sqrt{\frac{16}{25}} = {\frac{4}{5}}d

2. Divide width resolution by physical width (or height resolution by physical height) to get PPI.

These steps can be summed up in one short formula:

PPI = \frac{r_w}{d} \sqrt{1 + \left (\frac{a_h}{a_w}\right )^2}

where

  • d is diagonal size in inches (12.1"),
  • rw is width screen resolution (1024),
  • aw aspect ratio width part (4),
  • ah aspect ratio height part (3).

For example, for a 12.1" screen with 4:3 aspect ratio, the width is 9.68" and with a resolution of 1024×768 we get 1024/9.68 ≈ 105.79 PPI.

Note that these calculations are not very precise. Frequently, screens advertised as "X inch screen" can have their real physical dimensions of viewable area differ, for example:

  • HP LP2065 20" monitor — 20.1" viewable area[1]

[edit] References

  1. ^ Toshiba Portégé G900 Official Page. Toshiba Portege G900 Review (2007-07-27). Retrieved on 2008-05-01.
  2. ^ Kopin unveils smallest color SVGA display. optics.org (2008-01-11). Retrieved on 2008-06-06.
  3. ^ Company Debuts World’s Smallest Color SVGA Display. SID, Information Display magazine May 2008 Vol. 24, No. 05 (2008-05-31). Retrieved on 2008-06-06.
  4. ^ Electronic displays for information technology. IBM Journal of Research and Development Volume 44, Number 3, 2000 (1999-11-10). Retrieved on 2008-06-06.

[edit] See also

[edit] External links