Laser Scanning - The Moving Mirror: The Heartbeat of Laser Barcode Readers |
Executive Summary |
This article provides a comprehensive exploration of the moving mirror mechanism, the core component that brings life to a laser barcode scanner. We examine how the controlled deflection of a laser beam, achieved through oscillating or rotating mirrors, enables the scanner to sweep a focused spot of light across a barcode, translating spatial patterns into time-varying electrical signals. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real patent disclosures from industry leaders including Symbol Technologies, Metrologic Instruments, and Honeywell. We explore the fundamental principles of resonant scanning and rotating polygon mirrors, the integration of collection optics and folding mirrors for compact module design, and the practical challenges of creating high-density scan patterns for omni-directional reading. The article covers both the optical and mechanical aspects of scanner design, with special attention to the trade-offs between size, speed, and durability. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing or selecting laser scanning systems for barcode reading applications. |
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Chapter 1: The Principle of Laser Scanning |
A laser barcode reader is fundamentally different from an imaging-based reader. Instead of capturing a picture of the entire barcode at once, a laser scanner uses a moving mirror to sweep a concentrated beam of light across the symbol in a rapid, repetitive pattern. The reflected light intensity varies as the beam crosses the dark bars and light spaces, and this time-varying signal is then digitized and decoded. |
The key to this operation is the moving mirror. As a patent from Symbol Technologies explains, a scanner includes a moving mirror assembly driven by a motor, which causes the light beam and a field of view for the reflected light to move along a scan line across the bar code symbol . The mirror's motion translates the laser's static point of light into a dynamic line that can cross the entire barcode. |
The steering of the beam can be one-dimensional, as in a simple handheld scanner, or two-dimensional, as in a presentation scanner that projects a complex raster pattern . Additionally, the mirrors can lead to a periodic motion---like the rotating polygon mirror in a barcode scanner or so-called resonant galvanometer scanners---or to a freely addressable motion, as in servo-controlled galvanometer scanners . For most barcode applications, the periodic motion of a resonant scanner or rotating polygon provides the speed and simplicity needed. |
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Chapter 2: The Resonant Oscillating Mirror |
The most common type of scanning mechanism in handheld laser barcode readers is the resonant oscillating mirror. This is a small mirror mounted on a flexure that is driven by an electromagnetic coil. The mirror oscillates back and forth at its natural resonant frequency, typically around 50 to 100 Hz. |
The patent from Symbol Technologies describes a scanner where the moving mirror is a planar mirror mounted at the rear side of the support between the frame wall and the base . The mirror is driven by a motor, and the scan mirror and curved mirror are jointly supported and driven jointly by the motor . This joint mounting of the scan mirror and the collection mirror ensures that the optical alignment between the transmitted and received beams is maintained. |
The resonant scanning mirror is simple, robust, and consumes relatively little power. The mirror is driven electromagnetically and is often made of a metal-coated polymer . However, because it is a resonant system, its scan angle and frequency are fixed by the mechanical design. It is well-suited for handheld scanners where the user aims the device and provides the gross motion to cover the barcode. |
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Chapter 3: The Rotating Polygon Mirror |
For high-speed or omni-directional scanning, the rotating polygon mirror is the mechanism of choice. This is a multi-faceted mirror (often with 4, 5, or 6 sides) that is spun continuously by an electric motor. As each facet rotates through the laser beam, it sweeps the beam across the scan field. |
A patent from Metrologic Instruments describes a laser scanning system with a centrally-positioned double-sided rotating mirror that produces two independent scanning raster fields from a single scanning element . This double-sided spinner has a first and a second reflecting surface disposed on opposites sides thereof and being rotatable about a rotational axis . Laser beams projected onto wobbling mirrors are redirected onto each reflecting surface of the double-sided rotating spinner, producing a pair of rotating scanning beams . |
The polygon mirror offers several advantages. It can sustain very high scan speeds because the mirror rotates continuously rather than oscillating. It is also capable of 360-degree scanning, which is essential for fixed-mount 'presentation' scanners that must read barcodes presented in any orientation. |
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Chapter 4: The Compound Collection Mirror |
The reflected light from the barcode must be captured efficiently and directed onto the photodetector. This is the job of the collection mirror. In advanced designs, the collection mirror is not a simple parabolic reflector but a compound structure with different zones optimized for different reading distances. |
A patent from a barcode reader manufacturer describes a compound collection mirror which includes two concentric areas: an inner mirror used to focus incoming light to detect long range objects, and an outer ring mirror used to focus incoming light to detect short range objects . The top surface of the inner mirror may have a parabolic or elliptical profile to maintain maximum efficiency for long ranges (e.g., 2 or more meters), while the top surface of the outer ring mirror has a curvature tailored to cover the short range . |
The design is based on the geometry of the reflected beam. When an object is far away, reflected rays are substantially parallel and only the inner mirror is illuminated. As the object gets closer, the reflected beam becomes diverging, and a larger area of the collecting mirror is illuminated . By tailoring the focal lengths of the inner and outer zones, the scanner can maintain a relatively flat signal response over a wide range of distances. |
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Chapter 5: Folding the Optical Path for Compactness |
Handheld scanners must be compact and ergonomic. To achieve this, the optical path of the laser beam is often 'folded' using a series of mirrors. This allows the laser source and the scanning mirror to be placed in a more efficient layout within the housing. |
A Symbol Technologies patent describes a scan module with a pair of beam-folding reflectors . One of the reflectors is mounted on a raised support wall at the rear of the module, and the other is mounted at the front side. The other reflector is operative for reflecting the light beam reflected from the scan mirror through an opening in the frame wall to the first reflector, which then reflects the beam exteriorly of the support . |
This folded beam path allows the module to be very compact---approximately 42 mm by 24 mm by 11 mm . The design also places the collection optics and sensors at the front of the module, close to the scan window, to maximize the collection of reflected light. The integration of the laser, scan mirror, and folding mirrors on a single support structure simplifies manufacturing and ensures precise alignment. |
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Chapter 6: The Collection Mirror and Fold Mirror Module |
An even more integrated approach to the optical system is the mirror module described in a Symbol Technologies patent. This module combines the collection mirror and the fold mirror into a single, unified structure . |
The patent describes a mirror module comprising a collection mirror with an opening and a first reflective surface, and a fold mirror coupled to the collection mirror with a second reflective surface lying in a plane not parallel to the first plane . The fold mirror extends behind the collection mirror at an angle so that a laser impinging on the fold mirror is reflected through the opening in the collection mirror . |
This unified structure simplifies assembly and ensures that the relative alignment between the outgoing laser beam and the incoming collection optics is precisely maintained. The collection mirror's opening allows the folded laser beam to pass through to the scanning mirror, while the rest of the reflective surface captures and focuses the reflected light onto the sensor. |
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Chapter 7: The Scan Motor with Shock Protection |
The scan motor is a critical component that determines the reliability and durability of the scanner. Handheld scanners are subject to drops and impacts, so the motor must be able to withstand shocks. A Symbol Technologies patent describes an innovative scan motor design with integrated shock protection . |
The scan motor comprises a spring module, a magnet, and a reflective element. The spring module consists of a static substrate and a dynamic substrate coupled together by a molded flexible spring, such as a silicone spring . This flexible spring provides the restoring force for the oscillating mirror and also acts as a shock absorber, protecting the delicate mirror from damage during a drop. |
The dynamic substrate has an extending member with a magnet on one side and a mirror on the other . A drive coil positioned opposite the magnet excites the oscillation. The use of thermoplastic substrates and silicone springs makes the motor low-cost and relatively easy to manufacture . |
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Chapter 8: Aiming and Tracking the Beam |
For handheld scanners, the ability to aim the invisible or visible laser beam is crucial for the user. Early laser diodes emitted infrared light, which is invisible to the human eye. This made aiming difficult, as the operator could not see where the beam was pointing. |
A patent from Symbol Technologies describes a hand-held bar code scanner with a trigger-actuated aiming light arrangement for visually locating and tracking each symbol . The scanner emits and receives non-readily-visible laser light but also provides a visible aiming light that is coincident with the invisible scanning beam. This enables the operator to locate the laser beam with respect to the code symbols, ensuring that the beam is properly aimed. |
Modern scanners often use visible laser diodes (such as those emitting at 670 nanometers) that are bright enough to be seen in ambient light. The visible beam serves the dual purpose of illuminating the barcode and providing a visible aiming pattern. |
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Chapter 9: Scan Patterns and Omni-Directional Reading |
In fixed-mount 'presentation' scanners, a single simple scan line is not sufficient. The user may present the barcode in any orientation, so the scanner must project a complex pattern of intersecting lines that can read a barcode regardless of its angle. |
A patent from Metrologic Instruments describes a laser scanning system with a double-sided rotating spinner and wobbling mirrors that produces complex, high-density scanning patterns . The system produces two independent scanning raster fields from a single optically and mechanically simple scanning element . By using two laser sources and a double-sided mirror, the scanner can project a dense pattern of scan lines that increases the effectiveness of barcode reading. |
Honeywell's patent on a user-adaptive presentation scanner describes a system that can alter its baseline scan pattern to a second scan pattern responsive to a user profile . This allows the scanner to adapt to the preferences or typical usage patterns of different operators, improving efficiency at the point of sale. |
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Chapter 10: The Laser Scanner Camera System |
An interesting hybrid approach combines a laser scanner with a camera system for dimensional measurement. This type of system, sometimes used in logistics, uses a laser line scanner to measure the dimensions of objects as they move along a conveyor. |
A patent on a laser scanner camera system describes a laser transmitter and receiver sharing a common rotating polygonal mirror . The polygonal mirror performs a scan, sweeping the laser beam across the object, while also receiving the reflected light from the object surface. The laser transmitter and receiver are arranged coaxially, sharing the common optical path via the polygonal mirror . |
This system uses the laser scanner not just to read barcodes but to profile the object's shape, measuring its dimensions for shipping or sorting purposes. The integration of the scanning and receiving functions through the same mirror mechanism simplifies the optics and improves alignment. |
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Chapter 11: The Dual-Axis Scanner |
In addition to the single-axis oscillating mirror, more complex scanners use dual-axis mechanisms. These can project complex patterns or raster scans. The Metrologic double-sided spinner is effectively a dual-axis system, using wobbling mirrors to introduce a second axis of motion to the beam reflected from the rotating polygon . |
Dual-axis scanning allows the creation of Lissajous patterns, rosettes, and other complex 2D patterns. These patterns are produced by varying the number, type, placement, and orientation of the mirrors . They are more effective at reading barcodes in arbitrary orientations because the beam covers a larger area of the scan field. |
The dual-axis approach is also used in laser shows and other applications, but for barcode reading, the complexity is often traded for simpler, more reliable single-axis mechanisms combined with multiple pattern mirrors. |
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Chapter 12: Summary --- The Moving Mirror in Perspective |
The moving mirror is the mechanism that makes laser barcode scanning possible. By translating a static laser beam into a dynamic sweep, it enables the reader to capture the barcode pattern as a time-varying signal. Whether it is the simple resonant scanner in a handheld gun or the complex double-sided spinner in a presentation scanner, the moving mirror defines the performance and capabilities of the reader. |
We have examined how different companies and technologies have approached the challenges of laser scanning: |
Symbol Technologies developed compact scan modules with folded optical paths and integrated collection mirrors. Their patents describe resonant scanners with joint mounting of scan and collection mirrors, and robust shock-protected scan motors using silicone springs . |
Metrologic Instruments developed double-sided rotating spinners and wobbling mirrors to produce high-density scan patterns from a single scanning element . This technology is used in both handheld and fixed-mount scanners to improve omni-directional reading. |
Honeywell developed user-adaptive presentation scanners that can alter their scan patterns based on user profiles . The company also designed compound collection mirrors that use inner and outer reflective zones to maintain signal strength over a wide range of distances . |
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The key lessons from our exploration are: |
The scanning mechanism defines the reader. The moving mirror determines the scan speed, pattern, and coverage area. |
Resonant scanners are simple and robust. They are ideal for handheld devices where the user provides the gross motion. |
Rotating polygon scanners are fast and allow 360-degree scanning. They are essential for high-speed and omni-directional applications. |
Folding the optical path enables compact form factors. Beam-folding mirrors allow the laser source and scanning mirror to be placed efficiently within the housing. |
Compound collection mirrors optimize performance over depth of field. Different zones of the collection mirror are optimized for different distances. |
Mechanical shock protection is critical for handheld devices. Flexible spring elements in the scan motor can protect the mirror from drop damage. |
Scan patterns can be adapted to users and environments. User-adaptive scanners can optimize their patterns for specific operators. |
In the end, the moving mirror is a testament to the power of simple mechanical ingenuity. It is the humble component that, by its rhythmic motion, brings the precision of a laser to the practical task of reading barcodes. The art of laser scanner design lies in the careful balance of optical efficiency, mechanical robustness, and electronic control, creating a reader that can operate reliably in the hands of a user or in the heart of a high-speed conveyor system. |
cs@easiersoft.com