Pulsed vs. Continuous Illumination: The Art of Lighting in Barcode Readers |
Executive Summary |
This article provides a comprehensive exploration of the fundamental choice between pulsed and continuous illumination in barcode readers. We examine why this decision is critical for reader performance, affecting everything from image quality and motion tolerance to power consumption and user experience. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real products from industry leaders including Symbol Technologies, Texas Instruments, Smart Vision Lights, Opticon, and Code Corporation. We explore the physics of LED overdriving, the practical implementation of capacitor-based flash drivers, the challenge of flicker perception, and the hybrid approaches that combine both operating modes. The article covers both handheld and fixed-mount readers, with special attention to the practical trade-offs between image quality, battery life, and user comfort. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing or selecting barcode reader illumination systems. |
|
Chapter 1: The Fundamental Choice |
Every barcode reader designer faces a fundamental choice: should the illumination be continuous or pulsedThis decision affects nearly every aspect of the reader's performance, from the quality of captured images to the device's battery life and even the comfort of the user operating it. |
Continuous illumination is the simpler approach. The LEDs are powered on steadily, providing a constant light output. This is straightforward to implement and eliminates the complexities of timing and synchronization. However, continuous illumination has significant limitations. The LEDs cannot be driven beyond their continuous current rating without overheating, which limits the maximum light output. This can be problematic in low-light conditions or when reading barcodes from a distance. |
Pulsed illumination offers a compelling alternative. By driving the LEDs with brief, high-current pulses, the reader can achieve light intensities far greater than would be possible with continuous operation. This 'overdriving' technique allows the reader to freeze motion, improve signal-to-noise ratio, and extend reading range. However, pulsed operation introduces new challenges: the pulses must be synchronized with the imager's exposure, the power supply must deliver high peak currents, and the pulsing must be managed to avoid distracting flicker. |
This article explores the engineering decisions behind this choice, drawing on real-world examples from leading companies in the barcode reader industry. |
|
Chapter 2: The Physics of LEDs - Continuous vs. Overdriven Operation |
Understanding the physics of LEDs is essential to appreciating the difference between continuous and pulsed operation. An LED's brightness is directly proportional to its forward current. Under continuous operation, the current is limited by the LED's thermal dissipation capability. If the current is too high, the LED will overheat and its lifespan will be dramatically reduced. |
However, LEDs can be operated at much higher currents if the current is applied in short pulses. This is because the LED has thermal mass; a brief pulse of high current raises the junction temperature only slightly. The LED can be driven at currents many times its continuous rating, producing bursts of light that are significantly brighter than continuous operation. |
This principle is known as overdriving. In machine vision applications, overdriving can achieve light pulses that are many times brighter than the LED's continuous output. Smart Vision Lights, for example, describes their OverDrive technology as providing 'greater than 10 times brighter light pulses compared to standard continuous mode.' This capability is particularly valuable for high-speed barcode reading applications, where a bright pulse of light can freeze motion and improve image quality. |
Chapter 3: Continuous Illumination - The Simple Approach |
Continuous illumination is the default approach for many barcode readers, particularly those designed for lower-cost applications where extreme performance is not required. The LEDs are powered continuously, providing a steady light output. |
The primary advantage of continuous illumination is simplicity. The LED driver circuit is straightforward, and no timing or synchronization is required. The imager can capture images at any time, and the illumination will be consistent. |
However, continuous illumination has significant drawbacks. As described in a patent from a barcode reader manufacturer, 'continuous lighting is limited to what light output is available at 100% power. The only way to increase intensity of continuous lighting is to increase the size and wattage of the LED array. This also increases initial expense, operating costs and produces undesirable heat.' |
For handheld readers, continuous illumination also consumes significant power. The LEDs must be driven continuously, draining the battery even when no barcode is being scanned. This is why Code Corporation recommends enabling continuous illumination 'only with cabled units due to increased power consumption.' |
|
Chapter 4: Pulsed Illumination - The Performance Enhancer |
Pulsed illumination addresses the limitations of continuous operation by driving the LEDs with brief, high-current pulses. This technique, sometimes called 'strobing,' delivers intense bursts of light that can dramatically improve reader performance. |
The benefits of pulsed illumination are numerous. First, the high intensity of the light pulse improves the signal-to-noise ratio, making it easier to decode barcodes even in challenging conditions. Second, the short duration of the pulse effectively 'freezes' motion, allowing the reader to capture clear images of fast-moving barcodes. Third, the average power consumption is reduced because the LEDs are only active for a small fraction of the time. |
A Symbol Technologies patent describes how a high-intensity illumination source uses a capacitor-based drive circuit to deliver a current pulse to the LED. 'In a so called flash mode a short duration pulse causes the light emitting diode to emit a high intensity light pulse to illuminate the target.' This approach allows the LED to be driven at currents far higher than the battery could sustain continuously. |
Chapter 5: The Flicker Problem - A User Experience Challenge |
While pulsed illumination offers significant performance advantages, it introduces a critical user experience challenge: flicker. If the light pulses at a rate that is visible to the human eye, it can be distracting, uncomfortable, and even disorienting for users. |
The human visual system can perceive flicker at frequencies up to about 60 Hz. This is known as the flicker fusion frequency, the rate at which a flickering light appears to be continuous. If the LED pulses at a frequency below this threshold, the user will perceive the light as flashing. |
This is a significant concern for barcode readers used in retail environments. Cashiers and customers are exposed to the reader's illumination for extended periods, and flickering light can cause eye strain, headaches, and general discomfort. A patent from a barcode reader manufacturer notes that 'repetitive flashing illumination... has been observed to be highly distracting to users.' |
Smart Vision Lights' 'Hidden Strobe' technology addresses this exact problem. The technology 'allows LEDs to internally self-trigger thousands of times per second, pulsing faster than the human eye can perceive and creating the illusion of continuous light.' |
|
Chapter 6: The Flicker Fusion Frequency - Engineering the User Experience |
The flicker fusion frequency is a critical design parameter for pulsed illumination systems. To avoid perceived flicker, the pulse rate must be at least 60 Hz, and preferably higher. However, the imager's frame rate may be lower than this, creating a conflict. |
A patent on illumination pulsing methods describes the challenge: 'Visible illumination... must be pulsed at a lower luminous power and at a rate that is higher than the flicker fusion frequency of human vision (typically about 60 Hz) to avoid distracting users and bystanders. Unfortunately, imager frame rates may need to be slower (less) than the flicker fusion frequency, due to design constraints.' |
The solution described in the patent is to generate additional pulses that are not synchronized with the imager's exposure. The illumination pulses at a constant rate above the flicker fusion frequency, and the imager is synchronized to capture images during some of those pulses. This approach ensures that flicker is not perceived while still providing illumination for image capture. |
This technique is particularly important when using multiple wavelengths of illumination. The patent describes a system that alternates between visible and infrared illumination. Since infrared illumination is not visible, it can be pulsed only when needed. However, visible illumination must pulse at a rate above the flicker fusion frequency, even if the infrared pulses occur at a lower rate. |
Chapter 7: Symbol Technologies' Capacitor-Based Flash Driver |
One of the most comprehensive examples of a pulsed illumination system is described in a Symbol Technologies patent. This design uses an energy storage capacitor to provide the high current pulse required for flash mode. |
The drive circuit includes a DC-to-DC boost circuit that charges a capacitor to a voltage in the range of 10 to 16 volts. When the LED is triggered, the capacitor discharges through the LED, producing a brief, intense flash. The discharge current is controlled by a programmable current source, allowing the controller to adjust the intensity of the flash. |
The design also supports a 'torch' mode, where the LED is driven at a lower current for continuous illumination. This mode is useful for aiming and for low-light operation where a flash is not required. The torch mode consumes much less power than the flash mode, extending battery life. |
The patent describes how the controller can monitor the capacitor voltage to determine when charging is complete and when the capacitor is ready for discharge. This real-time monitoring allows the system to optimize the charge and discharge cycles, maximizing the number of flash events that can be delivered in a given period. |
|
Chapter 8: The Programmable Current Source - Controlling the Pulse |
The programmable current source is a key element of the capacitor-based flash driver. It allows the controller to adjust the LED current on the fly, adapting to different operating conditions. |
The circuit is coupled to a digital-to-analog converter that receives control signals from the host microprocessor or controller. The analog output from the DAC provides an analog input to an amplifier, which drives a transistor that controls the current through the LED. |
The current is determined by the voltage from the DAC divided by a resistance, as long as a minimum voltage headroom is maintained across the programmable current source. This allows the controller to set the LED current to any value within the operating range. |
The programmable current source is used in both flash mode and torch mode. In flash mode, the discharge current is controlled by the DAC. In torch mode, the DAC controls the current that is supplied to the LED by the boost converter. This flexibility allows the reader to adapt to different operating conditions, balancing performance and power consumption. |
Chapter 9: Hybrid Operation - Flash and Torch Combined |
Many barcode readers support both flash and torch modes, sometimes within a single scan. The Symbol Technologies patent describes a typical LED activation that 'uses an initial high intensity, flash of light output followed by a subsequent lower level constant LED output (torch).' |
This hybrid approach provides the best of both worlds. The flash provides the high intensity needed to capture a clear image, even in low-light conditions or at long range. The torch mode then provides continuous illumination for aiming and for subsequent scans. |
The timing of the flash and torch modes is controlled by the controller. The flash is triggered in synchronization with the imager's exposure, ensuring that the image is captured during the high-intensity pulse. The torch mode can be activated immediately after the flash, or it can be used independently when a flash is not required. |
This approach is particularly useful in handheld readers, where the user may need to scan multiple barcodes in quick succession. The torch mode provides a preview of the field of view, allowing the user to aim the reader accurately. The flash then provides the illumination needed to capture the image. |
|
Chapter 10: Smart Vision Lights' Hidden Strobe Technology |
Smart Vision Lights, a global leader in LED illumination solutions for machine vision, has developed an innovative approach to pulsed illumination called 'Hidden Strobe' technology. This technology addresses the flicker problem while delivering the benefits of strobing. |
The technology 'allows LEDs to internally self-trigger thousands of times per second, pulsing faster than the human eye can perceive and creating the illusion of continuous light.' This approach ensures that the benefits of pulsed illumination are realized without the distracting effects of visible flicker. |
The company's Lightgistics series lights are specifically designed for high-speed barcode reading and optical character recognition (OCR) applications. They feature Dual OverDrive technology, which 'combines SVL's Deca OverDrive and standard OverDrive engines and allows end users to attach polarizers that retain exceptional light output while handling any speed.' |
The JWL150-DO light includes an onboard charging capacitor 'designed to ensure powerful bursts of energy with a low consistent electrical draw.' This capacitor-based approach is similar to the Symbol Technologies design, but optimized for high-speed logistics applications. |
Chapter 11: Opticon's Illumination Control System |
Opticon, a manufacturer of barcode readers, provides a practical example of user-configurable illumination settings in their MDI-5350 imager engine. The device supports multiple illumination modes that can be configured via software commands. |
The MDI-5350 supports three detection modes: warm white illumination detection, green aiming detection, and no illumination detection. The warm white illumination mode is preferred in dark environments, while the no illumination mode uses ambient light and is most effective in well-lit areas. |
The device also supports configurable LED illumination brightness: standard brightness and minimum brightness. The minimum brightness setting 'provides minimum illumination brightness required for reading. Motion tolerance performance will be degraded. Use this setting for a built-in system when the LED illumination area faces the operator.' |
This configurability allows system integrators to optimize the illumination for their specific application. In a retail environment, where operators are near the reader, the minimum brightness may be preferred to avoid glare. In a warehouse environment, where the reader is mounted on a moving vehicle, standard brightness may be required for reliable reading. |
|
Chapter 12: Code Corporation's Continuous Illumination Setting |
Code Corporation, another barcode reader manufacturer, provides a simple user option for continuous illumination in their CR3 portable data terminal. The device supports enabling or disabling continuous illumination via configuration codes. |
The user manual notes that 'Code recommends only using this feature with cabled units due to increased power consumption.' This is a practical acknowledgment of the power trade-off: continuous illumination provides a steady light for easier aiming, but it drains the battery much faster than pulsed operation. |
The CR3 is a rugged handheld device with Bluetooth wireless capability. Continuous illumination would reduce the device's battery life significantly, so the default setting is disabled. Users who need continuous illumination can enable it, but they are advised to use the device with a cable to avoid running down the battery. |
This simple example illustrates the practical trade-offs that designers must consider. Continuous illumination is convenient, but it comes at the cost of power consumption. Pulsed operation is more efficient but requires careful synchronization and control. |
Chapter 13: Overdriving LEDs - The 10 Times Brighter Pulse |
Overdriving is a technique where LEDs are driven at currents much higher than their continuous rating for brief periods. This can produce light pulses that are significantly brighter than continuous operation. |
The Smart Vision Lights Lightgistics series 'offers greater than 10 times brighter light pulses compared to standard continuous mode.' This dramatic increase in brightness is achieved by pulsing the LEDs at high current for short durations. |
A machine vision industry article describes the physics behind this technique: 'Continuous lighting is limited to what light output is available at 100% power. The only way to increase intensity of continuous lighting is to increase the size and wattage of the LED array. This also increases initial expense, operating costs and produces undesirable heat. Pulsing or strobing the LED array, especially overdriving it, provides high intensity illumination at short duty cycle.' |
The article also notes that overdriving has no detrimental impact on the lifespan or performance of the LED array, provided the duty cycle is short enough that the junction temperature does not rise excessively. This makes overdriving an attractive option for applications requiring high-intensity illumination. |
|
Chapter 14: Synchronization with Imager Exposure - Timing is Everything |
For pulsed illumination to be effective, the light pulse must be synchronized with the imager's exposure. The imager captures light during the exposure period, so the illumination must be active during that time. |
A patent on illumination pulsing describes a method for synchronizing illumination pulses with the imager's frame rate. The method involves counting row signals generated by the imager during each frame. When the count reaches a value corresponding to the desired pulse frequency, the illumination is pulsed. |
This approach ensures that the illumination is synchronized with the imager's exposure while maintaining a constant pulse rate above the flicker fusion frequency. The pulse frequency can be higher than the frame rate, allowing multiple pulses per frame, or lower, allowing pulses to skip some frames. |
The patent also describes a method for alternating between different wavelengths of illumination on consecutive frames. This allows the reader to use visible and infrared illumination in sequence, adapting to different barcode surfaces and conditions. |
Chapter 15: Multiple Wavelengths - Alternating Illumination |
Different barcode surfaces have different reflectance characteristics in different wavelengths. A barcode that is readable with red illumination may be unreadable with infrared, and vice versa. To handle this variation, some readers support multiple wavelengths of illumination. |
The patent describes a system that alternates between visible and infrared illumination on consecutive frames. The visible illumination, typically red or yellow, is used for most barcodes. The infrared illumination, which is invisible to the human eye, is used for barcodes that have poor contrast in the visible spectrum. |
The challenge is that the infrared illumination must be pulsed at a rate above the flicker fusion frequency to avoid visible flicker. However, the imager's frame rate may be slower than the flicker fusion frequency. The solution is to pulse the infrared illumination at a constant rate above the flicker fusion frequency, independent of the frame rate, and synchronize the imager to capture some of those pulses. |
This approach is described in the patent as a way to 'pulse visible illumination at a rate in excess of the flicker fusion frequency of human vision, to thereby avoid a flicker effect.' The technique allows the reader to use multiple wavelengths without distracting users. |
|
Chapter 16: Thermal Management in Pulsed Operation |
One of the advantages of pulsed operation is reduced thermal stress on the LEDs. Because the LEDs are only active for a small fraction of the time, the average power dissipation is much lower than in continuous operation. This allows the LEDs to be driven at higher peak currents without overheating. |
However, thermal management is still an important consideration. The pulses may be brief, but they are intense. The LED's junction temperature rises during the pulse, and if the duty cycle is too high, the temperature may not have time to cool between pulses. |
The Symbol Technologies patent addresses this by monitoring the capacitor voltage to determine the state of charge. This allows the controller to manage the charge and discharge cycles, ensuring that the capacitor is not discharged too quickly and that the LED is not overstressed. |
The patent also describes a torch mode where the LED is driven at a lower current for extended periods. This mode produces less heat than the flash mode and is suitable for continuous operation. |
Chapter 17: Power Consumption - The Efficiency Advantage |
Pulsed illumination offers a significant efficiency advantage over continuous operation. The average power consumption is reduced because the LEDs are only active for a small fraction of the time. |
The Symbol Technologies patent describes a system where the capacitor is charged by a DC-to-DC boost converter. The capacitor stores energy, and the LED discharges it in a brief pulse. This allows the LED to be driven at a high current without requiring the battery to deliver that current continuously. |
The efficiency advantage is particularly significant for handheld readers, where battery life is a critical design parameter. Code Corporation's recommendation to use continuous illumination only with cabled units reflects this concern. |
A machine vision article notes that 'pulsing or strobing the LED array... provides high intensity illumination at short duty cycle.' The short duty cycle means that the average current is much lower than the peak current, resulting in lower average power consumption. |
|
Chapter 18: The Texas Instruments TPS61376 - A Modern Solution |
Texas Instruments has developed the TPS61376, an integrated circuit that simplifies barcode scanner designs, including the illumination subsystem. While details are limited, the device is designed to address the specific challenges of barcode scanner illumination. |
The TPS61376 is described as providing 'an overview of the integrated features... and how the features can help to simplify barcode scanner designs.' This suggests that the device integrates the power management and control functions needed for LED drive, reducing the component count and simplifying the design. |
This approach is consistent with the trend toward greater integration in barcode reader design. By combining the DC-DC converter, the current control, and the timing functions on a single chip, designers can create more compact and reliable readers. |
Chapter 19: Datalogic's Pulse Flicker Minimization |
Datalogic, a major barcode reader manufacturer, has addressed the flicker problem in a patent that describes a 'system and method for reading optical codes on reflective surfaces while minimizing flicker perception of pulsed illumination.' |
The patent describes methods and systems that 'include controlling image exposure and illumination pulse timing so as to avoid or minimize the perception of flicker of the pulsed illumination by a user or bystander while implementing methods for reading optical codes presented on electronic display screens or other highly reflective surfaces.' |
This is particularly important for reading barcodes from electronic displays, such as smartphones, where reflection can be a significant problem. The timing of the illumination pulses must be carefully controlled to avoid flicker while still capturing a clear image. |
The patent also addresses the challenge of reflective surfaces, which can cause bright spots and make barcode reading difficult. The illumination pulses may be timed to reduce the effect of these reflections, improving the readability of the barcode. |
|
Chapter 20: Optical Code Reading Device - Comprehensive Illumination Control |
A patent for an 'optical code reading device' describes a comprehensive illumination control system that addresses many of the challenges discussed in this article. The device is capable of producing illumination with both continuous and pulsed components. |
The device includes 'an illumination system and a processor that causes the illumination system to produce illumination according to an illumination period having a continuous or substantially continuous, illumination sub-period and at least one pulsed, illumination sub-period.' |
This hybrid approach allows the reader to benefit from both continuous and pulsed illumination. The continuous illumination provides a steady light for aiming and for detecting the presence of a barcode. The pulsed illumination provides the high intensity needed for image capture. |
The patent notes that this approach 'allows the image reading device to determine a proper exposure for imaged indicia by sectioning a single frame into individual exposures.' This is a sophisticated approach that optimizes exposure for different parts of the barcode. |
Chapter 21: CMOS and CCD Imagers - The Impact on Illumination |
The choice of imager technology, CMOS or CCD, affects the design of the illumination system. CMOS imagers are generally more power-efficient and can be integrated with the control circuitry, while CCD imagers offer better image quality but require more complex support circuits. |
The Symbol Technologies patent describes a reader that uses 'an active-pixel digital imaging sensor with a global shutter (simultaneous total-pixel exposure system---also referred to as 'frame shutter exposure') and good near infrared (NIR) sensitivity.' This is a CMOS imager, which supports global shutter operation, meaning that all pixels are exposed simultaneously. |
Global shutter is important for pulsed illumination because it allows the entire image to be captured during the brief flash. A rolling shutter, which exposes rows sequentially, would result in uneven illumination if the pulse is brief. |
The patent also notes that 'in other embodiments a different type of imager may be employed, such as a frame-shuttered interline transfer CCD.' The choice of imager affects the timing and synchronization of the illumination pulses. |
|
Chapter 22: Edge Detection - Triggering the Illumination |
Some readers use edge detection to determine when a barcode is in the field of view and trigger the illumination accordingly. This approach, sometimes called 'auto-triggering,' reduces power consumption by only activating the illumination when a barcode is detected. |
An optical reader patent describes a method where the control unit analyzes image information and 'changes the mode of operation of the reader if the image information indicates that machine readable indicia is in the field of view.' The analysis includes 'detecting for edges, or edge transitions in the image information.' |
If the control unit determines that the image contains more than a predetermined number of edge transitions, it changes the mode of operation. The second mode 'may be characterized, for example, by an increased illumination of the field of view.' |
This approach allows the reader to operate in a low-power mode, with reduced or no illumination, when no barcode is present. When a barcode is detected, the illumination is activated, providing the light needed for image capture. This reduces average power consumption and extends battery life. |
Chapter 23: Continuous Scan Mode - The Power Drain Trade-off |
Continuous scan mode is an operating mode where the reader is constantly scanning for barcodes, without requiring the user to pull a trigger. This mode is convenient but consumes significant power. |
An optical reader patent describes the trade-off: 'A continuous scan operating configuration requires repetitive illumination flashing of an LED array... Repetitive flashing illumination or laser scanning requires a high level of energy consumption and can result in premature component degradation. Furthermore, the repetitive illumination or laser scanning has been observed to be highly distracting to users.' |
The patent describes an alternative approach where the reader operates in a low-power mode when no barcode is present, and only activates the high-power illumination when a barcode is detected. This reduces power consumption and extends the life of the components. |
This approach uses edge detection to trigger the illumination, as described in the previous chapter. The reader continuously captures low-resolution images with low or no illumination, and only activates the high-power illumination when a barcode is detected. |
|
Chapter 24: Strobing for Motion Freeze - High-Speed Applications |
Strobing is particularly effective for high-speed applications, where the barcode is moving rapidly relative to the reader. The brief, intense pulse of light effectively 'freezes' the motion, allowing the imager to capture a clear image. |
A Symbol Technologies patent describes this application: 'A target is illuminated with high intensity light pulses to make the target appear stationary to a solid state imager of an imaging reader to improve reader performance at high swipe speeds of the target.' |
The patent notes that at swipe speeds greater than 50 inches per second, the symbol can enter and exit the field of view in less than 33 milliseconds, making it impossible for a conventional imager to capture a clear image. The strobing illumination overcomes this limitation by providing the light needed for a very short exposure. |
The reader uses a 'high-speed strobe illuminator to enable the image of the target to be acquired in a very short period of time, for example, on the order of 500 microseconds.' This short exposure, combined with the intense illumination, ensures that the image is not blurred even if there is relative motion. |
Chapter 25: Workstation Readers - Fixed Mount Applications |
Workstation readers, such as those used at checkout counters, have different requirements than handheld readers. They are typically cabled, so power consumption is less of a concern. However, they must be reliable and comfortable for operators who use them for extended periods. |
A Symbol Technologies patent describes a workstation reader that uses pulsed illumination for high-speed barcode reading. The reader is mounted on a countertop, and products are slid past a window. The pulsed illumination provides the high intensity needed to capture clear images of moving products. |
The patent notes that 'the strobe illumination is brighter than ambient illumination, especially close to the window, and assists autodiscrimination.' This ensures that the barcode is clearly visible even in bright ambient light. |
The workstation reader also includes features to assist the operator, such as an aiming pattern generator that projects a visible pattern onto the barcode. The aiming pattern helps the operator position the product correctly, improving the reading success rate. |
|
Chapter 26: Aiming Patterns - Guiding the User |
Aiming patterns are an important feature of many barcode readers, providing visual feedback to help the user position the reader correctly. The aiming pattern is typically generated by a laser or an LED, and it projects a pattern onto the barcode. |
The Symbol Technologies patent describes an aiming pattern generator that 'generates a visible aiming pattern to aid the user in properly aiming the reader at the target bar code.' The pattern may be a crosshair formed from a thick horizontal line and a perpendicular thin vertical line. |
The aiming pattern is typically activated when the user pulls the trigger, and it remains on while the reader is scanning. The pattern helps the user align the reader with the barcode, ensuring that the barcode is within the reader's field of view. |
The illumination system may be separate from the aiming pattern generator, or they may be integrated. Some readers use the same LEDs for both illumination and aiming, reducing the component count and simplifying the design. |
Chapter 27: Laser vs. LED Aiming - Different Approaches |
Aiming patterns can be generated with lasers or LEDs. Laser aiming provides a bright, focused beam that is visible in a wide range of lighting conditions. LED aiming provides a flood of light that illuminates the field of view, making it easier to see the barcode. |
The Symbol Technologies patent describes a reader that uses a laser aiming apparatus. The laser aiming apparatus 'includes a laser diode and a diffractive lens' that generates the aiming pattern. |
The Opticon MDI-5350 uses a green LED for aiming. The device supports 'green LED floodlight used for aiming' with configurable brightness (high, standard, or low). The aiming is typically enabled by default, but it can be disabled if it is not needed. |
The choice between laser and LED aiming depends on the application. Laser aiming provides a precise pattern that is visible in bright light, but it requires more complex optics. LED aiming provides a wider field of view and is simpler to implement. |
|
Chapter 28: The IADJ Pin - Controlling LED Current |
The Symbol Technologies patent describes the use of the IADJ pin on the LT1618 boost converter to control the LED current. This pin is controlled with a DC voltage that determines the output current of the converter. |
The patent explains that 'a second pin (pin 4) designated as IADJ is controlled with a DC voltage to control the output current of the voltage step up converter.' This allows the controller to adjust the LED current, and therefore the brightness, in real time. |
The controller can use the IADJ pin to set different current levels for flash and torch modes. In flash mode, the current is high, providing a brief burst of intense light. In torch mode, the current is lower, providing a steady, less intense illumination. |
The ability to adjust the LED current is essential for the hybrid operation described earlier in this article. The controller can switch between flash and torch modes simply by changing the voltage on the IADJ pin. |
Chapter 29: The Feedforward Function - Pre-Charging the Capacitor |
The Symbol Technologies patent describes a feedforward function that allows the controller to pre-charge the energy storage capacitor. The controller knows when a flash will be needed and starts charging the capacitor in advance. |
The feedforward function is described as a feature of the LT1618 boost converter. 'The LT1618 also provides a 3% accurate constant current sense amplifier. This function is used to limit or set the current delivered to the load. In addition, a feedforward function is also available to bring the output voltage up quickly.' |
The feedforward function allows the controller to bring the capacitor to the required voltage quickly, ensuring that the flash is available when needed. This reduces the delay between the trigger and the capture, improving the responsiveness of the reader. |
The patent notes that the controller can 'adjust the output voltage (Vo) such that the voltage at the output 178 is held constant at a minimal voltage as a constant current is continuously delivered to the LED(s).' This efficiency is crucial for battery-powered operation. |
|
Chapter 30: Progressive Disclosure - Multiple Exposure Frames |
Some readers use progressive disclosure, capturing multiple exposure frames with different illumination settings. This approach allows the reader to adapt to different barcode types and lighting conditions. |
The Datalogic patent on reading optical codes on reflective surfaces describes a method where 'controlling image exposure and illumination pulse timing' is used to optimize the image capture. The system may capture multiple frames with different exposure and illumination settings, and then select the best image for decoding. |
This approach is particularly useful for reading barcodes on reflective surfaces, such as smartphone displays, where the image quality can vary dramatically with the illumination angle and intensity. |
The progressive disclosure approach is more complex than a simple single-exposure capture, but it provides greater flexibility and can improve the reading success rate. |
Chapter 31: Thermal Print Barcodes - The Wavelength Challenge |
Thermal print barcodes, which are common on shipping labels and price tags, present a particular challenge for illumination design. These barcodes have 'little or no contrast in the infrared spectrum, but good contrast in the visible spectrum at around 590 nm.' |
This means that an infrared illumination source, while effective for many barcodes, would not work well for thermal print barcodes. The reader must use visible illumination, typically red or yellow, to read these barcodes. |
The patent describes a system that uses different wavelengths of illumination for different barcodes. The reader may alternate between visible and infrared illumination, or it may use visible illumination exclusively for thermal print barcodes. |
This highlights the importance of wavelength selection in the illumination design. The reader must provide the right wavelength for the barcode types it is expected to read. |
|
Chapter 32: LED Output Degradation - The Long-Term Problem |
LEDs degrade over time, losing brightness as they age. This is a particular concern for barcode readers, where consistent illumination is essential for reliable reading. |
A machine vision industry article notes that 'because LED output degrades over time, only a high-quality LED lighting controller can ensure consistently uniform lighting for the life of the project.' The lighting controller must compensate for the degradation by adjusting the LED current. |
The article describes a calibration process where the machine vision system is calibrated using a 'Calibrated Conformance Standard Test Card.' The system gain is adjusted until the Symbol Contrast measured by the sensor matches the SC recorded on the test target. |
This calibration process compensates for LED degradation and ensures that the barcode verifier continues to produce accurate grades over its lifetime. |
Chapter 33: The Hidden Strobe - Internal Self-Triggering |
Smart Vision Lights' Hidden Strobe technology uses internal self-triggering to pulse the LEDs at a very high rate. The LEDs are triggered thousands of times per second, pulsing faster than the human eye can perceive. |
The technology 'allows LEDs to internally self-trigger thousands of times per second, pulsing faster than the human eye can perceive and creating the illusion of continuous light.' This eliminates visible flicker while providing the benefits of pulsed illumination. |
The hidden strobe technology is particularly useful for machine vision systems where the strobe light could otherwise be a distraction to workers. The technology 'protects employees from the disorientation caused by flashing lights.' |
The JWL150-DO light includes an 'onboard charging capacitor designed to ensure powerful bursts of energy with a low consistent electrical draw.' This is the same principle as the Symbol Technologies capacitor-based driver. |
|
Chapter 34: The Dual OverDrive - Combining Engines |
Smart Vision Lights' Dual OverDrive technology combines two different overdrive engines: Deca OverDrive and standard OverDrive. This combination provides exceptional light output while handling any speed. |
The technology 'combines SVL's Deca OverDrive and standard OverDrive engines and allows end users to attach polarizers that retain exceptional light output while handling any speed.' This flexibility is particularly valuable for logistics applications where packages move at varying speeds. |
The light is designed for 'performing challenging high-speed, accurate barcode reading and optical character recognition (OCR) with packages in highly reflective plastic wraps or shipping bags.' These are difficult applications where traditional illumination may fail. |
The Dual OverDrive technology addresses these challenges by providing very bright light pulses that can overcome the reflections and variable speeds. |
Chapter 35: Reading from Displays - Smartphones and Tablets |
Reading barcodes from electronic displays, such as smartphones and tablets, presents unique challenges. The display surface is highly reflective, and the barcode may be displayed on a dark background. |
The Datalogic patent describes a system that addresses these challenges. The system controls the image exposure and illumination pulse timing 'while implementing methods for reading optical codes presented on electronic display screens or other highly reflective surfaces.' |
The system may use a combination of illumination techniques to overcome the reflections. The pulse timing may be adjusted to reduce the effect of reflections, and the exposure may be controlled to capture the barcode while minimizing glare. |
This is a growing application area, as more barcodes are being displayed on smartphones for mobile ticketing, loyalty cards, and other applications. |
|
Chapter 36: Specular Reflection - A Common Challenge |
Specular reflection, or glare, is a common challenge in barcode reading. When the light reflects directly from the surface into the imager, it creates a bright spot that can obscure the barcode. |
The Opticon MDI-5350 includes a setting for 'Prevent specular reflection' that disables illumination when specular reflection occurs. This reduces the glare and improves the readability of the barcode. |
The device also includes an 'Automatic switching' mode where 'illumination automatically switches between floodlight ON and OFF.' The illumination mode selected when a code was read is memorized and prioritized for subsequent reading. |
This adaptive approach addresses the specular reflection problem without requiring user intervention. The reader learns which illumination mode works best and uses that mode for future scans. |
Chapter 37: Brightness Adjustment - Standard vs. Minimum |
The Opticon MDI-5350 supports two brightness levels: standard brightness and minimum brightness. The standard brightness provides the illumination needed for reliable reading, while the minimum brightness 'provides minimum illumination brightness required for reading.' |
The minimum brightness setting is recommended 'for a built-in system when the LED illumination area faces the operator.' This is a safety consideration: when the reader is mounted in a fixed position and the operator is exposed to the illumination, the brightness should be minimized to avoid eye discomfort. |
The minimum brightness setting also reduces power consumption, which can be beneficial in battery-powered applications. However, the motion tolerance performance is degraded with the minimum brightness setting, so this setting should be used with caution. |
The ability to adjust the brightness is a useful feature for system integrators, allowing them to optimize the reader for their specific application. |
|
Chapter 38: Prevent Flickering - A User Setting |
The Opticon MDI-5350 includes a 'Prevent flickering' setting that prioritizes code reading performance on dark screens. This setting prevents flicker when the LED illumination is used to read barcodes on LCD displays. |
The device offers two options: 'LED illumination flicker prevention' and 'Give priority to LCD display reading.' The flicker prevention setting does not prioritize code reading performance on dark screens, while the priority setting prioritizes code reading performance, causing flicker in LED illumination. |
This illustrates the trade-off between reading performance and user comfort. The flicker prevention setting reduces eye strain for operators, but it may reduce the reading success rate for barcodes on dark screens. |
The default setting is flicker prevention, which prioritizes user comfort. The priority setting is available for applications where reading performance is more critical. |
Chapter 39: External LED Illumination - A Signal for External Lighting |
The Opticon MDI-5350 includes a 'L LED Output Mode' that provides a signal for external LED illumination. This is useful when the built-in illumination is not sufficient for the application. |
The device 'can provide an output signal on Pin-3' that can be used to control external LEDs. The external LEDs can provide additional illumination or can be positioned differently to improve the lighting of the barcode. |
The external LED signal can be enabled or disabled via software commands. When enabled, the external LED signal is active, and the Good Read Output Signal is disabled. This allows the same pin to be used for either function. |
This feature is useful for system integrators who need to add external lighting to their barcode reader system. |
|
Chapter 40: Green Aiming Detection - The Opticon Approach |
The Opticon MDI-5350 supports green aiming detection, where a green LED is used to detect the presence of a target. The green aiming light is projected onto the field of view, and the imager looks for the reflection. |
The green aiming detection mode is 'recommended for indoor use only, because target detection is reduced in environments with higher illumination levels than typically found indoors.' This is a limitation of green aiming: it relies on the contrast between the green light and the ambient illumination. |
The device also supports warm white illumination detection, which is preferred in dark environments. The warm white light is more effective than green aiming in low-light conditions. |
The choice between detection modes depends on the application environment. In a dark warehouse, the warm white illumination detection is preferred. In a well-lit indoor environment, the green aiming detection may be sufficient. |
Chapter 41: The Count Signal - Synchronizing the Illumination |
The illumination pulsing patent describes a method for synchronizing the illumination pulses with the imager's frame rate using a count signal. The imager generates row signals during each frame, and these signals are counted to determine when to pulse the illumination. |
The method involves 'operating an imager at a nominal frame rate, the imager generating a predetermined number of regularly occurring row signals during each frame; incrementing a row counter in response to each row signal; and, in response to the row counter reaching a count corresponding to an illumination pulse frequency greater than or equal to the minimum flicker fusion frequency, pulsing a source of illumination.' |
This approach ensures that the illumination pulses are synchronized with the imager's frame rate while maintaining a pulse rate above the flicker fusion frequency. The pulse rate can be set independently of the frame rate, allowing the reader to adapt to different operating conditions. |
This method is particularly useful for readers that support multiple wavelengths of illumination, where the pulses must be alternated between different sources. |
|
Chapter 42: Illumination Modes - Continuous, Pulsed, and Hybrid |
Many barcode readers support multiple illumination modes, allowing the user or the system to select the optimal mode for the application. The Opticon MDI-5350, for example, supports enabling or disabling the illumination, automatic switching, and prevent specular reflection. |
The 'Automatic switching' mode is particularly interesting. In this mode, the illumination automatically switches between floodlight ON and OFF based on the conditions. The illumination mode selected when a code was read is memorized and prioritized for subsequent reading. |
This adaptive approach is an improvement over a fixed illumination mode. The reader learns from its environment and adapts accordingly, improving the reading success rate and reducing user intervention. |
The 'Prevent specular reflection' mode disables illumination when specular reflection occurs. This reduces glare and improves the readability of barcodes on reflective surfaces. |
Chapter 43: The Hidden Strobe in Machine Vision - A Comparison |
The Smart Vision Lights Hidden Strobe technology is designed for machine vision applications, including barcode reading. It provides the benefits of strobing without the distracting flicker. |
The technology addresses a specific problem: 'In high-speed machine vision systems today, LEDs are pulsed to freeze images of fast-moving objects, but the strobing effect can create an uncomfortable and potentially unsafe working environment.' |
The Hidden Strobe technology 'maximizes machine vision systems while protecting employees from the disorientation caused by flashing lights.' This is a significant improvement over traditional strobing, which can be distracting and even dangerous. |
The technology is available in the JWL150-DO Lightgistics Series light, which is designed for logistics applications such as barcode reading and OCR. The light includes an 'onboard charging capacitor designed to ensure powerful bursts of energy with a low consistent electrical draw.' |
|
Chapter 44: The Capacitor Discharge - High Current Pulse |
The capacitor discharge is the key to the high-current pulse in the Symbol Technologies design. The capacitor stores energy at a high voltage, and when discharged through the LED, it produces a brief, intense current pulse. |
The patent describes how the discharge current is controlled by a programmable current source. The current is equal to the voltage from the DAC divided by a resistance, 'as long as a minimum voltage (sometimes referred to as head room) of about 0.4V volts is maintained across the programmable current source.' |
This ensures that the current is well-controlled and does not damage the LED. The programmable current source also allows the controller to adjust the discharge current, providing flexibility for different operating conditions. |
The capacitor discharge is also used in the Smart Vision Lights design, which includes an 'onboard charging capacitor designed to ensure powerful bursts of energy with a low consistent electrical draw.' |
Chapter 45: Summary - The Illumination Choice in Perspective |
The choice between pulsed and continuous illumination is one of the most fundamental decisions in barcode reader design. It affects image quality, motion tolerance, power consumption, user comfort, and system cost. |
We have seen how different companies and technologies have approached this challenge: |
Symbol Technologies developed a capacitor-based flash driver that supports both high-intensity flash mode and lower-power torch mode. The design includes a programmable current source that allows the controller to adjust the LED current on the fly. |
Smart Vision Lights developed Hidden Strobe technology that pulses LEDs thousands of times per second, creating the illusion of continuous light while providing the benefits of strobing. Their Dual OverDrive technology provides greater than 10 times brighter light pulses compared to standard continuous mode. |
Opticon provides a configurable illumination system with multiple detection modes, brightness settings, and flicker prevention options. The system allows the user or integrator to optimize the illumination for the specific application. |
Code Corporation provides a simple option for continuous illumination in their CR3 reader, but recommends only using it with cabled units due to power consumption. |
Datalogic has patented a system for minimizing flicker perception while reading optical codes on reflective surfaces, controlling both image exposure and illumination pulse timing. |
Texas Instruments has developed the TPS61376, an integrated circuit that simplifies barcode scanner designs, including the illumination subsystem. |
|
The key lessons from our exploration are: |
Continuous illumination is simpler but less efficient. It provides a steady light but cannot be overdriven to high intensity. It consumes more power and may be less effective in challenging conditions. |
Pulsed illumination provides higher intensity and better motion freeze. Overdriving the LEDs allows the reader to achieve light intensities far greater than continuous operation. This improves the signal-to-noise ratio, extends reading range, and freezes motion. |
Flicker is a significant user experience challenge. Pulsed illumination must be managed to avoid visible flicker, which can be distracting and uncomfortable. Techniques such as high-frequency pulsing and synchronization with the imager can minimize flicker. |
Hybrid approaches provide the best of both worlds. Many readers support both flash and torch modes, using a brief high-intensity flash for image capture and a lower-level continuous illumination for aiming and subsequent scans. |
Power consumption is a critical trade-off. Continuous illumination consumes more power, which is a significant concern for handheld readers. Pulsed illumination reduces average power consumption but requires more complex circuitry. |
Integration is the trend. Companies are integrating the illumination control functions with the power management and control functions on a single chip, reducing component count and simplifying the design. |
In the end, the choice between pulsed and continuous illumination depends on the application requirements. For high-speed logistics applications, where motion freeze is critical, pulsed illumination is the clear choice. For low-cost, low-power applications, continuous illumination may be sufficient. Many readers are now designed with configurable illumination modes, allowing the user to select the optimal mode for their specific application. |
cs@easiersoft.com