DPI, PPI, LPI – What is the difference?

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PPI, LPI and DPI, What is the difference?

DPI, PPI, LPI – What is the difference?

What’s the difference between dots per inch (DPI), pixels per inch (PPI) and line screens (LPI)?

Designers are often confused by the terms (DPI, PPI, LPI, SPI) when describing the resolution or quality of digital images.

One of the reasons for the confusion is that some of these terms, such as LPI pre-date desktop publishing. While others, such as PPI, are rooted firmly in the age of electronic publishing.

As such, many of these terms are used interchangeably in situations, while 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 print.

Lines per inch (LPI) 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 allow offset printers to simulate the shades and gradients of continuous tone images of 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 150lpi. Newspapers and billboard posters are likely to use even lower settings. Newspapers because they are printed on uncoated paper require a courser line screen. Posters and screen printed t-shirts use a lower LPI as they are designed to be viewed from a distance.

Dots per inch (DPI) define the printer resolution.

Dots per inch (DPI) refers to the smallest amount of ink that a printer can print. In other words, the more dots a printer can apply per inch, the higher the resolution and quality image a printer can reproduce.

Pixels per inch (PPI)

Pixels per inch (PPI) is often used interchangeably with DPI. PPI is, arguably, where the confusion starts from in the first place. PPI a somewhat relative term. Once a photograph is opened in a program such as Adobe Photoshop, its DPI is not always helpful. In many cases it is more helpful to talk about the width and height of the image in pixels, rather than PPI or DPI.

For printed images, the amount of DPI only becomes relevant when calculating the intended output size of the image. For example, if the image is 300dpi, to be output via a 150 line screen, then an image that is 1500×800 pixels will be printed at a size of 5×2.66 inches. Changing the resolution to 350dpi for a 175 line screen printing job, reduces the output size to 4.28×2.28 inches, however the actual dimensions of the on-screen image remain the same, 1500×800 pixels.

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. For example images on a 96dpi monitor, will display slightly smaller than those on a 72dpi monitor.

Samples per inch (SPI) from scanners and digital cameras

Samples per inch, or SPI refers to the resolution at which a scanner or digital camera captures an image. 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.

Related Links

Wikipedia: Dots Per Inch
Adobe: Image Size Resolution

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