Although Real World Scanning and Halftones and some other sources make a distinct separation between samples and pixels, in practice it is probably not reasonable to expect most users to trade in one term for the other. However, for the purposes of this article, for the purists, and for the benefit of the curious, PPI does differ from other terms of resolution. It may also be the failure to differentiate the terms that leads to the greatest confusion over how an image looks on screen and how it looks when printed.
How SPI, PPI, DPI, and LPI relate
PPI is a function of your monitor. A pixel or "picture element" is a specific x,y coordinate (dot) on your screen. A high-resolution setting displays more pixels per inch. In practical terms, the same picture on a low resolution monitor looks larger than it does on a higher resolution monitor because the pixels are spread out over a larger area.
One of the biggest challenges new (and even seasoned) desktop publishers face is reconciling image display size to the size of the printed image. Throw in resampling and resizing and it really gets confusing. Here's a little exercise I performed and the results. I freely admit that it confuses the heck out of me too but at least you know you're not alone. With practice and patience you'll be able to discern a pattern and won't be quite so surprised when the image you see on screen prints at a radically different size some of the time.
The following exercise refers to the illustration in the sidebar: Scanned a 1" x 1" (physical size of the photograph) image at 300 SPI Downsampled the 300 SPI image to 96 SPI at 1" x 1" (96px x 96px) Downsampled the 300 SPI image to 96 SPI at 3" x 3" (300px x 300px)
The first thing you might notice is the difference in size of the images. Why do image one and two appear so different in physical size when I only changed the resolution? Because the monitor displays in pixels and 300 pixels per inch takes up more screen space than 96 pixels per inch. (And they would take up differing amounts of screen real estate if your monitor is set to 640 x 480 or 800 x 600, etc.) So what happened with image 3? It's only 96 SPI but its physical size is 3" x 3" (300 pixels square) so it displays at the larger size.
So, do you want to guess what the printed images will look like? The first two (large 300 SPI and small 96 SPI) printed at the same 1" x 1" size. (The 300 SPI image looked better though). The large 96 SPI image with a physical size of 3" x 3" printed at 3" square.
As you can see, how it looks on screen can be quite different than how it prints. Check the actual dimensions, as well as resolution of your image, to know how it will print.
In practice, SPI and PPI are often used interchangeably. DPI is frequently used in place of one or both terms. However, even if you call it DPI, remember that each dot or "unit of measure" behaves differently depending on whether it is a scanner (or scanned image), a monitor (or on-screen image), or a printer (or printed image).
Next > DPI: Printer Resolution
Measurements Systems > Measuring Resolution > PPI: Display Resolution
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