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The most frequently
asked question about UPC symbols on publications is: "What colors, screens,
tints, etc. can be used as background for the bars in the UPC symbol?" To
answer this question, first accept the fact that there are no absolutes in
printing or scanning UPC symbols. There are only specifications, guidelines,
and experience. But a quick answer would be:
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Use no black screens; |
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Use no more than 10% screen in process blue;
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Use no more than 50% screen in yellow and/or
red, singly or in combination; |
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Acceptability of PMS colors varies with the
ink formulation of black and cyan, causing no-scans. |
Please read on to
understand the reasons for these limits and the exceptions to them.
What
the laser 'sees'
Practically all
supermarket scanners (and most wholesaler return scanners) are based on a
helium-neon laser light beam which "sees" the yellow and red process
printing colors as light areas, and the blue and black colors as dark areas.
(This is something of a reversal from photography.) Since
the UPC decode is based
on the time it takes the beam to cross the dark bars and the light spaces in
between, it is obvious there must be sufficient color contrast between the
bars and spaces. To the laser, a 100% process blue background would be
indistinguishable from the black bars; but yellow and red background would
be OK. So why 10% process blue as a "usually acceptable" screen in the
"light" spaces between the bars? First, the blue dots in screens higher than
10% are usually of such a size that when the laser beam crosses them, it
thinks it has begun to see a dark bar. This confuses the scanner, and it
reports a no-scan. Second, when the blue dots butt against an actual dark
bar, the scanner thinks the bar is wider than it actually is and again
reports a no-scan.
Black
and blue
Screens of black cause
similar, but even greater, problems and thus are to be totally avoided. To
attempt to use blue screens over 10% and any black is to take a high-risk
gamble on scannability. Why is there a limit on yellow and red? While it is
true that process yellow and red are good background colors for laser
scanning of black and blue dark bars, don't overlook the effect of laying
one ink down on another.
The
ink problem
In the making of the
film master used to generate the bars on the printing plate, the width of
the bars is reduced to allow for bar width gain caused by the spread of the
ink as it is absorbed into the paper. This bar width reduction (taken by
shaving the bar edges) varies with the printing process (offset,
letterpress, gravure) and the paper (coated, uncoated, weight, newsprint,
etc). It is also affected by humidity, ink viscosity, heat treatment and
other variables.
When the black bars are printed on top of the red and/or yellow ink, the
absorption capacity of the paper will have been reduced. The
black ink will tend to spread as it
goes onto the still wet yellow/red
ink background. While there will
be no contrast problem, the distance between the bars
may be so
reduced, or the bar edge become
so irregular, that the symbol will
not scan. This is known as an "ink
trapping" problem. It has been
found through experience that a
limit of 50% in yellow and red,
singly or in combination,
minimizes these problems.
Remember, a UPC scanner
needs:
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good contrast between the dark
bars and the light spaces; |
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Straight bar edges so the
distances (thus time intervals) across
the bars and spaces are exact. A
good color contrast (solid black or
blue on white) "excuses" and enables a scanner to
read some symbols which would otherwise be unscannable due to irregular,
too wide or
too narrow bars. By the
same token, straight edge bars of
correct width will often "excuse"
some otherwise unscannable color
contrasts. |
Scanner variance
Each brand, type, and model of
scanner—even identical units—
may vary in ability to "forgive"
and decode out-of-spec symbols.
Some non-laser beam wand scanners (used by retailers
and wholesalers) may recognize symbols on
which a laser unit completely
strikes out, and vice versa.
However the UPC specifications
were not written
around any one
scanner, but around
the entire
technology. All
scanners can read
an in-spec symbol,
and that is the criterion
to use—not what can be
gotten away with!
Summary of guidelines
A few other reminders about
UPC symbols:
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The bar width gain or loss tolerance
for the 80% magnification
factor symbol used on most magazines is only ± 1.4 mil. One
mil = one
thousandth of an inch. |
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1.4 mil is about 1/3 the thickness of
the negative film used for
the film master.
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No photographic process can
hold this tolerance. Thus the symbol should never be
used as "art"
Over- or under-exposure and over-or under-development can
cause
excessive gain or loss of bar width.
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Even contact duplication should
be avoided if at all possible.
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The scanner sees what the
human eye cannot, and acceptable
cover graphics may still produce an
unacceptable, unscannable symbol.
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Never "reverse" a symbol. This
converts the dark bars to light
spaces and vice versa, and thus
destroys the symbol,
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From the above it should be obvious
that process red and yellow
can never be used as "dark bars"
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UPC over cover graphics is
unacceptable unless the black is
dropped out, the blue is less than
10%, and the yellow and red do
not total over 50%.
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Never print blue (or any other
color) under the black bars. Registration is
impossible. |
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Make sure the film master you
start with is an original, not a
second or later generation film.
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Order and use either negative or
positive film masters (not art) with
emulsion up or
down to match the
black ink film after color separation and before
platemaking. |
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In ordering film masters specify
letterpress or gravure as appropriate.
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The 90° variation in symbol
"orientation" (bars parallel or
perpendicular to the spine) is permissible to enable
the bars no be
printed parallel to the direction of
paper movement into the press. It
is not an aesthetic option, but is
meant to be used to minimize bar
width gain and edge irregularity.
However, the centered "0" must
be on the left or at the bottom for
wholesaler scanning.
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The retail checkout scanner and
the wholesaler return scanner do
not have sufficient range to cover
the entire width of a regular size
magazine. Therefore the guidelines call for the UPC
symbol to be
located in the lower left corner, so
that it is not necessary to inspect
the magazine to determine where
the symbol is located.
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There must be a sufficient light
margin on either side of the symbol. The centered
"0" protects the
left side. At least 1/16 inch should
be on the right. Always respect and
observe the cropmarks on the film.
If there are no cropmarks on the
film, it is probably not first generation film.
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