Multi-bit screening

Screening for most analog printing processes such as offset litho, flexography or silk-screen uses dot patterns where every pixel is rendered as black or white. Thus every area on the printed medium is, at least theoretically, either completely covered in ink at the target density, or has no ink at all placed onto it. Gravure printing is unusual for conventional printing processes in that it enables areas of the output to be printed with an amount of ink that falls mid-way between none and complete coverage, yielding a result that can be photographic quality, even when viewed extremely closely.

With the advent of digital printing it’s now possible to achieve that same result on a far wider variety of work, without the slow pre-press and high make-ready costs associated with gravure. This is made possible by using “multi-bit screening”. The resulting screen is no longer a pattern of solid ink and no ink, but includes a number of different levels of ink density. Thus a single location on the output might be paper colored (no ink), 25% ink density, 50% density, 75% density or solid ink. Different print technologies support 2-bit screening (giving 4 levels of coverage), 3-bit (8 levels), 4-bit (16 levels), or some intermediate number of levels.

Multi-bit screening allows a device at a relatively low resolution (e.g. 600 dpi) to produce very smooth images, graduations and flat tints without any screening artifacts visible at normal viewing distance.

It’s most common in inkjet solutions, although it’s occasionally found in printers using other marking technologies. Inkjet systems can achieve this result by a variety of methods including jetting different sizes of ink droplet, or a chain of droplets, each droplet the same size, but with a different number of them in the chain.

It’s very common for the contribution of each level of ink to be different, thus a 2-bit screen may not achieve 25%, 50%, 75% and 100% density with the different ink levels. Instead it may deliver 50%, 75%, 87%, 100%, so it’s important to be able to apply suitable calibration curves to generate accurate tone reproduction.