The Photodetector: Converting Light into the Language of Electronics |
Executive Summary |
This article provides a comprehensive, accessible exploration of the photodetector --- the critical component in a barcode reader that transforms reflected light into a measurable electrical signal. We examine why the choice of photodetector is fundamental to reader performance, affecting everything from signal quality and reading speed to circuit complexity and cost. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real products from industry leaders including Hamamatsu Photonics, ams OSRAM, and various pioneering patent holders. We explore the fundamental differences between photodiodes and phototransistors, the practical considerations of speed versus sensitivity, and the specific requirements for barcode reading applications. The article covers both simple discrete designs and sophisticated integrated solutions, with special attention to the practical trade-offs between performance, cost, and reliability. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing or selecting photodetectors for barcode reader systems. |
|
Chapter 1: The First Translator |
At the heart of every barcode reader lies a fundamental transformation. Light, which carries the pattern of bars and spaces from the printed symbol, must be translated into an electrical signal that can be processed by electronic circuits. This translation is performed by the photodetector. |
The photodetector is the first active element in the signal chain after the light has interacted with the barcode. It is the component that converts optical energy into electrical current, creating the raw signal from which all subsequent processing derives. If this conversion is noisy, slow, or non-linear, no amount of clever amplification or sophisticated decoding can fully compensate. |
The demands on the photodetector are significant. It must respond quickly enough to capture the barcode pattern as it moves past the reading window. It must be sensitive enough to detect light reflected from low-contrast or distant barcodes. It must be linear enough that the output current is proportional to the light intensity, preserving the information encoded in the bars and spaces. And it must do all this reliably, over years of use in environments ranging from clean retail counters to dusty warehouses. |
This article examines how engineers have addressed these challenges, drawing on real-world examples from leading companies in the optoelectronics and barcode reader industries. |
|
Chapter 2: The Photodiode --- Precision and Speed |
The photodiode is the most common photodetector in modern barcode readers. It is a semiconductor device that generates an electrical current when exposed to light. When photons strike the photodiode's PN junction, they create electron-hole pairs, producing a current that is directly proportional to the incident light intensity . |
This proportionality --- the linearity of the photodiode's response --- is one of its greatest advantages for barcode reading. The reflected light from a barcode varies continuously between the high reflectance of white spaces and the low reflectance of black bars. A linear response ensures that this variation is faithfully reproduced in the electrical signal, preserving the information encoded in the pattern. |
Photodiodes also offer exceptional speed. Their response times are measured in nanoseconds, making them suitable for high-speed applications such as optical communication and barcode scanning . The fast response is achieved by operating the photodiode in reverse bias mode, known as photoconductive mode. The reverse bias increases the width of the depletion region, reducing the junction capacitance and allowing the device to respond more quickly to changes in light . |
However, the photodiode produces a relatively small current, typically in the microamp range, which requires amplification by a transimpedance amplifier before it can be used by subsequent circuits . This added circuitry is a trade-off for the photodiode's precision and speed. |
|
Chapter 3: The Phototransistor --- Sensitivity with Simplicity |
The phototransistor is an alternative photodetector that offers a different set of characteristics. It is structurally similar to a standard bipolar transistor, but the base terminal is exposed to light instead of being driven by an electrical current . |
When photons strike the phototransistor, they generate a primary photocurrent at the collector-base junction. This small current is then internally amplified by the transistor's current gain, producing a much larger output current for the same amount of incident light . This internal gain is the phototransistor's key advantage. |
The phototransistor offers two significant benefits for barcode reader designers. First, its high sensitivity allows it to detect lower light levels than a photodiode without additional amplification. Second, the higher output current can often drive subsequent circuits directly, eliminating the need for a separate amplifier stage. This simplifies the circuit design and reduces component count and cost . |
However, the phototransistor's internal gain comes at a price. The device is slower than a photodiode, with response times in the microsecond range rather than nanoseconds. It is also less linear, particularly near saturation, meaning the output current is not perfectly proportional to the light intensity. For precision applications like barcode decoding, where accurate measurement of contrast is essential, this non-linearity can be problematic . |
|
Chapter 4: The Fundamental Trade-off --- Speed vs. Sensitivity |
The choice between a photodiode and a phototransistor is ultimately a choice between speed and sensitivity. The photodiode excels in speed and linearity, making it ideal for applications that require accurate measurement of rapidly changing light. The phototransistor excels in sensitivity and simplicity, making it suitable for applications where signal strength and circuit simplicity are paramount . |
For barcode reading, this trade-off is particularly important. The photodiode's speed allows the reader to capture barcodes that are moving quickly, as in high-speed conveyor belt applications. Its linearity ensures that the contrast between bars and spaces is accurately represented, which is essential for reliable decoding. These advantages generally make the photodiode the preferred choice for modern barcode readers. |
However, there are applications where the phototransistor's sensitivity is valuable. In low-light conditions, where the reflected light from the barcode is weak, the phototransistor's internal gain can boost the signal without requiring high-gain amplifiers that might introduce noise. In cost-sensitive applications, eliminating the amplifier stage can reduce the bill of materials. |
The key point is that the choice depends on the specific application requirements. A high-performance industrial reader will likely use a photodiode. A low-cost consumer reader may use a phototransistor. Understanding this trade-off is the first step in choosing the right photodetector for a given design. |
|
Chapter 5: Hamamatsu Photonics --- A Leading Supplier |
Hamamatsu Photonics is a leading manufacturer of optoelectronic components, including photodetectors for barcode readers. The company has a long history of providing components for the barcode reading industry, from the earliest pen-type readers to modern imaging scanners . |
Hamamatsu's product portfolio includes a wide range of silicon photodiodes and PIN photodiodes designed for barcode scanning applications. The S8385, for example, is a large-area Si PIN photodiode housed in a miniature plastic SIP package that is 75% smaller in cubic volume than conventional types . This compact size is valuable for handheld readers where space is at a premium. |
The S8385 features a photosensitive area of 2 by 2 millimeters, high sensitivity, and fast response times. Its spectral response ranges from 320 to 1100 nanometers, covering both visible and infrared light . This wide range makes it suitable for barcode readers that use red LEDs, as well as those that use infrared illumination. |
Hamamatsu also offers CMOS linear image sensors for barcode readers, which integrate multiple photodiodes in a linear array . These sensors capture the entire barcode pattern in a single exposure, allowing the reader to decode the barcode without moving the sensor. |
|
Chapter 6: ams OSRAM --- Broad Portfolio of Photodetectors |
ams OSRAM is another major supplier of optoelectronic components for barcode readers. The company offers a broad portfolio of photodetectors, including PIN photodiodes, avalanche photodiodes, and ambient light sensors . |
The BPW34 series of PIN photodiodes is one of ams OSRAM's most popular product lines for barcode scanning applications. The BPW34FAS, for example, offers a peak sensitivity at 940 nanometers, a response time of just 12 to 20 nanoseconds, and a low dark current of only 2 nanoamps . The device has a wide spectral range of 730 to 1100 nanometers and a large active area of 7 square millimeters . |
The BP104FS is another ams OSRAM photodiode designed for barcode scanners. This device features a fast response time of 20 nanoseconds, excellent spectral sensitivity, and a viewing angle of 120 degrees . Its spectral range of 800 to 1100 nanometers makes it suitable for near-infrared applications. |
ams OSRAM's product portfolio also includes CMOS image sensors that can be easily integrated into scanners, with global shutter options that provide power-efficient readout and no image distortion . These sensors are ideal for 2D barcode reading applications. |
|
Chapter 7: The PIN Photodiode --- The Workhorse of Barcode Reading |
The PIN photodiode is the most common type of photodiode used in barcode readers. The name stands for P-type, Intrinsic, N-type, referring to the structure of the device. The intrinsic layer between the P and N regions increases the width of the depletion region, reducing the junction capacitance and improving the response speed . |
The PIN photodiode's fast response time is one of its key advantages for barcode reading. The BP104FS from ams OSRAM, for example, has a response time of 20 nanoseconds . This allows the photodiode to capture the rapid variations in reflected light as the barcode moves past the reading window. |
The PIN photodiode's spectral response is also important. Most silicon PIN photodiodes have a peak sensitivity in the near-infrared range, around 900 to 960 nanometers. This makes them well-suited for use with red LEDs and laser diodes, which have wavelengths around 630 to 660 nanometers, as well as with infrared illumination. |
The spectral range of the BP104FS extends from 800 to 1100 nanometers, covering the wavelengths used by most barcode reader illumination sources . The S8385 from Hamamatsu has an even wider range, from 320 to 1100 nanometers, covering visible light as well . |
|
Chapter 8: Photodiode Operating Modes --- Photovoltaic vs. Photoconductive |
Photodiodes can be operated in two modes: photovoltaic and photoconductive. The choice of mode affects the device's speed, noise, and linearity. |
In photovoltaic mode, the photodiode is operated with zero bias. The light generates a voltage across the diode, and this voltage can be measured directly. This mode offers the lowest noise because there is no dark current, but it is relatively slow because the junction capacitance is not reduced by reverse bias. Photovoltaic mode is typically used in applications where speed is not critical, such as solar cells and low-light precision measurements . |
In photoconductive mode, the photodiode is operated with a reverse bias. The reverse bias increases the width of the depletion region, reducing the junction capacitance and allowing the device to respond more quickly to changes in light. However, the reverse bias also introduces dark current --- a small current that flows even when no light is present. The dark current contributes noise to the signal, but the trade-off is acceptable for high-speed applications. |
For barcode readers, photoconductive mode is generally preferred. The fast response time is essential for capturing barcode patterns, especially at high scanning speeds. The dark current can be compensated by circuit design, as described in the patent by Knapp et al. , which includes an average DC offset negative feedback loop to keep the signal within the dynamic range of the amplifier. |
|
Chapter 9: Dark Current and Offset Cancellation |
Dark current is a small current that flows through a photodiode even when it is in complete darkness. This current is caused by thermal generation of electron-hole pairs within the semiconductor material. In a silicon photodiode, the dark current is typically in the nanoamp range, but it increases with temperature and with the reverse bias voltage. |
The BP104FS, for example, has a typical dark current of 2 nanoamps . The S8385 from Hamamatsu has a similarly low dark current . These low dark currents are important for barcode reading because the photodiode is often used in a reverse-biased mode, where the dark current is present. |
If left uncorrected, the dark current appears as a DC offset in the photodiode's output, shifting the baseline of the signal and potentially causing decoding errors. The patent by Knapp et al. describes a circuit that addresses this problem with an average DC offset negative feedback loop . |
The circuit uses an RC circuit with a slow time constant, about one second, to settle to the average DC level of the amplifier output. This average level is fed back to the amplifier input, decreasing the DC gain while maintaining high AC gain. The result is that the signal stays within the dynamic range of the amplifier, regardless of the DC level of the photodiode pulse stream . |
|
Chapter 10: The Transimpedance Amplifier --- Converting Current to Voltage |
The photodiode produces a current, not a voltage. To use this current in further processing, it must be converted to a voltage. This conversion is performed by a transimpedance amplifier, or TIA. |
The transimpedance amplifier is a specialized operational amplifier configuration that uses a feedback resistor to convert the input current to an output voltage. The output voltage is equal to the input current multiplied by the feedback resistance. A feedback resistor of 100 kilohms, for example, produces 0.1 volts from a 1 microamp current. |
The transimpedance amplifier is a critical component in the barcode reader signal chain. Its design determines the gain, bandwidth, and noise of the photodetector interface. A well-designed TIA preserves the linearity and speed of the photodiode, while a poorly designed TIA can introduce distortion and noise that degrades the reader's performance. |
The patent by Knapp et al. describes a transimpedance amplifier with a gain of about 300, implemented with a bi-polar op amp . The circuit includes a feedback resistor that sets the gain and a compensation capacitor that stabilizes the amplifier. The detailed design of this stage is critical for achieving the sensitivity and bandwidth required for reliable barcode reading. |
|
Chapter 11: The RCA CA3240 --- A Practical Op-Amp Example |
The patent by Knapp et al. provides a concrete example of a transimpedance amplifier design for a barcode reader. The input amplifier is a MOSFET front-end op-amp, preferably the RCA CA3240, which has high input impedance and low noise . |
The CA3240 is a dual operational amplifier that combines the low input bias current of a MOSFET input stage with the speed and output drive of a bipolar amplifier. Its high input impedance is essential for connecting to the photodiode without loading the signal. Its low noise ensures that the small photodiode current is not swamped by amplifier noise. |
The amplifier is connected with the photodiode across its non-inverting and inverting inputs. A feedback resistor between the output and the inverting input sets the gain. The non-inverting input is coupled to a reference voltage, establishing the DC operating point . |
The patent notes that any amplifier with high input impedance and low noise may be employed, but the CA3240 provides a good balance of performance and cost . This is a common theme in barcode reader design: selecting components that meet the performance requirements while remaining cost-effective for high-volume production. |
|
Chapter 12: The Summing Amplifier --- Combining Signals |
After the transimpedance amplifier, the signal is often passed through a summing amplifier. This stage combines the photodiode signal with feedback signals that correct for DC offset and peak noise. |
In the patent by Knapp et al., the summing amplifier is implemented with a bi-polar op amp, preferably an LM358-type, which provides sufficient bandwidth and noise performance at a low cost . The amplifier is DC coupled to the transimpedance amplifier, allowing good sensitivity to signals produced by very slowly swiped barcoded badges . |
The summing amplifier is connected with its inverting input to the output of the transimpedance amplifier, and its non-inverting input to the same reference voltage. A feedback resistor sets the gain, and feedback loops from the output provide correction signals. |
The average DC offset negative feedback loop keeps the signal within the dynamic range of the summing amplifier, independently of the particular characteristics of the circuit components. The information peak detect negative feedback loop removes peak noise from the amplified pulse stream and adjusts for data DC average differences . Both loops are manual-adjustment-free, making the circuit easy to manufacture and calibrate. |
|
Chapter 13: The Average DC Offset Negative Feedback Loop |
The average DC offset negative feedback loop is a clever circuit that ensures the signal remains within the dynamic range of the amplifier, regardless of the DC level of the photodiode pulse stream. |
The circuit consists of a resistor and capacitor that provide an RC network with a very slow time constant, preferably about one second . This RC network settles to the average DC level of the summing amplifier output and feeds it back to the inverting input through a buffer. |
The effect is to decrease the DC gain of the summing amplifier while maintaining high AC gain. The circuit keeps the average DC output within a few millivolts of its input, ensuring that the signal is within the dynamic range of the amplifier . |
This feedback loop is particularly important for barcode readers because the photodiode signal can have a wide range of DC levels. The ambient light, the distance to the barcode, and the reflectance of the barcode surface can all affect the DC level. The feedback loop automatically compensates for these variations, providing a consistent signal to the subsequent stages. |
|
Chapter 14: The Information Peak Detect Negative Feedback Loop |
The information peak detect negative feedback loop is another clever circuit that removes peak noise from the amplified pulse stream and adjusts for data DC average differences . |
The circuit consists of a diode and a capacitor with a resistor to ground. The positive-going signals at the output of the summing amplifier cause the diode to charge the capacitor to within about 0.8 volts of their maximum amplitude. This peak DC voltage is fed back to the inverting input of the summing amplifier through a resistor . |
The effect is to compress all signals to the same amplitude and flatten the often erratic average DC components that accompany them. The capacitor holds the peak DC voltage throughout the length of the barcode signal, allowing the signal's low-going AC components to be passed without attenuation . |
This feedback loop is particularly important for handling the 'quiet zones' at the beginning and ends of a barcode data stream. The light reflection from the surface of the card upon which the barcode label is fixed can cause problems at the digitizing stage. The feedback loop flattens the tops of these DC plateaus, making the signal easier to digitize. |
|
Chapter 15: The Comparator --- Digitizing the Signal |
After amplification and conditioning, the analog signal must be digitized --- converted from a continuous voltage to a stream of ones and zeros. This is performed by a comparator. |
The comparator compares the amplified signal to a threshold voltage. If the signal is above the threshold, the output is a one; if below, the output is a zero. The comparator must have hysteresis to prevent chatter when the signal is near the threshold. |
In the patent by Knapp et al., the comparator is a bi-polar comparator of the LM393 type . The comparator's non-inverting input is fed with the signal from the summing amplifier through a pair of back-to-back diodes and a resistor. The inverting input is fed directly from the summing amplifier output. |
The back-to-back diodes charge a capacitor to the positive and negative peaks of the information signal, providing a tracking threshold. The hysteresis is set by a feedback resistor and the series resistor, typically at about 20 millivolts peak-to-peak . |
The comparator squares up the amplified pulse stream around a DC level that approximately tracks the halfway point between its peaks, regardless of the erratic nature of the signal . |
|
Chapter 16: Hysteresis --- Preventing Chatter |
Hysteresis is a technique that prevents chatter --- the rapid bouncing of the comparator output when the signal is near the threshold. The comparator's switching threshold is different for rising and falling signals, creating a 'dead zone' that filters out noise. |
In the patent by Knapp et al., the hysteresis is set by the feedback resistor and the series resistor. The feedback resistor connects the comparator output to the non-inverting input, providing positive feedback. The series resistor limits the current. |
When the comparator output is high, the feedback resistor pulls the non-inverting input above the threshold, making it harder for the input signal to pull it down. When the output is low, the feedback resistor pulls the non-inverting input below the threshold, making it harder for the input signal to pull it up. |
The hysteresis level is typically set at about 20 millivolts peak-to-peak . This level is low enough to preserve the barcode signal's detail but high enough to prevent chatter from noise. The result is a clean, well-behaved digital pulse stream at the comparator output. |
|
Chapter 17: Anti-Latching --- Ensuring a Reliable Output |
The comparator's output can sometimes latch, remaining in a low state even when the signal would normally drive it high. This can happen at the end of a corrupted barcode read, where the feedback network discharges the capacitor further than during the data stream. |
The patent by Knapp et al. addresses this problem with an anti-latching resistor, R12. This resistor counteracts the effects of the feedback resistors, charging the capacitor back toward the DC feedback voltage. This ensures that the comparator's output eventually returns high . |
The anti-latching resistor is connected between the buffer output and the comparator's non-inverting input. The buffer output provides a stable reference voltage, and the resistor provides a path for charging the capacitor. The value is chosen so that the capacitor charges at a controlled rate, preventing the comparator from latching. |
This simple addition ensures that the comparator's output is always a well-behaved digital pulse stream, ready for the decoder to process. |
|
Chapter 18: The BPW34 Family --- Popular Photodiodes for Barcode Scanners |
The BPW34 family of PIN photodiodes is among the most popular for barcode scanning applications. These devices offer a good balance of speed, sensitivity, and cost. |
The BPW34FAS from ams OSRAM has a peak sensitivity at 940 nanometers, a response time of just 12 to 20 nanoseconds, and a low dark current of only 2 nanoamps . The device has a wide spectral range from 730 to 1100 nanometers and a large active area of 7 square millimeters . |
The BPW34 family's fast response time is essential for barcode reading. The 20 nanosecond rise time allows the photodiode to capture the rapid variations in reflected light as the barcode passes the reading window. The low dark current ensures that the signal is not swamped by noise. |
The BPW34 family is available in both through-hole and surface-mount packages, providing flexibility for different PCB designs. The surface-mount version is particularly suitable for compact handheld readers where space is limited. |
|
Chapter 19: The S8385 --- Hamamatsu's Compact Photodiode |
Hamamatsu Photonics offers the S8385, a large-area Si PIN photodiode designed for barcode scanners and other optical applications. This device is notable for its compact size: a miniature plastic SIP package that is 75% smaller in cubic volume than conventional types . |
The S8385 has a photosensitive area of 2 by 2 millimeters, high sensitivity, and fast response times. Its spectral response ranges from 320 to 1100 nanometers, covering both visible and infrared light . The device has a maximum reverse voltage rating of 960 volts and can operate in a temperature range of -25 to 85C . |
The compact package is a significant advantage for handheld barcode readers, where space is at a premium. The SIP package is easy to mount on a PCB, and its small size allows the reader to be more compact and ergonomic. |
Hamamatsu also offers custom solutions, allowing designers to tailor the photodiode's specifications to their specific requirements . |
|
Chapter 20: CMOS Image Sensors --- The Future of Barcode Reading |
While discrete photodiodes are still widely used, many modern barcode readers use CMOS image sensors. These sensors integrate an array of photodiodes with readout and processing circuitry on a single chip. |
The advantage of a CMOS image sensor is that it captures the entire barcode pattern in a single exposure, eliminating the need for scanning motion. This makes the reader more reliable and easier to use, as it is not dependent on the user's scanning speed. |
ams OSRAM offers CMOS image sensors that can be easily integrated into scanners. The sensors feature global shutter options, providing power-efficient readout with no image distortion . Global shutter is important for barcode reading because all pixels are exposed simultaneously, capturing the entire pattern without the motion artifacts that can occur with rolling shutter. |
Hamamatsu also offers CMOS linear image sensors designed for barcode readers. These sensors integrate multiple photodiodes in a linear array, capturing the barcode pattern in a single line scan . |
|
Chapter 21: Global Shutter vs. Rolling Shutter |
The choice between global shutter and rolling shutter is an important consideration for CMOS image sensors used in barcode readers. |
In a global shutter sensor, all pixels are exposed simultaneously. The light is collected by all pixels at the same time, and then the charge is read out sequentially. This is the ideal mode for barcode reading because the entire pattern is captured at a single instant, with no motion artifacts. |
In a rolling shutter sensor, the rows are exposed sequentially. The light is collected by one row at a time, and the rows are read out in a rolling sequence. This can cause distortion if the barcode or the reader is moving during the exposure. |
Global shutter is preferred for barcode reading, especially when the barcode is moving, as in a conveyor belt application. However, global shutter sensors are typically more expensive than rolling shutter sensors. The choice depends on the application requirements and cost constraints. |
|
Chapter 22: The Line Sensor --- Capturing the Barcode in a Single Scan |
A line sensor is a linear array of photodiodes that captures the barcode pattern in a single scan. The sensor is typically several hundred pixels long, and the barcode pattern is imaged onto the sensor by a lens system. |
The line sensor captures the entire barcode pattern at once, eliminating the need for scanning motion. This makes the reader simpler and more reliable, as it has no moving parts. However, the line sensor requires a lens system to image the barcode onto the sensor, which adds cost and complexity. |
Line sensors are used in both handheld and fixed-mount barcode readers. They are particularly suitable for reading barcodes that are moving on a conveyor belt, as they can capture the pattern quickly and accurately. |
Both Hamamatsu and ams OSRAM offer line sensors for barcode reading applications . |
|
Chapter 23: The 2D Imager --- Capturing Two-Dimensional Codes |
For reading two-dimensional codes, such as QR codes or Data Matrix codes, a two-dimensional imager is required. This is typically a CMOS image sensor with an array of pixels, such as 640 by 480 or 1280 by 1024. |
The 2D imager captures an entire image of the barcode, and then software processing locates and decodes the symbol. This allows the reader to read barcodes in any orientation, and it can also read multiple barcodes in a single image. |
The processing required for 2D decoding is more intensive than for 1D decoding, requiring a more powerful processor. However, the advantages of 2D reading --- flexibility, speed, and reliability --- have made these readers increasingly popular. |
ams OSRAM offers CMOS image sensors for 2D barcode reading, with global shutter options and high-speed readout . Hamamatsu also offers area image sensors for barcode applications . |
|
Chapter 24: Hamamatsu's Role in Barcode Reading |
Hamamatsu Photonics has been a major supplier of optoelectronic components for barcode readers since the earliest pen-type readers. The company's expertise in photodetectors and image sensors has made it a trusted partner for barcode reader manufacturers. |
Hamamatsu offers a wide range of products for barcode reading, including silicon photodiodes, PIN photodiodes, CMOS linear image sensors, and area image sensors. The company also offers custom solutions, allowing designers to tailor the specifications to their specific requirements . |
The company's commitment to quality and reliability is reflected in the performance of its components. The S8385, for example, offers high sensitivity and fast response in a compact package . Hamamatsu's CMOS image sensors provide the resolution and speed needed for 2D barcode reading. |
Hamamatsu's technical expertise extends to application support, helping designers integrate its components into their barcode reader designs . |
|
Chapter 25: ams OSRAM's Complete Portfolio |
ams OSRAM offers a complete portfolio of optical components for barcode readers, including illumination sources, photodetectors, and image sensors . |
The company's photodetector portfolio includes PIN photodiodes, avalanche photodiodes, and ambient light sensors. The BPW34 series of PIN photodiodes is widely used in barcode scanners, offering a good balance of speed, sensitivity, and cost . |
ams OSRAM's CMOS image sensors are designed for easy integration into scanners, with global shutter options that provide power-efficient readout and no image distortion. The sensors are suitable for both 1D and 2D barcode reading applications . |
The company also offers application support, helping designers select the right components and integrate them into their barcode reader designs. |
|
Chapter 26: The Photodiode's Spectral Response |
The spectral response of the photodiode is a key specification for barcode reading. The photodiode must be sensitive to the wavelength of the illumination source, typically red or infrared. |
Silicon photodiodes have a spectral response that ranges from about 300 to 1100 nanometers. The peak sensitivity is typically in the near-infrared range, around 900 to 960 nanometers. This makes them well-suited for use with red LEDs and laser diodes, which have wavelengths around 630 to 660 nanometers, as well as with infrared illumination. |
The BP104FS from ams OSRAM has a spectral range of 800 to 1100 nanometers, covering the wavelengths used by most barcode reader illumination sources . The S8385 from Hamamatsu has an even wider range, from 320 to 1100 nanometers, covering visible light as well . |
The spectral response can be tailored by the photodiode's design. Some photodiodes include a filter that blocks visible light, making them suitable for infrared applications. The BP104FS includes a filter that blocks visible light, improving the signal-to-noise ratio in infrared applications . |
|
Chapter 27: Active Area and Signal-to-Noise Ratio |
The active area of the photodiode is another important specification. A larger active area captures more light, producing a stronger signal. However, a larger active area also increases the junction capacitance, reducing the response speed. |
The BPW34FAS has an active area of 7 square millimeters . This is a good compromise between signal strength and speed for barcode reading applications. The S8385 has a smaller active area of 2 by 2 millimeters, which allows for a more compact package . |
The signal-to-noise ratio is determined by the signal strength and the noise sources. The signal strength is determined by the active area, the illumination intensity, and the reflectance of the barcode. The noise sources include the dark current, the amplifier noise, and the ambient light. |
The photodiode's design optimizes the trade-off between signal strength and speed. The choice of photodiode depends on the specific application requirements, such as the reading distance and the scanning speed. |
|
Chapter 28: Dark Current and Temperature Sensitivity |
The dark current of a photodiode is a noise source that must be minimized for reliable barcode reading. The dark current is typically in the nanoamp range for silicon photodiodes, but it increases with temperature. |
The BP104FS has a typical dark current of 2 nanoamps . The BPW34FAS also has a dark current of 2 nanoamps . These low dark currents are important for barcode reading, especially when the reader is used in warm environments. |
The dark current doubles with every 10C increase in temperature. At high temperatures, the dark current can become significant, especially if the photodiode is operated in reverse bias mode. The reader's design must account for this by providing temperature compensation or by using a photodiode with low dark current. |
The temperature sensitivity of the photodiode also affects the dark current offset cancellation. The feedback loops described in the patent by Knapp et al. compensate for DC offset variations, including those caused by temperature changes . |
|
Chapter 29: Integrated Optical Scanner --- Omron's 2D Approach |
Omron has developed a two-dimensional micro optical scanner integrated with a photodiode and piezoresistors for barcode reading applications. The scanner, with a size of 7 by 7 millimeters, is fabricated by silicon bulk-micromachining and is capable of 2D optical scanning, photodetection, and scanning position detection . |
The integration of the photodiode with the scanning mirror is a significant advance. The photodiode is co-located with the mirror, providing a compact optical sensor. The piezoresistors provide position feedback for the scanning mirror, ensuring accurate scanning. |
Omron has also developed a miniature 3D vision sensor for pipe inspection using the same technology. This demonstrates the versatility of the integrated optical scanner for various sensing applications . |
This approach represents a move toward highly integrated optical sensors, where the photodetector is combined with other optical and mechanical components on a single chip. |
|
Chapter 30: The Optical Reader Patent --- A Comprehensive Design |
The patent by Knapp et al. describes a comprehensive barcode reader design that includes two photodetectors. The optic assembly includes a light-emitting diode and two photodetectors . |
The use of two photodetectors is significant. The two photodetectors receive light along separate optical paths, providing redundancy and improving the signal-to-noise ratio. The light generated by the photodiode is modulated by the electronic circuit to produce a series of light pulses. |
The photodetectors produce electrical signals which indicate the information contained in the bar code symbol being read. The circuit assembly includes electronic components used to process the electrical signals produced by the two photodetectors . |
The patent describes a specific implementation where the photodiode's signal is processed by a transimpedance amplifier, a high-pass gain block, and a band-pass filter. The signal is then demodulated by a synchronous detector and filtered by a low-pass filter before being digitized. |
|
Chapter 31: The Modulated Light Approach |
The patent by Knapp et al. describes a barcode reader that uses modulated light. The LED is pulsed with a 40 kHz carrier, and the photodiode signal is synchronously demodulated. This approach rejects ambient light and provides high sensitivity. |
The LED is pulsed with a peak current of up to 80 mA at a 50% duty cycle in normal operation. In standby mode, the duty cycle is reduced to 0.8%, saving power. The light from the LED reflects from the barcode and is focused on the photodetectors, resulting in an amplitude modulated 40 kHz current . |
The modulated current is amplified by a series of band-pass amplifiers with a bandwidth centered around 40 kHz. The maximum frequency components in the bar code signal are about 7 kHz, which is below the carrier frequency, allowing the signal to be separated from the carrier . |
The receipt of a light pulse is detected by a demodulator, which serves as a synchronous detector. The demodulator compares the signal to a reference signal derived from the LED drive, extracting the barcode information . |
|
Chapter 32: The Phase Shift Compensation Circuit |
The demodulator in the patent by Knapp et al. includes a phase shift compensation circuit. This circuit compensates for phase shifts through the signal path, maximizing the demodulated signal amplitude. |
The phase shift compensation circuit produces a programmable time delay signal that is applied to the reference signal of the demodulator. The delay has a range of 0 to 12.5 microseconds and a resolution of 0.78 microseconds . |
The delay is programmed by data read from the PROM (programmable read-only memory). This allows the phase shift compensation to be tuned during manufacturing, compensating for component variations. |
The phase shift compensation circuit is important because any phase error reduces the demodulated signal amplitude. By compensating for the phase shift, the circuit ensures that the demodulated signal is maximized, improving the signal-to-noise ratio . |
|
Chapter 33: The Paper Detector --- Saving Power |
The patent by Knapp et al. describes a paper detector that determines whether a surface is present from which the light pulses can be reflected. This feature saves power by reducing the duty cycle of the light pulses when no surface is present . |
The paper detector receives the output signal of the amplifier in the demodulator and processes it to determine whether any pulses correspond to light pulses reflected from a surface. If no surface is detected, the LED's duty cycle is reduced from 50% to 0.8%, significantly reducing power consumption . |
When a surface is detected, the LED's duty cycle returns to normal, and the reader begins processing the barcode signal. This ensures that the reader is responsive when a barcode is present while conserving power when it is not. |
The paper detector is implemented in the wave shaper logic circuit, which also processes the barcode data and produces the video output signal . |
|
Chapter 34: The ASIC --- Integration for Compactness |
The patent by Knapp et al. describes that as much as possible of the electronics is contained in an ASIC (application-specific integrated circuit). External components are used only for the optoelectronic devices and those components whose values, accuracies, or power dissipation preclude their inclusion within the ASIC . |
The use of an ASIC allows the reader to be more compact, reliable, and cost-effective. The ASIC integrates the analog and digital processing functions, reducing the component count and simplifying the PCB layout. |
The ASIC includes the high-pass gain block, the band-pass filter, the demodulator, the low-pass filter, and the wave shaper logic. The external components include the photodetectors, the transimpedance amplifier, and the LED driver transistor . |
The transimpedance amplifier is included on the optic assembly, separate from the ASIC, to minimize the length of the lines connected to the highly sensitive node at the cathodes of the photodetectors . |
|
Chapter 35: The Photodiode's Role in Laser Radar |
Photodiodes are also used in laser radar applications, which share many features with barcode readers. Laser radar uses a laser beam to measure distances, and the photodiode detects the reflected light. |
The Omron micro optical scanner described earlier is intended for both barcode readers and laser radar . The scanner includes a photodiode integrated with the scanning mirror, providing a compact optical sensor. |
The photodiode's fast response time is essential for laser radar, where the time of flight of the laser pulse is measured. The photodiode's sensitivity is also important, as the reflected light can be weak. |
Hamamatsu's S8385 photodiode is also designed for laser radar applications . The device's high speed and low dark current make it suitable for time-of-flight measurements. |
|
Chapter 36: The Phototransistor's Advantages in Simpler Applications |
While photodiodes are preferred for high-performance barcode readers, phototransistors have advantages in simpler applications. The phototransistor's internal gain provides high sensitivity without external amplification, simplifying the circuit. |
Phototransistors are suitable for low-cost readers where the reading distance is short and the scanning speed is low. They are also used in applications where the ambient light is low, such as in warehouses. |
The phototransistor's slower response and non-linearity are acceptable in these simpler applications. The cost savings from eliminating the transimpedance amplifier can be significant, making the phototransistor an attractive choice for high-volume consumer products. |
However, for applications that require high speed, long reading distance, or high accuracy, the photodiode is the preferred choice. |
|
Chapter 37: The Ambient Light Sensor --- A Related Application |
Ambient light sensors are related to photodetectors, but they are designed to measure the overall light level rather than detect rapid variations. These sensors are used in barcode readers to adjust the illumination brightness based on the ambient light. |
ams OSRAM offers ambient light sensors as part of its product portfolio . These sensors can be used to control the LED current, reducing power consumption in bright environments and increasing it in dark environments. |
The ambient light sensor is typically a photodiode with a spectral response that mimics the human eye. The output is a current that is proportional to the ambient light level, and this current is used by the microcontroller to adjust the illumination. |
The integration of ambient light sensing with barcode reading is a trend in modern readers, providing improved performance and power efficiency. |
|
Chapter 38: Proximity Sensors --- Another Application |
Proximity sensors are another related application. These sensors use an LED and a photodetector to detect the presence of an object. The LED emits light, and the photodetector detects the reflected light. |
Proximity sensors are used in barcode readers to detect when a barcode is in the field of view, triggering the reader to start scanning. This saves power by keeping the reader in a low-power state until a barcode is detected. |
ams OSRAM offers proximity and presence sensing solutions that enhance autofocus and controlled system wake-up . These solutions integrate an LED driver and a photodetector on a single chip, providing a compact and efficient solution. |
The photodetector in a proximity sensor is typically a photodiode or a phototransistor, depending on the application requirements. |
|
Chapter 39: The Photodiode's Linearity --- A Key Advantage |
The photodiode's linearity is one of its key advantages for barcode reading. The output current is directly proportional to the incident light intensity over a wide range . |
This linearity ensures that the barcode's contrast is accurately represented in the electrical signal. The high reflectance of white spaces produces a high current, and the low reflectance of black bars produces a low current. The transitions between bars and spaces are faithfully reproduced. |
The linearity is maintained even at high frequencies, making the photodiode suitable for high-speed scanning. This is in contrast to the phototransistor, which can become non-linear at high light levels or high frequencies. |
The photodiode's linearity is also important for decoding algorithms that use the signal amplitude, such as those that use adaptive thresholding. |
|
Chapter 40: The Phototransistor's Non-Linearity --- A Limitation |
The phototransistor's non-linearity is its main limitation for precision barcode reading. The internal amplification is not perfectly linear, especially at high light levels or near saturation . |
The non-linearity means that the output current is not perfectly proportional to the incident light intensity. This can distort the barcode signal, making it more difficult to decode. The transitions between bars and spaces may be blurred, and the contrast may be compressed. |
The non-linearity is particularly problematic for barcodes with low contrast, where the signal amplitude is small. The phototransistor's non-linear gain can amplify the signal unevenly, making it difficult to distinguish bars from spaces. |
For these reasons, phototransistors are generally not recommended for high-performance barcode readers that require high accuracy and long reading distances. |
|
Chapter 41: The Photodiode's Low Noise --- Enabling Weak Signal Detection |
The photodiode's low noise is another key advantage for barcode reading. The photodiode's minimal output current has a very low and predictable noise floor . |
The low noise allows the photodiode to detect weak signals, such as the reflected light from a distant or low-contrast barcode. The signal-to-noise ratio is maximized, making it easier to extract the barcode information. |
The low noise is achieved by the photodiode's design. The photodiode's dark current is low, and the thermal noise is minimized. The transimpedance amplifier adds some noise, but this can be minimized by careful design. |
The photodiode's low noise is essential for high-performance barcode readers that must operate in challenging conditions. |
|
Chapter 42: PIN Photodiode Applications --- Beyond Barcode Reading |
PIN photodiodes are used in many applications beyond barcode reading, including optical communication, laser range finders, and medical imaging . |
In optical communication, the PIN photodiode's fast response time allows it to detect high-speed optical signals. In laser range finders, the PIN photodiode's sensitivity allows it to detect weak reflected light pulses. In medical imaging, the PIN photodiode's linearity allows it to accurately measure light intensity. |
The BP104FS, for example, is used in barcode scanners, pulse detection systems, laser range finders, and opto-electronic coupling devices . This versatility demonstrates the PIN photodiode's usefulness in a wide range of applications. |
The common thread is the need for a fast, linear, and low-noise photodetector. The PIN photodiode meets these requirements, making it a versatile component for optical sensing. |
|
Chapter 43: The Photodiode's Spectral Range --- Matching the Illumination |
The photodiode's spectral range must match the wavelength of the illumination source. The spectral range is determined by the photodiode's semiconductor material and design. |
Silicon photodiodes have a spectral range of about 300 to 1100 nanometers, making them suitable for visible and near-infrared light. For longer wavelengths, germanium or indium gallium arsenide (InGaAs) photodiodes are used . |
The BP104FS has a spectral range of 800 to 1100 nanometers, matching the wavelength of red LEDs and laser diodes . The S8385 has a range of 320 to 1100 nanometers, covering both visible and infrared light . |
Choosing a photodiode with the right spectral range is important for maximizing the signal strength. A photodiode that is not sensitive to the illumination wavelength will produce a weak signal, reducing the reader's performance. |
|
Chapter 44: Photodiode vs. Phototransistor --- A Summary of Trade-offs |
The choice between a photodiode and a phototransistor for a barcode reader is a trade-off between speed and sensitivity, precision and simplicity. |
The photodiode offers: |
- High speed and bandwidth |
- Excellent linearity |
- Low noise |
- Broad wavelength range |
- Small output current (requires amplification) |
The phototransistor offers: |
- High sensitivity (internal gain) |
- Simpler circuit (no external amplifier) |
- Higher output current (can drive loads directly) |
- Lower cost |
- Slower response |
- Non-linear response |
For most modern barcode readers, the photodiode is the preferred choice due to its speed and linearity. However, for low-cost, low-speed applications, the phototransistor may be a viable alternative. |
|
Chapter 45: Summary --- The Photodetector in Perspective |
The photodetector is the first and most critical stage in the barcode reader signal chain. It is the component that transforms reflected light into an electrical signal, preserving the information encoded in the bars and spaces. |
We have examined how different companies and technologies have approached the challenges of photodetection for barcode reading: |
Hamamatsu Photonics offers a wide range of silicon photodiodes and PIN photodiodes for barcode scanning, including the compact S8385 and CMOS linear image sensors. Their components are designed for high sensitivity, fast response, and reliability . |
ams OSRAM provides a complete portfolio of optical components for barcode readers, including the BPW34 series of PIN photodiodes, CMOS image sensors, and ambient light sensors. Their products are designed for easy integration and high performance . |
Omron has developed a two-dimensional micro optical scanner integrated with a photodiode for barcode reading and laser radar applications. This integration demonstrates the trend toward compact, multifunctional optical sensors . |
Patent literature reveals a wealth of innovative designs, including the transimpedance amplifier, the average DC offset negative feedback loop, the information peak detect negative feedback loop, and the anti-latching resistor . |
|
The key lessons from our exploration are: |
The photodiode is the preferred choice for high-performance barcode readers. Its speed, linearity, and low noise make it ideal for capturing barcode patterns accurately. The small output current requires amplification, but this trade-off is acceptable for the performance benefits. |
The phototransistor is suitable for simpler, low-cost applications. Its internal gain provides high sensitivity without external amplification, simplifying the circuit and reducing cost. However, its slower response and non-linearity limit its use in high-performance readers. |
The PIN photodiode is the most common type of photodiode used in barcode readers. Its fast response, wide spectral range, and low dark current make it ideal for barcode scanning. Products like the BP104FS and the S8385 are specifically designed for this application. |
Integration is the trend. Companies are integrating photodetectors with other optical and electronic components on a single chip, reducing size and cost. The Omron scanner is one example, and CMOS image sensors are another. |
The photodetector's characteristics --- speed, sensitivity, linearity, and spectral response --- must match the application requirements. There is no single best photodetector; the choice depends on the specific design requirements. |
In the end, the photodetector is the gateway between the optical and electronic worlds. Its performance determines the quality of the signal that enters the rest of the signal chain, from the transimpedance amplifier to the decoder. A well-chosen photodetector, combined with a well-designed signal chain, is the foundation of a reliable and accurate barcode reader. |
cs@easiersoft.com