Testing and Calibration: The Final Steps to a Reliable Barcode Reader |
Executive Summary |
This article provides a comprehensive exploration of testing and calibration procedures for barcode readers, the essential final steps that transform a working prototype into a reliable, production-ready device. We examine how systematic verification ensures that the analog gain, digital threshold, illumination, and optics are all properly adjusted to decode barcodes accurately in real-world conditions. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real products from industry leaders including Cognex, Omron Microscan, Datalogic, and NEC. We explore the use of NIST-traceable calibration cards, the importance of regular recalibration to account for environmental changes, the procedures for testing symbol contrast and reflectance values, and the difference between verification and calibration. The article covers both the fundamental principles and the practical implementation details that make testing and calibration an indispensable part of barcode reader design and deployment. The closing summary synthesizes the key lessons and offers practical guidance for anyone implementing or maintaining barcode reading systems. |
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Chapter 1: The Final Frontier --- From Prototype to Product |
A barcode reader that has been carefully designed, assembled, and programmed is not yet ready for the field. The component tolerances, the aging of the LED, the variations in the photodetector, and the countless other variables mean that the reader must be tested and calibrated before it can be trusted to decode barcodes reliably. |
Testing and calibration are the final steps in the barcode reader development process. They ensure that the reader meets its specifications, that it can decode the required symbologies at the specified distances and angles, and that it will continue to perform reliably over its lifetime. |
The calibration process is particularly important for barcode verification systems, which are used to grade barcode print quality according to ISO standards. As a Cognex FAQ explains, 'To be compliant with the International Organization for Standardization (ISO) standards, barcode verifiers need to be calibrated regularly.' For ordinary barcode readers, calibration ensures that the analog gain and threshold are set correctly for the specific optical and electronic components in the unit. |
The NEC Platforms patent on signal processing for readers describes the problem that calibration addresses: 'With respect to the conventional bar-code reader, there are cases where a bar-code cannot be decoded and an error in reading arises by the bar-code's defect such as unfulfillment of contrast ratio of black bars/white bars configuring a symbol in part or all of the bar-code.' Calibration helps the reader compensate for these variations and defects, maximizing the decoding success rate. |
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Chapter 2: The Calibration Test Card --- The Reference Standard |
At the heart of the barcode reader calibration process is the calibration test card. This card is a precisely manufactured reference standard that contains barcode symbols with known, certified characteristics. The reader is calibrated by scanning these symbols and adjusting its internal parameters until the measured values match the certified values. |
The calibration test cards used in the barcode industry are typically NIST-traceable, meaning that their measurements are linked to national standards maintained by the National Institute of Standards and Technology. This traceability ensures that the calibration is accurate and consistent across different readers and locations. |
Cognex explains the role of these cards: 'Conformance calibration cards are a widely used industry tool designed to support the proper calibration of verifiers. The card contains 'Conformance Test Symbols' with intentional imperfections that check the reporting capabilities of the verifier and document conformance to industry standards such as ISO 15415, ISO 15416, and GS1 specifications.' |
The Applied Image calibration card for UPC/EAN symbols is a typical example. It is 'calibrated using an extremely high-resolution reflection micro-densitometer as well as NIST Traceability, for reflectance and linear dimensions, individually serialized and is Judge Certified in accordance with ISO-15416 and ISO-15426-1 and ANSI X3.182 standards.' |
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Chapter 3: Calibrating the Analog Gain and Threshold |
For a barcode reader, the most critical calibration parameters are the analog gain and the digitization threshold. These determine how the reader converts the photodetector's analog signal into a digital pulse stream. |
The analog gain must be set so that the signal from the white spaces is within the operating range of the digitizer. If the gain is too low, the signal will be too small to detect. If the gain is too high, the signal will saturate, clipping the waveform and distorting the barcode pattern. |
The digitization threshold must be set so that the comparator switches correctly between bars and spaces. If the threshold is too high, narrow bars may be missed. If the threshold is too low, noise may be interpreted as bars. |
The NEC Platforms patent describes a sophisticated automatic calibration method that uses multiple thresholds to detect signal amplitude and offset variations. The system uses 'comparators [that] compare a level of the output signal of the variable amplifier with a plurality of thresholds' and then 'detects either or both of a shift of the offset of the variable amplifier and an excess or deficiency of the gain' based on the comparison results. |
The calibration test card provides the reference signals needed for this adjustment. By scanning the known symbols on the card, the reader can determine whether its gain and threshold are correct and make the necessary adjustments. |
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Chapter 4: Setting the Illumination Brightness |
The illumination brightness is another critical calibration parameter. The LEDs or laser must be driven at the correct current to provide sufficient light for reading without saturating the photodetector. |
A patent from NEC Platforms describes a system that adjusts the gain and offset of the variable amplifier based on the detected signal characteristics. The system includes 'an adjusting means configured to adjust either or both of the offset and the gain of the variable amplifier based on the result of the detection.' This same principle can be applied to adjust the illumination brightness. |
In practice, the illumination brightness is often set by measuring the signal from the calibration card's white spaces and adjusting the LED current until the signal reaches a target level. This ensures that the reader can read barcodes with varying contrast levels. |
The Omron LVS-95XX calibration procedure states: 'The software automatically adjusts to the proper brightness level.' This indicates that modern verifiers handle illumination calibration automatically, using feedback from the image sensor. |
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Chapter 5: The Calibration Procedure in Practice |
The calibration procedure for a barcode verifier follows a specific sequence. The Omron LVS-9580/9585 user manual provides a detailed example : |
1. The user clicks the 'Calibration' tab in the software. |
2. The Calibrated Conformance Standard Test Card is placed on a flat surface. |
3. The verifier window is placed over one of the Master Grade barcodes on the card, with the rubber feet resting on the surface. |
4. The user ensures the blue line on the screen travels through the middle of the 'PASS' portion of the barcode. |
5. The 'Goal' values in the software are checked against the values printed on the card. These include Decodability, Contrast, Modulation, and Rmax values. |
6. The user clicks the 'Calibrate' button. |
The system then performs the calibration. Successful calibration is indicated by a green 'Calibration OK' message. Failed calibration is indicated by a red 'Calibration Needed' message. |
The manual notes: 'The calibration score will hardly ever match exactly; this is normal and acceptable as long as the scores are within +/- 3 percentage points.' This tolerance accounts for minor variations in the positioning and the reader's optics. |
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Chapter 6: Verifying the Calibration Results |
After calibration, the results should be verified to ensure that the reader is performing correctly. This is done by scanning the calibration test card's symbols and checking that the measured values match the certified values. |
The Cognex calibration FAQ explains how to verify the results: 'Check to see that the verifier is returning the correct Rmin/Rmax values by verifying the test symbol as you would a normal code. In the verification software for Cognex verification products, the Rmin and Rmax values are found next to the Symbol Contrast grade in the Quality Detail section... These values should match those shown on the card and allow small tolerance variations.' |
If the measured values do not match the certified values, the calibration should be repeated. If the calibration repeatedly fails, it may indicate a problem with the reader's optics or electronics that requires service. |
The Omron LVS-9580/9585 manual notes: 'It may take two or three attempts before calibration is complete. If calibration continues to fail, contact Omron Microscan or an Omron Microscan representative for further instructions.' |
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Chapter 7: Calibration for Different Symbologies |
Different barcode symbologies require different calibration procedures and test cards. The most common calibration cards are for UPC/EAN, GS1-128, and Data Matrix symbols. |
The Omron LVS-95XX series uses different calibration cards depending on the field of view and the symbologies being verified : |
EAN/UPC Calibration Cards: Used for systems verifying EAN/UPC symbols. The 'EAN-13 Master Grade' code is used to calibrate systems in Europe, while the 'UPC-A Master Grade' code is used for systems within the United States. |
GS1-128 Calibration Cards: Used for larger fields of view, such as those for verifying wide linear barcodes. The 'PASS' portion of the GS1-128 barcode is used for calibration. |
Data Matrix Calibration Card: Used only for calibrating the LVS-9585-DPM-HD and LVS-9580-DPM-HD systems for direct part marking verification. Symbol 1 on the card is used for calibration. |
The calibration card compatibility is essential. As the Omron manual explains, 'The LVS-95XX system recognizes only these cards for calibration.' |
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Chapter 8: Calibration for Color Imaging Systems |
Color imaging barcode readers, which are increasingly used for product recognition applications, require color calibration in addition to the standard reflectance calibration. The Datalogic patent describes a method for real-time color calibration of imaging systems for item recognition . |
The method involves 'setting an RGB imager pixel color gain' and then 'capturing at least one image with active or ambient illumination using the color imager with a calibration target in view.' The system then 'calculates a mean value of R, G, B pixels for the at least one image' and uses a color plane as a reference to determine the color gain parameters. |
The patent notes that color calibration is particularly important for item recognition in retail environments, where 'illuminants estimation error from (AWB) may cause color shift, which... may limit item recognition accuracy which can vary in different illumination conditions.' |
While a traditional barcode reader does not require color calibration, readers that perform product recognition or that read color 2D codes may benefit from this capability. |
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Chapter 9: Calibration Frequency and Maintenance |
Barcode verifiers must be calibrated regularly to maintain their accuracy. The frequency of calibration depends on the usage and the environment. |
Cognex recommends 'calibrating every 30 days to account for environmental changes like ambient light and temperature.' The calibration cards themselves have a limited shelf life and must be replaced periodically. Cognex notes that 'calibration cards expire two years from the in-service date recorded by the end user.' |
The Omron LVS-9580/9585 manual states that 'the Calibrated Conformance Standard Test Card should be replaced every two years.' The cards should be stored away from dust and direct sunlight when not in use to prevent degradation of the printed symbols. |
For standard barcode readers, calibration may be less frequent. A one-time factory calibration may be sufficient for many applications, particularly if the reader uses a stable LED illumination source and has a well-designed analog front end. |
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Chapter 10: The Difference Between Verification and Calibration |
It is important to distinguish between calibration and verification. Calibration is the process of adjusting the reader's internal parameters so that its measurements match the reference standards. Verification is the process of checking that the reader is performing correctly by scanning a known reference and comparing the results. |
The Cognex FAQ explains: 'Calibration is the process of mapping a camera's measurement to actual reflectance levels, establishing a baseline for verification software reflectance levels. The calibration process adjusts reflectance levels and checks these levels to ensure the most accurate verification results.' |
In a barcode verification system, calibration is performed using the calibration test card. Verification is performed by scanning the conformance test symbols on the same card and checking that the results are within the expected tolerances. |
For a standard barcode reader, the distinction is less formal. The reader is calibrated at the factory, and its performance is verified by scanning a test barcode. |
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Chapter 11: Factory Calibration of Standard Readers |
For mass-produced barcode readers, calibration is typically performed at the factory. The reader is placed in a fixture, and a known reference barcode is scanned. The gain and threshold are adjusted automatically until the reader can decode the reference barcode reliably. |
The NEC Platforms patent describes an automated calibration process that uses 'a variable amplifier configured to output a signal, as an output signal thereof, corresponding to an input signal, an offset and a gain of the variable amplifier being able to be adjusted.' The system then uses 'a comparing means configured to compare a level of the output signal of the variable amplifier with a plurality of thresholds' to detect gain and offset errors and adjust them automatically. |
The factory calibration data can be stored in non-volatile memory on the reader. If the reader is later repaired or if its components drift, it may need to be recalibrated. However, for many low-cost readers, a factory calibration is sufficient for the product's lifetime. |
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Chapter 12: Customer-Specific Calibration |
In some applications, the barcode reader must be calibrated for a specific customer's environment or symbology. For example, a reader used in a pharmaceutical manufacturing facility may need to be calibrated for the specific labels used in that facility. |
The calibration process for customer-specific applications is similar to the standard process, but it uses the customer's own labels or a custom calibration card. The reader is adjusted to decode the customer's barcodes reliably, considering the specific print quality, contrast, and substrate. |
The Omron LVS-95XX systems support customer-specific calibration by providing configurable goal values. The user can 'enter the Decodability, Contrast, Modulation, and Rmax values from the UPC/EAN Calibration Card in the Goal column.' This allows the system to be calibrated to the specific values of the customer's test card. |
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Chapter 13: NIST Traceability and ISO Compliance |
NIST traceability and ISO compliance are essential for barcode verification systems. NIST traceability ensures that the calibration is linked to national measurement standards, providing confidence in the accuracy of the measurements. ISO compliance ensures that the verification is performed according to international standards. |
The Applied Image calibration card is 'NIST Traceable' and 'Judge Certified in accordance with ISO-15416 and ISO-15426-1 and ANSI X3.182 standards.' The card includes a 'Calibration Report' and a 'Certificate' that document the calibration results and the estimated uncertainty. |
The Omron LVS-95XX systems comply with 'ISO 15415, ISO 15416, ISO 15426-1, and ISO 15426-2 standards.' The calibration process is designed to ensure that the system meets these standards. |
The Cognex FAQ explains the importance of ISO compliance: 'To be compliant with the International Organization for Standardization (ISO) standards, barcode verifiers need to be calibrated regularly.' |
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Chapter 14: Environmental Effects on Calibration |
Environmental factors can affect the calibration of a barcode reader. Temperature, humidity, ambient light, and dust can all change the reader's performance, requiring recalibration. |
Cognex recommends recalibration 'every 30 days to account for environmental changes like ambient light and temperature.' The Omron LVS-9580/9585 manual notes that 'if calibration continues to fail... be sure there is no direct light shining on the viewing stage.' |
In a reader's final integration, the designer may need to account for environmental factors by calibrating the reader at the factory, storing the calibration data in non-volatile memory, and providing the user with a means of recalibration in the field. |
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Chapter 15: The Role of Software in Calibration |
Modern barcode readers and verifiers rely heavily on software for calibration. The software guides the user through the calibration process, validates the results, and stores the calibration data. |
The Omron LVS-9580/9585 software provides a 'Calibration' tab where the user initiates the calibration. The software displays the live image, the goal values, and the calibration status. When calibration is successful, the software displays a green 'Calibration OK' message. |
The Datalogic patent describes a software-based calibration method where 'a processor... set an RGB imager pixel color gain; capture at least one image... calculate a mean value of R, G, B pixels... determine a color gain to set calibration parameters; and save the calibration parameters if mean values of the R, G, B, pixels are within a predetermined range.' |
The software-based calibration is more flexible and can be updated as new symbologies and standards are introduced. |
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Chapter 16: Calibration of Wireless and Bluetooth Readers |
Wireless barcode readers that communicate via Bluetooth may require additional calibration to compensate for the wireless link's latency and packet loss. However, the core calibration of the optical and electronic components is the same as for wired readers. |
The Honeywell Voyager 1202g, a wireless laser scanner, does not require special calibration for its Bluetooth link. The calibration of the optical system is performed at the factory, and the wireless link is transparent to the calibration process. |
The ZKTeco ZKB104S, a wireless CCD scanner, also does not require special calibration for its wireless link. The reader is calibrated at the factory using the standard test card procedure. |
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Chapter 17: Calibration in a Production Environment |
In a production environment, barcode readers are often calibrated as part of the final test. A test fixture presents a known reference barcode to the reader, and the reader's gain and threshold are adjusted automatically. |
The NEC Platforms patent describes an automated calibration process that could be integrated into a production test environment. The system uses 'a variable amplifier configured to output a signal... an offset and a gain of the variable amplifier being able to be adjusted' and then detects and corrects gain and offset errors. |
The automated calibration reduces the time and cost of production testing, ensuring that every reader is correctly calibrated before it leaves the factory. |
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Chapter 18: The Cost of Calibration |
Calibration adds cost to the barcode reader manufacturing process. The test equipment, the calibration cards, and the time required for calibration all contribute to the product's cost. |
The calibration cards themselves are relatively expensive. The Omron 98-CAL020 EAN/UPC Conformance Calibration Standard Test Card is listed at $757.00, and the 98-CAL021 GS1-128 calibration card is $1,029.00. These costs are justified for verification systems, but they may be too high for low-cost barcode readers. |
For low-cost readers, the calibration is typically performed at the factory using a few test barcodes. The calibration is not NIST-traceable, but it is sufficient to ensure that the reader can decode barcodes in typical applications. |
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Chapter 19: Summary --- Testing and Calibration in Perspective |
Testing and calibration are the essential final steps in the barcode reader development process. They ensure that the reader meets its specifications, that it can decode the required symbologies, and that it will continue to perform reliably over its lifetime. |
We have examined how different companies and technologies have approached the challenges of testing and calibration: |
Cognex provides calibration FAQs and guidance for their barcode verification products. They recommend calibrating 'every 30 days to account for environmental changes like ambient light and temperature' and note that 'calibration cards expire two years from the in-service date.' |
Omron Microscan provides detailed calibration procedures for the LVS-95XX series verifiers. The procedure involves using a Calibrated Conformance Standard Test Card, verifying the goal values, and clicking the calibrate button. The system supports different calibration cards for EAN/UPC, GS1-128, and Data Matrix symbols. |
Datalogic has patented a real-time color calibration method for imaging systems used in item recognition. The method adjusts RGB color gain and uses a calibration target to set color correction parameters. |
Applied Image provides calibration cards that are 'NIST Traceable' and 'Judge Certified in accordance with ISO-15416 and ISO-15426-1.' The cards include calibrated UPC/EAN symbols and reflectance patches. |
NEC Platforms has patented a signal processing device that adjusts the gain and offset of a variable amplifier based on comparisons with multiple thresholds, enabling automatic calibration of barcode readers. |
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The key lessons from our exploration are: |
Calibration test cards are the reference standards. They contain barcode symbols with known, certified characteristics. The reader is calibrated by scanning these symbols and adjusting its internal parameters. |
NIST traceability ensures accuracy. NIST-traceable calibration cards are linked to national measurement standards, providing confidence in the accuracy of the measurements. |
Regular recalibration is essential for verifiers. Environmental changes, component aging, and other factors can affect the reader's performance. Regular recalibration ensures continued accuracy. |
Different symbologies require different calibration cards. UPC/EAN, GS1-128, and Data Matrix symbols each have their own calibration cards. |
The calibration procedure involves scanning the Master Grade barcode. The goal values in the software must match the values printed on the card. The calibration is successful when the measured values match the certified values within a tolerance. |
Calibration is different from verification. Calibration adjusts the reader's parameters; verification checks that the reader is performing correctly. |
Calibration adds cost but is essential for reliability. The cost of calibration is justified by the increased confidence in the reader's performance. |
In the end, testing and calibration are a testament to the importance of quality assurance in barcode reader design. They are the steps that transform a working prototype into a reliable, production-ready device. The art of calibration lies in the careful balance of cost and performance, creating a reader that can be trusted to decode barcodes accurately in the real world. |
cs@easiersoft.com