Lines Per Inch (LPI)
printing term used to measure the number of lines that appear within a vertical inch of a halftone image. The number of LPI is related to the resolution of this type of image.
Lines per inch (LPI) Details
Lines per inch (LPI) is a measure typically used in commercial offset printing. It measures halftone images' resolution—those composed of a series of dots instead of continuous tones. Graphic designers and software engineers often use this term.
The higher the lines per inch number, the denser the series of dots of the halftone image is. This means that a high LPI results in a higher image resolution. Similarly, a picture with a low LPI typically has a poor image resolution, depending on what its dimensions are.
It is essential to know that the LPI number is related to the type of paper used in the printing, just as it is associated with the final product (newspaper, magazine, book, etc.). Coated paper (the type that is coated in a mix of materials or a polymer to make the paper a higher quality), for instance, can be printed using up to 200 lines per inch—uncoated paper prints typically up to 150 LPI.
Real World Example of LPI
Real-world examples of LPI are newspapers and magazines and the different paper quality used by those two types of media. Usually, publishers print newspapers on uncoated paper, while they print magazines on higher-quality paper.
The typical resolution of newspapers is 85 LPI, making it easier for the reader to notice the paper's series of dots. That's because the naked eye can notice the halftone dots easily up to about 120 lines per inch. Unlike newspapers, magazines print in higher resolutions because of their content. Publishers don't want readers to be distracted by any pixelated advertisements or glamour shots. Although magazines are print in higher resolution—resulting in cleaner images—you may still be able to see the dots if you look hard enough. To see the halftone dots in better quality printed material, the viewer will have to take a much closer look at it.
LPI vs. DPI
Lines per inch (LPI) is a measure often confused with dots per inch (DPI). While LPI is used to measure the number of lines that appear within a vertical inch of a halftone image, DPI refers to the number of dots printed in an inch by an output device, such as desktop printers imagesetters.
The LPI range typically stays within 85 to 200. That number scope is much lower when compared to the DPI range. Desktop printers, for example, have resolutions of 600, 2,800, or even higher. In a parallel manner, people who use imagesetters to produce screen printing can print using DPI ranges of up to 4,000.
Although they have different ranges, both LPI and DPI measures work similarly—the higher their numbers, the cleaner, and sharper the images are. This means that you should aim for high standards of lines per inch and dots per inch if you want a higher-quality material.