Ambient Light Rejection: The Battle Against the Sun |
Executive Summary |
This article provides a comprehensive, accessible exploration of ambient light rejection techniques in barcode readers. We examine why ambient light is one of the most formidable enemies of reliable barcode reading and how engineers have developed a range of ingenious solutions to defeat it. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real products from industry leaders including Symbol Technologies, Hewlett-Packard, Datalogic, Texas Instruments, and Analog Devices. We explore the fundamental challenge of the DC offset, the elegance of AC coupling with high-pass filters, the sophistication of differential optical architectures, and the power of synchronous demodulation. The article covers both analog and digital approaches, with special attention to the practical trade-offs between cost, complexity, and rejection effectiveness. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing circuits that must distinguish signal from the overwhelming noise of the environment. |
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Chapter 1: The Enemy That Never Sleeps |
Every barcode reader faces a silent enemy. It is present in every environment, from the brightest supermarket under fluorescent lights to the sun-drenched loading dock. It is ambient light, the ubiquitous illumination that floods the photodetector along with the desired reflected light from the barcode. |
Ambient light is not just a nuisance; it is a fundamental threat to reliable barcode reading. The photodetector cannot distinguish between light that comes from the barcode and light that comes from the sun, the ceiling lights, or any other source. It simply produces a current proportional to the total light intensity. The barcode signal, which is the difference in reflected light between the black bars and white spaces, is superimposed on a large DC offset caused by ambient light. |
The problem is particularly acute because ambient light can be far brighter than the reader's own illumination. In direct sunlight, the ambient light level can exceed 100,000 lux. The reflected light from a barcode, even under bright illumination, is orders of magnitude smaller. If the ambient light is not rejected, it will saturate the amplifiers, reducing the dynamic range and distorting the signal. |
A patent from Symbol Technologies explains the challenge: 'The differentiator circuit effectively filters out the effects of ambient light that could otherwise cause the last amplifier stage to saturate, thereby improving ambient light immunity' . This observation highlights the central challenge: the ambient light creates a DC offset that can push the amplifier into saturation, making it impossible to recover the barcode signal. |
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Chapter 2: The Nature of the DC Offset |
The DC offset is an unwanted, constant voltage that sits on top of the barcode signal. It comes from several sources, but ambient light is the most significant. The photodetector receives not only the light from the scanner's illumination but also the ambient light from the environment. This ambient light creates a constant photocurrent that adds to the signal . |
The DC offset has several detrimental effects on the barcode reader's performance. First, it consumes the amplifier's dynamic range. If the offset is large enough, the amplifier will saturate, and the barcode signal will be clipped. Second, it shifts the baseline of the signal, making it difficult for the digitizer to set the threshold correctly. Third, it can cause the amplifier to operate outside its linear range, distorting the signal. |
The problem is compounded by the fact that the DC offset is not constant. It varies with the ambient light level, which changes as the reader is moved from one environment to another. It also varies with temperature, as the photodiode's dark current increases with temperature. A circuit that simply subtracts a fixed DC offset will not work reliably. |
The patent from Symbol Technologies notes that the differentiator circuit 'eliminates the transient response of other forms of high pass filtering or AC coupling that are ordinarily required to improve ambient light immunity or to eliminate the build-up of large offset voltages associated with DC coupled amplifiers' . This highlights the various approaches that have been developed to address the DC offset problem. |
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Chapter 3: AC Coupling --- The First Line of Defense |
The simplest and most common technique for rejecting the DC offset is AC coupling. An AC coupling circuit uses a capacitor to block the DC component of the signal while passing the AC component. The capacitor is placed in series with the signal path, and a resistor is connected from the signal side of the capacitor to ground. This forms a high-pass filter. |
The high-pass filter has a corner frequency, below which signals are attenuated. By choosing the corner frequency appropriately, the DC offset is blocked while the barcode signal, which varies at a higher frequency, is passed. The corner frequency is set by the values of the capacitor and the resistor. |
The challenge in designing the high-pass filter is choosing the corner frequency. If the corner frequency is too high, the low-frequency components of the barcode signal will be attenuated, distorting the signal. If the corner frequency is too low, the DC offset will not be fully rejected, and the amplifier may still saturate. |
The Symbol Technologies patent describes a differentiator circuit that effectively filters out ambient light 'without the transient response of other forms of high pass filtering or AC coupling' . The differentiator, which is placed at a very early stage in the amplifier string, provides high-pass filtering with minimal transient response, improving ambient light immunity. |
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Chapter 4: The Differentiator Approach |
The differentiator circuit is a clever alternative to conventional AC coupling. Instead of a simple capacitor-resistor high-pass filter, the differentiator circuit produces the first derivative of the analog signal. The derivative emphasizes the edges of the barcode signal while rejecting the DC offset. |
The Symbol Technologies patent describes a digitizer that 'obtains all of its information for digitizing, false transition rejections, and margin cleaning from the first derivative of an analog signal produced by a photodetector detecting the scanning light spot' . The original analog signal and the second derivative signal are not used. |
The advantage of this approach is that the differentiator can be placed at a very early stage in the amplifier string. Additional amplification stages can then amplify the differentiated signal until it is large enough to be digitized. This approach provides two benefits: it filters out the effects of ambient light that could otherwise saturate the amplifier, and it eliminates the transient response of other forms of high-pass filtering. |
The patent notes that 'all of these requirements of the prior art normally result in the need for large power supply voltages to eliminate the possibility of amplifier clipping' . The differentiator approach allows the digitizer and its associated amplifiers to work on low supply voltages, such as a single 5-volt supply. |
Chapter 5: Hewlett-Packard's HBCC-0500 Digitizer IC |
Hewlett-Packard's HBCC-0500 digitizer IC provides a practical example of ambient light rejection in a commercial product. The IC was designed for barcode wands and includes high-pass filtering to remove the DC component from the photodetector signal. |
The HBCC-0500 datasheet explains: 'The sensor output has two components; DC due to ambient light, and AC from the bar code label. Photocurrent from the sensor is amplified and high pass filtered to remove the ambient light signal' . The IC then recovers the barcode information using an AM detector and a low-pass filter. |
The HBCC-0500 was designed to operate with scan velocities from 7.6 to 76 centimeters per second, edge contrast of 40% or greater, and ambient light levels up to 100,000 lux . This high ambient light rejection capability made it suitable for use in a wide range of environments. |
The IC includes both positive and negative peak detectors and a comparator, with the comparator threshold generated from the peak detectors using a resistor ladder. This adaptive threshold technique ensures reliable digitization even when the signal amplitude varies . |
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Chapter 6: Differential Optical Architecture |
Differential optical architecture is a more sophisticated technique for ambient light rejection. Instead of using a single photodiode and filtering the signal, this approach uses two or more photodiodes and subtracts the signals. |
The basic idea is simple: one photodiode receives both the barcode signal and the ambient light, while a second photodiode receives only the ambient light. By subtracting the second signal from the first, the ambient light component is canceled, leaving only the barcode signal. |
A patent from Datalogic describes this approach in detail: 'The imaging signal and the ambient signal are mathematically manipulated to subtract the contribution of the first portion of the ambient light from the imaging signal' . The system uses a main photodiode that receives the laser barcode signal along with ambient light, and two smaller photodiodes that receive only the ambient light. |
The patent notes that 'the total of the active areas of the two smaller photodiodes is approximately equal to the active area of the main photodiode' . This ensures that the ambient light signals are matched, allowing accurate subtraction. |
This differential approach is particularly effective because it cancels the ambient light in the current domain, before it can saturate the amplifier. The patent explains that 'the collection optics are designed to optimize the field of view, maximize the collection area, and collect the light on a photodetector' . |
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Chapter 7: Symbol Technologies' Ambient Light Shield and Color Filter |
Symbol Technologies developed an ambient illumination shielding apparatus specifically for reading direct part marking (DPM) barcodes . DPM barcodes are marked directly on metal or plastic surfaces, creating craters or indentations. The shadows and highlights from multiple ambient light sources can make these barcodes difficult to decode. |
The shielding apparatus includes two components: an illumination shield and a filter. The illumination shield extends from the housing of the scanner along the path of the aiming pattern. It passes illumination within a predetermined wavelength range and blocks illumination outside that range. The predetermined wavelength range includes the wavelength of the aiming pattern . |
The filter is disposed adjacent to the imaging system's lens. It passes illumination within a predetermined wavelength range to the pixel array and blocks illumination outside that range. The predetermined wavelength range of the filter includes the wavelength of the illumination system . |
The two components work in tandem: 'the bandpass ranges of the shield and the filter are complementary, that is, ambient illumination wavelengths that are passed by the shield will be blocked by the filter and vice versa' . This effectively eliminates ambient illumination incident upon the photosensor array, making it possible to decode DPM barcodes. |
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Chapter 8: The Frequency Challenge of Modern Lighting |
The ambient light problem has become more complex in recent years. Traditional incandescent lights produce a steady light, but modern lighting technologies such as fluorescent lamps and LEDs are often pulsed at high frequencies. |
A patent from a barcode reader manufacturer notes that 'when fluorescent lamps and LEDs are operated at kilohertz frequencies,' the ambient light signal has 'a constant illumination DC component and a relatively large time-varying AC frequency component at kilohertz frequencies, typically anywhere from 30 kHz to 300 kHz' . |
This creates a new challenge: the time-varying component of the ambient light can be close in frequency to the barcode signal. The patent explains that 'there are circumstances where the time-varying frequency component of the ambient light signal is too close in frequency to the frequency of the information signal, in which event the ambient light signal can interfere and impede the decoding' . |
To address this challenge, the patent describes a system that measures the ambient light signal to determine a threshold, and then adjusts the scan angle or the bandwidth of the filter when the ambient light signal exceeds the threshold . This adaptive approach ensures reliable reading even under challenging lighting conditions. |
Chapter 9: Adaptive Bandwidth Filtering |
The concept of adaptive bandwidth filtering is a powerful tool for ambient light rejection. By adjusting the bandwidth of the filter based on the ambient light conditions, the system can optimize the trade-off between signal preservation and noise rejection. |
The patent describes an active low-pass filter with an adjustable bandwidth under the control of the microprocessor . When the ambient light is high, the bandwidth is reduced to reject more of the interfering signal. When the ambient light is low, the bandwidth is increased to preserve more of the barcode signal. |
The filter is part of a signal processing receiver circuit that includes a transimpedance amplifier, an automatic gain controller, and a digitizer. The microprocessor controls the gain and bandwidth of the various stages, adapting to the ambient light conditions . |
This adaptive approach is particularly valuable in environments where the lighting conditions vary, such as retail stores where sunlight may enter through windows at different times of day. |
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Chapter 10: Synchronous Demodulation |
Synchronous demodulation is a powerful technique for rejecting ambient light. The technique involves modulating the illumination source at a known frequency and then demodulating the photodetector signal at the same frequency. |
The basic principle is that the barcode signal is impressed on the modulated illumination. The photodetector signal includes both the modulated barcode signal and the unmodulated ambient light. By multiplying the photodetector signal by a reference signal at the modulation frequency, the barcode signal is recovered, and the ambient light is rejected. |
A patent from a barcode reader manufacturer describes the use of a demodulator in the receiver: 'If the laser light source is pulsed, then a demodulator is advantageously provided in the receiver, preferably between the amplifier and the AGC' . |
The demodulator extracts the barcode information by comparing the signal to a reference signal derived from the laser drive. Any phase errors between the signal and the reference are compensated by a phase shift compensation circuit . |
Synchronous demodulation is particularly effective for rejecting high-frequency ambient light, such as the 100 kHz pulsing of LED lights. Because the barcode signal is modulated at a specific frequency, the demodulator can reject all other frequencies. |
Chapter 11: The Laser Power Regulator |
In laser-based barcode readers, the laser power regulator plays an indirect but important role in ambient light rejection. The regulator maintains a constant optical output power by varying the applied forward current to the laser diode. |
A patent describes the laser power regulator as 'a closed loop feedback system that maintains a constant optical output power by varying the applied forward current to the laser light source' . The laser includes a monitor photodiode that senses a fraction of the output laser light and provides a negative feedback signal to regulate the laser's output power. |
The constant laser power ensures that the barcode signal has a consistent amplitude, making it easier to distinguish from the ambient light. The regulator also protects the laser from overheating and ensures safe operation. |
The laser drive circuit includes both continuous and pulsed modes of operation. In pulsed mode, the laser is modulated at a frequency 'sufficiently fast to accurately resolve the narrowest bar or space used in the type of bar code symbol to be read' . |
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Chapter 12: Low-Noise Op-Amps for Reduced Offset |
The operational amplifier used in the transimpedance amplifier contributes to the DC offset. The input offset voltage of the op-amp is amplified by the gain of the TIA, adding to the DC offset at the output. |
Zero-drift amplifiers, also known as chopper amplifiers, use an auto-zero circuit that continuously cancels the input offset voltage. These amplifiers have very low input offset voltages and extremely low offset drift over temperature. |
Analog Devices' AD8615 is a zero-drift CMOS op-amp with an input offset voltage of less than 10 microvolts and an offset drift of less than 0.02 microvolts per degree Celsius . It is used in some high-end scanners, such as the Zebra DS3500. |
The AD8615's zero-drift feature eliminates the temperature drift of the offset, which is important for scanners used in outdoor environments with large temperature variations. A stable offset ensures that the comparator's threshold does not drift . |
The low offset of the AD8615 allows the use of a larger feedback resistor (higher gain) without saturating the amplifier, improving sensitivity to weak barcode signals. |
Chapter 13: Digital Ambient Light Rejection |
Modern barcode readers increasingly use digital techniques for ambient light rejection. Instead of relying solely on analog filtering, these readers digitize the signal and process it in software. |
A patent from Datalogic describes a method of reducing noise in an optically read image using an optical collection device . The system includes an imaging signal and an ambient signal, where the imaging signal corresponds to indicia information and a first portion of the ambient light, and the ambient signal corresponds to a second portion of the ambient light. The system mathematically manipulates the signals to subtract the contribution of the ambient light from the imaging signal . |
This digital approach offers several advantages over analog techniques. It is more flexible, as the subtraction can be performed with arbitrary precision. It can also be adapted dynamically based on the ambient light conditions. |
The digital approach is particularly valuable for 2D imagers, where the ambient light can vary across the image. The processor can subtract the ambient light on a pixel-by-pixel basis, providing uniform compensation across the entire image. |
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Chapter 14: The TIA's Offset and the AC Coupling Network |
The transimpedance amplifier's output includes the amplified photodiode current plus the amplifier's own offset voltage. For a feedback resistor of 100 kilohms and a photodiode current of 1 microamp, the output is 0.1 volts. The offset voltage adds to this, potentially pushing the output outside the dynamic range of the following stage. |
The AC coupling network that follows the TIA blocks the DC component of the signal, including both the ambient light offset and the TIA's offset. The corner frequency of the high-pass filter must be chosen to pass the barcode signal while blocking the DC offset. |
The design of the AC coupling network involves a trade-off. A lower corner frequency provides better rejection of the DC offset but may attenuate low-frequency components of the barcode signal. A higher corner frequency preserves the barcode signal but may allow some DC offset to pass. |
The differentiator approach described in the Symbol Technologies patent avoids this trade-off by placing the differentiator at an early stage in the amplifier string, where it filters the ambient light before it can saturate the amplifier . |
Chapter 15: Automatic Gain Control and Ambient Light |
Automatic gain control (AGC) is an important technique for adapting to varying signal levels, including those caused by changing ambient light. The AGC adjusts the gain of the amplifier to keep the signal within the dynamic range of the following stages. |
The patent describes an automatic gain controller with an on/off control input under the control of the microprocessor . The AGC adjusts the gain based on the signal level, ensuring that the signal is neither too small (lost in noise) nor too large (saturating the amplifier). |
The AGC works in conjunction with the active low-pass filter and the digitizer. The microprocessor controls the gain and bandwidth of the various stages, adapting to the ambient light conditions . |
This adaptive approach ensures reliable reading across a wide range of ambient light conditions, from dimly lit warehouses to bright outdoor environments. |
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Chapter 16: The Digitizer and Threshold Adaptation |
The digitizer is the final stage before the decoder. It converts the analog signal into a digital pulse signal where the widths and spacings correspond to the bars and spaces of the barcode. |
The digitizer includes an edge detector or wave shaper circuit, with threshold points that determine what points of the output signal represent bar edges . The threshold must be set correctly to ensure accurate digitization. |
In ambient light rejection, the threshold must adapt to the varying signal level. If the threshold is too high, the barcode signal may be lost. If the threshold is too low, noise may be incorrectly digitized. |
The Symbol Technologies patent describes a digitizer that obtains all of its threshold information from the first derivative of the analog signal . This allows the threshold to adapt automatically to the signal level, providing robust digitization even with varying ambient light. |
The digitizer's output is applied to a decoder, which analyzes the widths and spacings to find and decode a legitimate barcode symbol . |
Chapter 17: The HP HBCC-0500's AM Detection |
The Hewlett-Packard HBCC-0500 digitizer IC uses an AM detection technique to recover the barcode information. The IC amplifies and high-pass filters the photodetector signal to remove the ambient light, and then applies the signal to an AM detector . |
The AM detector recovers the barcode information by extracting the envelope of the modulated signal. The detected signal is then low-pass filtered to eliminate the modulation carrier, and input to a digitizer consisting of positive and negative peak detectors and a comparator . |
The comparator threshold is generated from the peak detectors using a resistor ladder. This adaptive threshold technique ensures reliable digitization even when the signal amplitude varies due to changes in ambient light . |
The HBCC-0500's AM detection technique is a practical example of how commercial ICs implement ambient light rejection in a compact, cost-effective package. |
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Chapter 18: The Scanning Mirror and Ambient Light Rejection |
In laser-based barcode readers, the scanning mirror plays a role in ambient light rejection. The mirror scans the laser beam across the barcode, and the reflected light is collected by the photodetector. |
The patent describes a scanning mirror that is oscillated by a drive motor to sweep the laser beam back and forth over the barcode . The mirror may be a planar mirror or a concave mirror, depending on the optical design. |
The scanning mirror's operation affects the ambient light rejection in several ways. The scanning beam is focused to a small spot, which concentrates the reflected light on the photodetector. This improves the signal-to-noise ratio, making it easier to distinguish the barcode signal from the ambient light. |
The scanning mirror also allows the reader to operate in a de-energized state when no barcode is present, reducing power consumption. The patent describes a reader that measures the ambient light signal in the de-energized state of the laser light source . |
Chapter 19: The De-Energized State Measurement |
The de-energized state measurement is a clever technique for measuring the ambient light. The laser light source is turned off, and the photodetector output is measured. Since the laser is off, the output represents only the ambient light. |
The patent describes: 'When the laser light source is in the deenergized state, then the output signal is just the ambient light signal' . This measurement can be used to set the threshold for digitization or to adjust the gain of the amplifier. |
The ambient light measurement can be performed periodically to track changes in the ambient light level. The patent notes that the measurement can be performed 'in the deenergized state of the laser light source, or [by] measuring the output signal from the photodetector when the laser light source is pulsed or modulated' . |
This technique provides an accurate estimate of the ambient light, which can be used for adaptive thresholding and gain control. The ambient light measurement can also be used to detect when the reader is pointed at a surface that is too reflective, causing the photodetector to saturate. |
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Chapter 20: The Barcode 3 Click and LV30 Engine |
The Barcode 3 Click module, based on the LV30 area image engine from Rakinda, provides a modern example of ambient light management in a commercial barcode reader . The LV30 is designed to be easily integrated into OEM equipment such as handheld, portable, or stationary barcode scanners. |
The LV30 incorporates a CMOS image sensor and a sophisticated image recognition system-on-chip. It includes a red LED for additional lighting, allowing for barcode scanning even in complete darkness, with the option to turn the illumination ON or OFF. It also includes a laser aimer to help users accurately position the barcode within the scanning area . |
The LV30's documentation notes that 'while the LV30 performs well under ambient light, exposure to high-frequency pulsed light can negatively impact its performance' . This highlights the ongoing challenge of ambient light rejection in modern imagers. |
The LV30 is designed to be used with a clear cover, such as PMMA or optical glass, to protect against dust and contaminants. The cover must be clear to avoid degrading the scanner's performance . |
Chapter 21: The Analog-to-Digital Converter's Role |
The analog-to-digital converter (ADC) is a critical component in modern barcode readers. It converts the analog signal from the photodetector into digital values that can be processed by the microprocessor. |
The patent describes an ADC connected to the output of the amplifier: 'The controller also has an analog-to-digital converter (ADC) connected to the decoder and to the output of the amplifier' . The ADC digitizes the signal, allowing the microprocessor to measure the ambient light level and adjust the gain and bandwidth. |
The ADC's resolution and sampling rate are important for ambient light rejection. A higher resolution allows more precise measurement of the signal, while a higher sampling rate allows the capture of high-frequency signals. |
The ADC is also used to measure the ambient light signal during the de-energized state. The patent notes that the measurement of the ambient light signal can be performed by the controller using the ADC . |
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Chapter 22: The Microcontroller's Role in Ambient Light Rejection |
The microcontroller is the brain of the barcode reader. It controls the illumination, the gain of the amplifiers, the bandwidth of the filters, and the threshold of the digitizer. In modern readers, much of the ambient light rejection is performed in the microcontroller. |
The patent describes a microprocessor or controller that controls the laser light source, the automatic gain controller, the active low-pass filter, and the amplifier gain . The microprocessor measures the ambient light signal using the ADC and adjusts the parameters to optimize performance. |
The microcontroller also performs the decoding of the barcode signal. The pulse signal from the digitizer is applied to a decoder, 'typically incorporated as software in the programmed controller, which will also have associated program memory and random access data memory' . |
The microcontroller's ability to adapt to changing ambient light conditions is a key advantage of modern barcode readers. By continuously monitoring the ambient light and adjusting the signal chain, the microcontroller ensures reliable reading across a wide range of environments. |
Chapter 23: The Comparative Approach to Ambient Light Rejection |
The comparative approach uses a reference signal to cancel the ambient light. The reference signal is derived from a photodetector that is exposed to the ambient light but not to the barcode signal. |
A patent from Datalogic describes this approach: 'an imaging signal and an ambient signal are received by the system where the imaging signal corresponds to indicia information and a first portion of the ambient light, and the ambient signal corresponds to a second portion of the ambient light. The imaging signal and the ambient signal are mathematically manipulated to subtract the contribution of the first portion of the ambient light from the imaging signal' . |
The comparative approach is particularly effective for rejecting modulated ambient light. Because the reference signal is derived from the ambient light, it includes the modulation components. By subtracting the reference signal from the imaging signal, the modulated ambient light is canceled. |
The patent describes three photodiodes: 'a first main photodiode that receives the laser bar code signal along with ambient light and two smaller photodiodes located on either side of the main photodiode that receive only the ambient light' . |
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Chapter 24: The Problem of Specular Reflection |
Specular reflection is a type of ambient light problem that occurs when the light from the illumination source reflects directly from the barcode surface into the photodetector. This creates a bright spot that can obscure the barcode signal. |
Specular reflection is particularly problematic for DPM barcodes, where the surface of the metal or plastic part can be highly reflective. The Symbol Technologies patent on the ambient light shield and color filter addresses this problem by using a shield and filter that block ambient light while passing the illumination . |
The patent explains that DPM bar codes 'have no intrinsic or natural contrast at the site of the markings, the markings consist of shallow indentations or craters in the background surface of the item' . When ambient light creates multiple shadows and highlights, the decoding of the barcode becomes difficult. |
The shield and filter work together to eliminate ambient illumination incident upon the photosensor array. The bandpass ranges of the shield and the filter are complementary, ensuring that all ambient light is blocked . |
Chapter 25: The Future of Ambient Light Rejection |
The future of ambient light rejection in barcode readers lies in a combination of improved optics, more sophisticated analog circuits, and advanced digital signal processing. |
In the optical domain, improved filters and shields can block more ambient light while passing the desired illumination. The Symbol Technologies patent on the ambient light shield and color filter is an example of this trend . |
In the analog domain, zero-drift amplifiers and adaptive filters can provide more precise rejection of the ambient light. The AD8615 zero-drift amplifier, used in the Zebra DS3500, is an example of this trend . |
In the digital domain, more powerful processors can perform more sophisticated algorithms for ambient light rejection. Digital signal processing allows the use of matched filters, adaptive thresholds, and other techniques that are difficult to implement in analog hardware. |
The combination of these approaches will enable barcode readers that work reliably in even the most challenging ambient light conditions, from direct sunlight to the flickering light of a warehouse. |
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Chapter 26: The LV30's Integration into OEM Equipment |
The LV30 area image engine from Rakinda is designed for easy integration into OEM equipment, including handheld, portable, and stationary barcode scanners . Its integration illustrates the practical considerations of ambient light management in real products. |
The LV30 communicates via UART or USB, and it includes an onboard buzzer and an LED indicator to provide user feedback. The READ button activates scanning until a barcode is decoded or the decode session timeout expires . |
The LV30 operates at 3.3 volts and includes a low-dropout regulator to convert the USB voltage to 3.3 volts . This low-voltage operation is typical of modern barcode readers, which must be power-efficient for battery-powered handheld devices. |
The LV30's documentation emphasizes the importance of protecting the module from dust and contaminants, which can degrade performance. The cover through which scanning occurs 'must be made of clear material; it is recommended to use cell-cast plastics or optical glass such as PMMA' . |
Chapter 27: The TLV700 LDO Regulator |
The TLV700 low-dropout (LDO) regulator is used in the Barcode 3 Click to convert the USB voltage to the 3.3 volts required by the LV30 engine . This component illustrates the importance of clean power for reliable ambient light rejection. |
The LDO regulator provides a stable, low-noise power supply for the imaging engine. A clean power supply is essential for ambient light rejection because noise on the power supply can couple into the signal path, degrading the signal-to-noise ratio. |
The TLV700 is a low-dropout regulator, meaning it can regulate the output voltage even when the input voltage is close to the output voltage. This is important for USB-powered devices, where the input voltage may vary. |
The choice of a low-dropout regulator reflects the importance of power supply design in barcode readers. A noisy power supply can introduce noise that interferes with the ambient light rejection circuits. |
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Chapter 28: The Laser Aiming System |
The LV30 includes a laser aiming system that projects a pattern to help users position the barcode within the scanning area . The aiming pattern is an important feature that improves user efficiency. |
The aiming system is a laser, which must meet safety standards. The LV30's documentation notes that the laser meets the IEC 60825:2014 standard for laser safety . |
The aiming system's operation affects the ambient light rejection in several ways. The aiming laser is typically turned on when the reader is triggered, providing a visual indication of the scanning area. The aiming laser also helps the user avoid scanning areas with high ambient light, such as direct sunlight. |
The aiming system's optical path is separate from the imaging system's optical path, but both must be protected from ambient light. The Symbol Technologies patent on the ambient light shield and color filter includes a shield that extends along the path of the aiming pattern to block ambient light . |
Chapter 29: The CMOS Image Sensor |
The LV30 incorporates a CMOS image sensor, which is the photodetector for the imaging system . CMOS sensors are widely used in modern barcode readers because they are low-cost, power-efficient, and can be integrated with other circuitry. |
The CMOS image sensor includes an array of pixels, each of which converts light into an electrical signal. The sensor is read out sequentially, generating an analog signal representative of the captured image frame. |
CMOS sensors are more susceptible to noise than CCD sensors, but they offer advantages in power consumption and cost. The LV30's CMOS sensor is designed to provide reliable barcode decoding even under varying ambient light conditions. |
The sensor's performance under ambient light is a key specification for any barcode reader. The LV30's documentation notes that the sensor performs well under ambient light, but exposure to high-frequency pulsed light can negatively impact performance . |
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Chapter 30: The Decoding System-on-Chip |
The LV30 includes a sophisticated image recognition system-on-chip (SoC) . This SoC integrates the image sensor, the processing logic, and the communication interface on a single chip. |
The SoC's processing logic includes the algorithms for ambient light rejection. The SoC can analyze the captured image, identify the barcode, and decode it, even in the presence of ambient light. |
The integration of the image sensor and processing logic on a single chip reduces the complexity of the overall system. The SoC handles the ambient light rejection in software, allowing the rest of the system to be simpler and more cost-effective. |
The SoC's algorithms for ambient light rejection likely include digital filtering, adaptive thresholding, and other techniques. The specific algorithms are proprietary to Rakinda, but they reflect the state of the art in barcode decoding. |
Chapter 31: The Corner Frequency of the High-Pass Filter |
The corner frequency of the high-pass filter is a critical parameter in ambient light rejection. The corner frequency must be chosen to block the DC component of the ambient light while passing the AC component of the barcode signal. |
The corner frequency is set by the values of the capacitor and resistor in the AC coupling network. A lower corner frequency provides better rejection of the DC offset, but may attenuate low-frequency components of the barcode signal. |
The optimal corner frequency depends on the scanning speed and the barcode density. A faster scanning speed produces higher-frequency signals, allowing a higher corner frequency. A slower scanning speed requires a lower corner frequency to avoid attenuating the signal. |
The Hewlett-Packard HBCC-0500 digitizer IC includes a high-pass filter with a corner frequency optimized for the scanning speed and barcode density . The IC is compatible with different sensors, allowing the corner frequency to be adjusted for the specific application. |
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Chapter 32: The Pulse Modulation Technique |
Pulse modulation is a technique for reducing power consumption and improving ambient light rejection. The LED is pulsed at a high frequency, and the photodetector signal is sampled during the pulse. |
The Hewlett-Packard HBCC-0500 digitizer IC uses pulse modulation to reduce power consumption and sensitivity to ambient light. The LED is pulsed every 33 microseconds with a 1-microsecond pulse . |
The pulse modulation technique has several advantages. The short pulse reduces the average power consumption of the LED, extending battery life. The pulsed operation also allows the photodetector to sample the signal during the pulse, reducing the effect of ambient light. |
The pulse modulation technique is described in US Patent 4,682,015, which is referenced in the HBCC-0500 datasheet . The technique is a key feature of the HBCC-0500's ambient light rejection capability. |
Chapter 33: The AM Detector and Low-Pass Filter |
The AM detector and low-pass filter are used to recover the barcode information from the modulated signal. The AM detector extracts the envelope of the signal, and the low-pass filter removes the modulation carrier. |
The Hewlett-Packard HBCC-0500 digitizer IC includes an AM detector that recovers the barcode information from the signal . The recovered signal is then low-pass filtered to eliminate the 30 kHz carrier. |
The low-pass filter's corner frequency must be chosen to pass the barcode signal while rejecting the modulation carrier. The corner frequency is set by the values of the capacitor and resistor in the filter network. |
The AM detection and low-pass filtering techniques are well-established in analog signal processing. They are used in the HBCC-0500 to provide robust ambient light rejection in a compact, cost-effective package. |
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Chapter 34: The Peak Detectors and Comparator |
The peak detectors and comparator form the digitizer stage of the HBCC-0500. The peak detectors capture the positive and negative peaks of the signal, and the comparator generates a digital pulse signal based on these peaks. |
The HBCC-0500 datasheet describes: 'The signal is input to a digitizer consisting of positive and negative peak detectors and a comparator. The comparator threshold is generated from the peak detectors using a resistor ladder' . |
The peak detectors provide an adaptive threshold that tracks the signal level. This ensures reliable digitization even when the signal amplitude varies due to changes in ambient light. |
The comparator output drives an external output transistor, providing a digital signal that can be decoded by the following logic. The output is high when the sensor is looking at black bars and low when looking at white spaces . |
Chapter 35: The Threshold Adjustment Feature |
The HBCC-0500 includes a threshold adjustment feature that allows the digitizer to be optimized for different barcode characteristics. The standard circuit uses a threshold designed for most barcodes, but the threshold can be adjusted if needed. |
The datasheet notes: 'If the bar codes to be read consistently have narrow bars that are lighter than the wide bars, then the 470 k and the 390 k resistors attached to pin 18 should be swapped' . |
This threshold adjustment feature is an example of how analog circuits can be tuned to optimize performance for specific applications. The adjustment changes the threshold level, ensuring reliable digitization for barcodes with unusual characteristics. |
The threshold adjustment is particularly useful for barcodes with poor contrast, where the signal amplitude is low. By adjusting the threshold, the digitizer can reliably detect the edges of the barcode even when the signal is weak. |
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Chapter 36: The Black State Return Feature |
The HBCC-0500 includes a black state return feature that forces the output to the black state after a timeout period. The normal operation returns to the white state after 100 milliseconds, but the black state return extends this timeout to about 1.5 seconds. |
The datasheet explains: 'The HBCC-0500 normally returns to the white state 100 milliseconds after the last transition. The extra circuitry forces the black state after a time out period set by the 1.0 F cap. The normal time out period is about 1.5 seconds' . |
The black state return feature is useful for applications where the output must remain in the black state for an extended period. This may be required for certain types of decoders or communication protocols. |
The feature is implemented with additional circuitry that forces the output to the black state after the timeout period. This circuitry is optional and can be added if needed for the specific application. |
Chapter 37: The Inverted Output Option |
The HBCC-0500 includes an inverted output option that provides an inverted version of the digitized signal. The standard output is high for black bars and low for white spaces, but the inverted output reverses this relationship. |
The datasheet explains: 'If inverted output is needed, add the extra circuitry in the inverted output block. Make sure that the 10 k pull up resistor on the normal output transistor is loaded' . |
The inverted output option is useful for applications where the decoder expects the opposite polarity. The option allows the HBCC-0500 to be easily integrated into a wide range of systems. |
The inverted output is generated by adding a transistor and associated components to the standard output. This additional circuitry is simple and cost-effective. |
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Chapter 38: The Bin Resistor and Sensor Matching |
The HBCC-0500 includes a bin resistor that is used to match the digitizer IC to the specific sensor. The bin resistor value depends on the sensor's bin number, which is marked on the sensor. |
The datasheet explains: 'Sensors are marked with an eight digit code, the last digit being the 'bin' number. The bin number is used to determine the bin resistor, Rb, using the Bin Table' . |
The bin resistor compensates for variations in the sensor's characteristics, ensuring consistent performance across different sensors. This is an example of how manufacturing tolerances are addressed in the design. |
The bin resistor is connected to the sensor and the digitizer IC. The resistor value is chosen to equalize the output of different sensors by varying the amplitude of the LED drive current . |
Chapter 39: The Construction Tips for the HBCC-0500 |
The HBCC-0500 datasheet includes construction tips for implementing the digitizer circuit. These tips are important for achieving reliable ambient light rejection. |
The first tip is to 'place the sensor, 2N4403, bin resistor, and the 0.47 F cap close to each other to minimize loop area' . This minimizes the parasitic inductance and capacitance, which can introduce noise into the signal. |
The second tip is to 'place the 0.22 F cap attached to pins 4 and 23 close to the IC' . This ensures that the IC's power supply is well-decoupled, reducing noise. |
The third tip is to 'use a single point ground close to pin 23' . A single point ground reduces ground loops, which can introduce noise. |
These construction tips are essential for achieving the ambient light rejection performance specified in the datasheet. |
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Chapter 40: The Recommended Operating Conditions |
The HBCC-0500 datasheet specifies the recommended operating conditions for the digitizer IC. These conditions include the scan velocity, edge contrast, ambient temperature, and ambient light level. |
The scan velocity is specified as 7.6 to 76 centimeters per second . This range covers the typical scanning speeds for handheld barcode readers. |
The edge contrast is specified as 40% or greater . This is the minimum contrast between the bars and spaces that is required for reliable reading. |
The ambient temperature is specified as -20 to 65 degrees Celsius . This wide temperature range allows the digitizer to be used in a variety of environments. |
The ambient light level is specified as up to 100,000 lux . This is the maximum ambient light level that the digitizer can reject while still reading barcodes reliably. |
Chapter 41: The Power Supply Requirements |
The HBCC-0500 datasheet specifies the power supply requirements for the digitizer IC. The power supply is critical for ambient light rejection, as noise on the supply can couple into the signal path. |
The recommended operating voltage is 5 volts . The power supply ripple and noise should be less than 100 mV peak-to-peak . |
The low noise requirement is typical for analog circuits, which are sensitive to power supply noise. The 100 mV peak-to-peak limit is relatively stringent, requiring good decoupling and regulation. |
The datasheet includes a schematic for the recommended power supply decoupling. The decoupling capacitors are placed close to the IC to minimize noise. |
The power supply requirements highlight the importance of careful power supply design in barcode readers. A noisy power supply can degrade the ambient light rejection performance, making it difficult to read barcodes reliably. |
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Chapter 42: The Bin Table and Sensor Selection |
The HBCC-0500 datasheet includes a bin table that maps the sensor's bin number to the required bin resistor value. The bin table is essential for selecting the correct resistor for the specific sensor. |
The bin table is not shown in the excerpt, but it is part of the datasheet. The table provides the resistor value for each bin number, ensuring consistent performance across different sensors. |
The bin table is an example of how manufacturing variations are managed in the design. By measuring each sensor and assigning a bin number, the manufacturer ensures that the digitizer IC can be matched to the sensor. |
The bin resistor is typically a 39-ohm resistor, but the value may vary depending on the bin number. The resistor is connected in series with the LED to adjust the drive current . |
Chapter 43: The Amplifier String and Ambient Light Immunity |
The Symbol Technologies patent describes an amplifier string that includes the differentiator at a very early stage. The differentiator filters out the ambient light before it can saturate the later amplifier stages. |
The patent explains: 'Since the original analog signal is not needed, it is possible to put the signal differentiator circuit at a very early stage in an amplifier string. Additional amplification stages can then be used to amplify the differentiated signal until it is large enough to be digitized' . |
This approach provides two benefits. The differentiator filters out the effects of ambient light that could otherwise cause the last amplifier stage to saturate, improving ambient light immunity. It also eliminates the transient response of other forms of high-pass filtering or AC coupling . |
The patent notes that 'all of these requirements of the prior art normally result in the need for large power supply voltages to eliminate the possibility of amplifier clipping' . The differentiator approach allows the digitizer and its associated amplifiers to work on low supply voltages. |
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Chapter 44: The Peak Locating Comparator and False Transition Rejection |
The Symbol Technologies patent describes a peak locating comparator circuit that detects the crossings of the first derivative signal and a delayed version of the same signal. These crossings correspond to the edges of the barcode. |
The patent explains: 'a peak locating comparator circuit means receiving the first derivative signal as a first input and the delayed first derivative signal as a second input, and detecting crossings of the first derivative signal and the delayed first derivative signal when the signals are equal, which result in transitions in the output signal' . |
The patent also describes a false transition gating circuit that 'discriminates against false transitions in the output signal of the peak locating comparator circuit means and changes states only upon the detection of transitions in the output signal of the peak locating comparator circuit means about a given threshold' . |
The false transition gating circuit is important for ambient light rejection because ambient light can cause false transitions in the signal. By gating the false transitions, the circuit ensures that only valid barcode edges are passed to the decoder. |
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Chapter 45: Summary --- Ambient Light Rejection in Perspective |
Ambient light rejection is one of the most important and challenging aspects of barcode reader design. The ambient light creates a DC offset that can saturate the amplifiers and distort the signal, and it can also introduce AC noise that interferes with the barcode signal. |
We have examined how different companies and technologies have approached the challenge of ambient light rejection: |
Symbol Technologies developed a differentiator-based digitizer that obtains all of its information for digitizing from the first derivative of the analog signal . The differentiator filters out the ambient light before it can saturate the later amplifier stages, improving ambient light immunity. |
Hewlett-Packard developed the HBCC-0500 digitizer IC, which uses AC coupling and AM detection to reject ambient light . The IC was designed for barcode wands and provides high ambient light rejection up to 100,000 lux. |
Datalogic developed a differential optical architecture using three photodiodes to subtract the ambient light from the signal . The main photodiode receives the barcode signal along with ambient light, while two smaller photodiodes receive only the ambient light. |
Symbol Technologies also developed an ambient light shield and color filter for imaging-based readers, specifically for reading DPM barcodes . The shield and filter work together to block ambient light while passing the desired illumination. |
Texas Instruments and Analog Devices provide components such as zero-drift amplifiers that reduce the DC offset in the transimpedance amplifier, improving ambient light rejection . |
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The key lessons from our exploration are: |
AC coupling is the simplest approach. A high-pass filter blocks the DC component of the signal while passing the AC component. The corner frequency must be chosen carefully to pass the barcode signal while blocking the DC offset. |
Differentiation provides a robust alternative. The first derivative of the analog signal eliminates the DC offset and provides a signal that is easy to digitize. |
Differential optical architecture cancels ambient light. By using a reference photodetector that receives only ambient light, the system can subtract the ambient light from the signal. |
Optical filtering blocks ambient light. Shields and filters can block ambient light while passing the desired illumination, improving the signal-to-noise ratio. |
Adaptive techniques are essential. The ambient light level varies, and the rejection circuits must adapt to the changing conditions. This includes adaptive thresholds, automatic gain control, and adjustable bandwidth filters. |
Integration is the trend. Modern barcode readers integrate ambient light rejection into the imager or the processor, reducing cost and improving performance. |
In the end, ambient light rejection is a testament to the ingenuity of engineers who have developed a range of techniques to solve a fundamental problem. The choice of technique depends on the application requirements, the cost constraints, and the specific challenges of the environment. The art of ambient light rejection lies in the careful balance of circuit design, component selection, and system-level integration. |
cs@easiersoft.com