Resolution Printing Knowledge

Digital image technology strives to capture finer detail when an image is either created or scanned. Capturing greater pixel depth results in more colors delivered on output, and thus a higher quality reproduction of the original image. Maintaining the colors of the image through a pixel spatial resolution as close to the original as possible, results in more bits of information being generated and saved as digital files. Increasingly, digital image creation and capture are requiring larger volumes of data and data storage.

When selecting an image for print or Web publishing, the intended use of the image must be considered in order to determine the proposed resolution and the size of the file. Web publishing requires considerably less resolution and no output concerns, unlike images intended for print publishing. Therefore, when publishing to print it is important to understand some of the considerations necessary as the image goes through the various stages from capture to output.

Resolution Considerations

There are two types of images used in graphic arts: vector and raster images. Vector images use mathematical formulas to create graphic objects, lines, or shapes. When outputting to print or Web, vector images are resolution independent and the final output occurs using the dpi setting of the output device.

For the purposes of calculating file sizes, we are concerned only with raster images (also called bitmap images) which use dots per inch (dpi), lines per inch (lpi), or pixels per inch (ppi), when creating and outputting the reproduced image. To explain the differences between dpi, lpi, and ppi, consider the following:

  • Dots per inch (dpi): A gauge used for stating the resolution that is to be achieved from an output device such as a laser printer, image processors, or a digital printing press. 300 dpi is a common printer resolution while imagesetters and digital presses can achieve significantly higher dpi outputs.
  • Lines per inch (lpi): A measurement used to define the resolution of the image to be reproduced through a lithographic printing process. The resolution is referred to as a line screen (also line frequency or line ruling) and is determined by the number of lines per inch used to create the dot pattern in an image. Using the correct lpi screen enables the four transparent ink colors for color printing (cyan, magenta, yellow, and black) to accurately print the reproduced image with clarity, definition, and quality.
  • Pixels per inch (ppi): Video monitor or on-screen resolution uses ppi as a measure of the resolution established by the number of dots or pixels per linear inch. 72 ppi is a common screen resolution with high definition screens displaying above 200 ppi. Scanners and digital cameras may have a ppi measurement in the thousands.





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