Definition of DPI vs PPI vs LPI, halftone screens, dots per inch, resolution myths and even samples per inch.

What's the difference between dots per inch, pixels per inch and line screens?

What is the definition of the different terms that designers and printers use to describe photographic image resolutions, scans & imagesetter output?

Many designers are confused by the, often misused, terms applied when describing the resolution or quality of digital photographs and other bitmap images.

One of the reasons for the confusion (or even myths as some might say) is that some of these terms, such as half-tone line-screens (or lines per inch - LPI) pre-date the advent of computers and desktop publishing. Whilst others, such as pixels per inch (PPI), are planted firmly in the age of electronic publishing.

As such, many of these terms are used interchangeably in situations that, whilst being technically incorrect, have become commonplace. In many respects this is understandable as all these terms are linked, to varying degrees, with definitions about the reproductive quality of images – whether digital or in print.

Dots versus pixels, the difference between printers and monitors

Lines per inch or halftone screens

Lines per inch (LPI) refers to the lines, or rows, of halftone dots used predominantly in commercial offset-litho printing. By varying the size of evenly spaced halftone dots, traditional halftone screens (as opposed to stochastic screening) allow printing machines to simulate the shades and gradients of continuous tone images, like photographs.

The finer the screen, the more detailed the reproduced image. Art books may use halftone screens up to 300lpi, whereas magazines are more likely to use 150 lines per inch. Newspapers and billboard posters are likely to use even lower settings. Newspapers because they are printed on rough, uncoated paper require a courser line screen. Posters and screen printed t-shirts can afford a lower line screen as they are designed to be viewed from a relative distance and so the size of the dots will be less noticeable.

Dots per inch define the printer resolution

Dots per inch (DPI) in this context, refers to the smallest amount of ink that a given printer can print. Put another way, the more dots that a printer (or imagesetter) can apply per inch, the higher the resolution (and therefore quality of image reproduction) an imaging device can reproduce.

Pixels per inch

Pixels per inch (PPI) is often used interchangeably with DPI. PPI is, arguably, where the confusion started from in the first place. PPI a somewhat relative term. Once a photograph is opened in a program such as Adobe Photoshop and displayed on a computer screen, its relation to print paradigm concepts such as per inch are not always helpful. In many cases it is actually more helpful to talk about the total width and height of the digital image in pixels, rather than pixels per inch or dots per inch.

For printed reproduction, the amount of dots per inch only becomes relevant when calculating the amount of digital information against the intended output size of the image. For example, if the printed requirement of an image is 300dpi, to be output via a 150 line screen, then an image that is 1500 pixels wide and 800 pixels in depth can be printed at a size of 127mm by 67.73mm. Changing the resolution to 350dpi for a 175 line screen printing job, reduces the acceptable output size to 108.86mm by 58.06mm, however the actual dimensions of the on-screen image remain the same – 1500 pixels by 800 pixels.

Note that we are not talking about resampling the image here, which is different in that it involves actually either reducing or adding information to the image, rather than simply defining its attributes from the digital world to the print paradigm.

For on-screen use, such as web design, the PPI of an image is largely irrelevant. It is the PPI of the computer screen which defines the display size of an image on screen. For example images on a 96dpi monitor, will display slightly smaller than those on a 72dpi monitor.

Samples per inch from scanners and digital cameras

Samples per inch, or SPI only really refer to the resolution at which a scanner or digital camera is able to capture an image. In practice, as soon as the image has been digitized by the input device, it would be fair to refer to the data as PPI. However, it is possible to interpolate scanned image resolutions upwards, as well as rasterise downwards the amount of information captured by an input device. Therefore, as mentioned above, it may still be more helpful to think about the total width and height of an on-screen image, until such point as it is targeted at a particular printing size and output method.