Method and apparatus for capturing an image of a moving object

A scanned beam imager or laser scanner is operable to scan an object moving through its field-of-view. The system may include means for detecting direction and/or speed of the object. The velocity detection means may include sensors, an interface for receiving velocity information from other system elements, or image analysis that examines the skew, stretch, or compression in images. Responsive to object movement direction and speed, the scanned beam imager may alter its pixel capture rate and/or its scan rate to compensate. Alternatively or in combination, the imager may perform software-based image motion compensation. In some embodiments, the system may allow the image capture region to pace objects moving rapidly through its field-of-view.

FIELD OF THE INVENTION

Embodiments according to the invention relate to scanned beam image capture devices, and more particularly to scanned beam image capture devices that capture an image or series of raster lines from a moving field-of-view.

BACKGROUND

Bar code symbols are used throughout our economy and by the government to track goods, transactions, and people. Within logistics applications, for example, goods are frequently transported past fixed-mount bar code scanners. The scanners read symbols on the goods and report the scanned symbols to a computer system that records the transaction and initiates appropriate processes. In some applications, it is desirable to move objects past fixed mount scanners at relatively high speeds. Examples include cross docking during package shipping and mail sorting, where packages are often moved on conveyor belts.

Linear charge-coupled device (CCD) cameras have been used in some high speed scanning applications. In many of these applications, the axis of the linear CCD array is placed at a right angle to the motion of the conveyor belt. A high intensity lighting system is set up to brightly illuminate the field of view (FOV) of the CCD array. Data is intermittently or continuously read out of the CCD array as the conveyor moves objects with bar code symbols past its FOV. When data is read continuously, the motion of the conveyor acts to create a vertical scan and the system may be used to capture a two-dimensional (2D) image. In some cases, the conveyor must maintain a constant velocity past the CCD FOV for the system to properly image. One drawback of the linear CCD systems is the high intensity of the illuminators and resultant high power consumption. Maximum conveyor speed is frequently determined by the lighting intensity, the distance the lights and camera must be placed from the surface, and/or the data rate out of the CCD array.

Scanned beam systems in the form of conventional linear (1D) bar code scanners have been in use since the early 1970s as fixed mount devices such as those used in grocery stores. By the early 1980s, scanned beam systems had been adapted to hand held form as several bar code companies introduced helium-neon laser based hand held scanners. Most commonly called laser scanners, such systems scan a laser beam over a surface and measure the light reflected from the beam. The pattern of received light is called a scan reflectance profile and may be processed to decode bar code symbols through which the beam is scanned.

Laser scanners are generally regarded to have several advantages over CCD scanners. Because the laser beam provides its own illumination, a separate, power consuming bank of lights is not required. Collimation of the beam results in a power loss proportional to the inverse square of the distance to the surface rather than proportional to the inverse fourth power as in flood-illuminated cameras. Collimation can also result in greater depth of field (DOF) than CCD systems that must operate with large apertures to maximize light gathering efficiency. Finally, since laser scanners provide intrinsic illumination, alignment of the illuminator FOV with an illuminator FOV is not a problem, allowing for faster installation and greater system mobility.

While laser scanners have been used extensively in low speed fixed-mount environments, they have not heretofore proved successful in high speed imaging applications such as the high speed conveyor applications described above. Due in part to lack of beam position feedback, they have also frequently suffered from scan rates insufficient to capture all lines in a FOV.

With respect to hand held applications, lasers have often proved superior to focal plane sensor-based technologies such as charge-coupled device (CCD) and complementary metal oxide semiconductor (CMOS) sensor arrays, particularly with respect to aiming, depth-of-field, motion blur immunity, and low peak power consumption. Unfortunately, lasers have not been widely adapted to image capture applications such as reading (2D) matrix symbols, signature capture, etc. Instead, they have been relegated to reading only linear or 2D stacked bar code symbols. This again is due in part to lack of beam position information, and scan rates to slow to capture all pixels in a FOV.

OVERVIEW

Embodiments according to the present invention relate to beam scanning systems and particularly imagers or raster scanners based on scanned laser beams.

In one set of related aspects, methods are taught for compensating for relative motion between the imager and its FOV. Depending upon the relative directions of the nominal scan axes and the apparent motion, the methods used may be changed.

In another aspect, an apparatus includes one or more embodiments of the motion compensation methods.

In one embodiment, a laser scanner includes a scan axis parallel to the relative motion of the FOV. The rate of scan in that axis can be modified to maximize image quality and readability of bar code symbols in the FOV.

In another embodiment, a material handling system includes a 2D laser scanner that enables it to vary and even reverse conveyor direction without negatively impacting the image quality captured by the scanner.

In another embodiment, a fixed mount laser scanner allows a high relative motion of the FOV, and hence higher throughput of objects passing therethrough.

DETAILED DESCRIPTION

FIG. 1shows a simplified block diagram of a scanned beam imager102according to an embodiment. An illuminator104creates a first beam of light106. A scanner108deflects the first beam of light across a field-of-view (FOV) to produce a second scanned beam of light110. Taken together, illuminator104and scanner108comprise a beam scanner109. Instantaneous positions of scanned beam of light110may be designated as110a,110b, etc. The scanned beam of light110sequentially illuminates spots112in the FOV. Spots112aand112bin the FOV are instantaneously illuminated by scanned beam position110aand110b, respectively, scattering or reflecting the light energy. One or more detectors116A receive a portion of the scattered light energy. The one or more detectors116convert(s) the light to electrical signals from which controller118builds up an image and transmits it for further processing, decoding, archiving, and/or display via interface120.

Light source104may include multiple emitters such as, for instance, light emitting diodes (LEDs), lasers, thermal sources, arc sources, fluorescent sources, gas discharge sources, or other types of illuminators. In one embodiment, illuminator104comprises a red laser diode having a wavelength of approximately 635 to 670 nanometers (nm). In another embodiment, illuminator104comprises three lasers; a red diode laser, a green diode-pumped solid state (DPSS) laser, and a blue DPSS laser at approximately 635 nm, 532 nm, and 473 nm, respectively. Light source104may include, in the case of multiple emitters, beam combining optics to combine some or all of the emitters into a single beam. Light source104may also include beam-shaping optics such as one or more collimating lenses and/or apertures. Alternate light sources may use different or additional optical components for beam shaping, filtering, and other optical parameters.

Light beam106, while illustrated as a single beam, may comprise multiple beams converging on one or more scanners108.

Scanner108may be formed using many known technologies such as, for instance, a rotating mirrored polygon, a mirror on a voice-coil as is used in miniature bar code scanners such as used in the Symbol Technologies SE 900 scan engine, a mirror affixed to a high speed motor or a mirror on a bimorph beam as described in U.S. Pat. No. 4,387,297 entitled PORTABLE LASER SCANNING SYSTEM AND SCANNING METHODS, an in-line or “axial” gyrating, or “axial” scan element such as is described by U.S. Pat. No. 6,390,370 entitled LIGHT BEAM SCANNING PEN, SCAN MODULE FOR THE DEVICE AND METHOD OF UTILIZATION, a non-powered scanning assembly such as is described in U.S. patent application Ser. No. 10/007,784, SCANNER AND METHOD FOR SWEEPING A BEAM ACROSS A TARGET, commonly assigned, a MEMS scanner, all incorporated herein by reference, or other type. Alternatively, the scanner108may be a MEMS scanner, such as those described in U.S. Pat. Nos. 5,629,790 and 5,648,618, to Neukermans et al., U.S. Pat. No. 5,867,297 to Solgaard, et al., and U.S. Pat. No. 6,057,952 to Dickensheets, et al., each of which is incorporated herein by reference. The scanner108may include acousto-optic components, electro-optic components, spinning polygons or other types of scanning elements.

Scanner108may also include a pair of orthogonally positioned scanning devices, each including a respective reflective component that oscillates at a respective frequency.

Alternatively, illuminator104, scanner108, and/or detector116may comprise an integrated beam scanning assembly as is described in U.S. Pat. No. 5,714,750, BAR CODE SCANNING AND READING APPARATUS AND DIFFRACTIVE LIGHT COLLECTION DEVICE SUITABLE FOR USE THEREIN, incorporated by reference herein. Other beam scanning technologies may be usable or preferable, depending upon the application and system.

In the case of a 2D scanned-beam imager, scanner108is driven to scan output beams110along a plurality of axes so as to sequentially illuminate a 2D FOV111.

For the case of 2D imaging, a silicon MEMS scanner is one embodiment, owing to the high frequency, durability, repeatability and energy efficiency of such devices. In such a case, a single-crystal silicon MEMS scanner may be preferable because of its lack of fatigue life, among other things.

A 2D MEMS scanner108scans one or more light beams at high speed in a pattern that covers an entire 2D FOV within a frame period. A typical frame rate may be 60 Hz, for example. In some embodiments, it may be advantageous to run one or both scan axes resonantly. In one embodiment, one axis is run resonantly at about 19 KHz while the other axis is run non-resonantly in a sawtooth pattern so as to create a progressive scan pattern. For example, in an imager using progressively scanned bi-directional lines and a single beam, a horizontal bi-sinusoidal scan frequency of approximately 19 KHz and vertical sawtooth pattern at 60 Hz can approximate SVGA resolution. The horizontal scan motion may be driven electrostatically and the vertical scan motion driven magnetically. In alternative embodiments, both axes may be driven magnetically. Other actuation technologies will be apparent to one skilled in the art.

Detectors116may comprise several different forms. In one embodiment, a simple PIN photodiode connected to an amplifier and digitizer may be used. In the case of multi-color imaging, the detector116may comprise more sophisticated splitting and filtering to separate the scattered light into its component parts prior to detection. As alternatives to PIN photodiodes, avalanche photodiodes (APDs) or photomultiplier tubes (PMTs) may be preferred for certain applications, particularly low light applications.

Simple photodetectors such as PIN photodiodes, APDs, and PMTs may be arranged to stare at the entire FOV, stare at a portion of the FOV, collect light retrocollectively, or collect light confocally, depending upon the application. In some embodiments, the photodetector116collects light through filters to eliminate much of the ambient light.

The present device may be embodied as monochrome, as full-color, and even as a hyper-spectral. In some embodiments, it may be desirable to add color channels between the conventional RGB channels used for many color cameras. Herein, the term grayscale and related discussion shall be understood to refer to each of these embodiments as well as other, unmentioned embodiments. In the control apparatus and methods described below, pixel gray levels may comprise a single value in the case of a monochrome system, or may comprise an RGB triad or greater in the case of color or hyperspectral systems.

FIG. 2is an isometric view of an imaging engine according to an embodiment.FIG. 2shows a scanned beam imager201having a single non-pixelated detector. Chassis202carries illuminator assembly104. Illuminator assembly104comprises an emitter203that emits raw beam204. Emitter203may be a laser diode such as a monochrome laser diode with peak emission at approximately 635 to 670 nm. Raw beam204is shaped by beam optics206, which may for instance comprise a collimating lens and an aperture, to produce first beam106. First beam106is reflected by scanning mirror108, here shown as deflecting beam106in two axes, as indicated by rotation angles208, to produce two dimensionally scanned beam110. Two instantaneous positions of scanned beam110are shown as beam110aand110b. Instantaneous reflected or scattered beam114, is collected by optional collection optic210which focuses scattered beam114onto detector116, here shown as a photodiode.

Return beam114is shown as having width. This is indicative of the gathering effect of optional collection optic210, which serves to increase the subtended angle over which the scattered light may be collected, increasing the numerical aperture and therefore the intensity of the collected signal. It may thus be seen that detector116is non-imaging in this embodiment. That is, detector116is a staring detector that simply collects all light scattered from the FOV. To improve the signal-to-noise ratio (SNR), it is often advantageous for collection optic210to include a filter to exclude wavelengths not scanned, fluoresced, or otherwise indicative of FOV response to scanned beam110.

As an alternative or in addition to a staring detector116, scanned beam imager201may use confocal or retrocollective collection of return beam114. Confocal and retrocollective collection schemas may de-scan the return signal with the scanning mirror or a synchronized scanning mirror, thus using spatial filtering to ensure the return signal is dominated as much as possible by the scanning spot. Confocal systems detect through an aperture that is arranged confocally to the beam source, thus detecting over a reduced DOF for maximum resolution. Retrocollective systems collect light from around and on the scanning spot, resulting in a balance between maximum signal isolation and maximum DOF.

FIG. 3is an electrical block diagram showing one embodiment, drawing particular attention to the distribution of controller118into microprocessor118aand memory118bblocks connected to one another and to emitter104, scanner108, and detector116by buss302. Interface block120may for instance include wired and wireless data interfaces; visible indicators, audio indicators, tactile indicators, and/or displays; input means such as temperature sensors, ambient light sensors, trigger, orientation and/or location sensors, remote memory or storage, a keyboard, mouse, microphone, and/or other devices for communicating operatively relevant information.

Optional remote unit304may be connected to interface120by wired or wireless means including a networked or Internet connection. The nature of remote unit304may be determined based on application requirement. For example, a remote expert comprising artificial intelligence and/or human intelligence may be useful when the embodiment involves image analysis or abstract decision-making. A remote decoder may be used to advantage in bar code or OCR reading applications. One embodiment particularly relevant to the present invention is the remote unit including a sortation system comprising diverters and other material handling equipment. Multiple imagers may be networked to one or a plurality of remote units.

FIG. 4is a block diagram of an embodiment that uses a synchronous illuminator and detector, according to an embodiment. Timer-controller401controls the synchronization of the illuminator(s)104and detector(s)116.

Embodiments related toFIG. 4pulse the illuminator. The detector is then “tuned” to the pulse rate of the illuminator. Timer-controller401comprises an RF source402that may be controlled by controller118. RF source402modulates the illuminator104, which outputs a modulated beam106that is deflected by scanner108to produce scanned beam110. In some embodiments, illuminator104is a red laser diode of the type typically used in bar code scanners, for example a red laser diode having a wavelength between 635 and 670 nanometers with a rated power output of 10 to 30 milliwatts.

Scanner108may be one or a combination of several types of scanners if capable of producing an appropriate scan rate. In some embodiments, scanner108is a MEMS mirror.

Scanned beam110scans FOV111and is reflected or scattered back as reflected beam114to synchronous detector116. Synchronous detector116is tuned to detect the pulse modulation frequency of illuminator104. Light may be collected at detector116by collection optics (not shown). Collection may be made retrocollectively, wherein the beam is de-scanned by the scanning mirror108, or may be made via staring optics. The staring optics may use reflectors, lenses, filters, vignetting structures, or combinations thereof. Non-imaging collection optics are described in the book entitled “High Collection Nonimaging Optics” by W. T. Welford and R. Winston, 1989, Academic Press, incorporated herein by reference.

One way to tune a detector to a pulse modulation frequency is to use lock-in amplifier406, which amplifies a signal at one or more particular frequencies. Lock-in amplifier406may include circuitry to convert the detected modulated signal to base band or, alternatively, may pass a modulated signal to the controller. The controller converts the signal into an image and performs other necessary functions appropriate for the application.

Lock-in amplifier406is of the type that is sensitive to one or more particular frequencies. The frequency or frequency range to which the lock-in amplifier is sensitive may be pre-determined or the RF source402may optionally be fed back to the lock-in amplifier. The RF source may be fed back to the lock-in amplifier via optional delay line408. Optional delay line408may optionally be a variable delay line controlled by controller118. Lock-in amplifier406may be of a design that locks into the frequency of the illuminator pulse modulation and follows it through any variation. One type of such design is a phase-locked loop, which may be implemented as a heterodyne design.

Optional modulator404may apply modulation to the RF signal generated by RF source402, thus driving illuminator104with a modulated RF signal. Some or all of RF source401, optional modulator404, illuminator104, scanner108, synchronous detector116, lock-in amplifier406, optional delay line408, and optional interface120may be under control of controller118, for example via control lines302.

The RF drive signal produced by timer-controller401effectively produces a carrier frequency that tunes illuminator104and synchronous detector116to each other and helps to reject ambient noise and provide other benefits, some of which are described herein.

Scanned beam imagers often have data rates on the order of 20 MHz. One way to operate a synchronous detector with a scanned beam imager is to pulse the beam at a frequency that is high compared to the data rate. For instance, the beam may be modulated at a rate of 20 to 200 times the data rate, resulting in a pulse rate of 400 MHz to 4 GHz. Such high pulse rates can be a challenge for detectors, however, often resulting in significant photon shot noise as well as practical design difficulties. In some embodiments, the pulse rate may be run at a small multiple of data rate, for example at 1 to 10 times the data rate, resulting in a more manageable pulse rate of 20 to 200 MHz.

The device ofFIG. 4may operate at a pre-determined pulse frequency. It may desirable, particularly in low frequency multiple embodiments, to maintain a constant phase relationship between pixel clocking and synchronous pulse modulation in order to ensure an equal number of pulse modulation cycles. However, preferred resonant scanning technologies do not have constant rotational velocities.

For resonant scanning systems, constant frequency pulse modulation may be used with constant pixel clock rate and variable pixel spacing. In this mode, it may be desirable to apply image processing to interpolate between actual sample locations to produce a constant pitch output. In this case, the addressability limit is set at the highest velocity point in the scan as the beam crosses the center of the FOV. More peripheral areas at each end of the scan where the scan beam is moving slower are over-sampled. In general, linear interpolation, applied two-dimensionally where appropriate, has been found to yield good image quality and have a relatively modest processing requirement. U.S. Provisional patent application Ser. No. 10/441,916 filed May 19, 2003 and entitled APPARATUS AND METHOD FOR BI-DIRECTIONALLY SWEEPING AN IMAGE BEAM IN THE VERTICAL DIMENSION AND RELATED APPARATI AND METHODS, incorporated herein by reference, teaches methods of interpolating pixel values, particularly with respect to bi-sinusoidal scanning.

Alternatively, constant pixel spacing may be maintained by varying pixel clocking and synchronous pulse modulation frequency. Methods and apparatus for varying pixel clocking across a FOV are described in U.S. patent application Ser. No. 10/118,861 entitled ELECTRONICALLY SCANNED BEAM DISPLAY, incorporated herein by reference. By using a clock divider (for frequency ratios greater than 1:1), one may use the apparatus disclosed therein to also control pulse modulation frequency synchronously with pixel clocking.

Varying the pulse modulation frequency sinusoidally produces a chirp that may be useful for further improving noise immunity. In effect, this creates frequency diversification that acts in a manner similar to spread spectrum radio systems. This may be particularly useful when two or more of the systems ofFIG. 4are used in proximity to one another.

Pulse modulation frequency diversification may also or alternatively be implemented by varying the ratio of modulation frequency to pixel frequency. This may be done on a frame-by-frame, line-by-line, or even a pixel-by-pixel basis. This type of modulation frequency diversification is particularly akin to frequency hopping spread spectrum radio systems. A programmable clock divider may be used to set the frequency ratio.

FIG. 5shows an electronic imager102in which a first light source104projects a first light beam106toward a scanner108, according to an embodiment. Additionally, a second light source104′ projects a second light beam106′ toward the scanner108.

Scanner108receives the first light beam106and the second light beam106′ and redirects them in respective first and second scanned beams110and110′. First and second scanned beams110and110′ are scanned in first and second scan patterns502and502′, respectively, across combined FOV111. In some applications, scanned beam imager102is oriented such that the scan patterns502and502′ illuminate a FOV111comprising all or a portion of an outer surface of a target object504. While the patterns502and502′ shown inFIG. 5are raster patterns, it will be appreciated by those of skill in the art that the light beams can be redirected in other scan patterns including linear, circular, vector, and other patterns.

The outer surface of the target object504bears a machine readable indicia506that may be one or more conventional linear or one-dimensional (1D) bar code symbols such as CODE 30, Code 128, UPC/EAN, Interleaved 2/5, etc.; one or more two dimensional (2D) bar code symbols including stacked 2D symbols such as PDF-417, Code 49, Code 16K, or Codablock, for example, and 2D matrix symbols such as Data Matrix, Code One, MaxiCode, QR Code, etc; composite symbols such as those in the RSS family for example; OCR; bumpy bar code; a laser card, or others. Typically, such symbols have regions of differing optical properties that are patterned according to information to be represented. While the embodiment described with respect toFIG. 5depicts a 1D or 2D symbol, the concepts herein are not limited to such symbols. Instead, many of the concepts described herein can apply to capturing other types of images.

Returning to the structure ofFIG. 5, the symbol506reflects a portion of the light from the first and second light beams110and110′, depending upon the respective reflectivities of the regions struck by the beams. As represented by beams114and114′, portions of the reflected light strike a pair of detectors116and116′.

The amount of reflected light incident upon the detectors is dependent upon several factors including wavelength, scanner position, detector position, any associated gathering optics, and the reflectivity of the symbol506. Responsive to the light114and114′, each of the detectors116and116′ produce a respective electrical signal508and508′. Detectors116and116′ may be conventional electronic devices, such as a PIN photodiode, avalanche photodiode, photomultiplier tube, or a CCD.

A controller118receives the signals508and508′ and converts the received signals into a digital image of the symbol506, i.e., the areas of the symbol that reflect the scanned light beams110and110′ onto the detectors116and116′. The controller or another component such as a digital signal/image processor employs the digital representation of the received signals to identify information represented by the symbol506. For example, the controller may identify the target object504or may determine characteristics such as a shipping date, destination, or other information. Alternatively, the identified information may not pertain directly to the target object504. For example, where the target object504is an identification card, the symbol may provide information about the holder.

The first and second light sources104and104′ may be oriented such that each of the light beams106and106′ converge upon the scanner108along slightly different angles or vectors. In one embodiment, first and second light sources104and104′ are wavelength division multiplexed (WDM). Scanning beams110and110′ then diverge as they travel away from the scanner108to strike the target object504at slightly offset locations. As scanner108sweeps beams110and110′ onto symbol506, their respective scan patterns are offset (interlaced). The interlacing of the two scan patterns can increase the resolution of the digital image by sampling the reflectivity of two locations simultaneously.

While the embodiment ofFIG. 5includes two light sources, the invention is not so limited. Some configurations may employ three or more light source/detector pairs in a “multi-line” reading approach, as will be described below with reference toFIGS. 6 and 7. For a given scan angle and frequency, the inclusion of additional light sources and detectors can further increase the resolution of the image. Also, while the above-described embodiment incorporates a single scanner, structures with more than one scanner are within the scope of the invention. For example, the light beams106and106′ could be directed toward separate scanners having reflective components that oscillate at the same frequency or at different frequencies.

FIG. 6is a block diagram of an alternative embodiment of an electronic imager. The outer surface of target object504bears a reflective symbol506. Symbol506reflects a portion of the light from light beams110, depending upon the respective reflectivities of the regions struck by beams110. As represented by arrows114,114′, and114″, portions of the reflected light strike detector116.

Responsive to light114, detector116produces a respective electrical signal508. In one embodiment, detector116uses synchronous demodulation to distinguish the reflected light. A detector that can discriminate between the reflected light may be used. For example, the detector may be a conventional electronic device, such as a photodiode or a CCD configured to discriminate the reflected light.

Light sources104are oriented such that each of light beams106converge upon scanner108along slightly different angles or vectors. According to one embodiment, light sources104are frequency modulated to provide separable frequency division multiplexed signals, or FDM signals. Alternatively, the light sources are time-sliced modulated to produce TDM signals. The reflected beams110then diverge as they travel away from the scanner108to strike the target object504. In one embodiment, the reflected beams110are slightly offset from each other. In another embodiment, the reflected beams110are not offset. As the scanner108sweeps the beams110onto the symbol506, their respective scan patterns are interlaced. The interlacing of the scan patterns can increase the resolution of the digital representation of the symbol by sampling the reflectivity of two locations simultaneously.

FIG. 7is a block diagram of another alternative embodiment of a scanned beam imager102having a plurality of light sources. Each of the “n” light sources104projects a beam of light106towards at least one scanner108. As with the previously described embodiment, scanner108may utilize oscillating reflective components rotating about two orthogonal axes, although other types of scanners may be utilized. The scanner108redirects the “n” light beams106towards a target object504such that the “n” light beams scan “n” patterns702,702′, etc. simultaneously on a target object504in accordance with the oscillations and disposition of the reflective component of the scanner.

The target object504includes a symbol506with areas of differing reflectivity. The “n” detectors116are oriented to receive at least a portion of the light114reflected from the symbol506. The amount of light received by each detector116may correspond to the reflectivity of the symbol areas illuminated by all or most of the respective beams110.

To permit the detectors116to discriminate between light reflected from each respective area of the target object504, each of the “n” light sources104emits light at a respective wavelength different from the other light sources. Accordingly, each of the “n” detectors116has a wavelength selectivity tuned to its corresponding light source, such that each detector116can selectively detect light from its corresponding light source.

Responsive to the light reflected from its corresponding area of symbol506, each detector116produces a signal508indicative of the reflectivity of the corresponding illuminated area. A controller118receives the signals and converts them into digital representations of the areas scanned by each respective beam. Further signal processing can then reconstruct, from the various digital representations, information represented by the symbol506.

As shown, the “n” light sources104are oriented such that each of the “n” light beams106converges at the scanner108along a respective vector. As the beams110exit the scanner108, they diverge slightly according to their respective arrival vectors. The slightly diverging beams strike the target object504substantially simultaneously and, as the scanner sweeps through its scan pattern, each of the “n” beams traces a respective scan pattern702,702′, etc. on the target object504. The “n” scan patterns702are therefore effectively interlaced and the “n” light sources104produce “n” light beams106that are scanned as “n” light beams110to illuminate separate areas of the symbol506located on target object504.

The wavelengths of different light beams projected by the light sources described herein may be visible light. Other non-visible light wavelengths, such as ultraviolet or infrared, may be used in place of or in conjunction with visible light.

To permit the detectors116to respond to their respective light sources, a respective one of “n” filters702is positioned in an optical path between the corresponding detector116and the target object504. Each filter is transmitting at a desired set of wavelengths and non-transmitting at undesired wavelengths. In this way, each of the “n” detectors can respond selectively to light of interest and ignore light of less interest. A decode module704, may be located separately or within the controller118to process the digital representations of the signals and identify information associated with the symbol506.

While the embodiment described herein uses wavelength of the light sources and detectors as a basis for discriminating between light illuminating each respective area of the target object504, other approaches may be used. For example, the light sources may be modulated at respective frequencies. The outputs of the detectors can then be synchronously demodulated to indicate the reflectivity of the respective illuminated areas. In some applications of this approach, a single detector with a sufficiently broad frequency response may receive the reflected light. Synchronous demodulation can then isolate each of the modulated components.

FIG. 8is a hardware block diagram illustrating an imager102that provides motion compensation by automatically adjusting to the movement of an object within the field of view of the imager, according to an embodiment. A light source104projects light at a scanner108that is positioned by a scanner driver802, optional second scanner driver802′, and an electronic controller118. The scanner driver802(and optionally second scanner driver802′) causes the scanner108to reflect scanned light beams from the scanner onto a symbol (not shown). Separate scanner drivers may be desirable when two scanning axes are caused by separate scanning devices, for example. Alternatively, the imager may integrate both driving functions into a single driver, or further segregate scanner driving into more than two drivers.

An image detector116receives light reflected off the reflective areas of the symbol and produces an electrical output corresponding to the reflectivity of a region being illuminated by the scanned beam. The controller118provides electrical signals that control the operation of the scanner108, scanner driver802(and optionally to second scanner driver802′, light source104, and image detector116. Additionally, the controller118may provide information regarding these components to the motion detector804. In turn, the motion detector804provides feedback information to the controller, which uses this information to adjust the operation of the scanner. The motion detector804may comprise various components including an image processor to detect change in a position of the target object from one frame to the next, manual input keys for an operator to input field-of-view velocity, a transducer to measure velocity, for example from a moving conveyor belt, an interface to receive velocity information from a device that controls velocity, such as a printing press for example, etc. Transducers may include many known velocimeters or velocity measurement devices including a an encoder wheel, a Doppler device, a magnetic or optical pickup, etc. Additionally, other technologies not listed may be used depending upon the system and application.

In another embodiment, a separate motion detector804may not be included and, instead, software could be employed to provide substantially the same functionality to detect motion of the target object and/or adjust the operation of the scanner. For some applications, it may be preferable to detect motion directly from the image. In some applications, feature locations of the image of the symbol can be repeatable and certain features of interest have, or may be assumed to have, straight lines by the invention. Also, in other applications, feature location information can be employed by the invention to determine and adjust for movement of the image of a target object.

FIG. 9illustrates an exemplary symbol that may be within the field-of-view111of an imager. Symbol506is a Data Matrix symbol that is nominally square. That is, printing specifications for the symbology provide a priori knowledge that the nominal shape of the symbol is square. The outer dimensions of the symbol are equal to one another in both the x- and y-axes and the cell size is equal in both axes.

FIG. 10illustrates an idealized scan pattern of a 2D scanner across the field-of-view111. The imager scans a beam of light along scan path702. Individual reflectance values are determined at each of pixels1002. The time for the beam to be scanned back-and-forth from the top left to the bottom right of the field-of-view, plus the time required for the beam to fly back to the starting position at the upper left is one frame time. In some applications, typical frame times may be between about 0.016 seconds (60 Hz) and 0.05 seconds (20 Hz). Other embodiments may have different frame times depending upon application and system requirements.

The beam scans horizontally across the entire field-of-view each time a line is incremented vertically. Thus, the horizontal scan may be called the fast scan and the vertical scan the slow scan. This arrangement of axes may be varied according to application and system requirements.

The example ofFIG. 10illustrates a pixelated sampling embodiment, wherein reflected light is sampled at discrete points to build a bitmap of the image. Alternatively, some applications us a continuously measured horizontal scan. One such example is the typical laser bar code scanner, which may sample substantially continuously to create a scan-reflectance-profile. It is intended that aspects of the invention relating thereto are applicable to either schema. For purposes of clarity, much of this discussion will draw examples from the pixelated sampling modality.

FIGS. 11athrough11fillustrate the distortion that can occur in the symbol ofFIG. 9when it is passed through the field-of-view of a scanned beam imager (scanning in the manner described in the example ofFIG. 10) in various directions. Specifically,FIG. 11ashows the distortion of symbol506when the object504(not shown) on which the symbol is affixed moves through the field-of-view at a relatively high velocity in a left-to-right direction during the frame period.FIG. 11bshows distortion of symbol506when the object is moving in the right-to-left direction.FIG. 11cshows distortion of symbol506when the object is moving in a bottom-to-top direction.FIG. 1dshows distortion of symbol506when the object moves in a top-to-bottom direction.FIG. 11eshows the distortion of symbol506when the object is moving directly away from the scanned beam imager andFIG. 11fshows the distortion of symbol506when the object is moving directly toward the imager.

While referred to as motion of objects in the field-of-view, it should be understood that motion is relative. The distortion ofFIGS. 11athrough11fas well as the compensation methods described herein also apply to cases where the imager is moving, the objects themselves being either at rest or in motion at a relative speed to the imager.

In some embodiments, the geometric distortion caused by movement of an object through the field-of-view may be compensated for by altering the pixel capture frequency, the vertical line spacing, or making other modifications to the scan pattern.

FIG. 12shows an idealized scan pattern corresponding toFIG. 10that has been compensated to account for the object movement ofFIG. 11a, according to an embodiment. As may be seen, the pixel timing has been altered relative to the nominal scan pattern ofFIG. 10. This causes the pixels to track along with the movement of the symbol left-to-right through the field of view. The top row of pixels1002aare shown in their nominal positions. The second row of pixels1002bhave been shifted to the right by an amount corresponding to the object velocity. In this case, the second row is a right-to-left scan so the pixel placement change may be made, for example, by adding a negative phase delay to the pixel clock. The third row of pixels1002cis shifted farther to the right. Because the third row is shown as a left-to-right scan, the required pixel shift may be accomplished, for example, by adding a positive phase delay to the pixel clock. For the case of constant velocity, the magnitude of the phase delay for the third line is double that for the second line (although it is of opposite sign). For non-steady motion, the relationship between the magnitudes of phase delay per line may vary.

The actual field-of-view captured by the imaging device may change when the scan pattern is varied as shown. For example, the left edge of the field-of-view no longer follows the vertical line of field-of-view111, but rather follows the slanted edge shown by line1202. Right-to-left movement through the field-of-view may be compensated for in an analogous manner.

In a further refinement of the technique, the amount of pixel phase delay may be varied across each horizontal scan line to account for movement during the scan.

FIG. 13illustrates a scan pattern that has been perturbed to compensate for the bottom-to-top movement of the symbol ofFIG. 11c, according to an embodiment. In this case the angular distance between horizontal scan lines may be reduced, recognizing that the bottom-to-top movement of the object through the field-of-view will provide the extra distance needed to maintain desired vertical spacing across the symbol. Scan path702scans back-and-forth down the nominal field-of-view111. The top row of pixels1002aoccupy the first line. The second row of pixels1002b, comprising the second line of the image, are scanned closer to the first row than the nominal scan pattern ofFIG. 10. Similarly, the third row of pixels1002cand other rows (not shown) are scanned at a relatively close vertical pitch. Taking the point of view of the moving reference frame of the object, this results in vertical pixel spacing on the moving image substantially equal to the vertical spacing resulting from the scan pattern ofFIG. 10when scanning a stationary image. A single frame, comprised of a specified number of rows and ending with the bottom row pixels1002z, thus has a bottom limit at line1302when viewed from the stationary reference frame of the imager. In contrast, the field-of-view projected onto the moving object has the same extent as that of the scan pattern ofFIG. 10projected onto a stationary object.

FIG. 14illustrates a scan pattern that has been modified from that ofFIG. 10to compensate for the top-to-bottom object movement through the field-of-view ofFIG. 11d, according to an embodiment. Nominal field-of-view111is defined by pixels sampled along scan path702. The top row of pixels1002amay be captured in their normal position. Compared to the nominal scan pattern, however, the second row of pixels1002b, third row of pixels1002c, etc., culminating with the bottom row of pixels1002zare captured at an expanded vertical pitch. That is, there is extra angular distance between scan lines. When projected onto an object moving from top-to-bottom through the field-of-view, this extra vertical pitch results in the scan lines “keeping up” with the motion of the object. This keeps the vertical pitch of the scan lines projected onto the moving object equal to the vertical pitch of the scan lines ofFIG. 10projected onto a stationary object. The desired number of horizontal lines may thus be maintained by scanning beyond the normal (fixed reference frame) vertical limits of the field-of-view, culminating for example at line1402. Alternatively, the vertical extent of the image could be truncated at another point, for example at a point approximately corresponding to the bottom edge of nominal field-of-view111.

FIG. 15illustrates an alternative scan pattern that has been modified from that ofFIG. 10to compensate for the top-to-bottom object movement through the field-of-view ofFIG. 11d, according to an embodiment. In this example, the frame may be imaged from bottom-to-top. Scan path702defines a first row of pixels1002a, a second row of pixels1002b, a third row of pixels1002c, etc., culminating with the last row of pixels1002z. For a stationary object, the pixel rows may be spaced at the nominal spacing ofFIG. 10, resulting in the desired projected spacing on the object. For an object moving from top-to-bottom through the field-of-view, however, it may be desirable to space the horizontal scan lines closer together vertically as indicated byFIG. 15. When the moving object provides the remainder of the nominal vertical pitch, this results in an image that is captured from the moving object at the desired vertical pitch. Because of the extra vertical distance provided by the moving object, the vertical extent of the stationary field-of-view may end with line1502coincident to the new upper edge.

FIGS. 16aand16bare side views of a scanner102scanning an object504moving at a high velocity past the scanner in the direction of arrow1602, according to embodiments. InFIG. 16a, scanned beam110is shown impinging upon a second row of pixels1002bbelow a first row of pixels1002a. The beam is scanned at a vertical deflection angle, which in this case is horizontal. InFIG. 16b, the object504has moved upward relative to the scanner102in the direction of arrow1602. At this instant, scanned beam110impinges upon a third row of pixels1002cimmediately below the second row of pixels1002b. In this case, the velocity of the object past the scanned beam imager102is sufficient that the vertical deflection angle of the scanning beam110has remained horizontal while the object has moved a distance equal to pixel pitch.

FIGS. 17aand17bare side views of a scanned beam imager102scanning an object504moving at a very high velocity past the scanner in the direction of arrow1602, according to embodiments. InFIG. 17a, scanned beam110is shown impinging upon a second row of pixels1002bbelow a first row of pixels1002a. The beam is scanned at a vertical deflection angle, which in this case is horizontal. InFIG. 17b, the object504has moved upward relative to the scanner102in the direction of arrow1602. At this instant, scanned beam110impinges upon a third row of pixels1002cimmediately below the second row of pixels1002b. In this case, the very high velocity of the object past the scanned beam imager102is sufficient that the scanning beam110has deflected upward to impinge upon pixel row1002c. That is, the object has moved a distance greater than pixel pitch and the scanning beam110has had to scan upward to impinge upon a line of pixels below the previous line.

In other embodiments elements1002a,1002b, and1002cmay represent rows of scanned elements that are not lines of adjacent pixels per se. Rather, it may be preferable to scan the surface somewhat more sparsely and element rows1002a,1002b, and1002cmay represent a plurality of linear bar code symbols or a stacked 2D bar code symbol.

FIGS. 18athrough18care isometric views of a scanned beam imager102scanning a symbol506on an object504moving past at a very high velocity on a conveyor belt1802, according to embodiments.FIG. 18ashows optional motion detector804, here depicted as a velocimeter that may be used to track belt speed. The example ofFIGS. 18athrough18cmay correspond, for example, to the example ofFIGS. 16a,16b,17aand17bwhere the vertical scan rate of the imager is used to pace the moving object. In each ofFIGS. 18a,18b, and18c, the projected line on the symbol has moved “up” the symbol while the actual scan angle of the scanning beam110has moved “down” and the object has moved “down” through the field-of-view even faster. In pacing the movement of an object, the vertical scan rate may be made somewhat faster than the speed of the object or somewhat slower than the speed of the object. In the former case, the object is scanned from top-to-bottom and in the latter case the object is scanned from bottom-to-top.

The table below illustrates the compounding effects of scanned beam vertical movement with object vertical movement for a subset of possible object velocities. The particular velocities shown are exemplary of a particular embodiment and scan range but are indicative of the principle. In this example, we will assume the maximum vertical scan rate to equal 22 inches per second (ips) at the surface to be imaged and that 22 ips is the linear vertical scan velocity required to maintain appropriate vertical pixel spacing given a constant (resonant) horizontal scan rate, horizontal scan angle, approximately SVGA resolution, and distance to object. Negative numbers indicate bottom-to-top motion and positive numbers top-to-bottom motion.

It can be seen from inspection of the table that according to the present invention it is possible to maintain the desired 22 ips vertical scan rate with object speed ranging from zero (stationary) up to 44 ips in either direction along the vertical axis of the scanner (which, as could be seen inFIGS. 18a-18cmay not be literally vertical). This range may be increased by many variables including increasing the distance to the surface, increasing the maximum vertical scan angular rate, increasing the desired or allowable pixel vertical spacing, adding multiple beam scanning, or other changes that may be appropriate for various application or system requirements.

FIG. 19illustrates a scan pattern that has been modified from that ofFIG. 10to compensate for the near-to-far object movement through the field-of-view ofFIG. 11e, according to an embodiment. The top row of pixels1002amay be captured in their normal position. Compared to the nominal scan pattern, however, the second row of pixels1002b, third row of pixels1002c, etc., culminating with the bottom row of pixels1002zare captured at a monotonically decreasing vertical pitch by varying the vertical spacing of scan path702. Horizontal placement of pixels is also squeezed. The net effect of these changes is that the geometric spacing of pixels projected onto the object is held constant even though the object is moving away from the scanner. The desired number of horizontal lines may be maintained by ending the vertical scanning early, above the nominal bottom of the field-of-view. Alternatively, the vertical extent of scan could be maintained at the nominal limits as shown in the example or another vertical extent could be chosen.

In some applications, such as the example pictured, it may be advantageous to maintain a constant maximum horizontal scan angle but simply change the pixel sampling along that axis. This may be advantageous for resonant horizontal scanners such as some MEMS scanners for example, or for scanners, such as rotating polygons where the scan angle cannot be readily adjusted.

To maintain a constant fill factor (the ratio of sampled area to pixel spacing) and therefore constant geometric resolution on the object, the beam shape may be altered synchronously with pixel spacing decreases. For example, the beam waist may be reduced in size and/or moved out with the object. Alternatively, the waist position may be chosen such that an object moving away from the scanner is impinged by a substantially constant beam diameter. This may be done by setting the waist beyond the maximum object range, such that the object moves radially along the tapering portion of the beam between the scanner and the waist. Alternatively, the beam shape may be set constant such that it has a reasonable size throughout the working range of the scanner.

FIG. 20illustrates a scan pattern that has been modified from that ofFIG. 10to compensate for the far-to-near object movement through the field-of-view ofFIG. 11f, according to an embodiment. The top row of pixels1002amay be captured in their normal position. Compared to the nominal scan pattern, however, the second row of pixels1002b, third row of pixels1002c, etc., culminating with the bottom row of pixels1002zare captured at a monotonically increasing vertical pitch. To do this, the vertical scan rate may be increased during the frame while the horizontal scan rate is held constant. Horizontal placement of pixels is also increased. The net effect of these changes is that the geometric spacing of pixels projected onto the object is held constant even though the object is moving toward the scanner. The desired number of horizontal lines may be maintained by extending the vertical scan beyond the nominal bottom of the field-of-view, as illustrated by line2002. Alternatively, the vertical extent of scan could be maintained at the nominal limits or another vertical extent could be chosen by varying the number of scan lines.

Another way to compensate for object motion through the imager's field-of-view is to maintain a substantially constant matrix of captured pixels, but to adjust the pixel spacing in software. For example, adjusted pixel values may be determined by interpolating between or combining captured pixels. The inventors have discovered that this is made practical by a scanned beam imager because the individual pixel capture time may be short enough, for example at a few nanoseconds to a few tens of nanoseconds, to avoid individual pixel blur. Motion is instead exhibited as the type of object skew illustrated byFIGS. 11athrough11f.

The motion distortion illustrated byFIGS. 11athrough11fmay be combined. That is,FIGS. 11athrough11fwere each drawn showing only a single motion vector. In some applications, motion may be so dominated by one vector that other directions of motion may be ignored. In other applications, it may be advantageous to simultaneously compensate for motion in two or more axes. The motion compensation techniques shown inFIGS. 12 through 20or additional techniques not shown may be combined to account for multi-axis motion.

FIG. 21is a flowchart illustrating an overview of the operation of an imager with motion compensation, according to an embodiment. At a decision operation2102, the logic flow determines if the object is moving relative to the scanner's field-of-view. In one embodiment, the invention will determine the object is moving if the object follows any one of the patterns illustrated inFIGS. 11athrough11f. In other embodiments, a motion detector (such as motion detector804ofFIG. 8) may inform the controller that the object is moving. In other embodiments, the imager may be informed from an outside source, such as a conveyor belt controller or printer controller, for example through an interface (such as interface120ofFIG. 1and elsewhere).

When the determination at the decision operation2102is true, the logic flow advances to an operation2104where the rate of movement of the object is determined. The rate of movement may be determined in many different ways including: (1) comparing the positions of the target object in two images taken at two distinct times; (2) determining the displacement of the target object between the two images; and (3) calculating the rate of movement in accordance with the amount of displacement and the amount of time between the images. In other embodiments, the motion detector may provide the rate of movement information. In still other embodiments, rate of movement information may be provided manually or through an interface.

After moving to an operation2106, the imager compensates for the determined movement of the target object. For example, the imager may adjust the operation of the scanner by changing the scanner's oscillation frequency, by changing the pixel capture phase or frequency, by changing the vertical scan rate, by changing the pixel map, or other methods.FIGS. 12 through 20illustrate several compensation techniques. The imager may also compensate for the movement of the target object in software. After the movement of the object is motion compensated for by the imager, the logic flow moves to an operation2108where the target object is scanned by the imager. Alternatively, when the decision operation2102determines that the target object is not moving, the logic flow may jump to the operation2108and scan the target object. Moving from operation2108, the logic flow returns to executing other operations.

In practice it may be desirable to combine two or more of the steps ofFIG. 21. For example, when object movement rate is provided by an encoder or is pre-set, the determination of whether the object is moving and its movement velocity takes place substantially simultaneously. When set for objects of substantially constant movement velocity, such as in a conveyor scanning application for example, the decision step2102may be eliminated and the scan always synchronized to the set or determined velocity. In other cases, the compensation and object scanning steps2106and2108may be performed substantially simultaneously.

FIG. 22is a flowchart that illustrates adjusting the scan rate (oscillation frequency) of an imager's scanner in response to determining that a target object is moving, according to an embodiment. A decision operation2202determines the direction of movement of the target object. The decision operation2204determines when the target object is moving vertically. If true, the logic flow moves to the operation2206where the vertical scan rate of the imager's scanner is adjusted to compensate for the determined vertical movement of the target object.

The vertical scan rate is adjusted when the target object is determined to be moving in either an upward or downward direction. If the target object is moving upward, the downward vertical scan rate of the scanner may be decreased or the upward vertical scan rate increased. Similarly, when the target object is determined to be moving downward, the downward vertical scan rate may be increased or the upward vertical scan rate decreased.

The logic flow moves to an operation2208where the imager may further compensate for the moving target object. In some embodiments, operation2208may be inherent in the vertical scan rate adjustment2206. In other embodiments, operation2208may be used in place of or in conjunction with vertical scan rate adjustment2206. For example, image processing and signal processing software could be used to compensate for the determined movement of the image of the target object. Various algorithms for object recognition, to determine orientation of objects, and to determine speed of the object may be used.

When the decision operation2204determines a vertical image movement did not occur or the vertical scan rate was adjusted at the operation2206and/or optionally further compensated at the operation2208, the logic flow will advance to a decision operation2210and determine if the target object is moving in a horizontal direction. If so, the logic flow moves to an operation2212and the horizontal pixel phase of the scanner is adjusted.

When it is determined that the target object is moving in a left-to-right horizontal direction, the pixel sampling method shown ifFIG. 12may be used. For right-to-left horizontal movement, the method may be reversed, instead sampling pixels from successive rows slightly to the left of the corresponding pixel in the row above. As described forFIG. 12, this may result in positive or negative phase delays depending upon scan direction. Horizontal movement in either direction may analogously compensated for with an upward vertical scan.

When pixel phase has been adjusted, or instead of pixel phase adjustment, the logic flow advances to an operation2214where the imager may further compensate for the moving target object with image processing software.

From the operation2214, the logic flow steps to an operation2108where the target object is scanned by the imager and images of the object are processed. Also, when the determinations at either of the decision operations2204and2210are false, the logic flow will jump to the operation2108and perform substantially the same operations discussed above. Next, the logic flow returns to executing other operations.

When physical adjustments (phase, rate, etc.) are made to the scan pattern and the object scanning step2108is subsequently performed, optional processing steps2208and2214may by performed thereafter rather than in the order depicted by the flow chart ofFIG. 22. Conversely, for applications where motion compensation is performed in software and scanner physical parameters (phase, rate, etc.) are not adjusted, the system may jump directly to step2108with image processing steps2208and/or2214performed thereafter.

In other embodiments where object motion is known or may be predicted in advance, the flow chart ofFIG. 22may represent an initiation routine for the system. In that case, the scanner may be operated according to the determined parameters with minimal or no further adjustment of the parameters on a scan-by-scan basis. For such pre-set applications (examples include constant speed conveyor belt or printing press or moving motor vehicle scanning), it may be desirable to maintain a statistical matching of scanning parameters with residual image distortion. Such a system may be used to compensate for operational variations in the speed of the objects being scanned. For instance, two compressed top-to-bottom vertical scans in a row may indicate a change in conveyor speed that necessitates an adjustment in vertical scan rate.

While the cases for motion toward or away from the imager are not shown directly inFIG. 22, they may be compensated for similarly using the sampling methodologies shown inFIGS. 19 and 20, or in combination or alternative, using software compensation.

FIG. 23is a flowchart illustrating use of inherent knowledge of a reflective symbol in a scanned target object to determine motion in the image of the object, according to an embodiment. This embodiment may be particularly useful when the imager is intended to scan target objects with known geometries such as 1D or 2D bar codes, OCR, etc.

When the logic flow advances to an operation2108, a reflective symbol on a target object is scanned and an image of the symbol is saved. A decision operation2302determines if the scanned image of the symbol on the target object is skewed. Typically, the image of the symbol will be skewed if it moved significantly during the scan of the target object. When the inherent knowledge of this type of symbol indicates a square shape and the scanned image of the symbol includes a rectangular shape, the invention will determine that the target object moved during scanning. Any one of several image processing techniques can be used to determine if the image shape of the symbol is incorrect, i.e., skewed.

When the decision operation2302determines that the image of the symbol is skewed, the inherent knowledge of the symbol structure is employed to determine the direction(s) and/or speed of the motion at an operation2304. Various transforms may be applied to the image of the symbol to transform it back to its known shape. At an operation2106, the particular transformation used to convert the “skewed” shape of the image of the symbol back to its known shape is employed to compensate for the motion of the target object through the field-of-view. The skewed image may be also compensated for by physically adjusting the scanning rates of the scanner or with other image processing software.

The logic flow moves to an operation2306and the image of the symbol is decoded. Also, when the decision operation2302is false (image not skewed), the logic flow jumps to the operation2306. Next, the logic flow returns to executing other operations.

When the motion of a decodable object such as a 2D symbol is known or may be determined, knowledge of that motion may be used to modify decode parameters and thereby enhance decoding performance. For example, linear (1D) bar code decoders frequently have band-pass filters to aid in rejecting noise. An upper band-pass filter (a low pass filter) may be used to determine the minimum x-dimension and maximum range at which the unit will decode. A lower band-pass filter (a high pass filter) may be used to determine the maximum element size that will be fed into the decoder. Knowledge of object motion may be used to modify the pass-bands, for example increasing the frequency of the upper pass-band and/or decreasing the frequency of the lower pass band to compensate for the changed apparent x-dimension of an object moving rapidly in a horizontal direction. Similarly, 2D matrix symbols are characterized by a cell size that is nominally equal in both the x- and y-dimensions. A moving symbol may exhibit a cell size that is apparently different in the two axes. Thus, one way to compensate for motion of 2D symbols is to allow for differing cell sizes in each axis during decode.

Although the embodiments according to the invention described herein are described above as including one scanner, it is envisioned that a plurality scanners may be used with this invention (not shown). One or more light beams could be projected at the plurality of scanners that would reflect the light onto a target object in scan patterns and at frequencies that may or may not be substantially similar.

The preceding overview of the invention, brief description of the drawings, and detailed description describe exemplary embodiments of the present invention in a manner intended to foster ease of understanding by the reader. Other structures, methods, and equivalents may be within the scope of the invention. As such, the scope of the invention described herein shall be limited only by the claims.