Pixels per inch (ppi) and pixels per centimeter (ppcm or pixels/cm) are measurements of the pixel density (resolution) of an electronic image device, such as a computer monitor or television display, or image digitizing devices such as a camera or image scanner. Horizontal and vertical density are usually the same, as most devices have square pixels, but differ on devices that have non-square pixels.
Pixels per inch (or pixels per centimeter) can also describe the resolution, in pixels, of an image file. A 100×100 pixel image printed in a 1 inch square has a resolution of 100 pixels per inch. Used this way, the measurement is meaningful when printing an image. It has become commonplace to refer to PPI as DPI, even though PPI refers to input resolution. Industry-standard, good quality photographs usually require 300 pixels per inch, at 100% size, when printed onto coated paper stock, using a printing screen of 150 lines per inch (lpi). This delivers a quality factor of 2, which is optimum. The lowest acceptable quality factor is considered 1.5, which equates to printing a 225 ppi image using a 150 lpi screen onto coated paper.
Screen frequency is determined by the type of paper the image is printed on. An absorbent paper surface, uncoated recycled paper, for instance, lets ink droplets spread (dot gain)—so requires a more open printing screen. Input resolution can, therefore, be reduced to minimize file size without a loss in quality, as long as the quality factor of 2 is maintained. This is easily determined by doubling the line frequency. For example, printing on an uncoated paper stock often limits printing screen frequency to no more than 120 lpi, therefore, a quality factor of 2 is achieved with images of 240 ppi.
The PPI/PPCM of a computer display is related to the size of the display in inches/centimeters 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 (38 cm) display whose dimensions work out to 12 inches (30.48 cm) wide by 9 inches (22.86 cm) high, capable of a maximum 1024×768 (or XGA) pixel resolution, can display around 85 PPI, or 33.46 PPCM, in both the horizontal and vertical directions. This figure is determined by dividing the width (or height) of the display area in pixels by the width (or height) of the display area in inches. It is possible for a display to have different horizontal and vertical PPI measurements (e.g., a typical 4:3 ratio CRT monitor showing a 1280×1024 mode computer display at maximum size, which is a 5:4 ratio, not quite the same as 4:3). The apparent PPI of a monitor depends upon the screen resolution (that is, the number of pixels) and the size of the screen in use; a monitor in 800×600 mode has a lower PPI than does the same monitor in a 1024×768 or 1280×960 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, though desktop monitors have exceeded 200 PPI, and contemporary small-screen mobile devices often exceed 300 PPI, sometimes by a wide margin.
In January 2008, Kopin Corporation announced a 0.44 inch (1.12 cm) SVGA LCD with a pixel density of 2272 PPI (each pixel only 11.25 μm). In 2011 they followed this up with a 3760-DPI 0.21-inch diagonal VGA color display. The manufacturer says they designed the LCD to be optically magnified, as in high-resolution eyewear devices.
Holography applications demand even greater pixel density, as higher pixel density produces a larger image size and wider viewing angle. Spatial light modulators can reduce pixel pitch to 2.5 μm, giving a pixel density of 10,160 PPI.
Some observations indicate that the unaided human generally can't differentiate detail beyond 300 PPI. However, this figure depends both on the distance between viewer and image, and the viewer’s visual acuity. The human eye also responds in a different way to a bright, evenly lit interactive display from how it does to prints on paper.
High pixel density display technologies would make supersampled antialiasing obsolete, enable true WYSIWYG graphics and, potentially enable a practical “paperless office” era. For perspective, such a device at 15 inch (38 cm) screen size would have to display more than four Full HD screens (or WQUXGA resolution).
The development of a display with ≈900 ppi allows for three pixels with 16-bit color to act as sub-pixels to form a pixel cluster. These pixel clusters act as regular pixels at ≈300 ppi to produce a 48-bit color display.
The PPI pixel density specification of a display is also useful for calibrating a monitor with a printer. The software can use the PPI measurement to display a document at "actual size" on the screen.