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
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
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.]