Differential Optical Architecture: The Art of Subtraction in Barcode Reading |
Executive Summary |
This article provides a comprehensive, accessible exploration of differential optical architecture for ambient light rejection in barcode readers. We examine how this elegant technique uses matched photodetectors and subtraction to eliminate the unwanted ambient light that plagues single-ended designs. Rather than focusing on abstract theory, we ground every concept in concrete design examples from Datalogic, Symbol Technologies, AT&T, and other pioneers. We explore the fundamental principle of common-mode rejection, the use of area-matched photodiodes, the architecture of three-detector systems, and the practical circuit implementations for current subtraction. The article covers both the optical design considerations and the electronic processing techniques that make differential architecture effective. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing ambient-light-tolerant barcode readers. |
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Chapter 1: The Concept of Common-Mode Rejection |
The differential optical architecture is built on a simple but powerful idea: if two photodetectors receive the same unwanted ambient light, and only one receives the desired barcode signal, then subtracting the two signals will cancel the ambient light and leave the barcode signal behind. This is the optical analog of common-mode rejection in differential amplifiers. |
The concept is elegantly simple. Imagine two photodiodes placed close together. Both see the same ambient light from the sun, the fluorescent lights, and all other sources. But only one sees the light reflected from the barcode. If we subtract the current from the second photodiode from the current from the first, the ambient light components cancel, and only the barcode signal remains. |
This approach is powerful because it cancels ambient light in the current domain, before the signal is amplified. This prevents the ambient light from saturating the transimpedance amplifier, which is a common problem in single-ended designs. The differential architecture also rejects ambient light that is modulated, such as the high-frequency pulsing of LED lights, because the modulation is common to both photodiodes. |
A patent from Datalogic explains the challenge: 'When modulated ambient light is superimposed upon the laser light reflected off a bar code, the signal to noise ratio at the photodetector of the bar code signal is degraded, especially when the reading distance between the bar code and the bar code reader increases' . The differential approach addresses this by 'capturing the ambient light near the bar code and subtracting it from the light reflected from the bar code that includes ambient light' . |
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Chapter 2: The Single-Ended Problem |
To appreciate the differential architecture, it helps to understand the limitations of single-ended designs. In a conventional barcode reader, a single photodiode receives both the desired barcode signal and the unwanted ambient light. The photodiode produces a current that is the sum of both components. |
This creates several problems. The ambient light component acts as a DC offset, consuming the amplifier's dynamic range. If the ambient light is bright enough, the amplifier saturates, and the barcode signal is lost. Even if saturation does not occur, the ambient light adds noise and reduces the signal-to-noise ratio. |
The problem is particularly acute for laser-based barcode readers, where the reading distance can be large. A patent notes that 'the ambient light component severely degrades the signal to noise ratio of the reflected bar code signal when the reading distance between the bar code reader and the bar code increases' . This is because the reflected laser signal decreases with the square of the distance, while the ambient light remains constant. |
The conventional solution to this problem is to use filters and AC coupling to reject the ambient light. However, these techniques have limitations. AC coupling cannot reject ambient light that is modulated at frequencies close to the barcode signal. Filters can reject some wavelengths of ambient light but not all. |
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Chapter 3: The Datalogic Three-Photodiode System |
Datalogic developed a particularly elegant implementation of the differential optical architecture using three photodiodes . The system includes a primary photodiode that receives both the barcode signal and ambient light, and two secondary photodiodes that receive only ambient light. |
The Datalogic patent describes the arrangement: 'three photodiodes may be used, including 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' . |
The key to this design is 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 currents from the secondary photodiodes match the ambient light current from the primary photodiode, allowing accurate subtraction. |
The optical system uses imaging optics to project different parts of the field onto the photodiodes. The primary photodiode receives light from the area around the barcode, while the secondary photodiodes receive light from areas adjacent to the barcode that are not illuminated by the laser. This ensures that the secondary photodiodes do not receive any barcode signal. |
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Chapter 4: The Optical Collection Geometry |
The effectiveness of the differential architecture depends on careful optical design. The imaging optics must project the field of view onto the photodiodes in a way that maximizes the barcode signal on the primary photodiode and minimizes any barcode signal on the secondary photodiodes. |
The Datalogic patent describes the optical collection geometry in detail. The collection optics 'image the field of view onto an image plane' . The field of view is divided into areas: a central area containing the barcode, and top and bottom areas adjacent to the barcode . |
The primary photodiode is positioned to receive light from the central area, which contains the barcode and the laser illumination. The secondary photodiodes are positioned to receive light from the top and bottom areas, which are not illuminated by the laser. |
The patent emphasizes that 'the light source must not illuminate the photodiodes that sense ambient light' . This is achieved by focusing the laser beam to illuminate only the central area of the field of view, leaving the adjacent areas dark. |
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Chapter 5: The Multiplier for Active Area Matching |
Even with careful optical design, the ambient light current from the primary photodiode and the secondary photodiodes may not be perfectly matched. This is because the active areas of the photodiodes may not be exactly equal, or the imaging optics may have different efficiencies for different parts of the field. |
The Datalogic patent describes a technique for compensating for this mismatch: the system calculates a multiplier for weighting the secondary currents . The multiplier is calculated by dividing the active area of the primary photodetector by the sum of the active areas of the secondary photodetectors. |
The patent explains: 'the multiplier can also be dependent upon the spatial efficiency of the imaging optics and/or the surface reflection coefficients of the indicia and the areas near the indicia' . This allows the system to compensate for any variations in the optical system. |
The weighted secondary currents are then subtracted from the primary current to obtain the barcode signal, free of ambient light. The patent describes the process: 'the system subtracts the weighted secondary electric current sum from the primary electric current to obtain the electric current generated by the light reflected from the indicia, free of the influence of ambient light' . |
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Chapter 6: The Electronic Subtraction Circuit |
The subtraction of the primary and secondary currents can be performed in the analog domain using a differential amplifier circuit. The Datalogic patent describes a circuit where 'both photodiode circuit elements are DC-biased through resistors, transistors, or impedance elements' . |
The currents from the photodiode elements 'pass through the capacitors located near the input terminals of the amplifier' . This allows the unwanted parasitic current generated by the primary photodiode to be 'effectively amplified and cancelled electronically at the output to the amplifier without the introduction of any additional noise or the use of any other amplifiers or circuits that might decrease the signal to noise ratio' . |
The patent emphasizes that 'because the same optical collector operates upon the same local field, the efficiency of the cancellation of the parasitic ambient light is maximized' . This highlights a key advantage of the differential architecture: the ambient light cancellation is performed using signals that are generated by the same optical system and the same environment. |
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Chapter 7: The Need for Differential Design |
The need for differential optical architecture is driven by the increasing prevalence of modulated ambient light. Traditional incandescent lights produce a steady light, but modern lighting technologies such as fluorescent lamps and LEDs are often pulsed at high frequencies. |
A Symbol Technologies patent explains the challenge: 'when fluorescent lamps and LEDs are operated at kilohertz frequencies, the analog electrical ambient light signal has a constant illumination DC component and a relatively larger time-varying AC frequency component at kilohertz frequencies, typically anywhere from 30 kHz to 300 kHz' . |
This creates a problem because 'if the time-varying frequency component of the ambient light signal is too close in frequency to the frequency of the information signal, then the ambient light signal can interfere and impede the decoding of the information signal' . By way of example, 'an information signal of about 50 kHz and its harmonic at about 100 kHz can be generated during reading of a low density symbol. If the ambient light source includes LEDs operated to have a frequency of about 100 kHz, then the 100 kHz frequencies of the ambient light signal and the information signal are too close and will cause an interference' . |
The differential optical architecture is particularly effective against this type of interference because the ambient light modulation is common to both the primary and secondary photodiodes and is canceled by the subtraction. |
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Chapter 8: Symbol Technologies' Adaptive Approach |
Symbol Technologies developed an adaptive approach to ambient light rejection that can be used in conjunction with differential architecture . The system measures the ambient light signal and adjusts the reader's parameters when the ambient light exceeds a threshold. |
The Symbol Technologies patent describes a reader that 'measures the ambient light signal to determine a threshold, processes the output signal by passing the output signal through a filter having a bandwidth, and rejects the ambient light signal from the output signal by adjusting the scan angle and/or the bandwidth when the ambient light signal exceeds the threshold' . |
This adaptive approach is valuable because the ambient light conditions can vary widely. In some environments, the ambient light is low, and the reader can operate with a wide bandwidth and a large scan angle. In other environments, the ambient light is high, and the reader must reduce the bandwidth or the scan angle to reject the interference. |
The Symbol Technologies patent notes that 'there is a need to reject such interference caused by such ambient light to enhance reader performance' . |
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Chapter 9: AT&T's Differential Free-Space Optical Transmission |
The concept of differential optical transmission was not invented for barcode readers. AT&T developed a differential approach for free-space optical signal transmission, demonstrating the broader applicability of the technique . |
The AT&T patent describes a system where 'a complementary signal is developed electronically, and the signal and its complement are each applied to a light emitting device' . The light from the two devices is transmitted and received by separate lenses and photodetectors, and the outputs of the photodetectors are applied to a differential amplifier. |
The advantage of this arrangement is that 'the alignment of the receiving lens with respect to the transmitting pair is not as stringent as before because of the complementary nature of the signal transmission' . This is because the differential detection cancels common-mode alignment errors. |
The AT&T patent notes that 'a number of directed, free-space, optical interconnection arrangements are known' but that 'the positioning and alignment problem may be overcome' in simpler arrangements, while 'in denser utilizations of the optical backplane, or in an optical computer environment that uses free-space to pass thousands of closely positioned signal paths,' the alignment problem is more severe . |
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Chapter 10: Intel's Differential Image Barcode Communication |
Intel has extended the differential concept to the realm of 2D barcodes displayed on screens . The Intel patent describes 'techniques for communicating barcodes using differential images' . |
The technique involves capturing a barcode image and an alternative barcode image, and then performing an operation on the two images to generate comparative barcode image data . The alternative image may be a pure black image, and the operation may be an absolute value difference. |
The Intel patent explains the challenge: 'producing, capturing, and/or decoding a barcode may not be stable due to noise caused by over/under exposure, ambient light, shadows, or reflections' . This is 'particularly problematic for 2D barcodes and/or for barcodes presented via a display screen such as, for example, a liquid-crystal display (LCD) display' where 'reflections may be stronger than when a barcode is presented via paper' . |
The differential approach addresses this problem by canceling the common-mode noise components. The patent describes a process where 'the comparative barcode image data is denoised, binarized, and decoded' . |
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Chapter 11: The Prismatic Lens Detector Design |
Another approach to differential optical architecture uses a prismatic lens to shape the light signal before it reaches the photodiodes. This design, described in a patent, uses a lens of pyramidal frusta-like shape to concentrate light and a pair of photodiodes to detect it . |
The patent describes the lens as 'an optically transparent lens of high index of refraction that is generally prismatic... to conduct light reflected from the label to a pair of conventional photodiodes' . The lens condenses the signal in area, allowing the photodiode area to be minimized and reducing shot noise. |
The patent explains that 'shotnoise is a function of total photodiode area, so shotnoise will thereby be decreased' . This is an important consideration for barcode readers, where the signal-to-noise ratio is critical. |
The prismatic lens design also includes cross-polarization and bandpass filtering to reject specular reflections and ambient light. The patent notes that 'the filters and the input side of the lens are angled to the sides of the label, rather than the center, to ensure that light is received over the entire scan width' . |
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Chapter 12: Cross-Polarization for Specular Rejection |
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. |
The prismatic lens patent describes a cross-polarization technique to reject specular reflections. The light directed to the barcode is polarized, and a polarizer in the return path is oriented orthogonal to it. This means that 'specular reflections, which are of the same polarity as the light directed to the label,' are filtered out, while the diffuse light from the barcode is passed . |
The patent explains: 'This allows the detector to receive a predominantly diffuse light signal from the white background between the black bars. Specular reflections from the black bars are attenuated by the cross (orthogonal) polarization so as not to overpower the diffuse light read from the white spaces between bars' . |
This technique is a form of optical signal conditioning that complements the electrical differential architecture. By rejecting specular reflections optically, the photodetector receives a cleaner signal, reducing the burden on the electronic processing. |
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Chapter 13: Bandpass Filters for Wavelength Selection |
Bandpass filters are another optical technique for rejecting ambient light. A bandpass filter passes light within a narrow wavelength range and blocks light outside that range. If the barcode reader's illumination is monochromatic (as from a laser or LED), a bandpass filter centered at that wavelength will reject most ambient light. |
The prismatic lens patent describes the use of bandpass filters that 'absorb wavelengths outside a narrow band centered around the laser wavelength, thereby eliminating reflected ambient light' . The filters are angled towards the left and right sides of the barcode to allow them to receive light from the entire scan width . |
The patent notes that the filters are 'spaced apart with the space being covered by a strip of opaque material. This opaque material further attenuates the signal reflected from the central region of the label ensuring that a more uniform intensity of light is gathered across the scan width' . |
This combination of bandpass filtering, angled filters, and opaque material provides a complete optical treatment of the return light signal, improving the signal-to-noise ratio before the light reaches the photodiodes. |
Chapter 14: The Diffractive Optical Barcode |
A truly differential approach to barcode reading can be implemented at the barcode label itself. A patent describes a 'differential barcode label' that uses diffractive relief structures to create optical signals that can be detected differentially . |
The patent describes a label with 'at least one machine-readable diffractive bar code consisting of narrow rectangular fields occupied by the optically active structures and intermediate surfaces' . The diffractive relief structure 'diffracts and polarizes incident light and scatters the diffracted light into a half-space above the diffractive relief structure' . |
A second diffractive relief structure 'differs at least in respect of the polarization of the polarizedly backscattered light with respect to the first diffractive relief structure' . This allows the light from the bars and spaces to be distinguished by polarization, enabling differential detection. |
This approach is innovative because it moves the differential encoding from the reader to the label. The reader can then use polarization-sensitive detection to read the barcode, with the differential detection providing rejection of ambient light and other common-mode noise. |
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Chapter 15: Differential Optical SAR |
The differential optical concept has also been applied to synthetic aperture radar (SAR), demonstrating its broader utility in optical imaging systems . A patent from the University of California describes a 'differential synthetic aperture radar method to produce an image having a spatial resolution corresponding to a synthetic aperture' . |
The technique uses 'a single aperture to obtain unique (N) phases that can be processed to produce a synthetic aperture image at points along a trajectory' . This is accomplished by dividing the aperture into two equal 'subapertures' and collecting return signals from each. |
The differential technique is designed to 'cancel common-mode errors, trajectory deviations from a straight line, and laser phase noise' . This is analogous to the cancellation of common-mode ambient light in barcode readers. |
The patent notes that the differential approach 'would relax stability requirements, compared to conventional SARs, such as: deviations from a straight line platform trajectory during the image formation time, both line-of-sight and out-of-plane, laser frequency stability (bandwidth), and speckle and turbulence distortion' . |
Chapter 16: The Differential Amplifier in Practice |
The electronic implementation of the differential optical architecture typically uses a differential amplifier or a subtractor circuit. The differential amplifier amplifies the difference between two input signals while rejecting signals that are common to both inputs. |
In the Datalogic implementation, the currents from the primary and secondary photodiodes are subtracted in the current domain before being converted to a voltage by a transimpedance amplifier. This is the most efficient approach because it removes the ambient light component before it can saturate the amplifier. |
The patent describes that 'the unwanted parasitic current generated by the photodiode is effectively amplified and cancelled electronically at the output to the amplifier without the introduction of any additional noise' . This is achieved by 'DC-biasing through resistors, transistors, or impedance elements' and passing the currents through capacitors at the amplifier input. |
The subtraction can also be performed in the voltage domain, after the currents have been converted to voltages by separate transimpedance amplifiers. This approach is simpler to implement but may be less effective because the ambient light component is amplified along with the signal. |
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Chapter 17: The Spatial Independence of Ambient Light |
A key assumption of the differential optical architecture is that the ambient light is spatially independent over the field of view. This means that the ambient light per unit area is the same in the central area (where the barcode is) and the adjacent areas (where the secondary photodiodes are aimed). |
The Datalogic patent relies on this assumption: 'Because the ambient light is substantially spatially independent, the ambient light per unit area imaged from the first area is substantially the same as the ambient light per unit area imaged from the one or more additional areas near the indicia that do not overlap the first area' . |
This assumption is generally valid because ambient light sources are typically distant from the barcode and illuminate the entire field of view uniformly. However, in some situations, such as when there is a strong point source of light near the barcode, the ambient light may not be spatially independent. In these cases, the differential architecture may be less effective. |
The patent acknowledges that 'the amount of current generated by parasitic ambient light actually removed... depends upon the level of the signal detected... and the signal detected depends upon the spatial efficiency of the optical collection imagers and the surface reflection coefficients' . |
Chapter 18: The Active Area Matching Constraint |
For the differential architecture to work effectively, the ambient light current from the primary photodiode must be matched to the ambient light current from the secondary photodiodes. This requires that the active areas of the photodiodes be carefully matched. |
The Datalogic patent specifies 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 secondary photodiodes generate approximately the same ambient light current as the primary photodiode. |
If the active areas are not matched, the ambient light currents will not cancel completely, leaving a residual ambient light component in the output signal. This residual component will act as a DC offset, reducing the dynamic range of the amplifier. |
The patent also notes that 'the multiplier can also be dependent upon the spatial efficiency of the imaging optics and/or the surface reflection coefficients' . This allows the system to compensate for any residual mismatch by weighting the secondary currents. |
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Chapter 19: The Opaque Material for Uniform Intensity |
The prismatic lens patent describes the use of an opaque strip to ensure uniform light intensity across the scan width. This is a clever optical technique that addresses the problem of uneven illumination. |
The patent explains: 'An opaque material is used to prevent light from entering the center of the lens' . This is because 'considerably more light is returned from the central area of the scan than from the periphery' due to the inverse square law and the geometry of the scan. |
By blocking the light from the center of the lens, the opaque material ensures that the light reaching the photodiodes is more uniform across the scan width. This improves the linearity of the signal and simplifies the electronic processing. |
The opaque material is particularly important for differential optical architectures because uneven illumination can cause mismatches between the primary and secondary photodiode signals, reducing the effectiveness of the ambient light cancellation. |
Chapter 20: The Pair of Photodiodes |
The prismatic lens patent uses a pair of photodiodes connected in parallel to detect the optical signal. The pair of photodiodes 'transform the light signal into an electrical signal' and are 'connected in parallel to produce an electrical signal representative of the entire light signal received' . |
The use of a pair of photodiodes, rather than a single photodiode, provides several advantages. The total active area is larger, capturing more light and increasing the signal strength. The signal from the two photodiodes can also be processed differentially, providing ambient light rejection. |
The patent notes that 'the signal concentration performed by the lens allows the size of the photodiodes to be minimized, thereby reducing shotnoise' . This is a key benefit of the prismatic lens design: it concentrates the light signal, allowing smaller photodiodes with lower shot noise. |
The patent claims that 'conventional circuitry... processes, digitizes, and decodes the electrical signal without resorting to any special circuitry that would be needed had the novel optical processing described above not been employed' . This is a testament to the effectiveness of the optical signal conditioning. |
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Chapter 21: The Filter Angle and Scan Angle Matching |
The prismatic lens patent describes a careful matching of the filter angle to the scan angle to ensure that the filters receive light from the entire scan width. The filters are angled to the sides of the barcode, rather than the center. |
The patent explains: 'The filters used in the preferred embodiment each have an angle rating of approximately 20 to 25 degrees for a total of 40 to 50 degrees. The scan angle is approximately 60 to 70 degrees. Thus, the filter angle needs to be approximately 10 to 15 degrees for a total of 20 to 30 degrees' . |
The relationship is that 'the total of the filter angle rating plus the filter angle equal the scan angle' . This ensures that the filters are properly oriented to receive light from the full width of the scan. |
This careful optical design ensures that the photodiodes receive a consistent signal across the entire scan width, improving the linearity and accuracy of the barcode reading. |
Chapter 22: The Fresnel Reflection and Polarization |
The prismatic lens patent also addresses the issue of Fresnel reflection, which occurs when light passes through an interface between materials with different refractive indices. The patent uses the lens's high index of refraction to bend and concentrate the light. |
The patent notes that 'the invention employs an optically transparent lens of high index of refraction' . The high index of refraction allows the lens to be smaller and more compact while still providing the required light concentration. |
The patent also uses polarization to reject specular reflections. The cross-polarization technique is based on the principle that specular reflections preserve polarization, while diffuse reflections do not. By orienting the polarizer in the return path orthogonal to the transmitted polarization, the specular reflections are blocked. |
The patent explains that 'specular reflections from the black bars are attenuated by the cross (orthogonal) polarization so as not to overpower the diffuse light read from the white spaces between bars' . |
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Chapter 23: The Pyramidal Frusta-Like Shape |
The prismatic lens in the patent has a specific shape: a pyramidal frusta-like shape. This shape is designed to concentrate the light signal while minimizing the photodiode area. |
The patent describes: 'the width of the input side... is larger than the width of the output side. The preferred range of narrowing... is from 15 to 25 degrees' . This narrowing concentrates the light as it passes through the lens, reducing the cross-sectional area of the signal at the photodetector. |
The patent explains that 'this diminishment of cross sectional area is what allows lens to concentrate the signal leaving its output side' . The concentrated signal allows the photodiodes to be smaller, reducing shot noise. |
The pyramidal frusta-like shape also allows the lens to be attached to the subhousing and to provide recessed input side planes for seating the filters. The lens is a single piece of cast plastic, making it inexpensive to manufacture. |
Chapter 24: The Subhousing and Lens Attachment |
The prismatic lens patent describes a subhousing that holds the lens and the filters. The subhousing provides the mechanical support for the optical components and ensures their proper alignment. |
The patent describes 'rectangular protuberance... which is used to attach lens to lens subhousing. The protuberance also serves as a spacer separating recessed input side planes that receive filters affixed between the lens subhousing and lens planes' . |
The subhousing also includes 'pins projecting from the lens subhousing for locating and holding the lens in place. The lens mounting pins and adjacent area of the subhousing also serve as opaque material which blocks reflected light from entering the central area of the lens' . |
This integration of mechanical and optical functions in a single subhousing simplifies the manufacturing and assembly of the barcode reader. |
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Chapter 25: The Photodiode Mounting |
The prismatic lens patent describes the mounting of the photodiodes on a detector wall, typically a printed circuit board. The photodiodes are positioned to receive the concentrated light signal from the lens. |
The patent describes: 'This concentrated light signal passes to the photodetection means, preferably a pair of photodiodes, mounted on detector wall, such as provided by a printed circuit board' . |
The photodiodes transform the light signal into an electrical signal, which is then processed by conventional circuitry. The patent notes that 'conventional circuitry, mounted on base and detector wall, processes, digitizes, and decodes the electrical signal without resorting to any special circuitry that would be needed had the novel optical processing described above not been employed' . |
This demonstrates the power of the differential optical architecture: by processing the signal optically, it simplifies the electronic circuitry and reduces the cost of the reader. |
Chapter 26: The Datalogic System Operation |
The Datalogic patent provides a detailed description of the system operation. The process involves imaging light from the barcode and adjacent areas onto the primary and secondary photodiodes, converting the light to currents, and subtracting the currents. |
The patent describes the process: 'the system images the light reflected from a first area immediately surrounding and including the indicia onto the active area of a primary photodetector using imaging optics. The light reflected from the first area includes light reflected from the indicia and also parasitic ambient light' . |
In parallel, 'the system can use the same imaging optics to image ambient light from one or more additional areas near the indicia that should not be overlapping the first area onto one or more secondary photodetectors' . |
The system then 'converts the imaged light to a primary electric current' and 'converts the ambient light that impinges on their respective active areas into secondary electric currents' . The secondary currents are added together, weighted by a multiplier, and subtracted from the primary current. |
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Chapter 27: The Subtraction Process |
The subtraction process in the Datalogic system is performed electronically. The primary current and the weighted sum of the secondary currents are combined in a way that cancels the ambient light component. |
The patent describes: 'the system subtracts the weighted secondary electric current sum from the primary electric current to obtain the electric current generated by the light reflected from the indicia, free of the influence of ambient light' . The resulting signal is then amplified for further processing. |
The patent emphasizes the efficiency of this process: 'the unwanted parasitic current generated by the photodiode is effectively amplified and cancelled electronically at the output to the amplifier without the introduction of any additional noise or the use of any other amplifiers or circuits that might decrease the signal to noise ratio' . |
The key to this efficiency is that 'the same optical collector operates upon the same local field, the efficiency of the cancellation of the parasitic ambient light is maximized' . |
Chapter 28: The Limitations of the Differential Architecture |
While the differential optical architecture is powerful, it has limitations. The most significant limitation is that it relies on the spatial independence of the ambient light. If the ambient light is not uniform across the field of view, the cancellation will not be perfect. |
The Datalogic patent acknowledges this limitation: 'the amount of current generated by parasitic ambient light actually removed... depends upon the level of the signal detected by the photodiode relative to the levels of the signals detected by the other photodiodes, and the signal detected depends upon the spatial efficiency of the optical collection imagers used to image the light onto the photodiodes and the surface reflection coefficients of the bar code and the area near the bar code' . |
The patent notes that 'for the dimensions of the photodiodes in the prototype, where the photodiodes are approximately half the width of the main photodiode, spatial efficiency variations are very small. Also, typically the surfaces above and below the bar code usually have the same reflection coefficient as the bar code itself' . |
This suggests that the differential architecture is most effective when the ambient light is uniform and the adjacent surfaces have the same reflectance as the barcode surface. |
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Chapter 29: The Alternative Configurations |
The Datalogic patent notes that 'other dimensions and configurations of the photodiode active areas and/or different ratios of the width to the length of the photodiode active areas may be used' . This flexibility allows the differential architecture to be adapted to different applications. |
One alternative configuration uses a non-laser light source, such as an LED, to illuminate the barcode. The patent explains that 'a non-laser light source may be used to illuminate the bar code as long as the light source is focused to illuminate only the rectangular area around the bar code and not the neighboring areas' . |
Another alternative uses a scanning mechanism to spread the light from the laser into a narrow line. The patent explains: 'a scanning mechanism may use optics to spread light from a light source, such as a laser, into a narrow line of light and project the line of light onto the bar code, while remaining entirely within the area' . |
These alternative configurations provide flexibility for different barcode reading applications and form factors. |
Chapter 30: The Photodiode Current Subtraction Circuit |
The Datalogic patent describes the electronic circuit for subtracting the photodiode currents. The circuit uses capacitors near the amplifier input to couple the photodiode currents to the amplifier, blocking the DC component. |
The patent explains: 'the currents of the photodiode elements pass through the capacitors located near the input terminals of the amplifier. Consequently, unwanted parasitic current generated by the photodiode is effectively amplified and cancelled electronically at the output to the amplifier' . |
The use of capacitors at the amplifier input is significant. It allows the AC component of the photodiode currents to pass to the amplifier while blocking the DC component. This ensures that the DC bias of the photodiodes does not affect the amplifier's operation. |
The patent notes that the subtraction is performed 'without the introduction of any additional noise or the use of any other amplifiers or circuits that might decrease the signal to noise ratio' . This is a key advantage of the differential architecture: it cancels the ambient light without adding noise. |
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Chapter 31: The Symbol Technologies Adaptive Filtering |
Symbol Technologies' adaptive approach to ambient light rejection can be combined with differential architecture to provide even better performance. The adaptive system measures the ambient light signal and adjusts the reader's parameters to optimize reading. |
The Symbol Technologies patent describes a reader that 'includes a laser for emitting a laser beam, a scan component for scanning the laser beam over a scan angle across the target, a photodetector for generating an output signal by detecting return laser light from the target to generate an information signal bearing information related to the target, and by concomitantly detecting the ambient light to generate an ambient light signal' . |
The reader's 'signal processing circuitry measures the ambient light signal to determine a threshold, processes the output signal by passing the output signal through a filter having a bandwidth, and rejects the ambient light signal from the output signal by adjusting the scan angle and/or the bandwidth when the ambient light signal exceeds the threshold' . |
This adaptive approach is particularly effective against modulated ambient light, which is common from fluorescent lamps and LEDs operating at kilohertz frequencies. |
Chapter 32: The Frequency Interference Problem |
The Symbol Technologies patent highlights the frequency interference problem that modern ambient light sources create. When the ambient light is modulated at a frequency close to the barcode signal frequency, it can interfere with the decoding. |
The patent provides an example: 'an information signal of about 50 kHz and its harmonic at about 100 kHz can be generated during reading of a low density symbol located relatively close to the reader, e.g., about 10 inches away. If the ambient light source includes LEDs operated to have a frequency of about 100 kHz, then the 100 kHz frequencies of the ambient light signal and the information signal are too close and will cause an interference' . |
This interference 'may cause the symbol not to be successfully decoded and read' . The patent emphasizes that 'there is a need to reject such interference caused by such ambient light to enhance reader performance' . |
The differential optical architecture is particularly effective against this type of interference because the ambient light signal is common to both the primary and secondary photodiodes and is canceled by the subtraction. |
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Chapter 33: The Fully Differential Optical Link |
A paper on fully differential optical interconnects provides additional insight into the benefits of differential optical signaling . The paper proposes a 'fully differential optical link for board-to-board and onboard interconnections within digital systems' . |
The paper notes that 'a fully differential link has essentially the same SNR as a comparable single-ended link, but the differential system is less susceptible to the common-mode noise such as the switching noise generated at the transmitter or at the receiver input' . |
This is the key advantage of differential signaling: it rejects common-mode noise while preserving the signal. In the context of barcode readers, the common-mode noise is the ambient light, and the signal is the barcode reflection. |
The paper also notes that the differential link can be 'fully DC-coupled, to produce a low-latency, high-bandwidth parallel synchronous interconnection' . This is another advantage of differential signaling: it does not require AC coupling, which can limit the bandwidth. |
Chapter 34: The Application to 2D Barcodes |
Intel's differential image approach extends the differential concept to 2D barcodes . This is important because 2D barcodes are increasingly used for mobile payments and other applications where the barcode is displayed on a screen. |
The Intel patent describes a method where 'a barcode image and an alternative barcode image are captured, and an operation is performed on the captured barcode image data and the captured alternative barcode image data to generate comparative barcode image data' . |
The alternative barcode image may be a pure black image, and the operation may be an absolute value difference. The patent explains that the comparative barcode image data is then 'denoised, binarized, and decoded' . |
The patent notes that 'color barcodes such as color 2D barcodes, which may offer greater data capacity than black and white barcodes, may have instability due to... color shifting during the production and capturing of the 2D color image' . The differential approach can address this instability by canceling common-mode color shifts. |
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Chapter 35: The Diffractive Barcode Label |
The diffractive barcode label patent describes a truly differential approach where the barcode itself encodes the signal differentially . This is a radical departure from conventional barcodes, which encode data in the widths of bars and spaces. |
The patent describes a label with 'at least one machine-readable diffractive bar code consisting of narrow rectangular fields occupied by the optically active structures and intermediate surfaces' . The diffractive relief structure 'diffracts and polarizes incident light and scatters the diffracted light into a half-space above the diffractive relief structure' . |
A second diffractive relief structure 'differs at least in respect of the polarization of the polarizedly backscattered light with respect to the first diffractive relief structure' . This allows the bars and spaces to be distinguished by polarization, enabling differential detection. |
The patent notes that the 'light which is polarizedly backscattered at the diffractive bar code can be detected by means of one of the known commercially available reading apparatuses for bar codes produced by printing' . This means that existing barcode readers could be adapted to read differential barcodes with the addition of polarization-sensitive detection. |
Chapter 36: The Practical Implementation Challenges |
Implementing a differential optical architecture in a commercial barcode reader presents several practical challenges. The photodiodes must be carefully matched, the optical system must be precisely aligned, and the subtraction circuit must be well-designed. |
The Datalogic patent acknowledges that the 'spatial efficiency variations' must be considered . The patent also notes that 'the surfaces above and below the bar code usually have the same reflection coefficient as the bar code itself,' which simplifies the matching . |
The patent notes that 'other dimensions and configurations of the photodiode active areas and/or different ratios of the width to the length of the photodiode active areas may be used' . This provides flexibility for designers to optimize the system for their specific application. |
The implementation of the subtraction circuit also requires care. The circuit must cancel the ambient light without adding noise or reducing the signal-to-noise ratio. The Datalogic patent describes a circuit that achieves this using capacitors and a differential amplifier . |
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Chapter 37: The Advantages Over Filtering |
The differential optical architecture offers several advantages over filtering for ambient light rejection. Filtering rejects only certain wavelengths or frequencies of light, while differential architecture rejects all ambient light regardless of wavelength or frequency. |
Filtering is also wavelength-specific. A bandpass filter centered at the laser wavelength will reject ambient light at other wavelengths, but it will not reject ambient light at the laser wavelength. If the ambient light contains the same wavelength as the laser, the filter is ineffective. |
Differential architecture, on the other hand, rejects all ambient light regardless of wavelength. The cancellation is based on the spatial distribution of the light, not its spectral content. This makes differential architecture more robust and versatile. |
The Datalogic patent emphasizes that differential architecture provides 'efficiency of the cancellation of the parasitic ambient light' that is 'maximized' because 'the same optical collector operates upon the same local field' . |
Chapter 38: The Signal-to-Noise Ratio Improvement |
The differential optical architecture significantly improves the signal-to-noise ratio of the barcode reader. By canceling the ambient light, the architecture removes a major source of noise and DC offset. |
The Datalogic patent notes that 'the ambient light component severely degrades the signal to noise ratio of the reflected bar code signal when the reading distance between the bar code reader and the bar code increases' . By canceling this component, the differential architecture improves the SNR, especially at long reading distances. |
The patent claims that the differential architecture provides 'improved light sensing performance over a range of environmental parameters, such as varying volume and light source temperatures' . This robustness is a key advantage of the differential approach. |
The SNR improvement is particularly important for reading low-contrast barcodes or barcodes at long distances, where the reflected signal is weak and the ambient light is relatively strong. |
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Chapter 39: The Power Consumption Considerations |
The differential optical architecture can also reduce power consumption. By canceling the ambient light, the architecture reduces the DC offset and prevents the amplifier from saturating. This allows the amplifier to operate with lower gain and lower power. |
The differential architecture also allows the use of a lower-intensity illumination source. Because the ambient light is canceled, the barcode signal does not need to be as strong relative to the ambient light. This can reduce the power consumption of the LED or laser. |
The prismatic lens patent emphasizes that the optical signal conditioning 'alleviates the need for expensive electrical signal conditioning components' , which can also reduce power consumption by eliminating active components. |
The lower power consumption is particularly important for handheld barcode readers, which are battery-powered. |
Chapter 40: The Integration with Digital Processing |
The differential optical architecture can be integrated with digital processing for even better performance. The analog subtraction of the photodiode currents can be followed by digital signal processing to further improve the signal quality. |
The Intel patent describes a digital approach where 'a captured barcode image and a captured alternative barcode image' are processed by 'performing an operation on the captured barcode image data and the captured alternative barcode image data to generate comparative barcode image data' . |
The Datalogic patent also mentions that the subtraction can be 'mathematically manipulated' , suggesting that the processing can be performed in the digital domain. This allows for more complex algorithms, such as adaptive subtraction and noise filtering. |
The integration with digital processing is a trend in barcode reader design, as processors become more powerful and less expensive. |
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Chapter 41: The Calibration Requirements |
The differential optical architecture requires calibration to ensure that the primary and secondary photodiodes are properly matched. The calibration may involve adjusting the multiplier, aligning the optics, or compensating for variations in the photodiodes. |
The Datalogic patent describes a multiplier that is used to weight the secondary currents before subtraction. The multiplier is 'calculated by dividing the active area of the primary photodetector by the sum of the active areas of the secondary photodetectors' . |
The patent notes that 'the multiplier can also be dependent upon the spatial efficiency of the imaging optics and/or the surface reflection coefficients of the indicia and the areas near the indicia' . This suggests that the multiplier may need to be adjusted based on the specific optical system and the barcode surface. |
The calibration may be performed during manufacturing or may be performed dynamically during operation. The Symbol Technologies patent describes an adaptive approach that adjusts the reader's parameters based on the ambient light . |
Chapter 42: The Mechanical Alignment Tolerance |
The differential optical architecture provides some tolerance for mechanical misalignment. Because the cancellation is based on the relative signals from the primary and secondary photodiodes, small misalignments that affect both photodiodes equally are canceled. |
The AT&T patent notes that 'the alignment of the receiving lens with respect to the transmitting pair is not as stringent as before because of the complementary nature of the signal transmission' . This is because the differential detection cancels common-mode alignment errors. |
This tolerance is a significant advantage for commercial barcode readers, which must be manufactured with reasonable tolerances and survive rough handling. The differential architecture reduces the precision required in the optical assembly, reducing manufacturing costs. |
The tolerance also applies to the photodiode placement. If the photodiodes are slightly misaligned, the ambient light cancellation will still be effective as long as both photodiodes receive similar ambient light. |
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Chapter 43: The Comparison with Single-Ended Design |
Comparing the differential architecture with a single-ended design highlights the advantages of the differential approach. In a single-ended design, a single photodiode receives both the barcode signal and the ambient light. The amplifier must handle the full range of the combined signal, which can be large. |
In a differential design, the ambient light is canceled before the amplifier. The amplifier only needs to handle the barcode signal, which is much smaller. This allows the amplifier to operate with higher gain and lower noise. |
The differential design also provides better rejection of modulated ambient light. The ambient light modulation is common to both photodiodes and is canceled by the subtraction. In a single-ended design, the modulated ambient light would pass through the amplifier and interfere with the barcode signal. |
The prismatic lens patent notes that the optical signal conditioning allows 'conventional circuitry... to process, digitize, and decode the electrical signal without resorting to any special circuitry that would be needed had the novel optical processing described above not been employed' . |
Chapter 44: The Future of Differential Optical Architecture |
The future of differential optical architecture in barcode readers looks promising. As ambient light sources become more varied and more modulated, the need for robust ambient light rejection will only increase. |
The combination of differential optical architecture with digital signal processing offers the potential for even better performance. The analog subtraction can remove the bulk of the ambient light, while the digital processing can remove any residual interference. |
The Intel patent on differential images shows how the differential concept can be extended to 2D barcodes and screen-based applications . This suggests that differential architecture will remain relevant as barcode reading evolves. |
The diffractive barcode label patent demonstrates that the differential concept can be applied at the label itself, opening up new possibilities for barcode design . This could lead to barcodes that are inherently more robust to ambient light and other environmental noise. |
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Chapter 45: Summary --- Differential Optical Architecture in Perspective |
The differential optical architecture is a powerful technique for ambient light rejection in barcode readers. By using matched photodetectors and subtracting their signals, the architecture cancels the ambient light while preserving the barcode signal. |
We have examined how different companies and technologies have approached the challenges of differential optical architecture: |
Datalogic developed a three-photodiode system using a primary photodiode that receives both the barcode signal and ambient light, and two secondary photodiodes that receive only ambient light. The active areas of the photodiodes are matched to ensure accurate cancellation . |
Symbol Technologies developed an adaptive approach that measures the ambient light and adjusts the reader's scan angle and filter bandwidth when the ambient light exceeds a threshold. This can be combined with differential architecture for even better performance . |
AT&T developed a differential free-space optical transmission system that demonstrates the broader applicability of the technique. The system uses a signal and its complement, transmitted and received by separate optical paths, with the outputs applied to a differential amplifier . |
Intel extended the differential concept to 2D barcodes displayed on screens. The technique captures a barcode image and an alternative image, and subtracts them to remove common-mode noise . |
A prismatic lens patent describes an integrated optical detector that uses cross-polarization, bandpass filtering, and a prismatic lens to condition the light signal before it reaches the photodiodes. This optical signal conditioning simplifies the electronic processing . |
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The key lessons from our exploration are: |
The differential architecture cancels ambient light by subtraction. The ambient light is common to both photodiodes, so subtracting the signals removes it. |
The active areas of the photodiodes must be matched. The ambient light current from the primary photodiode must be matched to the ambient light current from the secondary photodiodes for effective cancellation. |
The subtraction can be performed in the current domain or the voltage domain. Current domain subtraction is more efficient because it removes the ambient light before the transimpedance amplifier. |
The differential architecture rejects all ambient light regardless of wavelength or frequency. This makes it more robust than filtering, which is wavelength-specific. |
The differential architecture provides tolerance for mechanical misalignment. Common-mode misalignments are canceled by the subtraction. |
The differential architecture can be combined with digital processing for even better performance. The analog subtraction removes the bulk of the ambient light, while the digital processing removes any residual interference. |
In the end, the differential optical architecture is a testament to the power of common-mode rejection. It is a simple yet elegant technique that addresses one of the most challenging problems in barcode reader design. The art of differential architecture lies in the careful matching of photodiodes, the precise alignment of optics, and the skillful design of subtraction circuits. |
cs@easiersoft.com