BARCODE
QUALITY PARAMETERS:
Bar code print quality is not as simple as verifying bar code scan-ability. Most users will not have scanners as sophisticated as the verifiers used in manufacturing plants to validate bar code readability. Also, bar codes verified in-plant are new, unused and presumably in perfect condition. Bar codes in live applications may be damaged, stained, etc. Verifying symbol quality only tells you how well the symbol was printed. It doesn’t determine whether the correct symbology or data was printed, whether it was formatted correctly, etc.
There are a number of parameters that make up a successful bar code scan. Within the bar code itself, these parameters include:
Decodeability:
Decodability indicates the amount of tolerance remaining in the width of the most deviant element in the symbol. Bar codes are made up of elements (bars and spaces) of different widths. For example, Code 39 has two element widths…one narrow and one wide. Each element within a bar code symbol should be consistent across the symbol. In other words, all of the narrow elements (bars) should be consistent in width and all the wide elements (bars) should be consistent in width.
The decodability grade indicates the amount of error in the width of the most deviant element in the symbol. The lower the deviation, the higher the grade. Optimally, each element size in a bar code symbol would be consistent across the symbol. A decodability failure means that the width of the bars and/or spaces in one of the elements is so inconsistent that it cannot be evaluated. A decodability failure results in an overall symbol failure regardless of the quality of the other parameters.
NOTE:
- Barcodes printed in the horizontal orientation (picket fence) will
be more consistent that barcodes printed in the vertical orientation
(ladder), which tend to have dimensional errors.
Quiet Zone:
This is the area immediately preceding the start character and following
the stop character in a barcode symbol as specified in a particular application
and/or symbology specification. The quiet zone immediately preceding the
start character and following the stop character must each be a minimum
of ten times the narrow element for all symbologies except UPC, EAN, and
2-D symbologies. Scanners expect the quiet zone to be free of any low
reflectance objects (dark areas). If a low reflectance object is detected
in the quiet zone then the scanner may not distinguish the start or stop
signal resulting in an unsuccessful scan.
NOTE:
- Do not print barcodes too close to the edge of the label, tag, or
document.
- Do not print other test or lines too close to the leading edge or
trailing edge of the barcode.
- Quiet zone requirements for 2D (two dimensional) barcodes very with
each Symbology, but can be as little as .01 inch.
- 2D barcodes typically require quiet zones on all four sides of the
symbol.
Print Contrast Signal:
Symbol contrast is the difference in reflectance values on the "lightest"
space (including the quiet zone) and the "darkest" bar of the
symbol. The greater the difference between light and dark contrast, the
higher the performance of the code (barcode colors and/or background colors
can have a large effect on this parameter).
NOTE:
- Substrates with metallic surfaces will reflect as black and are therefore
not recommended for use with barcodes.
- Scanners read barcodes by distinguishing between the bars and spaces.
In order to create the best contrast, the bars should be black and the
spaces and background should be white. Other combinations work including
different bar colors and different background colors, as long as the
contrast between the bars and spaces is distinguishable by the scanner.
When the contrast gets too low, the scanner will not distinguish between
the bars and spaces and will therefore not see the barcode.
Reflectance:
The reflectance value of the "lightest" space must be at least
twice as great as the reflectance value of the "darkest bar."
Edge Contrast:
Edge contrast is the difference of the reflectance values of the dark
and light components of an edge. Each element of a barcode has two edges
that go from dark to light. Therefore, each edge has a dark and light
reflectance value component.
Modulation:
Modulation is based on the relationship between the minimum edge contrast
and the symbol contrast. Ideally, the edge contrast should be equal to
symbol contrast but as an aperture size approaches an element size the
amplitude of the signal received will decrease and the edge contrast will
decrease. The greater the difference between minimum edge contrast and
symbol contrast, the lower the grade.
NOTE:
- Aperture size has the greatest affect on this parameter.
- The substrate material can also have a major affect on this.
Defects:
Defects are irregularities in bars, spaces and quiet zones. Defects
are measured based upon the relationship between the largest defect in
the symbol and the symbol contrast. The smaller the defect, the better
the grade. In general, a small aperture used to analyze very wide elements
will cause the largest defects.
NOTE:
- Substrates having recycled content can pose problems. Some recycled
content may be darker or protrude slightly from the surface. Such characteristics
can create misreads of barcodes.
- Maintenance;
Barcode quality degrades as miscellaneous deposits (paper dust, core,
adhesive debris) build up in the printer and on the print head. Because
of the density of barcode printing, it is probable that printer maintenance
be required more frequently that outlined in the manufacturer's recommendations.
Worn print heads or ribbon creases can result in barcode symbols having
missing bars or horizontal white streaks through the bars.
Printed barcodes are denser than standard text. As a result more printer
consumables are used and few labels/tags/documents can be printed before
maintenance is again required. More frequent maintenance will minimize
print defects.
BARCODE TROUBLE SHOOTING:
There are a number of general steps that can be performed to identify
or isolate a barcode problem. They include:
- Inspect the label for obvious damage including tears, dirt, or ink/pencil
marks.
- Determine if there are quiet zones at each end of the barcode. There
should be a clear area approximately .25 inch or ten times the narrow
bar width at the before and after the barcode. For 2D (two dimensional)
barcodes, refer to the symbol specifications provided with your software.
- If there is no obvious damage and there are quiet zones, you will
need to verify that the materials and scanner are compatible. Infrared
scanners require substrates that are designed for use with infrared
scanners.
- Verify that the scanner you are using is operating correctly, by
scanning another similar barcode symbol. If the second barcode symbol
can be scanned, then the scanner is not the problem. If the barcode
cannot be scanned, it may be the cable, if your scanner is attached
to a decoder via a cable. If you can disconnect the cable from the scanner
and the decoder, use a different scanner or a different cable in order
to isolate whether the cable and/or scanner is operational.
- If you're using a scanning terminal or an integrated scanner/decoder
and all of the previous steps have failed, make sure the scanner is
set to the parameters you need for your application.
- Depending on the scanner manufacturer, the dip switches in the
scanner may have to be reset. Refer to your scanner manual.
- Confirm that the Symbology used to encode the barcode is compatible
with scanner and decoder.
- Confirm the scanner's optical settings are compatible with the
barcode you are trying to scan.
NOTE:
The aperture size of the scanner can be adjusted to read barcodes
more accurately. However, there is not a "one size fits"
all solution. An aperture setting appropriate for one barcode size
will not necessarily result in the successful scan of a different
bar code x dimension.
- If there is no visible problem with the barcode and the scanner is
confirmed as operating satisfactorily, you will need to confirm that
the scanner and decoder are configured to read that barcode Symbology.
- If the scanner/decoder is configured to read that Symbology, the
next step would be bar code analysis. This will require a barcode verifier.
- If the verifier cannot decode the barcode, check for print head defects,
printer speed variations, toner variations, ribbon defects, and the
printer heat settings.
- Print quality consists of a number of factors and procedures. If
the problem remains unsolved, it may be caused by the choice of Symbology,
the format (including the x dimension, the ratio, code height, check
digit, etc.), the substrate, and even the data file that was encoded.
It may be the printing software did not correctly encode the barcode.
This may be the result of converting to new printing software, or utilizing
printing software programmed incorrectly. In such cases, a common oversight
is the exclusion of start/stop characters.
- If the barcode is scannable by a verifier, the parameters causing
the problem will be identified. A verifier will identify the decodability,
print contrast, reflectance, edge contrast, modulation, and defects
(see detail above on these subjects).
PRINTER CONSIDERATIONS:
Barcode scanning success can be improved by the following:
- Thermal/Thermal Transfer:
- Printer speed:
Check speed and heat settings, confirm that the substrate is compatible
with the printer, that the ribbons are compatible with the stock.
- Print-head temperature:
Since thermal transfer printing involves imaging through tiny heated
prints contacting a ribbon onto a label, the print head temperature
has a dramatic effect on symbol quality. If the heat setting is
too high, the bars will print too heavy. Conversely, if the setting
is too cool, the bars within a symbol will print too light. The
heat setting can be adjusted either as a printer function or through
a software setting. Adjustment control will vary between software
and printers.
- Printer Speed:
The rated print or run speed (normally stated in inches per second)
of thermal/thermal transfer printers is rarely achieved when printing
barcodes. If the print speed is too fast, the heating elements do
not have enough dwell time at the substrate surface to fully transfer
an image, typically resulting in bars that are too narrow and/or
too light.
- Laser:
Check the toner and density setting, as well as the minimum x dimension
(should not be less than 10 mil).
- Dot Matrix:
Check for ribbon/print head spacing to substrate. Spacing needs to be
adjusted based upon the thickness of the substrate or document. Barcodes
will not be scannable on multiple plies.
The problem solver sections on thermal transfer, thermal, and laser has
more specific bar code printing problem solutions as relates specifically
to their printing processes.
BARCODE PROBLEMS:
- The barcode will not scan.
Review Barcode Quality Parameters.
See the Barcode Trouble Shooting Guidelines
Review Printer Considerations.
Go to the printer specific problem
solver sections (Thermal Transfer, Thermal, Laser).
- The barcode scans at a distance, but not up close.
This can frequently happen in a warehouse setting where both large and
small barcodes are utilized.
See depth of field information.
Potential Cause:
Different scanners are built to scan barcodes of different x-dimensions
from a certain range of distance (depth of field). The scanner may not
set or suited to scan a barcode of that x-dimension from the distance
attempted.
Potential Solution:
Try scanning a barcode with a smaller x-dimension from the desired distance.
If this works, the problem was the size of the barcode x-dimension.
(see link to depth of field) See depth of field information.
- The barcode scans up close, but not at a distance.
This can frequently happen in a warehouse setting where both large and
small barcodes are utilized.
Potential Cause:
Different scanners are built to scan barcodes of different x-dimensions
from a certain range of distance (depth of field). The scanner may not
set or suited to scan a barcode of that x-dimension from the distance
attempted.
Potential Solution:
Try scanning a barcode with a larger x-dimension from the desired distance.
If this works, the problem was the size of the barcode x-dimension.
(see link to depth of field)
- I have relabeled my cartons (new barcode label over old barcode label)
and the new codes are not reading correctly now.
See depth of field information.
Potential Cause:
The label stock is too transparent. The scanner is reading graphics
and/or barcode data off the original label (the label that is covered)
as well as bar codedata off the new label. The "overlay" of
dark symbols from both labels creates a nonreadable field.
Potential Solution:
The new label should be manufactured with a more opaque material and/or
a blockout (black or gray tinted adhesive) that will increase opacity
and cover the data from the original label.
