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Where Do Haltones Come From?

Begin by considering the the word Halftone. What is it supposed to mean...half of a tone?

Yes, it is a partial tone. Remember that printing machines are able to print ink onto paper. That is, a consistent coating of ink...not a lot of ink here and a little ink there, it is all or nothing.

Because the ink film can't be varied, or modulated, we have resorted to optical illusions to trick the eye into seeing tones of gray or colors. This is the age old trick of making patterns of many small marks on paper. At a distance, these patterns appear to our eyes to be partial tones.

Some handmade halftone methods are woodcuts, pen and ink crosshatching and stippling, scratchboard,splattering ink with a toothbrush.


The image at left uses pen and ink lines to give us the impression of gray tones.

Since the lines are all about the same thickness and dark areas have more lines per inch, you might call this a Frequency Modulated halftone technique.

The lilly was printed with CMYK process inks but it was not color separated from a colored painting. Look at the closeup; it was drawn as color separations with pen and ink. The artist used some stippled dots and some crosshatching and drew the individual CMY and K plates.

The invention of photographic processes led to many new techniques in printing. First, photographs made it possible to capture a realistic image. The photographs consisted of smooth tones of grays so they could not be printed in their native form. Artists were hired to copy the photos as pen and ink or woodcut drawings but this was a slow, expensive process.

Printers began experimenting with photographic methods and found that a photo image projected through a piece of gauze onto film or photo paper would produce a reasonable copy of the original image, but this copy image was made of dots resulting from the light passing through the grid of the gauze threads. Using photo film with very high contrast characteristics yielded a pattern of sharp dots which could be photoengraved onto a printing plate. The film was exposed in large Process or Copy Cameras built to hold art work and large sheets of film. Refining the process, they tried wire screens and screens etched onto glass. The glass screens gave good results when placed a short distance off the film surface during exposure, which put the screen grid slighly out focus.

After breaking a few of those extremely expensive glass screens, someone discovered that you could expose a piece of low contrast film through the glass screen and produce a fuzzy grid pattern. If you placed this sheet of gridded film in tight contact with the unexposed film and exposed it to the artwork image the halftone dots were produced. These film screens became the standard and were called Contact Screens.

Examples of mechanically generated screen patterns
The standard dot screen. The huge majority of print jobs use these round(ish) dots. They are usually slightly oval, to give smoother transitions through the 50% zone.
The straight line screen is a good attention getter but not good for rendering details.
A mezzotint-like pattern. These screens use varied concentrations of same-sized splats to make tones. These are often called Stochastic screens, the most common example of FM, frequency modulated, screens.
A round line screen. Many patterns can be used make halftones. They special effect tools to be used very sparingly.


Enter the Digital Revolution

Typesetting was the first area to use digital imaging. To avoid the laborious typesetting process using metal characters, various technologies were invented. They tried photo typesetting which was a miniature photo enlarger which projected pictures of each letter. They first put every letter in a font onto a negative which was enlarged onto film a letter at a time. They then used a small cathode ray tube (TV tube) to display a letter at a time to be projected onto film. These TV type images of letters were stored as digital information using the early computers.

It wasn't long before they realized it was possible to generate any image with these typesetters, not just letters.When they had to figureout how to scan art images into a digital form, an entire page of type, borders and line art graphics could be exposed onto photo film or paper. It was logical to rename these new typesetting machines as Imagesetters.

The first imagesetters could not produce halftone dots. They did not have enough resolution to create the tiny halftone dots, so the printers continued to shoot haltones in their copy cameras and put them in during the platemaking process.

As the imagesetters became more powerful it was feasible to generate the halftone dots using the much smaller imagesetter spots. They could scan existing photographic halftones but that would be limited by optical devices and dust and scratches. They decided to scan original photos and convert them to a grid of samples, or pixels, containing the color and brightness information (as digital numbers...Bits).

This grid of pixels, the Raster image, is a map of the tones from the orginal image...a Bit Map. It is just a collection of numbers. It is not a halftone. In fact, the files you create in Quark and Illustrator, etc. do not contain any halftone dots. They can describe the halfone dots, but it is the Raster Image Processor which makes the halftones.

The RIP is a computer. It takes your instructions and your raster image and figures out how to build a pattern of halftone dots to your specs. using the map of tones (pixels) in your raster image.

The RIP builds a pattern of dots; a certain number of rows of dots per inch (lines per inch) and it builds the lines of dots at a specific angle. For one-color halftones it is best to generate the dots at a 45 degree angle. This gives you cleaner details on the edges of horizontal and vertical objects.

The RIP will build the halftone pattern from whatever raster image you provide. You should provide enough pixels to describe the details in your image in greater detail than the halftone will produce. That is, give the RIP more pixels per inch than the halftone dots per inch you request. How many more pixels is described as the Quality Factor. You will get pretty good quality halftones if you provide 1.5 pixels for each 1 halftone dot. You get really good quality if you give the RIP twice as many the pixels as dots. The safe motto is "double the LPI for your PPI".

If you are in a demanding production environment, such as a newspaper, excess PPIs mean more storage on the computers and long RIP times. To streamline the production, you would experiment to see how low you can go with the PPI in you scans before the images look ugly. You may find that choosing a quality factor of 1.2 instead of 2 might shave 20 minutes off the RIP time, which gives the editors a little more time to squeeze in late-breaking stories.

Whoever runs the RIP controls the dot

Hear me now and believe me later when I tell you...the halftone dots are controlled by the RIP operator. The computer designer/artist does not control the size or shape of the dots.

At the RIP, the operator will set the LPI for the halftones to best work with the paper and press. The designer may discuss the screen ruling with the printer. It is a good idea to follow the printer's suggestion as to what works on his press.

How does a RIP make make halftone dots? Click here.
An explanation of Dots and Spots.
An explanation of the creation of halftone dots by the marking engine.