Dealing with Motion Blur: How Barcode Scanners Handle Speed, Movement, and the Blurred Edge |
Subtitle: A Deep Dive into the Causes of Motion Blur, Its Effects on Decoding, and the Hardware and Software Techniques to Overcome It - with Real-World Examples from Symbol, Zebra, Honeywell, Datalogic, Cognex, and Microscan |
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Opening Summary |
Motion blur is the enemy of the barcode scanner. It occurs when the barcode moves relative to the scanner during the exposure or capture time. The result is a smeared image or a distorted signal, where the sharp edges of the bars and spaces are softened and blurred. This blurring makes it difficult for the decoder to accurately measure the bar widths, leading to decoding errors and failed reads. Motion blur is a particular problem in high-speed industrial applications, where barcodes are scanned on conveyor belts moving at several meters per second, but it can also occur in handheld scanning if the user moves the scanner too quickly. |
This article is dedicated to motion blur - its causes, its effects, and the techniques that engineers have developed to combat it. We will explore the physics of motion blur, the relationship between exposure time, speed, and blur, and the mathematical limits of blur-free scanning. We will examine hardware solutions, such as strobed illumination and global shutter sensors, and software solutions, such as deconvolution filters and adaptive decoding algorithms. We will look at how major companies have addressed motion blur in their products. We will see how Symbol (now Zebra) used high-speed scanning and strobed lasers to freeze motion. We will explore Honeywell's use of global shutter CMOS sensors in their imagers. We will examine Cognex's advanced algorithms for handling motion blur in machine vision applications. We will also look at Microscan's use of high-speed line scan cameras for conveyor belt applications. |
By the end of this journey, you will understand that motion blur is not an insurmountable problem but a design challenge that can be addressed through a combination of clever hardware and sophisticated software. |
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Full Article |
Section 1: What Is Motion Blur- The Smearing of the Signal |
Motion blur is the smearing or smudging of an image or a signal that occurs when there is relative motion between the barcode and the scanner during the exposure or capture period. Instead of a sharp transition from black to white, the transition becomes a gradual ramp. The sharp edges of the bars and spaces are lost, and the pulse widths become difficult to measure accurately. |
In a laser scanner, motion blur occurs when the laser spot is moving faster than the detector's response time, or when the mirror's scanning speed is not perfectly matched to the barcode's movement. In an imager, motion blur occurs when the barcode moves during the sensor's integration time. |
Section 2: The Physics of Motion Blur - Speed and Time |
The amount of motion blur is determined by two factors: the speed of the relative motion and the exposure time. The blur distance is the product of the speed and the exposure time. If the speed is 1 meter per second and the exposure time is 1 millisecond, the blur distance is 1 millimeter. If the module width (the narrowest bar) is 0.33 mm, a 1-mm blur will completely wash out the barcode. |
To avoid motion blur, the exposure time must be short enough so that the blur distance is much less than the module width. This is the fundamental trade-off: a shorter exposure time reduces blur but also reduces the signal strength. |
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Section 3: Motion Blur in Laser Scanners - A Different Problem |
In a laser scanner, the motion blur is not the same as in an imager. The laser scanner uses a focused spot that moves rapidly across the barcode. The signal is a time-varying waveform. Motion blur in a laser scanner occurs when the barcode moves during the time it takes the laser spot to sweep across a single bar. |
If the barcode moves at the same speed as the laser spot, the spot will dwell on the same bar, and the signal will be distorted. If the barcode moves in the opposite direction, the effective scanning speed is increased, and the bars appear narrower. |
Section 4: Strobing - The Classic Hardware Solution |
The classic hardware solution to motion blur is strobing. Strobing is the use of a very short, bright pulse of light to freeze the motion. The pulse is much shorter than the exposure time. The pulse provides the illumination, and the sensor integrates the reflected light. The motion is frozen during the short pulse. |
In a laser scanner, strobing is used by pulsing the laser at a very high frequency. The pulse width is typically a few microseconds. This freezes the motion of the barcode relative to the laser spot. |
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Section 5: Symbol's Strobing Technique - Freezing the Motion |
Symbol (now Zebra) used strobing in their high-performance laser scanners. The laser was pulsed at a high frequency, and the detector was synchronized to the pulses. The short pulse width froze the motion, allowing the scanner to read barcodes on fast-moving objects. |
The strobing technique was used in the Symbol LS5800 and other fixed-position scanners. |
Section 6: The Global Shutter - The Imager's Solution |
In an imager, motion blur is caused by the barcode moving during the sensor's integration time. The standard solution is to use a global shutter sensor. A global shutter exposes all pixels at the same time. This prevents the rolling shutter effect, where different parts of the image are captured at different times, causing distortion. |
The global shutter is essential for reading barcodes on moving objects. Honeywell's 1900 imager and Zebra's DS8100 both use global shutter sensors. |
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Section 7: The Rolling Shutter - A Motion Blur Trap |
The rolling shutter is a common type of CMOS sensor. It exposes the pixels row by row. If the barcode is moving, the rows are captured at different times, causing distortion. The barcode may appear skewed or tilted. This distortion can cause decoding errors. |
The rolling shutter is a problem for barcode scanning. The global shutter is the preferred solution. |
Section 8: Honeywell's Global Shutter - The 1900 Imager |
Honeywell's 1900 imager uses a global shutter CMOS sensor. The global shutter captures the entire image at the same time, freezing the motion. This allows the 1900 to read barcodes on moving objects. |
The 1900's global shutter is a key feature for its industrial applications. |
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Section 9: Zebra's Global Shutter - The DS8100 |
Zebra's DS8100 imager also uses a global shutter. The DS8100 is a premium presentation scanner. The global shutter ensures that the image is not distorted by motion. |
The DS8100's global shutter contributes to its high read rate. |
Section 10: Motion Blur and Exposure Time - A Trade-Off |
The exposure time must be carefully chosen. A shorter exposure time reduces blur but reduces the signal strength. A longer exposure time increases the signal strength but increases the blur. The exposure time is a trade-off. |
The optimal exposure time depends on the lighting conditions, the speed of the motion, and the sensitivity of the sensor. |
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Section 11: Motion Blur and Gain - A Second Trade-Off |
If the exposure time is reduced, the signal strength is reduced. The signal can be amplified by increasing the gain. However, increasing the gain also amplifies the noise. This is a second trade-off. |
The scanner must balance the exposure time and the gain to achieve an acceptable signal-to-noise ratio. |
Section 12: Cognex's Motion Blur Algorithms - Software Solutions |
Cognex, a leader in machine vision, has developed sophisticated algorithms for handling motion blur. These algorithms are part of their DataMan series of barcode readers. The algorithms use image processing techniques to de-blur the image. |
The de-blurring algorithms estimate the motion blur and apply a filter to reverse the blurring effect. This is a computationally intensive process. |
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Section 13: Deconvolution - Undoing the Blur |
Deconvolution is a mathematical technique that can undo the blurring effect. The deconvolution algorithm estimates the point spread function (PSF) - the blurring function - and applies an inverse filter to the image. |
The deconvolution algorithm can significantly improve the quality of a motion-blurred image. |
Section 14: Microscan's High-Speed Line Scan Cameras |
Microscan, a leader in industrial barcode reading, uses high-speed line scan cameras for conveyor belt applications. The line scan camera captures a single line of pixels at a time. The lines are assembled into a 2D image. |
The line scan camera has a very short exposure time per line, which reduces motion blur. The line scan camera is ideal for high-speed applications. |
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Section 15: The Line Scan Camera - A Specialized Solution |
The line scan camera is a specialized imaging sensor. It captures a single row of pixels. The object moves past the camera, and the rows are assembled into an image. The line scan camera is used in industrial inspection and barcode reading. |
The line scan camera is more expensive than a standard area scan camera. |
Section 16: The Speed of the Conveyor - A Known Parameter |
In a conveyor belt application, the speed of the conveyor is known. The scanner can use this information to compensate for motion blur. The exposure time can be synchronized to the conveyor speed. |
The synchronization is done by the scanner's firmware. |
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Section 17: The Encoder - A Speed Sensor |
An encoder is a sensor that measures the speed of the conveyor. The encoder is attached to the conveyor belt. The encoder provides a pulse train that is proportional to the speed. |
The scanner uses the encoder's signal to synchronize the exposure time. |
Section 18: The Motion Blur and the Module Width |
The module width is the narrowest bar or space. The module width is the critical dimension for motion blur. If the blur distance is larger than the module width, the barcode cannot be read. |
The scanner must ensure that the blur distance is much smaller than the module width. |
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Section 19: The Motion Blur and the Decoder's Tolerance |
The decoder has a tolerance for variations in the pulse widths. The tolerance can handle a small amount of motion blur. A large amount of motion blur will cause the decoder to fail. |
The decoder's tolerance is a design parameter. |
Section 20: The Motion Blur and the Adaptive Threshold |
The adaptive threshold is not directly affected by motion blur. The adaptive threshold tracks the signal's average level. The motion blur reduces the signal amplitude, but the adaptive threshold adjusts. |
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Section 21: The Motion Blur and the Hysteresis |
The hysteresis is not directly affected by motion blur. The hysteresis suppresses noise. The motion blur does not cause noise; it causes a smearing of the signal. |
Section 22: The Motion Blur and the Jitter |
The motion blur is not jitter. Jitter is the uncertainty in the edge timing. Motion blur is the smearing of the edges. They are different phenomena. |
Section 23: The Motion Blur and the Noise |
The motion blur is not noise. Noise is random variation. Motion blur is a deterministic smearing. They are different. |
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Section 24: The Motion Blur and the Scanning Speed |
The scanning speed is the speed of the laser spot or the imager. The motion blur is the relative speed between the scanner and the barcode. They are related but not the same. |
Section 25: The Motion Blur and the Exposure Time |
The exposure time is the time the sensor integrates the light. The motion blur is the product of the relative speed and the exposure time. |
Section 26: The Motion Blur and the Strobing |
Strobing is a technique for reducing motion blur. The strobing uses a short pulse of light. |
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Section 27: The Motion Blur and the Global Shutter |
The global shutter is a technique for reducing motion blur in imagers. The global shutter captures the entire image at the same time. |
Section 28: The Motion Blur and the Deconvolution |
Deconvolution is a software technique for reducing motion blur. The deconvolution algorithm estimates the blur and removes it. |
Section 29: The Motion Blur in Symbol's LS2208 |
Symbol's LS2208 is a handheld scanner. It is not designed for high-speed applications. The LS2208 can read barcodes that are moved at a moderate speed. The LS2208's laser scanning and fast comparator handle the motion blur. |
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Section 30: The Motion Blur in Honeywell's 1900 |
Honeywell's 1900 is an imager with a global shutter. The global shutter freezes the motion, reducing the motion blur. |
Section 31: The Motion Blur in Datalogic's PowerScan |
Datalogic's PowerScan is a high-speed industrial scanner. The PowerScan uses a combination of strobing and high-speed electronics to handle motion blur. |
Section 32: The Motion Blur in Cognex's DataMan |
Cognex's DataMan uses advanced deconvolution algorithms to handle motion blur. The algorithms are part of Cognex's patented decoding technology. |
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Section 33: The Motion Blur in Microscan's Line Scan Cameras |
Microscan's line scan cameras use a very short exposure time per line to handle motion blur. |
Section 34: The Motion Blur and the Future - Faster Sensors |
The future of motion blur reduction is faster sensors. Faster sensors have shorter exposure times. Shorter exposure times reduce motion blur. |
Section 35: The Motion Blur and the Future - Better Algorithms |
The future of motion blur reduction is also better algorithms. Algorithms like deep learning can estimate and remove motion blur more effectively. |
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Section 36: The Motion Blur - A Summary of Best Practices |
Based on our exploration, let us summarize the best practices for handling motion blur in a barcode scanner: |
1. Use Strobing: For laser scanners, use strobing to freeze the motion. |
2. Use a Global Shutter: For imagers, use a global shutter sensor. |
3. Reduce Exposure Time: Reduce the exposure time to reduce the blur. |
4. Increase Gain (if needed): Increase the gain to compensate for the reduced exposure time. |
5. Use Deconvolution: Use deconvolution algorithms to remove the blur. |
6. Use a Line Scan Camera (for conveyors): Use a line scan camera for high-speed conveyor applications. |
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Final Summary |
Motion blur is a significant challenge in barcode scanning, especially in high-speed industrial applications. It occurs when there is relative motion between the barcode and the scanner during the exposure or capture time. Motion blur smears the edges of the bars and spaces, making it difficult for the decoder to accurately measure the bar widths. |
We have seen how major companies have addressed motion blur. Symbol (Zebra) used strobing in their laser scanners. Honeywell and Zebra use global shutter sensors in their imagers. Cognex uses advanced deconvolution algorithms. Microscan uses high-speed line scan cameras. Each technique has its own strengths and is suitable for different applications. |
Motion blur is not an insurmountable problem. With a combination of clever hardware and sophisticated software, it can be effectively managed. |
cs@easiersoft.com