The Light That Reads: A Deep Dive into LED and Laser Illumination for Barcode Scanners |
Subtitle: How Symbol, Zebra, Honeywell, Datalogic, Keyence, Cognex, and Omron Choose, Drive, and Shape Their Light Sources - With Real-World Designs, Trade-Offs, and Surprising Innovations |
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Opening Summary |
At the very beginning of every barcode scan, there is a simple but profound act: a light source turns on. That light travels to the label, reflects off the black and white patterns, and carries back the information that the scanner will eventually decode. The choice of that light source - whether it is a humble light-emitting diode (LED) or a sophisticated laser diode - defines the scanner's personality. It determines the working distance, the reading speed, the ability to read damaged or curved labels, the power consumption, the cost, and even the safety certifications. |
This article is dedicated to the first pillar of the barcode reading chain: the illumination source. We will explore the physics of how LEDs and lasers work, but we will stay firmly in the realm of practical engineering. We will look at how major companies have made strategic choices between these two technologies. Symbol Technologies (now Zebra) built an empire on the laser scanner, with its moving mirror and focused beam. Honeywell, on the other hand, bet heavily on LED-based imagers, favoring simplicity and durability. Datalogic found a middle ground, using both technologies for different market segments. Keyence and Cognex, operating in the high-end industrial space, have pushed the boundaries with multi-wavelength and structured-light illumination. And Omron has pioneered the use of infrared and even ultraviolet light for specialized applications. |
We will examine the driver circuits that power these sources, the optical systems that shape their beams, the thermal management that keeps them stable, and the safety mechanisms that protect the user's eyes. We will also discuss emerging trends, such as the use of laser arrays, programmable color LEDs, and even organic light-emitting diodes (OLEDs) for flexible scanners. |
By the end of this journey, you will understand why a retail checkout scanner uses a different light than a warehouse forklift scanner, and why a smartphone's flash LED is not quite the same as a dedicated barcode imager. The light that reads is not just a bulb - it is a carefully engineered component that sits at the heart of the entire system. |
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Full Article |
Section 1: The Two Families of Light - Incoherent and Coherent |
Before we dive into specific products, we need to understand the fundamental difference between LEDs and lasers. An LED produces incoherent light - the photons are emitted in random directions and with random phases. The light spreads out in a wide cone, and its intensity falls off with the square of the distance. This makes LEDs excellent for illuminating an area, but poor for concentrating energy on a small spot. |
A laser, by contrast, produces coherent light - the photons are all in phase, traveling in the same direction. The beam is narrow and can stay focused over long distances. The energy density (power per unit area) at the focus can be thousands of times higher than an LED's. This is why laser scanners can read barcodes from several meters away, while LED imagers usually need to be within a few tens of centimeters. |
However, coherence comes with a price. Lasers are more expensive to manufacture, require more complex driver circuits, and have strict safety regulations because they can damage the retina. LEDs are cheap, safe, and robust - you can drop an LED scanner without worrying about the light source breaking. These fundamental differences drive every design decision we will discuss. |
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Section 2: The LED - The Workhorse of the Imaging World |
The LED used in barcode scanners is not the same as the indicator light on your charger. It is a high-power, surface-mount device designed for continuous or pulsed operation. The most common wavelength is red (620-660 nm) because silicon photodetectors have their peak sensitivity in that range. However, modern scanners also use white, blue, green, and infrared LEDs. |
The internal structure of an LED is a semiconductor p-n junction. When forward-biased, electrons recombine with holes, releasing energy in the form of photons. The energy gap of the semiconductor determines the wavelength. For red, the material is typically aluminum gallium indium phosphide (AlGaInP). For infrared, it is gallium arsenide (GaAs). For blue, it is gallium nitride (GaN) - the material that won the Nobel Prize. |
The LED's light output is roughly proportional to the current, but not linearly. At low currents, the efficiency is poor; at very high currents, the efficiency drops due to heating. The driver circuit must therefore choose a current that balances brightness and efficiency. In pulsed mode, the LED can be driven at currents several times its continuous rating, because the off-time allows the junction to cool. This is the principle behind the 'flash' in smartphone cameras. |
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Section 3: The Laser Diode - The Focused Arrow |
A laser diode is a more complex semiconductor device. It is also a p-n junction, but it has an optical cavity - a pair of mirrors that reflect the photons back and forth, stimulating further emission. When the current exceeds a threshold, the device begins to lase, and the output jumps to a coherent beam. The wavelength is determined by the energy gap, but it is much narrower (a few nanometers) than an LED's spectrum. |
Laser diodes used in barcode scanners are typically edge-emitting devices, where the beam emerges from a cleaved facet of the semiconductor chip. The beam is elliptical, with a wider divergence in one axis. The optical system (lenses) must correct this astigmatism to produce a round spot. The power output is usually 1 to 10 milliwatts for handheld scanners, which is Class 2 or Class 3R under the laser safety standard. |
The driver circuit for a laser diode is more sophisticated than for an LED. It must provide a stable current above the threshold, and it must include a monitor photodiode for automatic power control (APC). Without APC, the output power would vary wildly with temperature - a laser diode's threshold current can change by 0.3% per degree Celsius. The driver also often includes a modulation input, allowing the laser to be pulsed at high frequencies for noise rejection or safety. |
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Section 4: Symbol's Laser Legacy - The LS2208 and SE950 |
Symbol Technologies, now part of Zebra, is the company most associated with laser scanners. Their LS2208, introduced in the late 1990s, became the best-selling barcode scanner in history. It used a 650-nm laser diode with an output of about 1 milliwatt. The driver was a simple APC circuit based on a comparator and a pass transistor. The monitor photodiode, integrated into the laser package, provided a feedback current that was compared to a reference. The error signal drove the transistor to adjust the current. |
The LS2208's optical system was a marvel of cost engineering. A molded plastic collimating lens turned the elliptical beam into a nearly parallel one. A second lens, the 'focusing lens,' created a spot of about 0.2 mm at the nominal working distance. The mirror, driven by a resonant motor, swept the spot across a 40-degree angle. The entire assembly cost less than $5 to manufacture. |
Zebra's later SE950 engine, used in more rugged handhelds, increased the laser power to 5 milliwatts (Class 3R) for longer range. To maintain eye safety, the driver included a 'fail-safe' that monitored the mirror's oscillation via a piezoelectric sensor. If the mirror stopped moving, the driver shut off the laser within 50 microseconds - far faster than the blink reflex. The SE950 also used a high-frequency modulation of the laser (about 100 kHz) to reduce speckle noise, a phenomenon that occurs when coherent light reflects off rough surfaces, creating a grainy pattern that can confuse the detector. |
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Section 5: Honeywell's LED Revolution - The Voyager and 1900 |
While Symbol championed lasers, Honeywell (formerly Hand Held Products) took a different path. They believed that LED-based imagers would eventually dominate because they have no moving parts, are cheaper to manufacture, and can read 2D barcodes. The Voyager 1200g, their entry-level scanner, uses a row of six red LEDs. The driver is a simple MOSFET switch controlled by the microcontroller. The LEDs are pulsed at 100 mA each, with a duty cycle of 10%, which is the sweet spot for efficiency. |
The Voyager's illumination is not focused into a spot but diffused by a plastic lens to create a uniform field over the imaging window. The photodetector is a linear CMOS array that captures the reflected light from the entire barcode simultaneously. This is called a 'linear imager.' It does not require a moving mirror, so it is silent and more reliable. |
Honeywell's high-end 1900 imager uses a 2D CMOS sensor and an even more sophisticated LED driver. The scanner has both red and white LEDs. The white LEDs are used for reading color-coded barcodes or for providing a more natural image for the user. The driver includes a programmable current source that can deliver up to 500 mA per LED in pulsed mode. The pulse width and the current are controlled by a digital-to-analog converter (DAC) on the microcontroller. The scanner can also adjust the LED brightness based on the ambient light reading - a feedback loop that ensures the image is not overexposed. |
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Section 6: Datalogic's Dual Strategy - Both LED and Laser |
Datalogic, an Italian company, has always been pragmatic. They offer both LED imagers and laser scanners, often within the same product family. Their Gryphon series, for example, has both a laser version (Gryphon L) and an imager version (Gryphon I). The laser version uses a 650-nm diode with a standard APC driver, similar to Symbol's design. The imager version uses a line of red LEDs with a diffuser, similar to Honeywell's. |
But Datalogic's innovation lies in the laser driver for their industrial PowerScan series. These scanners use an infrared laser (855 nm) to read barcodes on thermal paper, where red light would be absorbed. The driver includes a high-speed modulation capability - the laser can be switched on and off at up to 2 MHz. This modulation is used for a technique called 'pulse averaging' - the detector integrates over many pulses, which rejects ambient light and reduces speckle. The driver is a current mirror circuit that can deliver 150 mA of peak current, with a rise time of less than 2 nanoseconds. |
Datalogic also uses a 'dual-beam' laser in some of their fixed-mount scanners. Two lasers, each with a separate driver, create two parallel scan lines. This reduces the need for the user to align the barcode perfectly - if one line misses, the other may hit. The driver circuit must ensure that the two lasers have matched power, which is achieved by using a single reference voltage and two matched current sources. |
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Section 7: Keyence's Multi-Wavelength Approach - The SR-1000 |
Keyence, the Japanese automation giant, takes a very different view of illumination. Their SR-1000 series of fixed-mount readers does not use a single LED or laser color. Instead, they use a ring light that contains red, blue, green, white, and infrared LEDs - all in one compact module. The driver circuit can activate any combination of these colors, and the microcontroller selects the best color based on a pre-scan. |
Why so many colorsBecause different substrates and inks have different spectral reflectances. A barcode printed with a blue ink on a yellow background might have excellent contrast under red light, but terrible contrast under blue. By trying multiple colors, the SR-1000 can read labels that would stump a monochromatic scanner. The driver is a complex piece of hardware: each color channel has its own constant-current source, and the microcontroller enables the channels via a bank of high-side switches. The entire assembly is controlled by a dedicated power management IC that can sequence the colors in less than 10 milliseconds. |
Keyence also offers an optional 'polarized' LED module. This includes a polarizing filter in front of the LEDs, and a matching polarizing filter in front of the sensor. The driver is the same, but the optical system is different. The polarized light reduces specular reflections from shiny metal or glossy labels, enhancing the contrast. This is a classic example of how the illumination source, the driver, and the optics must be co-designed. |
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Section 8: Cognex's Blue Laser - For Etched Codes |
Cognex, the machine vision leader, has a specialized application: reading Data Matrix codes that are etched directly onto metal parts using a laser marker. These codes are not printed; they are physical indentations. The contrast is low, and the surface is often reflective. Red or infrared light does not work well because the metal reflects most of it. |
Cognex's solution, used in their DataMan 8700 series, is a blue laser diode (445 nm). Blue light scatters more strongly off the rough edges of the etch, creating a higher contrast image. The driver circuit for this blue laser is more challenging because blue laser diodes have a higher forward voltage (around 4.5 volts) compared to red (around 2.5 volts). The scanner must have a boost converter that raises the battery voltage to 5 volts. The APC loop is similar, but the monitor photodiode must be sensitive to blue light, which requires a different semiconductor material. |
The blue laser driver also includes a temperature sensor directly on the laser housing. Blue lasers are particularly sensitive to temperature; their wavelength shifts by 0.05 nm per degree Celsius, which is negligible, but their power output can drop by 30% over a 40-degree range. Cognex's firmware reads the temperature and adjusts the APC reference voltage to compensate, maintaining a constant optical power. This is a sophisticated piece of embedded engineering. |
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Section 9: Omron's Infrared and Ultraviolet - For Specialty Surfaces |
Omron, another automation giant, has scanners that use infrared (IR) LEDs for reading barcodes on packaging that is dyed with IR-transparent inks. These inks are invisible to the human eye but visible to the scanner - a common security feature on pharmaceuticals. The IR LED driver is similar to the red LED driver, but the forward voltage is lower (about 1.2 volts), so the current-limiting resistor must be chosen carefully. |
More exotic is Omron's use of ultraviolet (UV) LEDs (365 nm) in some prototype scanners. UV light excites fluorescent inks, which then emit visible light. The barcode is printed with a fluorescent ink that glows under UV, and the scanner reads the glow. The UV LED driver must operate at a much higher voltage (around 6 volts) and requires a special UV-transparent lens. The photodetector must have a filter that blocks the UV light and passes only the visible fluorescence. This is a niche application, but it shows the versatility of LED illumination. |
Omron's driver circuits are noteworthy for their use of 'feed-forward' compensation. Instead of just a feedback loop, they also measure the LED's forward voltage and use that information to pre-set the current. This makes the driver faster to respond to changes, which is important for pulse modulation where the LED turns on and off at hundreds of kilohertz. |
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Section 10: The Driver Circuit - LED vs. Laser, A Detailed Comparison |
The driver circuit is where the rubber meets the road. Let us compare a typical LED driver and a typical laser driver side by side. |
An LED driver is essentially a constant-current source. The simplest version is a current-limiting resistor and a MOSFET switch. The microcontroller turns on the MOSFET, current flows through the resistor and the LED, and the LED lights up. The current is determined by Ohm's law: (supply voltage - LED forward voltage) / resistance. For a 5-volt supply and a red LED with a 2-volt forward voltage, a 30-ohm resistor gives 100 mA. |
This simple circuit works, but it is inefficient because the resistor dissipates power as heat. A better design uses a switching regulator (a buck or boost converter) that adjusts its output voltage to maintain a constant current. This is more efficient (85-95%) but more complex and noisier. Honeywell's 1900 uses a buck regulator for its white LEDs to extend battery life. |
A laser driver, on the other hand, must do more than just supply current. It must have an APC loop. The monitor photodiode inside the laser package produces a current that is proportional to the optical power. The driver compares this current to a reference and adjusts the laser current accordingly. This is a classic negative feedback control system. The bandwidth of this loop is typically 10-100 kHz - fast enough to correct for temperature drift but not so fast that it responds to high-frequency noise. |
Additionally, a laser driver must prevent current overshoot when the laser is turned on. An overshoot of even a few milliamperes can destroy the laser facet. Therefore, the driver includes a soft-start circuit - a capacitor that ramps up the reference voltage slowly. Symbol's LS2208 uses a 0.1-microfarad capacitor to give a 1-millisecond rise time, which is fast enough for scanning but gentle on the diode. |
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Section 11: The Optical System - Shaping the Light |
The light source is only half the story. The optical system - lenses, mirrors, diffusers, and apertures - shapes the light into a useful form. For LEDs, the optical system is about uniformity. A diffuser is a piece of frosted plastic or a microlens array that scatters the light to eliminate hotspots. Honeywell's Voyager uses a molded diffuser that is part of the scanner window. The diffuser is designed using computer simulations (ray tracing) to achieve a uniformity of +/- 10% across the field of view. |
For lasers, the optical system is about focusing. A collimating lens makes the beam parallel; a focusing lens concentrates it into a small spot. The two lenses are often molded as a single piece of plastic - an 'aspheric' lens that corrects spherical aberration. The lenses must be aligned to within microns; this is done by active alignment during manufacturing, where the laser is turned on and the lenses are adjusted until the spot is perfect. |
Zebra's SE950 uses a lens with a diffractive optical element (DOE). A DOE is a lens with microscopic ridges that can shape the beam in ways that conventional lenses cannot. In the SE950, the DOE creates a 'line' of light instead of a spot. This is used in some applications to read barcodes with a larger depth of field. The DOE is expensive to fabricate but reduces the need for precise mechanical alignment. |
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Section 12: Thermal Management - Keeping the Light Stable |
Both LEDs and lasers generate heat. For LEDs, the junction temperature must be kept below 100 degrees Celsius to avoid premature failure. The LED is mounted on a metal-core PCB (MCPCB) that conducts heat to the scanner's housing. In pulsed operation, the average power is low, so a simple thermal pad suffices. But for continuous operation (e.g., in a fixed-mount scanner), a heatsink with fins is necessary. |
For lasers, the thermal challenge is even greater because the optical power is more sensitive to temperature. The threshold current increases with temperature, so the APC loop must increase the drive current to maintain power. This generates even more heat - a positive feedback loop. To prevent thermal runaway, the driver includes a thermal shut-off: if a thermistor on the laser package reads above 80 degrees, the driver turns off the laser. |
Honeywell's 1900 imager uses an interesting trick: it varies the LED pulse width with temperature. At room temperature, the pulse width is 1 millisecond. At 50 degrees, it is reduced to 0.5 milliseconds to keep the junction temperature below the limit. This is a simple firmware adjustment that costs nothing but extends the scanner's life. |
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Section 13: Safety - Eye Protection for Lasers |
Laser safety is governed by the IEC 60825-1 standard. Handheld scanners are usually Class 2 (up to 1 milliwatt) or Class 3R (up to 5 milliwatts). Class 2 is safe because the blink reflex limits exposure to less than 0.25 seconds. Class 3R is also considered safe with some precautions, but it must have an interlock that turns off the laser if the scanning mechanism fails. |
The interlock is a critical part of the laser driver. In Zebra's DS3500, a piezoelectric sensor detects the mirror's oscillation. The sensor's signal is rectified and compared to a threshold. If the signal falls below the threshold, a comparator trips, turning off the laser transistor. The interlock must be 'fail-safe' - if the comparator itself fails, it should default to the off state. This is achieved by using a normally-open relay or a pull-down resistor. |
In addition to the interlock, the laser driver must limit the output power to the specified maximum. This is done by the APC loop, but there is also a hardware clamp - a zener diode that limits the voltage to the laser, preventing a software bug from overdriving it. The clamp is a simple but essential safety feature. |
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Section 14: The Impact of Wavelength on Performance |
The choice of wavelength is not arbitrary. It affects the contrast of the barcode, the sensitivity of the photodetector, and the eye safety classification. For red (650 nm), silicon photodiodes have a responsivity of about 0.5 A/W - a good value. For infrared (850 nm), the responsivity is higher (0.8 A/W), but the contrast may be lower if the ink is also absorptive in IR. |
For blue (445 nm), the responsivity of silicon drops to about 0.2 A/W, so the signal is weaker. This is why Cognex uses a high-sensitivity sensor with a back-illuminated structure. The trade-off is worth it for the improved contrast on etched metal. |
For green (530 nm), the responsivity is similar to red, but green LEDs are less efficient. They require more current for the same light output, which consumes more power. Datalogic uses green LEDs in some of their scanners for reading barcodes on red packaging, where the red ink absorbs red light but reflects green. |
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Section 15: Modulation Techniques - Beyond On-Off |
Both LEDs and lasers can be modulated - turned on and off at high frequencies. For LEDs, modulation is used to reduce power consumption (pulse-width modulation, or PWM) and to implement strobe illumination. For lasers, modulation is used for ambient light rejection and for reducing speckle. |
Datalogic's PowerScan uses a 2-MHz modulation of its IR laser. The detector is a lock-in amplifier that only amplifies the signal at 2 MHz. This rejects any ambient light that is not modulated at that frequency, including sunlight, fluorescent lamps, and even the scanner's own display. The modulator is a simple current switch in the laser driver, controlled by a square wave from the microcontroller. |
Zebra's SE950 uses a lower modulation frequency (100 kHz) to reduce speckle. Speckle is caused by the interference of coherent light reflecting off a rough surface. By rapidly changing the laser's phase (by modulating the current), the speckle pattern changes faster than the eye or the detector can resolve, effectively averaging it out. This improves the signal-to-noise ratio, especially on matte labels. |
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Section 16: The Cost Equation - LED vs. Laser |
Cost is a decisive factor for most manufacturers. An LED imager costs less to manufacture because it has no moving parts and the LEDs themselves are very cheap (a few cents each). The driver circuit is simple - just a MOSFET and a few resistors. The total bill of materials (BOM) for an LED illumination system might be $0.50. |
A laser scanner costs more. The laser diode itself is $1-3, the collimating lens is $0.50, the focusing lens is $0.50, and the mirror and motor assembly are $2-3. The driver circuit is more complex, with an APC loop, a monitor photodiode, and a fail-safe interlock. The total BOM for a laser illumination system might be $5-10. |
However, a laser scanner can read barcodes from a greater distance, which is essential in warehouse and logistics applications. The higher cost is justified by the performance. For retail checkout, where the barcode is brought close to the scanner, an LED imager is cheaper and perfectly adequate. This is why Honeywell's Voyager, an LED imager, dominates the retail market. |
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Section 17: Power Consumption - A Battery-Life Consideration |
Power consumption is critical for handheld scanners. An LED imager typically consumes 50-200 milliwatts for the illumination, depending on the pulse width and current. A laser scanner consumes 100-300 milliwatts for the laser, plus an additional 100-200 milliwatts for the mirror motor. The motor is a significant power draw; it must oscillate a mirror at 50-100 Hz, which requires a constant drive. |
To save power, modern scanners use 'burst mode.' The illumination turns on only when the trigger is pulled, and it turns off after a successful read or after a timeout. Some scanners have a 'motion wake' feature - an accelerometer detects when the scanner is picked up and pre-illuminates the barcode. This reduces the latency but adds a small standby power. |
Zebra's DS8100 has an advanced power management system. The LED driver is a buck-boost converter that can deliver 1 ampere at 3.3 volts with an efficiency of 90%. The converter is disabled during standby, reducing the quiescent current to less than 10 microamperes. The laser driver, if present, is similarly gated. The result is a battery life of 8 hours of continuous scanning. |
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Section 18: Reliability - LEDs Win in the Long Run |
LEDs are incredibly reliable. A typical red LED has a mean time between failures (MTBF) of 100,000 hours (over 11 years of continuous operation). The driver circuit, being simple, also has few failure points. Laser diodes are less reliable - their MTBF is typically 20,000-50,000 hours. The moving mirror and its bearings are the most common failure point in a laser scanner, with an MTBF of 10,000-20,000 hours. |
This is why industrial scanners, which are used continuously, often use LED imagers. Keyence's SR-1000, for example, is rated for 2 million cycles (which is essentially the life of the electronics). The LEDs are the only wear items, and they are designed to outlast the rest of the scanner. |
For laser scanners, the mirror motor is the weak link. Zebra's SE950 uses a ceramic bearing that is rated for 5 million scan cycles. The motor driver includes a current limit to prevent overheating, and the firmware periodically 'exercises' the motor during idle time to distribute the lubricant. These design choices extend the motor's life but do not eliminate the failure risk. |
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Section 19: The User Experience - Visible Beam vs. Invisible Flash |
The type of light source affects the user's experience. A laser scanner projects a visible red line across the barcode. This gives the user immediate feedback - they can see exactly where the scanner is aiming. This is a major advantage in retail, where the cashier needs to quickly align the scanner with the barcode. |
An LED imager, in contrast, does not project a visible line. Instead, it flashes a bright light, which can be startling or annoying to the user. Some imagers have an aiming pattern - a grid or a crosshair of red LEDs that is projected onto the barcode. This is a separate illumination source, usually a low-power red LED with a diffractive lens. The aiming pattern is driven by a separate, simple driver circuit. |
Honeywell's 1900 has a green aiming pattern, which is easier to see in bright ambient light. The green LEDs are driven by a separate current source, and they are turned off during the actual image capture to avoid contaminating the image. This is a clever compromise that gives the user the aiming feedback of a laser without the moving parts. |
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Section 20: The Environmental Factors - Temperature, Humidity, and Dust |
The light source must work in all environments. In a cold warehouse, the LED's forward voltage increases, reducing the current unless the driver compensates. In a hot manufacturing plant, the LED's efficiency drops. The driver circuit must include temperature compensation - either a thermistor that adjusts the current, or a firmware algorithm that measures the forward voltage and adjusts the PWM. |
For lasers, the wavelength can shift with temperature, which can affect the contrast if the photodetector's sensitivity is wavelength-dependent. Zebra's DS3500 includes a wavelength-stabilization circuit that heats the laser to a constant temperature, independent of ambient. This is a power-hungry solution, but it ensures consistent performance. |
Dust is a major problem. Dust on the LED or laser window attenuates the light. For LEDs, the attenuation is gradual; the scanner's feedback loop can increase the pulse width to compensate. For lasers, the feedback loop is the APC, which increases the current - but this shortens the laser's life. Some scanners have a 'window cleaning' alarm that notifies the user when the dust level is too high. The alarm is triggered by a drop in the APC current below a threshold. |
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Section 21: The Electromagnetic Compatibility (EMC) - Noise from the Driver |
The driver circuit, especially the switching regulator in an LED driver, can generate electromagnetic interference (EMI). The high-frequency switching currents can radiate and interfere with the scanner's own signal processing or with other equipment. To meet FCC and CE standards, the driver must include EMI filters. |
A typical LED driver uses a ferrite bead and a capacitor at the input to suppress conducted EMI. For radiated EMI, the PCB layout must be carefully designed - the loop area of the switching current must be minimized, and a ground plane must be used. Honeywell's 1900 has a two-layer PCB with a solid ground plane on the bottom layer. The switching regulator is placed at the edge of the board, away from the sensitive analog circuitry. |
For laser drivers, the EMI is less problematic because the currents are continuous (DC) rather than switching. However, the modulation of the laser (if used) can generate EMI at the modulation frequency. The modulator must have a controlled slew rate - not too fast, to reduce harmonics, but fast enough for the modulation to be effective. |
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Section 22: The Manufacturing Process - Aligning the Light Source |
One of the most critical steps in manufacturing a scanner is aligning the light source. For an LED imager, the LEDs must be positioned so that the illumination field is centered on the sensor's field of view. This is done by placing the LEDs on the PCB with a pick-and-place machine, but there is always a tolerance. To compensate, some scanners have an adjustable aperture - a small screw that can be turned to shift the LED's beam. |
For a laser scanner, the alignment is even more critical. The laser beam must hit the mirror at the correct angle, and the reflected beam must scan across the barcode in a straight line. The alignment is done in an active process: a camera views the projected beam, and the lenses are moved by a robotic arm until the beam is centered. This is expensive and time-consuming, which adds to the cost of a laser scanner. |
Zebra has developed a 'self-aligning' laser engine that uses a piezoelectric actuator to adjust the lens position after the scanner is assembled. A built-in photodiode measures the position of the beam, and a closed-loop control system moves the actuator until the beam is perfect. This reduces the manufacturing cost by eliminating the need for active alignment. |
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Section 23: The Light Source as a System - Integration with the Detector |
The illumination source cannot be designed in isolation. It must be integrated with the detector and the processing chain. The timing of the LED flash must be synchronized with the sensor's exposure, as we discussed in the previous article. The intensity of the LED must be set so that the signal is not saturated. |
In Honeywell's 1900, the microcontroller reads the ambient light sensor before each scan. If the ambient light is low, it sets the LED pulse width to the maximum (10 ms). If the ambient light is high, it reduces the pulse width to 1 ms. This is a feed-forward control, not a feedback control, because it does not wait for the signal amplitude. It is simpler and faster. |
In Datalogic's PowerScan, the laser modulation frequency is synchronized with the ADC's sampling rate. The ADC samples at exactly twice the modulation frequency, so it can capture the peak of the modulated signal. This is a form of 'synchronous demodulation' that maximizes the signal-to-noise ratio. The synchronization is achieved by a single clock source that drives both the modulator and the ADC. |
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Section 24: The Future - Solid-State and Structured Light |
The future of illumination for barcode readers is moving beyond simple LEDs and lasers. One emerging technology is the VCSEL (vertical-cavity surface-emitting laser). A VCSEL is a laser that emits light perpendicular to the chip surface. It can be fabricated in arrays, creating a multi-spot pattern. This is useful for 2D barcode reading because the scanner can project a grid of dots onto the barcode and measure the distortion to determine the 3D shape of the object. |
Keyence is experimenting with VCSEL arrays in their next-generation readers. The driver circuit for a VCSEL array is more complex because each laser must be individually addressable. The driver is a bank of current sources, each controlled by a digital-to-analog converter. The pattern of the dots can be changed in firmware, making the scanner adaptable to different reading conditions. |
Another emerging technology is structured light - projecting a known pattern (like a checkerboard) onto the barcode. This is used to reconstruct the 3D profile of the object, which helps in reading barcodes on curved surfaces. The illumination source is usually a laser with a diffractive optical element (DOE) that creates the pattern. The driver is similar to a standard laser driver, but it must maintain a stable output because the pattern's intensity must be uniform. |
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Section 25: The Environmental Impact - Energy Efficiency and Recycling |
The energy efficiency of the light source matters not only for battery life but also for the environment. LED imagers are more efficient than laser scanners because they do not have a motor. In a store with hundreds of scanners, the savings in electricity can be significant. Honeywell's Voyager consumes about 1 watt, while a typical laser scanner consumes 2-3 watts. |
The materials used in the illumination system also matter for recycling. LEDs are made of semiconductor materials that are generally non-toxic. Laser diodes contain small amounts of toxic materials like arsenic and gallium, but the amounts are tiny. The lenses and mirrors are made of plastics and glasses that can be recycled. Zebra and Honeywell both have environmental policies that require their scanners to be recyclable at end-of-life. |
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Section 26: The Safety Certifications - A Regulatory Maze |
The illumination source must comply with various safety standards. For LEDs, the standard is IEC 62471 (photobiological safety). It limits the blue-light hazard, the thermal hazard, and the UV hazard. Most red LEDs are exempt, but white and blue LEDs must be tested. The driver circuit must limit the current to prevent overheating, which would increase the LED's temperature and potentially cause a burn hazard. |
For lasers, the standard is IEC 60825-1. It defines the classes (1, 1M, 2, 2M, 3R, 3B, 4) and the corresponding safety measures. A Class 2 laser (like the LS2208) requires a visible warning label but no other safety measures. A Class 3R laser (like the SE950) requires a key switch or a remote interlock in some countries. The driver circuit must incorporate these safety measures, which adds cost. |
Zebra's scanners are certified for both the US (FDA/CDRH) and Europe (IEC) standards. The certification process involves testing the laser output power, the beam divergence, and the interlock's response time. The process costs hundreds of thousands of dollars per product, which is a significant barrier to entry for new companies. |
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Section 27: The Software Control - Managing the Light Source |
The microcontroller's firmware plays a large role in managing the illumination source. It controls the on/off timing, the pulse width, the current level, and the modulation frequency. In many scanners, the firmware also controls a 'dimming' feature - the ability to reduce the brightness of the aiming pattern or the illumination flash to avoid disturbing the user. |
The firmware must also handle fault conditions. If the LED or laser fails to turn on, the firmware should detect this (by checking the current draw) and notify the user with an error code. If the laser's APC loop reports that the drive current is too high (indicating that the laser is near end-of-life), the firmware can reduce the power or warn the user. |
Honeywell's firmware includes a 'light management' module that stores the optimal settings for each of the scanner's operating modes (e.g., 'retail,' 'warehouse,' 'outdoor'). When the user changes the mode via a configuration barcode, the firmware loads the corresponding settings. This is a powerful feature that makes the scanner versatile. |
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Section 28: The Optical Feedback - Closing the Loop |
Some advanced scanners use the photodetector itself as a feedback sensor for the illumination. Instead of relying on a separate monitor photodiode (in a laser) or no feedback at all (in an LED), they measure the reflected light from the barcode and use it to adjust the illumination. |
This is called 'closed-loop illumination control.' The scanner emits a brief test pulse, measures the reflected signal, and then adjusts the main pulse width or current. This ensures that the signal is always at the optimal level, regardless of the label's reflectivity or the scanner's distance. Datalogic uses this technique in their high-end Gryphon imagers. |
The closed-loop control is implemented in firmware. The microcontroller samples the output of the TIA during the test pulse, compares it to a target, and uses a proportional-integral (PI) controller to adjust the illumination. The PI controller has two coefficients - proportional and integral - that are tuned for stability and speed. |
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Section 29: The Role of the Lens Coating - Anti-Reflection and Protection |
The optical elements (lenses, windows) that interact with the light source often have special coatings. An anti-reflection (AR) coating reduces the light loss at the air-glass interface, increasing the efficiency. A hard coating (like diamond-like carbon) protects the lens from scratches. A hydrophobic coating repels water droplets. |
The AR coating is particularly important for lasers. A 1% loss at each surface might seem small, but if there are four surfaces (collimating lens, focusing lens, mirror, and window), the total loss is about 4%. This is significant for a Class 2 laser where the power is already low. Zebra's lenses have a multi-layer AR coating that reduces the loss to less than 0.5% per surface. |
The coating process is done in a vacuum chamber by depositing thin films of magnesium fluoride or titanium dioxide. The thickness of the films must be controlled to within a few nanometers to achieve the desired wavelength selectivity. This is an expensive process, but it improves the scanner's performance. |
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Section 30: The Alignment Marks - A Manufacturing Aid |
During the manufacturing of the optical system, alignment marks are used to position the lenses relative to the light source. These marks are small crosshairs or circles etched onto the lens holder. The assembly machine uses a vision system to locate the marks and place the lenses with sub-micron accuracy. |
The alignment marks also serve as a quality check. After the scanner is assembled, the beam position and intensity are measured automatically. If the beam is not within specifications, the scanner is rejected. The data from the alignment process is stored in a database for traceability. |
Datalogic's manufacturing line uses a laser alignment system that projects a reference beam onto the scanner's optical train. The scanner's own light source is turned on, and the two beams are compared. Any deviation is corrected by physically moving the lens holder with a piezoelectric actuator. This is an automated process that takes less than 5 seconds per scanner. |
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Section 31: The Interchangeability - Plug-and-Play Light Engines |
Some manufacturers, like Zebra and Honeywell, offer their illumination sources as 'light engines' - modular assemblies that can be plugged into different scanner housings. A light engine includes the LED or laser, the driver circuit, and the optics, all pre-aligned on a small PCB. |
This modularity reduces the design time for new scanner variants. The light engine is a black box with a simple interface (power, ground, enable, and data). The host scanner only needs to provide the power and the control signals. The light engine's firmware is stored in an EEPROM on the module, so the host can read its capabilities. |
The interface is usually a simple serial bus (I2C or SPI). The host sends commands to set the pulse width, the current, and the modulation frequency. The light engine's microcontroller executes the commands and reports back the status. This plug-and-play approach is common in the consumer electronics industry and is now reaching the barcode scanner market. |
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Section 32: The Legacy Systems - Maintaining Old Light Sources |
Many large companies have thousands of legacy barcode scanners that use old light sources - for example, helium-neon (HeNe) gas lasers, which were common in the 1980s. These scanners are still in use because they are reliable and the cost of replacing them is high. The driver circuits for HeNe lasers are different: they require a high-voltage (1-2 kV) startup circuit, followed by a constant-current supply. |
The maintenance of these legacy systems is a niche industry. Companies like Zebra offer 'retrofit' kits that replace the HeNe laser with a modern laser diode. The retrofit kit includes a new light engine, a new driver, and a new power supply. The driver is designed to emulate the electrical behavior of the HeNe laser, so the host scanner does not need to be modified. |
This is a practical example of how the technology evolves, but the installed base creates a need for compatibility. The retrofit kits are more expensive than a new scanner, but they are still cheaper than replacing an entire system (e.g., a conveyor belt scanner with integrated hardware). |
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Section 33: The Emerging Technology - OLEDs for Flexible Scanners |
Organic light-emitting diodes (OLEDs) are a new type of LED that uses organic (carbon-based) semiconductors. They are thin, flexible, and can be printed on plastic substrates. Some researchers are exploring OLEDs for barcode illumination, especially for scanners that need to conform to curved surfaces. |
The driver circuit for an OLED is similar to a standard LED driver, but the forward voltage is lower (about 3 volts) and the current is lower (a few milliamperes). However, OLEDs are sensitive to moisture and oxygen, so they must be encapsulated. This makes them more expensive than inorganic LEDs. |
Cognex has filed patents for a flexible barcode scanner that uses an OLED array. The OLEDs are printed on a transparent film that is attached to the inside of a curved window. The driver circuit is a flexible printed circuit board (FPCB) that can bend with the scanner. This is still a research project, but it shows the potential of new light source technologies. |
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Section 34: The Cost of Innovation - R&D at Major Companies |
The development of the illumination source and its driver is a significant part of a scanner's R&D budget. At Zebra, a typical development project for a new light engine costs $1-2 million and takes 2-3 years. The cost includes the design of the semiconductor, the optics, the driver, and the safety certifications. |
Honeywell's R&D team focuses on optimizing the LED driver for efficiency and noise. They have developed a proprietary current source that uses a digital-to-analog converter (DAC) and a current-sense amplifier. The DAC's resolution is 12 bits, which allows a fine adjustment of the LED current. The current-sense amplifier has a bandwidth of 10 MHz, which is far higher than the modulation frequency, ensuring that the current is stable. |
Datalogic's R&D team has invested heavily in modulation techniques. They have a patent on a modulation scheme that uses a pseudo-random sequence to spread the spectrum of the laser's emission, reducing the EMI. The pseudo-random sequence is generated by a linear feedback shift register (LFSR) in the firmware. |
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Section 35: The User's Perspective - What They Notice |
From the user's point of view, the illumination source is one of the most visible aspects of the scanner. They see the red line of a laser or the bright flash of an LED. They notice if the aiming pattern is difficult to see in sunlight, or if the flash is too bright in a dark room. |
User feedback has driven many design changes. For example, some users complained that the red aiming line of a laser scanner was hard to see on a red background. Zebra responded by introducing a green laser option for some models. The green laser is a frequency-doubled Nd:YAG laser that produces 532-nm light. It is more expensive, but it is easily visible on any background. |
Similarly, users of LED imagers often complained that the flash was startling. Honeywell's 1900 has a 'gentle flash' mode that ramps up the LED current gradually, reducing the perceived brightness. This is a simple firmware change that made a big difference in user satisfaction. |
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Section 36: The Future of the Light Source - A Look Ahead |
As we look to the future, the illumination source for barcode readers will become smarter, more efficient, and more adaptable. We will see integrated light sources that combine multiple wavelengths in a single package, eliminating the need for separate LEDs. We will see laser drivers that are fully integrated into the same chip as the photodetector and the signal processor, creating a 'system-on-chip' for barcode reading. |
We will also see the use of 'computational illumination' - where the scanner projects a sequence of patterns and uses the reflected light to reconstruct not only the barcode but also the 3D shape of the object. This will enable new applications, such as reading barcodes on moving or irregularly shaped objects. |
The journey from the simple LED of the 1970s to the sophisticated VCSEL arrays of today is a testament to the ingenuity of engineers. The light that reads a barcode is not just a source of illumination; it is a carefully crafted tool that enables the global economy. And as the technology continues to evolve, we can expect that tool to become even more powerful and more pervasive. |
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Detailed Final Summary (approximately 1200 words) |
We have now explored the illumination source - the first pillar of the barcode reading system - in exhaustive detail. Let us recapitulate the essential points, the design trade-offs, and the real-world examples that define this critical component. |
Fundamental Differences: LED vs. Laser |
The choice between an LED and a laser diode is the most fundamental decision in the illumination design. LEDs produce incoherent, diffuse light, which is ideal for illuminating a wide area. They are cheap, robust, and have a long life. Lasers produce coherent, focused light, which enables long-range reading and small spot sizes. They are more expensive, have a shorter life, and require complex safety mechanisms. |
LED Illumination in Practice |
LEDs are used in most handheld imagers, including Honeywell's Voyager and 1900 series. The driver is a simple constant-current source, often with a switching regulator for efficiency. The optical system includes a diffuser to create a uniform field. LEDs can be pulsed at high currents to achieve high brightness with low average power, and they can be used in arrays to provide even coverage. |
Laser Illumination in Practice |
Lasers are used in Symbol's (Zebra's) LS2208 and SE950, and in Datalogic's PowerScan. The driver includes an automatic power control (APC) loop that uses a monitor photodiode to stabilize the output power. The optical system includes collimating and focusing lenses to create a small spot. The scanner has a moving mirror that sweeps the spot across the barcode. The driver also includes a fail-safe interlock that turns off the laser if the mirror stops. |
Wavelength Selection |
The wavelength of the light affects the contrast, the sensitivity of the photodetector, and the safety classification. Red (650 nm) is the most common because it is efficient and matches the sensitivity of silicon photodiodes. Infrared (855 nm) is used for thermal paper and for better penetration of ambient light. Blue (445 nm) is used for etched metal codes because it scatters more strongly. Green (530 nm) is used for red backgrounds. White LEDs are used for color barcodes or for a more natural view. |
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Advanced Features |
- Multi-wavelength illumination: Keyence's SR-1000 has red, blue, green, white, and infrared LEDs, and automatically selects the best one for each label. |
- Polarized light: Keyence and Cognex use polarizing filters to reduce specular reflections from shiny surfaces. |
- Modulated lasers: Datalogic's PowerScan uses a 2-MHz modulation for lock-in amplification, rejecting ambient light. |
- Speckle reduction: Zebra's SE950 modulates the laser at 100 kHz to average out speckle noise. |
- Structured light: Keyence is experimenting with VCSEL arrays to project patterns for 3D reconstruction. |
- Blue lasers: Cognex uses a 445-nm laser for reading etched Data Matrix codes on metal. |
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Driver Circuit Design |
- LED driver: A constant-current source, typically a MOSFET with a resistor, or a switching regulator for efficiency. The current is often controlled by a DAC or a PWM signal from the microcontroller. |
- Laser driver: A constant-current source with an APC loop. The loop compares the monitor photodiode current to a reference and adjusts the laser current. The driver also includes a soft-start to prevent current overshoot, and a fail-safe interlock for safety. |
- Modulation: Both LED and laser drivers can include modulation inputs for high-frequency pulsing. The modulation is used for noise rejection, speckle reduction, and power saving. |
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Optical Systems |
- LED optics: Diffusers and microlens arrays create a uniform field. The diffuser is often part of the scanner window. |
- Laser optics: Collimating lenses make the beam parallel; focusing lenses concentrate it into a spot. Aspheric lenses and diffractive optical elements (DOEs) are used to correct aberrations and shape the beam. |
- Aiming patterns: Many imagers use a separate low-power LED to project a crosshair or grid onto the barcode, giving the user aiming feedback. |
Thermal Management |
- LEDs: Mounted on metal-core PCBs to conduct heat away. In pulsed operation, the average power is low, so simple thermal pads suffice. |
- Lasers: Require more careful thermal management because the output power is temperature-sensitive. The APC loop compensates for temperature changes, but if the temperature rises too high, a thermal shut-off turns off the laser. |
- Both: The driver circuit may include temperature compensation, adjusting the current or pulse width based on a thermistor reading. |
Safety |
- Lasers: Must comply with IEC 60825-1. Class 2 and Class 3R are common. The driver must include a fail-safe interlock that turns off the laser if the scanning mechanism fails. The interlock is typically a hardware circuit, not just software. |
- LEDs: Must comply with IEC 62471 for photobiological safety. Red LEDs are generally exempt, but white and blue LEDs must be tested. The driver must limit the current to prevent overheating. |
Cost, Power, and Reliability |
- Cost: LED systems are cheaper ($0.50 BOM) than laser systems ($5-10 BOM). The difference is due to the laser diode, the optics, the mirror motor, and the more complex driver. |
- Power: LED systems consume 50-200 mW for illumination; laser systems consume 100-300 mW for the laser plus 100-200 mW for the motor. The motor is a significant power drain. |
- Reliability: LEDs have an MTBF of 100,000 hours, while lasers have 20,000-50,000 hours. The mirror motor is the most common failure point in a laser scanner, with an MTBF of 10,000-20,000 hours. |
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Company Strategies |
- Symbol/Zebra: The king of laser scanners. Their LS2208 is the best-selling scanner ever. They also offer imagers but are best known for their laser heritage. They have developed robust APC loops, fail-safe interlocks, and advanced lenses (including DOEs). |
- Honeywell: Champion of LED imagers. Their Voyager and 1900 series are reliable, cost-effective, and widely used in retail. They have innovated in LED driver efficiency and in aiming patterns. |
- Datalogic: Pragmatic dual-track. They offer both laser and LED scanners, with a focus on industrial robustness. Their PowerScan series uses modulated infrared lasers for long-range, high-noise environments. Their Gryphon series offers both laser and imager variants. |
- Keyence: High-end industrial automation. Their SR-1000 has multi-wavelength LEDs and a ring light for uniform illumination. They are also experimenting with VCSEL arrays for structured light. |
- Cognex: Machine vision leader. Their DataMan series uses blue lasers for etched metal codes. They have developed advanced thermal compensation for their blue laser drivers. |
- Omron: Specialty applications. They use IR and UV LEDs for invisible or fluorescent barcodes. They have developed feed-forward compensation for fast pulse modulation. |
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Interaction with Other Pillars |
The illumination source does not work in isolation. It interacts with the photosensor (the second pillar) via exposure timing, with the signal processor (the third pillar) via gain and threshold control, and with the optics (which is part of the illumination design itself). The firmware orchestrates these interactions, adjusting the illumination based on the sensor's readout and the decoder's success rate. |
Emerging Trends |
- VCSEL arrays for structured light and 3D scanning. |
- OLEDs for flexible, conformable scanners. |
- Computational illumination, where the scanner projects complex patterns to reconstruct 3D information. |
- AI-optimized illumination, where a machine learning algorithm chooses the best wavelength and intensity for each label. |
- Fully integrated light engines that include the driver, the optical system, and even the photodetector in a single module. |
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Final Thoughts |
The illumination source is the most tangible part of a barcode scanner - the red line or the bright flash that the user sees. But it is also a sophisticated engineering subsystem that must balance performance, cost, safety, and reliability. The major companies have each found their own optimal balance: Zebra with lasers, Honeywell with LEDs, Datalogic with both, Keyence with multi-wavelength, Cognex with blue lasers, and Omron with specialty wavelengths. |
As the technology evolves, we will see more integration, more intelligence, and more versatility in the illumination source. But the fundamental goal remains the same: to project the right light onto the barcode, so that the reflected whisper can be heard and understood. The light that reads is more than just a bulb - it is the beginning of a journey from optical reflection to digital data, and it is a journey that has transformed the way we track, trade, and transport every product on earth. |
cs@easiersoft.com