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Tech Papers

Understanding Machine Vision Verification of 1D and 2D Barcodes

by Barcode Quality for Reliable Process Operation, White Paper - OMRON Automation - Americas

Legible, accurate barcodes have never been more important than they are today, when automated supply chains depend on data accuracy to ensure the reliable performance of global operations. Machine vision verification is one tool that can be used to ensure that barcodes meet a consistent level of quality for readability in an automated process, and that bad codes are identified before they result in costly failures. This white paper introduces 1D and 2D barcode verification and identifies parameters for verifying codes against published standards.

  • Why Verify?
  • When Should You Verify?
  • Validation vs. Verification
  • What Kind of Solution Is Required?
  • Verification Evaluation Parameters
  • Verification Grades
  • Verification with AutoVISIONTM

Why Verify?
Barcode quality is integral to the success of an automated system. In a process where quality barcodes accurately store and communicate data – from code to reader to central system – little manual intervention is required. Thanks to quality bar­codes, the unique benefits of an automated system are realized: lower costs, higher productivity, and fewer errors. Poor quality barcodes, however, render the system almost as inefficient as using no automation at all. Unreadable barcodes may require re-labeling, re-scanning, or even manual entry of critical informa­tion by a human operator – disrupting the productivity of the process and causing a significant loss of time. Bad barcodes may prevent error-tracking, causing a domino effect of failures down the line and resulting in costly scrap and rework. All told, these effects completely counteract the benefits of implement­ing an automated system, the result being inflated cost, loss of productivity, and increased errors.

The purpose of barcode verification is to prevent this outcome and preserve the intended benefits of the automated system. Verification systems evaluate a barcode’s quality against pub­lished quality standards for 1D and 2D barcodes using precision instruments such as barcode verifiers or machine vision sys­tems. A verified barcode ensures consistent readability, support­ing 100% accurate automated data capture.

When Should You Verify?
To ensure that errors are prevented as early in the automated system as possible, verification must occur before a part enters the system. A verification step should occur after a part is marked or labeled with a barcode and before the part reaches the station where the barcode is first read.

 

When Should You Verify?


Proper verification ensures that every part is processed and shipped with a high-quality barcode, despite the fact that mark­ing and labeling systems will degrade over time. A verification system is much more accurate than a standard barcode reader at identifying low-quality barcodes early in the process, be­fore parts with bad barcodes make it through the line and are shipped to end customers. When barcode quality degradation is identified early, the marking or labeling system may be adjusted or replaced before unreadable barcodes are ever produced.
 

Reader to Check Mark Quality
 

Without verification, bad barcodes are not identified until they are unreadable. By the time a bad barcode is identified, con­ceivably thousands of poor-quality barcodes may have already escaped down the line.
With verification, bad barcodes are prevented from being applied to the product, eliminating the chance for future failures. 
 

Validation vs. Verification
Depending on the requirements of a particular process, industry, company, or customer, there are two levels of quality grading for ensuring barcode readability: validation (sometimes called process control) and true verification.
 
Validation/Process Control:Process control is a means of ensuring that barcodes are readable throughout a particular internal or internal/external process. Process control does not check barcodes for compliance to a published barcode qual­ity standard. Instead, it provides objective measurements for barcode quality when verification to a standard is not possible or not desired. If you are not concerned with meeting published barcode quality standards in your application, you may opt to use a subset of the default verification parameters in the verification system as the criteria for passing codes.
 
Verification: Verification ensures that a barcode complies with published barcode quality standards, such as ISO 15415, ISO 15416, and AIM DPM. To ensure compliance, all evaluation parameters in the machine vision system must be enabled dur­ing the verification process. Fully-conforming verification systems provide reports as evidence of barcode compliance, which can be sent to customers or other invested parties to provide the highest assurance of barcode quality and consistency.

What Kind of Solution Is Required?
True barcode verification relies on machine vision software that has been programmed for standards-based verification using parameters specific to ISO or AIM barcode quality standards. For compliance with each standard, codes must meet the minimum passing requirements of each parameter. Some commercially-available programs such as Microscan's AutoVISION allow the user to disable or modify the passing criteria of these parameters, enabling customized verification. With less exacting hardware requirements, and a number of easy-to-integrate smart cameras available to do the job, this type of solution can be a cost-effective way to ensure readable codes when compliance with published standards is not needed.

Selecting the right light and camera combination is key to the success of the application once an appropriate software platform has been chosen. More precise quality grading requires a higher-performance hardware solution. A smart camera with integrated optics and lighting will often be suitable for validation; however, barcodes that must comply with published barcode quality standards must be verified by a system with superior optics and with complete and uniform lighting by an ISO/AIM-compliant light to product an undistorted image.
 

Validation/Process Control:
Fully-Integrated Smart Camera
Vision System with Adaptable Software for Customized Validation

Verification: C-Mount Smart Camera Vision System with External Lighting and Standards-Based Verification Software
Verification:
 
 
Verification Evaluation Parameters
There are a number of verification evaluation parameters that determine barcode quality and they may be used for either true verification or validation/process control. Published barcode quality standards, such as ISO 15415, ISO 15416, and AIM DPM, require that a designated set of these parameters be met to ensure that a barcode is verified to the standard, while process control grading may require that a barcode meet only a subset of these parameters. Parameters for 1D and 2D barcode evaluation are shown below.
 
1D Verification Evaluation Parameters:
High Quality Symbol:

High Quality Symbol
 
Parameter
Description
Example
ISO 15416

Decodability

Legibility per a reference de­code algorithm
 

 
 


Defects

Voids in bars or spots in spaces
 

 
 


Edge Determination

Detection of all bars and spaces using a global threshold
 

 
 


Minimum Edge Contrast

Minimum reflectance difference for any bar/space combination
 

 


Minimum Reflectance

Reflectance of the darkest bar and the lightest space
 
 


Modulation

Relation between wide and narrow elements in the symbol
 
 

 


Symbol Contrast

Difference in reflectance between the darkest bar and the lightest space
 
 

 


Quiet Zone

Size of the quiet zone
 
 

 

 
 
2D Verification Evaluation Parameters:
High Quality Symbol:

High Quality Symbol
 
Parameter
Description
Example
ISO 15415
AIM DPM

Axial Non-Uniformity

Amount of de­viation along a symbol’s major axes
 
 

 
 



Symbol Contrast

Difference in reflectance be­tween light and dark symbol elements
 
 

 

 

Cell Contrast

Difference in grayscale value between light and dark sym­bol elements
 

 
 
 


Modulation

Difference in reflectance of light and dark symbol elements
 
 

 

 

Cell Modulation

Deviation in grayscale val­ues of symbol elements
 
 

 
 


Decodability

Legibility per a reference de­code algorithm
 
 

 



Fixed Pattern Damage

Damage to the quiet zone, finder pattern, or clock pattern
 


 



Grid Non-Uniformity

Amount of deviation of grid intersection
 
 

 



Minimum Reflectance

Minimum reflectance of light elements
 
 

 
 


Reflectance Margin

Degree to which each module is correctly distinguishable in comparison to the global threshold
 

 

 

Unused Error Correction

Remaining error correction avail­able
 
 

 



Print Growth

Variation of ele­ment size that could impede readability
 
 

 
For Reference Only
 
 
Verification Grades
Barcodes are graded by verification equipment like barcode verifi­ers and machine vision systems, which assign values 0-4/A-F to the barcode for each of the above-listed evaluation parameters. A barcode’s overall grade is determined by the worst result for each parameter, so the barcode is always as good as its poorest parameter. Typically, a barcode with a grade A, B, or C is consid­ered acceptable quality, while a grade D or F signifies a poorly-marked or poorly-printed barcode. It is possible that a barcode with grade D or F may still be readable within your system using certain equipment, but without verification there is no guarantee that this same barcode will be readable at other points in your supply chain, by different equipment, or by your customers. By verifying barcodes to an agreed-upon barcode quality standard, such as those put forth by ISO or AIM, it is no longer a question of a barcode's readability, but rather of a particular reader's ability to read a certain grade.
 
 
Verification with AutoVISION
Microscan’s AutoVISION Machine Vision Software provides powerful tools for both text quality verifi­cation (Optical Character Verification or OCV) and barcode quality verification. Through its intuitive user interface, AutoVISION enables easy setup of both offline verification and inline verification of products in production. Using AutoVISION and high-performance smart cameras and lighting, you can set up a complete verifi¬cation system to determine barcode quality for both process control and true barcode verification, including conformance to three published barcode quality standards:

ISO 15415                   AIM DPM                    ISO 15416

Clear and concise values are provided via the AutoVISION user interface to grade 1D and 2D barcodes for each parameter required by a particular barcode quality standard. AutoVISION assigns values 0-4/A-F to the barcode for each parameter and then the barcode receives an overall grade for meeting the quality standard.
 

 
A Data Matrix 2D barcode is verified against the AIM DPM quality standard using Microscan’s AutoVISION machine vision software.

Default parameters in AutoVISION are pre-set to grade barcodes against published barcode quality standards (ISO 15415, ISO 15416, and AIM DPM), but can be adjusted in the AutoVISION Symbol Quality Verification Tool to enable process control grading for barcodes that must meet application-specific criteria only.
 


Verification evaluation parameters are adjusted in the AutoVISION Symbol Quality Verification Tool to grade a barcode for internal process control.

 
Microscan Meets Verification Needs
From 2D barcode verifiers to complete, scalable machine vision systems like AutoVISION, Microscan offers a range of products to ensure that automation systems operate at peak performance thanks to quality and compliant barcodes. For engineers tasked with meeting quality control or global standards in marking and labeling, Microscan provides project evaluations to find the right barcode verification solution for any project.
 

 

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