Printing & Design Tips: MAY 2007, #70

Hybrid Screening

For many years we were taught that, since a printing press can only print 100 percent black ink, photos had to be created with halftones. Halftones are simulations of continuous-tone images (images with subtle gradations between black and white) composed of large and small dots arranged in a regular pattern or grid. Light areas contain small dots; dark areas contain large dots. You can observe this under a loupe. From a distance, the reader perceives this pattern of black dots as white, black, and various shades of grey even though only black dots are present on a white background.

Dots on a halftone grid vary in size but not in placement. On a 150-line halftone screen, for each inch of halftone image, there are 150 large, medium, or small dots across the image and 150 large, medium, or small dots along each vertical inch of the image. Again, you can observe this under a loupe. From a distance, the dot pattern is rendered invisible to the eye, and you see what appears to be a smooth gradation.

Historically speaking, at the beginning of halftone technology, this was called halftone screening. Then “FM” screening, also known as stochastic screening, was invented. FM screens contain many more, but much smaller, dots than do traditional halftone screens. This actually creates more detailed images. All FM dots are the same size, and there is no “grid,” or regular-spaced pattern of dots. Traditional halftone screening, as mentioned in the above two paragraphs, was then renamed “AM” (amplitude modulated) screening. Amplitude refers to size. A dark area of a photographic image in a traditional halftone screen would contain a number of large dots of ink in a regular pattern, some dots large enough to actually touch one another. In contrast, an FM-screened halftone of the same image would contain significantly more (but much smaller and all the same size) dots all placed at random. Dark portions of the image would contain many small dots grouped together. Light areas would contain a scatter-pattern of a few, small FM dots. All in all, this approach provides far crispier detail than traditional halftone screening.

AM and FM screening both have limitations, though. With AM screens, as shades of black get darker (as the halftone dots get larger in the range from white to black), there seems to be an abrupt shift at the point at which the dots actually touch one another. AM screens are said to have a rather dramatic tone shift in the middle range, causing a bothersome tone or color shift in flesh tones. FM screens, on the other hand, are hard to hold (or maintain without disappearing) on roll-fed web presses, since the FM dots are much smaller than AM dots. Therefore, neither option is always ideal.

Now, along comes “hybrid screening.”

Hybrid screening (also known as “cross modulated screening” and by many other names) places the miniscule FM dots on a regularly spaced AM grid. Initially, the AM dots are used until they can no longer be held on press (that is, until highlights lose all dots). Then they are replaced with FM dots, by using computer algorithms. Of course, since presses vary in their ability to print a dot at all levels from highlights to shadows, the particular characteristics of the press in use must be considered in making this transition from AM to FM screens.

The good news is that hybrid screening will hold a dot pattern in highlights, then it will allow for a smooth transition in the mid-tones, and finally it will continue to maintain detail in the shadows. (By their very nature, AM and FM screening used separately must sacrifice either smooth mid-tone transitions or detail in the highlights or shadows.) In addition, higher than usual halftone line screens can be printed with hybrid technology (such as 340 lpi, in contrast to 133 lpi for non-heatset web presses and 133-175 lpi for sheetfed work). Even under a loupe, the image looks almost like a continuous-tone photo because the screening pattern is so small and random.

In addition to flesh tones, food, and automotive images, hybrid screening is useful in rendering process tints, thin rules, and small process-build text. Whenever using halftone and hybrid technology, consult your print provider, and always get proofs during the course of your job.

What is Magnetic Ink?

If you look closely at the bottom of your checks, you will see a series of numbers, which banks refer to as MICR, and which render specific information related to the transaction. Inks used for printing these numbers and symbols have magnetic properties. These numbers and symbols are recognized by the bank’s automated “readers” or “sorters.” They often must be read up to thirty times without degrading during transactions. (This is in contrast to optical character recognition, which actually “reads” letter forms of text and has no relation to magnetic substances. In many cases, however, OCR is replacing, or being used along side of, MICR.)

When you feed preprinted checks into your laser printer or ink-jet printer, the information added by your computer software usually just includes the payee and the amount of the check. Therefore, this information can usually be printed with non-magnetic inks or toners. If, on the other hand, you plan to actually print the MICR information yourself, you need to use the special magnetic toners available for this purpose.

Why does this matter? For two reasons: First, the concept of magnetic ink is interesting and novel. Second, you need to keep in mind that MICR information printed in non-magnetic ink will be rejected by the automated equipment, and you may incur an extra fee (several dollars for each check) for manual processing.

The safest approach is to have your bank print the MICR information at their print shop in magnetic ink. Then, you need to only print the payee, check amount, etc., on your home-office laser printer.

[Steven Waxman is a printing consultant. He teaches corporations how to save money buying printing, brokers printing services, and teaches prepress techniques. Steven has been in the printing industry for thirty-three years working as a writer, editor, print buyer, photographer, graphic designer, art director, and production manager.]