TechNote (Archived)

Scan Resolution Guidelines

The number of samples per inch (spi) determines the scan resolution of an image. A scan resolution that is too low results in a low-quality image. A scan resolution that is too high increases the file size and printing time, without increasing the image's quality. A black-and-white (i.e., 1-bit) image's scan resolution should be equal to or greater than the final output device's printer resolution measured in dots per inch (dpi).

When scaling an image, the effective resolution changes. When you create a bitmap or scan image, the number of pixels is fixed. When you scale that image, the number of pixels does not change. Because the same number of pixels must fit into an area that is smaller or larger than the original, the pixel density of the scaled image changes. For example, when a photograph with a scan resolution of 300 spi is scaled to 200%, its scan resolution decreases to 150 spi.

Use the following guidelines to determine the resolution of a grayscale or color bitmap image that is high enough to avoid pixelization (jagged or digital image), but low enough for efficient files.

Scan Resolution Formula for Images at 100 Percent

Use a spi twice the line screen frequency you'll assign to the image. Screen frequency is measured in lines per inch (lpi).

lpi x 2 = spi

For example: 120 lpi x 2 = 240 spi

Scan Resolution Formula for Scaled Images

Line screen frequency times 2, times final width, divided by original width equals the spi.

lpi x 2 x (final width) / (original width) = spi

For example: 120 lpi x 2 x 5 inches (final) = 1200 / 4 = 300 spi