Let’s continue our look at resolution.
Samples per Inch (spi)
Although scanners are less common than they used to be, many professionals still use them to load sketches, photos, and original negatives. Manufacturers often tout the dpi capabilities of their scanner. This is inaccurate. Scanners don’t use dots, they use samples. A sample is when a scanner captures part of an image. Samples per inch (spi) is a measurement of how many samples are captured in the space of one inch. In general, an increase in sampling leads to a file that is truer to its analog original. However, there is a threshold: Once a certain amount of information is surpassed, human senses (and electronic output devices) cannot tell the difference.
Consumer-level scanners can capture optical resolution ranging between 300 spi and 4800 spi. Professional devices can capture significantly higher optical resolution. If you’re working with a large image, a lower number of samples is fine. If you’re enlarging a very small image, a large number of samples is crucial. More samples per inch translates into more information available as pixels, which can then be harnessed in output when they are converted to dots in the printer. So if your scanner’s software specifies dpi, it really means spi, but you can see how the two are closely related
Lines per Inch (lpi)
In professional printing environments, you’ll often hear the term lines per inch (lpi). This is from the traditional process where images with gradiated tones (such as photographs) are screened for printing to create a halftone. This was originally performed by laying film with dots printed on it over the film before exposure. In the digital age, this process and these terms are used less often, but it is still good for you to have a basic understanding. These days, the work of converting an image to lines is performed by an imagesetter. The dots are arranged in lines, and the lpi measurement refers to the number of lines per inch. An increase in lpi results in smoother images.
This post is from the book Understanding Adobe Photoshop CS6: The Essential Techniques for Imaging Professionals