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FAQ: Troubleshooting Problems Reading Codes

Problems Reading Barcodes

The thermal printing head may need to be replaced.



The figure on the right shows that the print is missing a line from the barcode through the number 1.
This is referred to as “missing dots”, which can cause a misread.

The barcode reader might be installed at a perpendicular angle to the target.

A barcode reader installed at a perpendicular angle to the barcode receives an intense reflection (specular beam) which results in a misread. This is why the barcode reader does not function properly.

Laser beam

Because glossy barcodes do not diffuse the reflected beam well.

Laminated barcode

Metallic or silver background

A laser beam reflects specularly off of a barcode with a glossy surface. It is difficult for the barcode reader to scan because such barcodes barely diffuse the reflection.

The quiet zone on the barcode label may be too narrow.

Since metal does not diffuse the beam, the barcode reader scans it as black. Because of this, the barcode’s quiet zone is too narrow (as shown here) and it is unreadable.

Laser beam
Metal surface

Ambient light may be the cause of the problem.

Sunlight may cause unstable reading when directed towards the reader.

Ambient light
Photoelectronic sensor

UPC/EAN and CODE128 have four types of bar and space widths.

Due to the nature of Dot Matrix printing, the width of each bar will vary throughout the length of the bar. Some positions will be wider, (1), while others will be narrower, (2). Such widths can be difficult to differentiate between, which may cause the barcode to be unreadable.

When printing a one-dot narrow bar.

Cardboard’s inconsistency may cause unstable readings.

A barcode is made up of bars (printing) and spaces (background). Both the print and the background can change due to inconsistent surface conditions that can cause unstable readings.

Problems Reading 2D Codes

The quiet zone may be too narrow.

Narrow quiet zone: The only white area is in the 2D code.
Influenced by the border: The code is surrounded by a black border.
No quiet zone: The code extends to the edge of the label.

Alignment pattern may be damaged.

DataMatrix codes are read by first locating the L-shaped frame called the alignment pattern. Due to this, the code cannot be read if the alignment pattern is damaged.

It may be difficult to read due to the color contrast.

Black Pattern
on a Blue Background

Black Pattern
on a Green Background

Red Pattern
on a White Background

2D code readers perform scans by looking at the contrast between the print pattern and the background. Therefore, scanning cannot be done if it is difficult to distinguish enough contrast.

The reader may not be installed at a proper angle, or a reader with polarizing filter may be needed.

Some 2D code readers are designed to be installed at an inclined angle, while others should be at a perpendicular angle to the target. If not properly installed, stable scanning cannot be performed.

Example of installation at an angle

Ambient light may be the cause of the problem.

2D code readers use lighting like a camera to perform scanning. Sometimes, when too much ambient light reaches the target or reader, the brightness becomes unequal and results in unstable scanning.

Photoelectronic sensor

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Want to learn more?

  • Barcode/Two-dimensional Code Verification Guide Book

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