Ed #12 Standards

Screen rules come into play in a large majority of commercial printing projects. Photos and artwork typically are printed using halftone screens, which produce a grid of dots that are opaque and distinct from one another. The size of the dots is determined by the number of lines per inch (lpi) or, in digital printing, dots per inch (dpi), with higher numbers representing finer screens that will capture subtler shifts in tone and more detail. Most commercial printing today relies on 150 to 175 line screens.

Not all images are printed using halftone screens, however. Some projects rely on stochastic screening techniques in which images are reproduced using microdots of the same size with random spacing between the dots or with variably sized dots and random spacing. Either way, the dots used are quite small and measured in microns or millionths of a meter. While a typical period at the end of a sentence is around 615 microns, the stochastic dots used to produce images typically measure around 20 microns, which is about the same size as a mold spore.
In addition to microns digital prepress and print production techniques have spawned another set of numbers that designers should know.

As discussed in Ed #10, the most basic element in digitized images is the bitmap, also known as a pixel, an abbreviation of “picture element.” Pixels are tiny black and white or colored squares that are arranged together like tiny mosaic tiles to form images. When an image is scanned or captured by a digital camera, the number of pixels that are captured (measured by pixels per inch or ppi) determines the resolution of the image, and the number of pixels contained in the original image cannot be increased. The greater the number of pixels, the higher the resolution of the image, and the more computer storage space it requires.