Digital Imaging Part 4 - Resolution

I'm sure you're familiar with the word resolution, but did you know that it can have many different meanings depending on the output you're referring to? In this article we'll be discussing only a few of the most relevant... PPI, LPI and DPI.
Published: May 15, 2006
Author: Stacy Reed

A key element in digital imaging is resolution. I'm sure you're familiar with the word, but did you know that it can have many different meanings depending on the output you are referring to? In short, resolution is a measurement that indicates the clarity and sharpness of an image. You may already be aware that there are several types of resolution but in this article we'll be discussing only a few of the most relevant, which are PPI, LPI and DPI.

PPI is an acronym for Pixels Per Inch and it refers to screen resolution. Your monitor is an output device that displays tiny specs of light called pixels. Most monitors display images at 72 PPI. Keep in mind that PPI does not refer to pixels per square inch so if we had an image that was 72 PPI, it would simply be 72 pixels in a single row. But if we had an image that was 72 pixels x 72 pixels, it would show up on your monitor as a 1 inch by 1 inch image. See the difference? This is why when people talk about their monitor resolution or images displayed on screen, they usually mention two numbers: one for width and one for height. A monitor that displays 1600 x 1200 actually has a total of 1,920,000 pixels. On a side note, in regards to digital cameras, that's almost two megapixels (two million pixels), while a resolution of 1280 x 960 has 1,228,800 pixels, or 1.2 megapixels. Generally, higher resolution means better image quality, but that also means the file size will be larger than a low resolution image.

Now let's talk about another output device, your printer! LPI stands for Lines Per Inch, which is one of the resolutions used in printing. This is tricky, because the word "lines" does not refer to actual lines. Don't let this confuse you, it's just old terminology reminiscent of the Linotype era. Instead, the value of LPI is a linear unit that refers to the size of the halftone cells, in essence, measuring how many of these cells are placed side by side in an inch. When you use a high LPI as opposed to a small LPI, the cells will be closer together because they are smaller and will therefore produce a smoother looking image.This kind of resolution is very important to take into consideration when you are choosing which type of paper stock you will be printing on. Ink spreads more when it is printed onto highly absorbent papers (like newsprint) so if the dots are spaced too closely together, the ink might spread too much resulting in a muddy mess. In this instance it would be wise to use a lower LPI. Higher quality stock does not absorb as much ink during printing so you can assign a higher LPI without experiencing any running colors.

DPI, or Dots Per Inch also refers to printer resolution, and this is the one you'll probably hear people talking about most. It's important to know not only the LPI, but also the number of halftone dots that can be printed per inch. Halftone dots, like pixels, are arranged in a grid of cells and every dot is printed in the middle of the cell. The variance in dot size is what gives the illusion of continuous tones because they blend with the surrounding white space in each cell. The grid of halftone cells is called the linescreen. The halftone dot size is determined by three values: the applied linescreen value, the resolution value of the printer and the pixel values of the image. The type of printer you have determines the size of the halftone dots it can print. For instance, most ink-jet printers range from 300 to 2400 DPI. A 300 DPI printer can produce a dot that is 1/300 of an inch but you cannot force that printer to print smaller dots than that. Like LPI, the higher the DPI, the smoother the printed image will appear. As you might have guessed, printers with higher DPI ranges are more expensive because they offer better quality output.

Sometimes people mistakingly interchange PPI with DPI, but just remember that scanners, digital cameras and monitors produce output (light) that is measured in terms of PPI while printer output (ink) is measured in DPI. Next week, we'll take a closer look at the many different types of printers that exist and discuss which printer(s) might be best for your home office or business needs.

About Stacy Reed

Stacy Reed is Tucows' resident software librarian and editor. She has been reviewing PC and mobile software as well as web services for over a decade. Helping developers improve and promote their products is only one of her areas of expertise. Stacy is also an advocate for Open Source, Creative Commons and freeware, taking special interest in educational resources, social media, cloud sharing, and mobile technology.

Digg This
Please login to add your comment
Leave A Comment