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Pixels per inch (ppi) and pixels per centimetre (ppcm or pixels/cm) are measurements of the pixel density of an electronic image device, such as a
computer monitor A computer monitor is an output device that displays information in pictorial or text form. A monitor usually comprises a electronic visual display, visual display, electronic circuit, some circuitry, a casing, and a power supply. The display de ...
or
television Television, sometimes shortened to TV or telly, is a telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cylindrical A cylinder (from Gre ...

television
display, or image digitizing device such as a
camera A camera is an optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and t ...
or
image scanner An image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting Handwriting is the writing done with a writing instrument, such as a pen or pencil, in the hand. Handwriting includes both ...
. Horizontal and vertical density are usually the same, as most devices have
square pixel Pixel aspect ratio (often abbreviated PAR) is a mathematical ratio In mathematics, a ratio indicates how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of o ...
s, but differ on devices that have non-square pixels. Note that pixel density is not the same as
resolution Resolution(s) may refer to: Common meanings * Resolution (debate), the statement which is debated in policy debate * Resolution (law), a written motion adopted by a deliberative body * New Year's resolution, a commitment that an individual make ...
, where the former describes the amount of detail on a physical surface or device, the latter describes the amount of pixel information regardless of its scale. Considered in another way, a
pixel In digital imaging Digital imaging or digital image acquisition is the creation of a representation of the visual characteristics of an object, such as a physical scene or the interior structure of an object. The term is often assumed to imp ...

pixel
has no inherent size or unit (a pixel is actually a sample), but when it is printed, displayed, or scanned, then the pixel has both a physical size (dimension) and a pixel density (ppi).


Basic Principles

Since most digital hardware devices use dots or pixels, the size of the media (in inches) and the number of pixels (or dots) are directly related by the 'pixels per inch'. The following formula gives the number of pixels, horizontally or vertically, given the physical size of a format and the ''pixels per inch'' of the output: :\text = \text * \text Pixels per inch (or pixels per centimetre) describes the ''detail'' of an image file when the print size is known. For example, a 100×100 pixel image printed in a 2 inch square has a resolution of 50 pixels per inch. Used this way, the measurement is meaningful when printing an image. In many applications, such as Adobe Photoshop, the program is designed so that one creates new images by specifying the output device and PPI (pixels per inch). Thus the output target is often defined upon creating the image.


Outputting to a Different Device

When moving images between devices, such as printing an image that was created on a monitor, it is important to understand the pixel density of both devices. Consider a 24″ HD monitor (20″ wide), that has a known, native resolution of 1920 pixels (horizontal). Let us assume an artist created a new image at this monitor resolution of 1920 pixels, possibly intended for the web without regard to printing. Rewriting the formula above can tell us the pixel density (PPI) of the image on the monitor display: :\text = \frac = \frac = 96\text Now, let us imagine the artist wishes to print a larger banner at 48″ horizontally. We know the number of pixels in the image, and the size of the output, from which we can use the same formula again to give the PPI of the printed poster: :\text = \frac = \frac = 40\text This shows that the output banner will have only 40 pixels per inch. Since a printer device is capable of printing at 300 ppi, the resolution of the original image is well below what would be needed to create a decent quality banner, even if it looked good on a monitor for a website. We would say more directly that a 1920 × 1080 pixel image does not have enough pixels to be printed in a large format.


Printing on Paper

Printing on paper is accomplished with different technologies. Newspapers and magazines were traditionally printed using a screen called a
halftone Halftone is the reprographicReprography is the reproduction of graphics through mechanical or electrical means, such as photography or xerography. Reprography is commonly used in catalogs and archives, as well as in the architectural reprograph ...
screen, which would print dots at a given frequency called the screen frequency in lines per inch (LPI) by using a purely analog process in which a photographic print is converted into variable sized dots through interference patterns passing through a screen. Modern inkjet printers can print microscopic dots at any location, and don't require a screen grid, so they use a metric called dots per inch (DPI). These are both different from pixel density or pixels per inch (PPI) because a pixel is a single sample of any color, whereas an inkjet print can only print a dot of a specific color either on or off. Thus a printer translates the pixels into a series of dots using a process called ''dithering''. The dot pitch, smallest size of each dot, is also determined by the type of paper the image is printed on. An absorbent paper surface, uncoated recycled paper for instance, lets ink droplets spread — so has a larger dot pitch. Often one wishes to know the image quality in pixels per inch (PPI) that would be suitable for a given output device. If the choice is too low, then the quality will be below what the device is capable of -- loss of quality -- and if the choice is too high then pixels will be stored unnecessarily -- wasted disk space. The ideal pixel density (PPI) depends on the output format, output device, the intended use and artistic choice. For inkjet printers measured in dots per inch it is generally good practice to use ''half or less'' than the DPI to determine the PPI. For example, an image intended for a printer capable of 600 dpi could be created at 300 ppi. When using other technologies such as AM or FM screen printing, there are often published ''screening charts'' that indicate the ideal PPI for a printing method. Using the DPI or LPI of a printer remains useful to determine PPI until one reaches larger formats, such as 36" or higher, as the factor of visual acuity then becomes more important to consider. If a print can be viewed close up, then one may choose the printer device limits. However, if a poster, banner or billboard will be viewed from far away then it is possible to use a much lower PPI.


Computer displays

The PPI/PPCM of a
computer display A computer monitor is an output device that displays information in pictorial or text form. A monitor usually comprises a electronic visual display, visual display, electronic circuit, some circuitry, a casing, and a power supply. The display dev ...
is related to the size of the display in
inch Measuring tape with inches The inch (symbol: in or ″) is a unit Unit may refer to: Arts and entertainment * UNIT Unit may refer to: Arts and entertainment * UNIT, a fictional military organization in the science fiction television s ...
es/
centimetre 330px, Different lengths as in respect to the Electromagnetic spectrum, measured by the Metre and its deriveds scales. The Microwave are in-between 1 meter to 1 millimeter. A centimetre (international spelling) or centimeter (American spellin ...
s and the total number of
pixel In digital imaging Digital imaging or digital image acquisition is the creation of a representation of the visual characteristics of an object, such as a physical scene or the interior structure of an object. The term is often assumed to imp ...

pixel
s in the horizontal and vertical directions. This measurement is often referred to as
dots per inch at draft quality. Actual size is approximately 0.25 inch by 0.25 inch (6.35 mm by 6.35 mm). Individual colored droplets of ink are visible; this sample is about 150 DPI. Dots per inch (DPI, or dpiThe acronym appears in sources as either "DPI" or lo ...
, though that measurement more accurately refers to the resolution of a
computer printer In computing, a printer is a peripheral machine which makes a persistent representation of graphics or text, usually on paper. While most output is human-readable, bar code printers are an example of an expanded use for printers. The differen ...
. 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 The graphics display resolution is the width and height dimension of an electronic visual display An electronic visual display, informally a screen, is a display device s, LED display and Vacuum fluorescent display, VF display, top to bottom. ...
) pixel resolution, can display around 85 PPI, or 33.46PPCM, 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 Dot pitch (sometimes called line pitch, stripe pitch, or phosphor pitch) is a specification for a computer display, computer printer, image scanner, or other pixel-based devices that describe the distance, for example, between dots (pixel#Subpix ...

dot pitch
of a computer display determines the absolute limit of possible pixel density. Typical circa-2000
cathode ray tube A cathode-ray tube (CRT) is a vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current An electric current is a stream of charged particles, such as electrons ...

cathode ray tube
or
LCD A liquid-crystal display (LCD) is a flat-panel display A flat-panel display (FPD) is an electronic display device s, LED display and Vacuum fluorescent display, VF display, top to bottom. A display device is an output device for presentation ...
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 The Kopin Corporation () is a Westborough, Massachusetts Westborough is a town A town is a human settlement. Towns are generally larger than villages and smaller than city, cities, though the criteria to distinguish between them ...
announced a 0.44 inch (1.12 cm)
SVGA Super VGA (SVGA) is a broad term that covers a wide range of computer display standard Computer display standards are a combination of Display aspect ratio, aspect ratio, display size, display resolution, color depth, and refresh rate. They are ...
LCD with a pixel density of 2272 PPI (each pixel only 11.25μm). In 2011 they followed this up with a 3760-DPI 0.21-inch diagonal VGA colour display. The manufacturer says they designed the LCD to be optically magnified, as in high-resolution eyewear devices.
Holography Holography is a technique that enables a wavefront In physics, the wavefront of a time-varying field is the set () of all where the wave has the same of the sinusoid. The term is generally meaningful only for fields that, at each point, v ...

Holography
applications demand even greater pixel density, as higher pixel density produces a larger image size and wider viewing angle.
Spatial light modulator A spatial light modulator (SLM) is an object that imposes some form of spatially varying modulation on a beam of light. A simple example is an overhead projector Transparency (projection), transparency. Usually when the phrase SLM is used, it mean ...

Spatial light modulator
s 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 Visual acuity (VA) commonly refers to the clarity of vision Vision or The Vision may refer to: Perception Optical perception * Visual perception Visual perception is the ability to interpret the surrounding environment (biophysical), ...
. 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
supersampled antialiasing
obsolete, enable true
WYSIWYG In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and sof ...
graphics and, potentially enable a practical “
paperless office A paperless office (or paper-free office) is a work environment in which the use of paper is eliminated or greatly reduced. This is done by converting documents and other papers into digital form, a process known as digitization. Proponents claim t ...
” era. For perspective, such a device at 15 inch (38 cm) screen size would have to display more than four
Full HD 1080p (1920×1080 progressive Progressive may refer to: Politics * Progressivism is a political philosophy in support of social reform Political organizations * Congressional Progressive Caucus, members within the Democratic Party in the United ...
screens (or WQUXGA resolution). 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. Software can use the PPI measurement to display a document at "actual size" on the screen.


Calculation of monitor PPI

PPI can be calculated from knowing the diagonal size of the screen in inches and the resolution in pixels (width and height). This can be done in two steps: # Calculate diagonal resolution in pixels using the
Pythagorean theorem In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no ...

Pythagorean theorem
, then the actual PPI: :PPI = \frac where # w_p is width resolution in pixels # h_p is height resolution in pixels # d_i is diagonal size in inches (this is the number advertised as the size of the display). For example: # For 15.6 inch screen with a 5120×2880 resolution you get \frac = 376.57 PPI. # For 50 inch screen with a 8192×4608 resolution you get \frac = 188 PPI. # For 27 inch screen with a 3840×2160 resolution you get \frac = 163 PPI. # For 32 inch screen with a 3840×2160 resolution you get \frac = 138 PPI. # For an old-school 10.1 inch
netbook The marketing term netbook identified small and inexpensive laptop computer, laptops that were sold from 2007 to around 2013; these were generally low-performance. While the ''name'' has fallen out of use, machines matching their descripti ...

netbook
screen with a 1024×600 resolution you get \frac = 117.5 PPI. # For 27 inch screen with a 2560×1440 resolution you get \frac = 108.8 PPI. # For a 21.5 inches (546.1 mm) screen with a 1920×1080 resolution you get \frac = 102.46 PPI; Note that these calculations may not be very precise. Frequently, screens advertised as “X inch screen” can have their real physical dimensions of viewable area differ, for example: *
Apple Inc. Apple Inc. is an American multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from multiple countries * Multinational state, ...
advertised their mid-2011
iMac iMac is a family of All-in-One PC#All-in-one, all-in-one Macintosh desktop computers designed and built by Apple Inc. It has been the primary part of Apple's consumer desktop offerings since its debut in August 1998, and has evolved through L ...

iMac
as a "21.5 inch (viewable) ..display," but its actual viewable area is 545.22 mm or 21.465 inches. The more precise figure increases the calculated PPI from 102.46 (using 21.5) to 102.63. * The
HP
HP
LP2065 20 inch (50.8 cm) monitor has an actual viewable area of 20.1 inch (51 cm). * In a more significant case, some monitors such as the
Dell Dell is an American company A company, abbreviated as co., is a Legal personality, legal entity representing an association of people, whether Natural person, natural, Legal personality, legal or a mixture of both, with a specific objecti ...

Dell
UltraSharp UP3216Q (3840×2160 px) are advertised as a 32 inch "class" monitor (137.68 PPI), but the actual viewing area diagonal is 31.5 inches, making the true PPI 139.87.


Calculating PPI of camera view screens

Camera manufacturers often quote view screens in 'number of dots'. This is not the same as the number of pixels, because there are 3 'dots' per pixel – red, green and blue. For example, the Canon 50D is quoted as having 920,000 dots. This translates as 307,200 pixels (×3 = 921,600 dots). Thus the screen is 640×480 pixels. This must be taken into account when working out the PPI. 'Dots' and 'pixels' are often confused in reviews and specs when viewing information about digital cameras specifically.


Scanners and cameras

"PPI" or "pixel density" may also describe
image scanner An image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting Handwriting is the writing done with a writing instrument, such as a pen or pencil, in the hand. Handwriting includes both ...
resolution. In this context, PPI is synonymous with samples per inch. In digital photography, pixel density is the number of pixels divided by the area of the sensor. A typical
DSLR A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on pho ...
, circa 2013, has 1–6.2 MP/cm2; a typical compact has 20–70 MP/cm2. For example, Sony Alpha SLT-A58 has 20.1 megapixels on an APS-C sensor having 6.2 MP/cm2 since a compact camera like Sony Cyber-shot DSC-HX50V has 20.4 megapixels on an 1/2.3" sensor having 70 MP/cm2. The professional camera has a lower PPI than a compact camera, because it has larger photodiodes due to having far larger sensors.


Smartphones

Smartphone A smartphone is a portable device A mobile device (or handheld computer) is a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern computers can per ...

Smartphone
s use small displays, but modern smartphone displays have a larger PPI rating, such as the
Samsung Galaxy S7 The Samsung Galaxy S7, Samsung Galaxy S7 Edge and Samsung Galaxy S7 Active are Android-based smartphone A smartphone is a mobile device that combines cellular and mobile computing functions into one unit. They are distinguished from feat ...

Samsung Galaxy S7
with a quad HD display at 577 PPI, Fujitsu F-02G with a quad HD display at 564 PPI, the
LG G6 The LG G6 is an Android smartphone developed by LG Electronics as part of the LG G series. It was announced during Mobile World Congress on February 26, 2017, as the successor to the 2016 LG G5. The G6 is distinguished by its 5.7-inch display, ...
with quad HD display at 564 PPI or – XHDPI or Oppo Find 7 with 534 PPI on 5.5" display – XXHDPI (see section below).
Sony , commonly known as Sony and stylized as SONY, is a Japanese multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from mult ...
's Xperia XZ Premium has a 4K display with a pixel density of 807 PPI, the highest of any smartphone as of 2017.


Named pixel densities

:''For the Bayesian statistic parameter HPDI ("highest posterior density interval"), see
Credible interval In Bayesian statistics, a credible interval is an interval within which an unobserved parameter A parameter (from the Ancient Greek language, Ancient Greek wikt:παρά#Ancient Greek, παρά, ''para'': "beside", "subsidiary"; and wikt:μέ ...
.'' The
Google Google LLC is an American multinational Multinational may refer to: * Multinational corporation, a corporate organization operating in multiple countries * Multinational force, a military body from multiple countries * Multinational stat ...

Google
Android Android may refer to: Science and technology * Android (robot), a humanoid robot or synthetic organism designed to imitate a human * Android (operating system), Google's mobile operating system ** Android (operating system)#Mascot, Unnamed Androi ...

Android
developer documentation groups displays by their approximate pixel densities into the following categories:


Metrication

The digital publishing industry primarily uses pixels per inch but sometimes pixels per centimeter is used, or a conversion factor is given. The PNG image file format only allows the meter as the unit for pixel density.


Image file format support

The following table show how pixel density is supported by popular image file formats. The cell colors used do not indicate how feature-rich a certain image file format is, but what density support can be expected of a certain image file format. Even though image manipulation software can optionally set density for some image file formats, not many other software uses density information when displaying images. Web browsers, for example, ignore any density information. As the table shows, support for density information in image file formats varies enormously and should be used with great care in a controlled context.


See also

*
Dots per inch Dots per inch (DPI, or dpiThe acronym appears in sources as either "DPI" or lowercase "dpi". See "Print Resolution Understanding 4-bit depth – Xerox" (PDF). Xerox.com. September 2012.) is a measure of spatial printing, video or image scanner d ...
* Computer monitor DPI standards – the origins of 96 DPI/PPI as Microsoft/Windows standard and 72 DPI/PPI as (former) Apple/Macintosh standard *
Dot pitch Dot pitch (sometimes called line pitch, stripe pitch, or phosphor pitch) is a specification for a computer display, computer printer, image scanner, or other pixel-based devices that describe the distance, for example, between dots (pixel#Subpix ...

Dot pitch
*
Resolution independence Resolution independence is where elements on a computer screen A computer monitor is an output device that displays information in pictorial form. A monitor usually comprises the visual display, circuitry, casing, and power supply A power su ...
* Apple
Retina Display
Retina Display
* Samsung
PenTile PenTile matrix is a family of patented subpixel In digital imaging Digital imaging or digital image acquisition is the creation of a representation of the visual characteristics of an object, such as a physical scene or the interior structure ...


References


External links


PPI calculator
– Calculate pixel density

– 4 Ways to Change Android PPI Density {{DEFAULTSORT:Pixel Density Digital imaging Density Digital photography Engineering ratios