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Pixels per inch 

"Ppi" redirects here. For other uses, see PPI.

Pixels per inch (PPI) or pixel density is a measurement of the resolution of devices in various contexts; typically computer displays, image scanners or digital camera image sensors.

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

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.

Contents

Computer displays

The PPI of a computer display is 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.

For example, a 15 inch display, whose dimensions work out to 12 inches wide by 9 inches high, capable of a maximum 1024 by 768 (or XGA) pixel resolution can display around 85 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 "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 displays having upwards of 300 PPI pixel densities, combined with their non-reflective, bright, 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 make supersampled antialiasing obsolete, 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.

Calculation of monitor PPI

Theoretically, PPI can be calculated from knowing diagonal size of screen in inches and resolution in pixels (width and height). This can be done in two steps:

1. Calculate diagonal resolution in pixels using the Pythagorean theorem:

d_p = \sqrt{w_p^2 + h_p^2}

2. Calculate PPI:

PPI = \frac{d_p}{d_i}

where

  • dp is diagonal resolution in pixels,
  • wp is width resolution in pixels,
  • hp is height resolution in pixels and
  • di is diagonal size in inches. (This is the number advertised as the size of the display.)

For example, for a 20" screen with a 1680x1050 resolution, we get a 99.06 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[5]

Scanners and cameras

"PPI" or "pixel density" may also be used to describe the resolution of an image scanner. In this context, PPI is synonymous with samples per inch.

In digital photography, pixel density is the area of the sensor divided by the number of pixels. A typical DSLR circa 2008 will have 1-4 MP/cm2, a typical compact will have 20-35 MP/cm2. Although sensitivity varies with technology, pixel density can be a useful rough guide to the likely image noise of a camera advertising many megapixels.[6].

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.
  5. ^ HP LP2065 20-inch LCD Monitor - Specifications and Warranty (Hewlett-Packard Company official website)
  6. ^ Pixel Density: when Moore is less (Digital Photography Review, 03 July 2008)

See also

External links
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