Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size or in spacing, thus generating a gradient like effect.
image not to scale. original image 3"x3" @ 600dpi. Click to enlarge
Halftones can be used to simulate gradients in screen printing art. Halftoning has it’s limitations, especially with screen printing onto garments. Here's what to expect when it comes to screen printing halftones.
Application
Halftones are used to give the appearance of different values of a particular ink color. For example, stock red printed on a white shirt as a 50% halftone will appear pink even though when viewed up close the ink is not pink but small red dots on a white background. Using halftones to reduce the number of print colors can save you money.
Halftones are used to reproduce photographic images. This is how newspapers print photographs. A high resolution grayscale photograph can be easily halftoned into a 1 color print with surprisingly good results.
Halftones make full color printing possible. Using software, a full color image can be separated into Cyan, Magenta, Yellow, and Black channels. These channels can be halftoned onto separate films, burned onto screens and the full color image can then be reconstructed on press.
Limitations
Good halftones require high resolution original art: at least 300-600 dpi at full print size. Lower resolution art can hurt the final print quality.
We print halftones with line counts ranging from 25 LPI to 65 LPI depending on the ink and the particular application. For a variety of reasons we generate our own halftones and prefer that designers leave any elements intended to be halftoned as vector gradients or raster objects in the art files.
A perfect, uninterrupted halftone print is not possible when printing onto fabric. Fabric surfaces are imperfect, and halftone dots are very small. Some printed dots will fall in the gaps between threads resulting in an imperfection that is usually only visible when viewed at very close up.
Halftones cannot be underbased. This means that color vibrancy on darker fabric colors is limited when printing halftones.
A halftoned image that looks correct when printed using a dark ink on a light shirt will likely look wrong when printed using a light ink on a dark shirt. Often if your image is going on both light and dark colored garments you will usually need to split the order into two jobs.