WO2001029576A1 - Rangefinder using collected spot spread and insert shadowing - Google Patents

Abstract

A rangefinder comprises a light collection system including a lens or mirror. Two or more photodetectors are positioned at a detector plane, with one of the photodetectors aligned with the optical axis, and the other photodetector (if one) adjacent thereto or (if several) arranged in a linear sequence emanating outward from the center photodetector. A light-blocking element, which may be a folding mirror for targeting an outgoing laser beam, is positioned along the optical axis so as to block light from reaching the center photodetector under certain conditions dependent upon the target range. When the target is distant, all or most of the collected light is focused on the center photodetector, and as the target approaches the lens or mirror the spot spread increases, and more light is focused on the side detector(s) and less on the center photodetector.

Description

- P_ E_ C_ -L F_ I_^ C_ A. T_ ^ O. N_

TITLE OF THE INVENTION

RANGEFINDER USING COLLECTED SPOT SPREAD AND INSERT SHADOWING

BACKGROUND OF THE INVENTION

The field of the present invention relates to rangefinding systems and methods that are particularly well suited for use in data reading devices such as optical scanning systems, and to data reading devices that incorporate such rangefinding systems.

In applications such as optical reading, it is convenient for the optical reader or device to have knowledge of the distance (i.e., range) to the target to be read. Optical readers that may benefit from knowing the range of the target include conventional flying- spot barcode readers or conventional imaging readers, such as optical readers using a CCD imaging device.

Conventional flying-spot barcode readers operate by scanning a beam of light across a target barcode, detecting reflections from the barcode using a photodetector, and processing and decoding the photodetector output to determine the value of the barcode .

When all of the target barcodes to be read are located at about the same distance from the scanner, the detected reflections from each barcode will have similar characteristics, and the processing and decoding of the photodetector output can be accomplished in a consistent manner. But when the distances to the target barcodes vary, the characteristics of the detected reflections can vary dramatically from one barcode to the next, and it becomes

LA-1588 7.1 more difficult to process and decode the photodetector output .

One characteristic that changes as a function of the range to the target is the amount of light power collected by the photodetector, which (for any given target) is inversely proportional to the target range, as shown m FIG. 1A. For an optical reader to be effective for reading targets at both short range and long range, the signal processing circuitry is generally required to handle a large dynamic range (due to the variation experienced m the amount of light collected at the photodetector) , which can be difficult or expensive to achieve. In some applications, an automatic gain control (AGC) circuit is employed m order to reduce the dynamic range to the signal processor. However, AGC circuits typically have a long settling time, which reduces the system performance by requiring additional scans of data.

Another characteristic that depends on the target range is known as "speckle noise." Speckle noise is caused by the interaction of the coherent light of the laser spot with the target barcode, which is typically printed on paper having minute surface or textural variations. The minute surface variations m the target barcode cause undesirable constructive and/or destructive interference m the light returned from the barcode, which manifests as low-level noise. FIG. IB shows how the magnitude of the speckle noise varies as a function of target range, with a maximum occurring when the target is at the laser waist (i.e., the distance where the laser spot is the smallest) . When the magnitude of the speckle noise is not known m advance, it is difficult to adjust the thresholds m the signal processor to reject the speckle noise: if the noise reduction threshold is too low, the speckle noise from a barcode located at the laser waist may be misinterpreted as an actual signal; and if the noise reduction threshold is too high, valid signals from distant barcodes will be squelched, which would reduce the operating range of the scanner.

Yet another characteristic that depends on target range is optical blurring. In a flying (or moving) spot scanner, the returned signal comprises the convolution of the laser spot with the barcode. When the laser spot is small with respect to the bars being scanned, the barcode is rendered with high fidelity, creating a waveform with sharp edges resembling a square wave. When laser spot is large with respect to the bars being scanned, the barcode waveform is rendered with low fidelity, experiencing convolution distortion - commonly known as blurring. This phenomenon is described m more detail m U.S. Patent No. 5,371,361 (Arends et al . ) , which is hereby incorporated by reference as if set forth fully herein. One way to quantify the blurring effect is to measure the time domain impulse response width of the optomechanical system - that is, the response of the system when it encounters a thin black line on a white field. When the time domain impulse response is converted to the frequency domain, the resulting frequency response will resemble a low-pass filter.

The effects of blurring can be minimized by a process called "equalization" , m which the blurred signal is processed by an equalization filter to compensate for the blurring. Thus, if the blurring function's frequency domain transfer function is F(s), the preferred transfer function for the equalization filter would be 1/F(s) . Typically, the equalization filter will boost the higher frequency components of the applied signal to compensate for the low- pass effects of the blurring.

Unfortunately, the blurring function is strongly dependent on range, as shown m FIG. IC (which shows how the width of the impulse response varies as a function of target range) . When the actual target range is sufficiently far from the optimum equalization distance, performing equalization will actually corrupt the blurred signal instead of correcting it.

Another characteristic affected by target range is spot speed, which causes the apparent size of barcode elements to vary depending upon the target range. In a flying spot laser scanner, the flying spot tends to sweep faster across a distant target than a closer target. As a result, barcode elements tend to appear larger when the target is closer to the scanner, and smaller when the target is farther from the scanner. FIG. ID is a plot of spot speed versus range, illustrating the linear increase m spot speed as the target range is increased. (The range is determined with respect to distance from the facet wheel . ) Rapid spot speeds at the larger target ranges tend to produce narrow element widths that require wide bandwidth signal processing and decoding. Moreover, on curved or filtered objects, barcode elements that are the same size may be rendered as different widths by the scanning system due to spot speed variation with range. These phenomena may prevent successful decoding of the barcode, or impose burdens on the design of the signal processing and decoding circuitry. Improved performance can be obtained by determining the range to the target and compensating for the range-variant parameters, such as those described above. Typically, devices which attempt to compensate for variations m target range rely upon a stand-alone distance measuring system to obtain the target range.

One proposed solution which attempts to compensate for a particular problem caused by varying target range (specifically, the blurring problem described above) is set forth m published PCT application No. WO 96/42064, which is incorporated herein by reference. That PCT application describes a technique for using a stand-alone radar system to measure the distance to the barcode and adjusting the signal processing to compensate for the expected distortion. However, a stand-alone radar system is too expensive and impractical for many applications, especially cost-sensitive products such as small, handheld barcode readers. Other prior art techniques involve measuring the intensity or other characteristics of the light signal returning from the target, such as by, for example, measuring the height and/or amplitude of a differentiated photodetector input signal, m order to estimate the range of the target. However, a problem with these techniques is that many variables affect the intensity and characteristics of the light signal returning from the target, resulting m accuracy problems .

Yet other devices that have been used for determining the range of targets are positional detectors or lateral effect detectors, which generally rely on parallax effects. Such devices detect the angle between the outgoing laser or light beam and the light returned from the target by detecting the center of the spot from the returned light. When the target is at far field, the angle between the outgoing beam and the return light path is very small, and so the center of the spot is close to the optical axis. As the target approaches near field, the angle between the outgoing beam and the return light path becomes larger, the returned light hits the lens off-angle, and the center of the spot strays from the center of the optical axis. A positional sensitive detector detects the center of the spot as being offset from the optical axis. However, lateral effect detectors may experience difficulty when the target is at near field as spot spread increases, making it difficult to detect the precise center of the spot. Further, lateral effect detectors are generally standalone devices, requiring circuitry separate from that used m the rest of the device to perform a rangefinding function, thus increasing the cost and components used m the device.

A variation of the lateral effect detector also relies on the parallax effect to determine a target range. Rather than using a positional sensitive detector, however, the device uses an array of detectors. However, these devices suffer from the same drawbacks as lateral effect detectors - that is, they suffer from potential inaccuracy when targets are at near-field due to spot spread making it difficult to pinpoint the center of the spot .

It would therefore be advantageous to provide a range finding system and/or method that does not require a standalone distance measuring system, that is inexpensive, accurate and easy to manufacture, that does not require AGC circuitry, and that is can handle a wide variation of target ranges without placing undue burdens on the dynamic range or bandwidth of the signal processing circuitry.

SUMMARY OF THE INVENTION

The present invention relates to rangefinding systems, data reading systems incorporating such rangefinding WO 01/29576 _n PCT/USOO/27344

systems, and to corresponding methods, wherein the range to a target is generally determined based on the intensity of light arriving at different measuring points at or near a detector plane. In a preferred embodiment, a rangefinding and data reading system and method uses two photodetectors to detect light arriving at a detector plane. The two photodetectors are preferably (but not necessarily) placed side-by-side, with one of the photodetectors being positioned along the center of the focal axis of a lens or light collection mirror. The lens or mirror focuses light on the detector plane, and the spread of the light on the detector plane varies with the range of the target. The rangefmder determines the range to the target by comparing the amount of light arriving at the two photodetectors, as the ratio of the amount of light received at the two photodetectors varies m proportion to the distance of the target due to the phenomenon of spot spread. Thus, when the target is located at certain distances (e.g., relatively far distances) , more light is received at one of the two photodetectors, and when the target is located at other distances (e.g., relatively close distances), more light is received at the other one of the two photodetectors. By comparing the amount of light received at the two photodetectors, the distance to the target can be determined.

In a variation of the above embodiment, a light - blocking element or other obstruction is positioned so as to shadow one of the two photodetectors, preferably the centrally positioned photodetector. The light-blocking element or other obstruction accentuates the effect of spot spread m that it substantially reduces the amount of light detected at the centrally positioned photodetector when the target is at certain distances (e.g., relatively close distances) . The effect is to provide a greater variation m the intensity of collected light from one extreme of target range to the other and, hence, greater resolution for the rangefmder. Further, the light-blocking element reduces parallax effects by providing an outgoing laser beam that is aligned with the optical axis of the light collection optics . In a preferred embodiment, the centrally positioned photodetector used for rangefinding is also used to provide a photodetector signal for edge detection and/or decoding. The photodetector thereby serves multiple functions.

In other embodiments, more than two photodetectors are utilized, and may be positioned (for example) m a linear array. The rangefmder may determine the distance to the target by comparing the relative intensity of light at the various photodetectors (i.e., measuring points), expanding upon the same principles of the effect of spot spread described above. In yet other embodiments, the location of the detector plane can be altered, thereby essentially reversing the effect of spot spread. In such embodiments, the relative intensity of light at the two or more photodetectors is again compared, but m opposite fashion, to determine the distance to the target.

When the rangefmder is incorporated an optical scanning system, parameters of the scanning system can be adjusted to compensate for range-dependent characteristics of the incoming signals. Further variations, enhancements and modifications are also described herein. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a graph that shows how the power collected by the photodetector varies with range m conventional barcode scanning systems . FIG. IB is a graph that shows how the speckle noise varies with range conventional barcode scanning systems.

FIG. IC is a graph that shows how the impulse response width varies with range conventional barcode scanning systems . FIG. ID is a graph illustrating the linear increase m spot speed of a laser spot swept across a target as the range to the target is increased.

FIG. 2 is a functional block diagram of a scanning system m accordance with a preferred embodiment . FIGS. 3A, 3B, and 3C are optical schematic diagrams of a preferred embodiment rangefmder for distant, intermediate, and close targets, respectively.

FIGS. 4A - 4D are diagrams illustrating the amount of collected light incident upon a pair of adjacent photodetectors m a system m accordance with a preferred embodiment for targets located at various distances.

FIG. 5 s a schematic block diagram of a rangefmder m accordance with a preferred embodiment .

FIG. 6 is a graph that shows how the voltage at two nodes m the circuit of FIG. 5 changes with respect to distance .

FIG. 7 is a graph that shows how the range output of the circuit of FIG. 5 changes with respect to distance and how it is affected by the color of the target . FIG. 8 is a graph that shows how the distance between the lens and the detector affects the rangefmder output. FIGS. 9A and 9B - 9C are diagrams of alternative embodiments using a collection mirror m place of a lens, and FIG. 9D is a diagram of a collection mirror with insert as may be used m the embodiments of FIGS. 9A and 9B - 9C. FIGS. 10A and 10B are graphs comparing the percentage of collected light attributable to the center detector of a two-detector rangefmder system, with and without a light - blocking element, respectively.

FIG. 11 is a diagram of a center/surround detector as may be used m various rangefmder embodiments described herein.

FIGS. 12A and 12B are diagrams of an alternative embodiment of a rangefmder, illustrating operation at near- field and far-field targets, respectively. FIG. 13 is a diagram of another alternative embodiment of a rangefmder m accordance with certain inventive techniques as described herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 is a block diagram of a preferred embodiment flying- spot scanning system. The scanner of this embodiment detects the information encoded m the target barcode, symbol, or other indicia by generating a beam of light, scanning that beam across the target, detecting the light reflected by the target using a photodetector, and processing and decoding the output of the photodetector.

For the sake of simplicity, this description assumes that the target is a one-dimensional barcode. It should be noted, however, that the preferred embodiments described herein may also be used to read other types of symbols or indicia . WO 01/29576 _{1 ±} PCT/USOO/27344

Preferably, the light source that generates the beam of light for scanning the target comprises a laser diode 121. Alternative lights sources, including but not limited to, a He-Ne laser, other types of lasers, and focused beams of non-laser light, may be used m place of the laser diode 121.

A laser driver 120 controls whether the light beam generated by the laser diode 121 is on or off, and optionally controls the brightness or intensity of the beam. Beam intensity control may be accomplished by controlling the current supplied to the laser diode 121, such as with a switchable current source, m a fashion well known to those skilled m the art. Preferably, the laser diode 121 is modulated m order to permit improved subsequent processing of the photodiode output signal (s) and, more particularly, to allow subtraction of the ambient light from the photodetector output signal (s) , thus providing more accurate measurement of the signal levels at each photodetector. Such techniques are described, for example, U.S. Patent No. 4,694,182 issued to P. Guy Howard on September 15, 1987, and entitled "Handheld Bar Code Reader with Modulated Laser Diode and Detector, " which is hereby incorporated by reference as if set forth fully herein.

Scanning of the beam is accomplished by a beam deflector 130 which may comprise a light-deflectmg element such as a rotating polygon having mirror facets, a motor, and a motor driver, all arranged m a conventional manner. Alternatively, an oscillating mirror and an oscillating beam dither driver may be used to move the scanning beam back and forth along a line-shaped path over the target barcode. Other alternative light deflector configurations include, for example, dithering the light source itself to form a scanning line and the use of holographic disks.

As the beam from the laser diode 121 scans across a target barcode (not shown) , the beam is reflected by the target . Because the bars of the barcode have lower reflectivity than the spaces between the bars, the amount of reflected light will vary depending on whether the laser spot is incident upon a bar or a space.

The reflected light is collected by appropriate collection optics, and directed to a photodetector such as the photodiode 101. The design of the collection optics and the selection of an appropriate photodetector are conventional m the field of flymg-spot scanners. The photodiode 101 converts the variations m the incident light level into an analog current signal with variations that correspond to the physical bar and space widths of the target barcode. Alternative photodetectors may be substituted for the photodiode 101, as will be apparent to persons skilled m the art. The signal from the photodiode 101 is then processed by the signal processor 110, and may also, m various embodiments as described herein, be provided to the rangefmder 140 for assisting with the determination of the distance to the target. The signal processor 110 preferably includes a transimpedance amplifier 102, a ga amplifier 103, an edge detector 104, and a noise reduction circuit 105. In a preferred signal processor configuration, the transimpedance amplifier 102 and the gam amplifier 103 amplify the signal from the photodiode 101. The edge detector 104 locates the edges of the amplified signal by, for example, detecting when the second derivative of the input signal is zero. A noise reduction circuit 105 eliminates edges attributed to noise by, for example, discarding or ignoring all edges detected whenever the first derivative of the input signal is below a threshold value. The foregoing techniques (i.e., amplification, edge detection and noise reduction) are well known m the art, and are described m various aspects, for example, m U.S. Patents No. 4,000,397 entitled "Signal Processor Method and Apparatus" issued m the name of Hebert et al . ; U.S. Patent No. 5,925,868 entitled "Method and Apparatus for Determining Transitions Between Relatively High and Low Levels m an Input Signal" issued the name of Arends et al . ; and U.S. Patent No. 5,923,023 entitled "Method and Apparatus for Detecting Transitions m an Input Signal" also issued m the name of Arends et al, all of which are hereby incorporated by reference as if set forth fully herein.

The output of the signal processor 110 is then fed into the decoder 115. Preferably, the decoder 115 measures the time between the edges of the signal arriving from the signal processor 110, which correspond to the distances between the edges m the target barcode (i.e., the widths of the bars and spaces) . Based on the widths of the detected bars and spaces, the decoder 115 determines the value of (or otherwise interprets) the target barcode, and formats that value as output data, using any of a variety of techniques that are well known m the art. Some or all of the decoder functions may be implemented m a single microcontroller or a digital signal processor (DSP) , by running appropriate software on the microcontroller or DSP. Alternatively, some or all of the width measurement, decoding, or formatting may be implemented m hardware, the form of an application- specific integrated circuit (ASIC) or other hardware circuitry.

The illustrated embodiment also includes a rangefmder 140, which measures the distance from the scanner to the target, and generates a range signal 142 that corresponds to that distance. The range signal 142 may be provided to other elements the system, including, for example, the gam amplifier 103, the noise reduction circuit 105, the laser driver 120, the deflector 130, and/or the decoder 115. The elements receiving the range signal 142 may utilize it to modify their operation, as described m more detail hereinafter .

FIGS. 3A, 3B, and 3C illustrate the optical behavior of a preferred embodiment rangefmder 300. The rangefmder 300 operates based on the principle that the collected spot size of the image on the detector plane changes (i.e., grows or shrinks) as a function of range. In the rangefmder 300, a center or region-of-focus detector 305 and an off-focus or side detector 301 are preferably located along the same plane 316, with the side detector 301 adjacent to the center detector 305. Preferably, the center detector 305 lines up with the optical axis 317 of the optical system (i.e., the lens 315) , and defines the center of the detector plane 316. The detector plane 316 is preferably positioned so as to provide the best focus on the center detector 305 for targets at far-field - that is, the detector plane 316 is preferably positioned slightly behind the focal point of the lens 315.

The side detector 301 may be immediately adjacent to the center detector 305, or else may be offset from the center detector 305, yet preferably is positioned on the same detector plane 316. In one embodiment, the two detectors 301, 305 are configured as a "center/surround" detector, with the side detector completely surrounding the center detector. Such a configuration is illustrated m FIG. 11, for example, with side detector portion 301' completely surrounding center detector portion 305'.

In operation, an outgoing light path or beam 311 is generated by a light source 321, which is preferably a red or infrared laser diode. The outgoing beam 311 bounces off a folding mirror 312, passes through the center of the lens 315, and arrives at the target 313. Additional beam shaping optics (not shown) may be included m the outgoing beam path, on either side of the lens 315. Preferably, the same beam used 311 is used for both rangefinding and scanning, and one or both detectors 301, 305 are used for both rangefinding and scanning the target, thus simplifying the design of the optics and reducing the number of components needed the device. However, alternative embodiments, different photodetectors may be used for rangefinding and scanning . Light from the outgoing beam 311 is reflected by the target 313. The reflected light 314 from the target 313 travels back toward the lens 315, and is focused by the lens 315 m the direction of the detector plane 316. The lens 315 preferably includes a central portion, referred to as the insert, which is lined up m the direction of the optical axis with the mirror 312. The lens 315 also includes a peripheral portion, referred to as the collection region, which surrounds the insert. In the embodiment illustrated m FIG. 3, incoming light arriving via the insert of the lens is blocked by the mirror 312 which, from the perspective of the incoming light, appears as a light - blocking element or an opaque object. When the target 313 is at the first extreme ranging distance, which is preferably at or near the maximum ranging distance as shown in FIG. 3A, all of the reflected light 314 that arrives via the peripheral portion of the lens 315 is focused on the center detector 305 by the lens 315. As a result, the side detector 301 receives no light.

When the target 313 is at the second extreme ranging distance, which is preferably at or near the minimum ranging distance as shown in FIG. 3C, none of the reflected light 314 reaches the center detector 305. This absence of light occurs because (a) the shadow of the mirror 312 blocks the incoming light from the central portion (i.e., insert) of the lens 315 before it reaches the center detector 305; and (b) the lens 315 does not bend the incoming light sufficiently to cause the incoming light from the peripheral portion of the lens 315 to reach the center detector 305 since the light is focused well behind the detector plane 316. On the other hand, because the images of near-range targets on the detector plane are relatively large, the reflected light 314 will reach the side detector 301.

When the target 313 is at mid-field, as shown in FIG. 3B, the image of the target at the detector plane 316 will be somewhere between the sizes show in FIGS. 3A and 3C, with the size varying as a function of the distance to the target 313. At some mid-field distances, both the center detector 305 and the side detector 301 will receive light 314 reflected from the target 313. Both detectors 301, 305 receive light at these mid-field distances because the lens 315 bends the incoming light sufficiently to cause some, but not all, of the incoming light to reach the center detector 305, but does not bend the light to such a degree that all of the collected light avoids the side detector 301. The relative intensity of the light incident upon the center detector 305 and side detector 301 will vary as the target 313 moves from far distance to near distance, with the light intensity on the center detector 305 decreasing as the target 313 moves mward towards the lens 315, and the light intensity on the side detector 301 increasing as the target 313 moves mward towards the lens 315. By examining the ratio of light intensity at the center detector 305 and the side detector 301, as further explained herein, the distance to the target 313 can be estimated.

FIGS. 4A-4D are diagrams illustrating the amount of collected light incident upon a pair of adjacent photodetectors 301, 305 m a system m accordance with a preferred embodiment as disclosed herein, for targets located at various distances. In each of FIGS. 4A-4D, the center square represents the center detector 305, the adjacent square, to the left of center, represents the side detector 301, and the light region 411 represents the image of the collected light (i.e., the spot spread) on the detector plane. The collected spot shape is a scaled version of the collection lens assembly, including the folding mirror shadow. In the instant example, a round collection lens is assumed.

In FIG. 4A, which corresponds to the situation shown m FIG. 3C, the range to target 313 is relatively close, and thus the center detector 305 is entirely shadowed by the mirror 312. As a result, the center detector 305 receives virtually no light from the target 313. This effect is illustrated m FIG. 4A by the shadow region 412 which entirely covers the center detector 305. The side detector 301, on the other hand, is located m the collected light region 411, and is not affected by the shadow region 412 caused by the mirror 312.

In FIGS. 4B and 4C, which both correspond the situation shown FIG. 3B, the range to the target 313 is greater than FIG. 4A, and is somewhere m mid-field. Consequently, the size of the light region 421 (FIG. 4B) and 431 (FIG. 4C) gradually shrinks as the target 313 moves farther away, so that a larger proportion of collected light arrives at the center detector 305, while a smaller proportion of the collected light arrives at the side detector 301. This effect continues until, at or near a maximum range, the side detector 301 no longer receives any light from the target 313, as illustrated m FIG. 4D. In the diagram of FIG. 4D, which corresponds to the situation shown m FIG. 3A, the collected light region 441 is entirely focused on the center detector 305, while the side detector receives virtually no collected light.

The graphs on the right hand side of each of FIGS. 4A- 4D compare the amount of light received at the side detector 301 with the amount of light received at the center detector 305. As shown those graphs, the proportion of collected light at the center detector 305 increases as the target 313 moves farther away, and the proportion of collected light at the side detector 301 correspondingly decreases. FIG. 5 is a schematic diagram which illustrates how the varying proportions of light arriving at the center detector 305 and the side detector 301 are used to generate a range output signal a preferred embodiment rangefmder. Preamplifiers 511 and 515 amplify the signals received from the side detector 301 and the center detector 305, respectively. Preferably, the preamplifiers 511, 515 are WO 01/29576 _{± 9} PCT/USOO/27344

transimpedance amplifiers that generate voltage outputs from the current that they receive from the detectors 301, 305.

In dark rooms or environments, where the laser diode 572 is the only source of light, the outputs of the detectors 301, 305 (and the preamplifiers 511, 515) would represent only the reflected light that originated from the laser diode 572. But m practical situations, the light received by the detectors 301, 305 contains two components: the reflected light that originated from the laser diode 572, and ambient light. Removing the effects of the ambient light improves the accuracy of the resulting range readings. A preferred approach for ambient light removal is to modulate the light emitted by the laser diode 572 (i.e., to switch it on and off alternately) , and to obtain readings both the on and off states. The readings m the off state are then subtracted from the readings m the on state. Such techniques, as previously noted, are well known the art, and are described m, for example, U.S. Patent No. 4,694,182, previously incorporated herein by reference. In a preferred embodiment, ambient light correction is accomplished using a set of sample-and-holds and a pair of subtractors. An oscillator 570 (e.g., an astable multivibrator) generates a rectangular wave, which is provided as a control signal to the laser driver 571. When the control signal is on, the laser driver 571 drives current through the laser diode 572, which causes the laser diode 572 to emit light. At the same time, the control signal from the oscillator 570 instructs the sample-and- holds 521, 525 to sample the amplified signals from the side preamplifier 511 and the center preamplifier 515, respectively. The sampled amplified signals then appear at the outputs of the sample-and-holds 521, 525. These outputs WO 01/29576 _{2 Q} PCT/USOO/27344

represent the amount of light arriving at the side detector 301 and the center detector 305 when those detectors are illuminated by both ambient light and light from the laser diode 572. When the control signal at the output of the oscillator 570 is off, the laser driver 571 will turn the laser diode 572 off, and it will not emit light. Meanwhile, the sample- and-holds 521, 525 continue to hold their previously stored output levels. The inverter 573 inverts the control signal, and the inverted control signal is provided to the sample input of the sample-and-holds 522, 526. This signal causes the sample-and-holds 522, 526 to sample the output voltages of the amplifiers 511, 515 at a time when the detectors 301, 305 are only receiving ambient light (and are not receiving any light from the laser diode 572) . These sampled voltages will then appear at the outputs of the sample-and-holds 522, 526, respectively. These outputs represent the amount of light arriving at the detectors 301, 305 when those detectors are illuminated by ambient light only. Summer 531 subtracts the output of sample-and-hold 522 from the output of the sample-and-hold 521, which subtracts the sideAMBiENT value from the sideASER+AMBiENT value. The result of this subtraction appears at the output of summer 531, which represents the amount of laser light arriving at the side detector 301 with the effects of any ambient light removed. Similarly, the summer 535 subtracts the output of the sample-and-hold 526 from the output of the sample-and- hold 525, which subtracts the centerAMBiENT value from the centerLASER₊AMBiET value. The result of this subtraction appears at the output of summer 535, which represents the amount of laser light arriving at the center detector 305 with the effects of any ambient light removed. A third summer 540 adds the side photodetector output signal 539 (i.e., the output of summer 531) to the center photodetector output signal 538 (i.e., the output of summer

535), resulting m a total photodetector output (i.e., side + center) signal 550.

In a preferred embodiment, the rangefmder comprises a normalization block 560 which receives as inputs the center photodetector output signal 538 and the total photodetector output signal 550, and outputs a range signal 561 indicative of the range of the target 313. Preferably, the range signal 561 is so calculated as to be unaffected by changes m target reflectivity. In one embodiment, the range signal 561 comprises a ratio of the light collected by the center photodetector to the total collected light -- that is, the quotient of the center photodetector output signal 538 divided by the total photodetector output (i.e., side + center) signal 550. The division operation to obtain the quotient may be implemented m an analog or digital fashion (including by A/D converting the center photodetector output signal 538 and total photodetector output signal 550 and carrying out a digital division calculation) , using techniques well known to those skilled m the art of analog or digital circuit design.

FIG. 6 is a graph illustrating how the center photodetector output signal 538 (represented by signal plot 602), side photodetector output signal 539 (represented by signal plot 603), and the total photodetector output (i.e., side + center) signal 550 (represented by signal plot 601) each vary as a function of range for an exemplary rangefmder constructed according to the techniques illustrated FIG. 5. For the exemplary rangefmder for which data was plotted m FIG. 6, the effective near range starts at slightly under 2" and ends at about 14" . The signal plot 601 for the total photodetector output signal 550 tracks signal plot 602 for the center photodetector output signal 538 after meeting at a target range of about 14" .

As illustrated FIG. 6, the signal plot 603 for the side photodetector output signal 539 is relatively flat until a target range of about 4" because the collected spot shrinks (thus raising power density) at nearly the same rate as the l/r² collection loss lowers total collected power. Between target ranges of 4" and 14", the collected spot gradually decreases m size, eventually (at about 14") missing the side detector 301 completely, while at the same time the signal plot 602 for the center photodetector output signal 638 is increasing as shadowing of the center photodetector 305 dissipates. At target ranges of about 14" to 20", the signal plot 601 for the center photodetector output signal 638 continues to increase, although the signal plot 603 for the side photo-detector output signal 639 has already reached zero, a phenomenon due to the fact that, m the exemplary photodetector for which this data was developed, the side detector 301 was physically offset from the center photodetector 305 by a small gap. Finally, at target ranges beyond about 20", the l/r² collection loss lowers the collected power on the center detector 305, as reflected m signal plot 602.

FIG. 7 is a graph illustrating how the range output signal 561 (see FIG. 5) varies as a function of range of the target 313. In particular, signal plot 701 corresponds to the range output signal 561 at a variety of different target ranges. As illustrated m FIG. 7, the center photodetector output signal 538 comprises an increasing percentage of the total photodetector output signal 550 as the target range increases. After a target range of about 14" is reached, all of the collected light is due to the center photodetector output signal 538, so a level of 100% is reached and maintained.

The range output signal 561 should be relatively unaffected due to variations or differences m target colors. Any differences m the range output signal 561 due to color should be small enough to be ignored m many applications.

FIG. 8 illustrates how changing the lens-to-detector distance affects the output response curve for an exemplary system. In the particular example given m FIG. 8, the output response curves for lens-to-detector distances of 2.222, 2.083, 2.000, and 1.900 inches (5.64, 5.29, 5.08, and 4.83 cm, respectively) are illustrated as plot lines 801, 802, 803, and 804, respectively. Each plot line 801, 802, 803 and 804 has a similar shape, with a 0% range output when the center detector 305 is shadowed or at near range, a generally linear region when each detector 301, 305 receives some fraction of collected light 305, and a 100% range output when the center detector 305 receives all of the collected light. Useful rangefinding operation occurs m the generally linear region. In embodiments wherein the rangefmder is incorporated into an optical scanner, the center photodetector 305 (possibly conjunction with the side detector 301) can also be used as the photodetector for scanning, thus advantageously combining a rangefinding and scanning function m the same photodetector elements, and reducing the total number of parts needed for the device. If both the center photodetector 305 and side photodetector 301 are used for scanning purposes, then their outputs can be summed to arrive at a single photodetector output signal to be used for scanning purposes, which is then sent to other circuitry for processing, edge detection and decoding according to any of a variety of techniques well known m the art.

Referring once again to FIGS. 3A - 3C, if the side detector 301 is not immediately adjacent to the center detector 301, there will be an effect on the useful output of the range signal. Operation of the rangefmder 300 depends upon the assumption that changing the range of the target will affect the ratio of light incident upon the center detector 305 and the side detector 301; thus, moving the side detector 301 away from the center detector 305 may lead to an effective "blind spot" over a certain range where changes m the target range will not yield corresponding changes the rangefmder output. The rangefmder 300 will not detect changes m the target range as the target moves away from the detector plane 316 until the spot spread becomes large enough for the collected light to reach the side detector 301. In the presently preferred embodiment, the side detector 301 is positioned immediately adjacent to the center detector 305, thereby providing the best performance and no "blind spot."

In one embodiment, no light-blocking element (such as mirror 312 m FIGS. 3A - 3C) is utilized. Rather, the range signal is obtained simply by comparing the ratio of the light incident on the center detector 305 with light incident on the side detector 301 (using, for example, the identical circuitry as that shown FIG. 5) . In such an embodiment, a useful range signal is still generated, since the two detectors 301, 305 still measure collected spot shape variation; however, the resolution of the signal will be less than if a light-blocking element were used. This is because the center detector 305 will not be shadowed at close target ranges, so it will continue to receive light (approximately one-half the total photodetector collected light) when the target is at the closest range. FIGS. 10A and 10B compare the amount of total collected light attributable to the center detector m a system with a light-blocking element (FIG. 10A) and without a light- blocking element (FIG. 10B) . As apparent from the graphs of FIGS. 10A and 10B, there is less total variation m the center detector signal without a light-blocking element (i.e., it varies only from 50% to 100% of the total collected light, as opposed to 0% to 100%) . Thus, there is greater resolution m the range signal with a light-blocking element, and its inclusion is preferred.

Other alternative embodiments relate to the optical elements. For example, the mirror 312 shown m FIGS. 3A - 3C and insert of lens 315 may optionally be offset from the center of the lens 315. However, m such an embodiment, parallax effects will occur m addition to collected spot changes, making rangefinding using the above-described principles potentially more difficult.

In another embodiment, neither detector 301, 305 (see FIGS. 3A - 3C) is positioned so as to directly align with the optical axis of the system; rather, both detectors 301, 305 are offset from the optical axis by differing amounts. While the same principles can be used to obtain the target range, such an embodiment is not as preferred as that shown m FIGS. 3A - 3C. In other embodiments, alternative opaque light blocking elements may be substituted for the mirror 312, as can non- opaque light blocking elements including, for example, light WO 01/29576 ₂ g PCT/USOO/27344

redirectors and diffusers. Other examples of light blocking elements may be found m copendmg U.S. Patent Application Ser. No. 09/127,399 filed July 31, 1998, hereby incorporated by reference as if set forth fully herein. In one embodiment, the detectors 301, 305 are not themselves placed on the detector plane 316, but rather at a remote location. Light from the center and side locations on the detector plane 316 where the detectors 301, 305 would otherwise be may be routed to the corresponding detectors 301, 305 using, for example, fiber optics.

In an alternative embodiment, the detector plane is adjusted such that the best focus on the center detector occurs at near field. Such an embodiment is illustrated FIGS. 12A and 12B, wherein the detector plane 980 (and thus detectors 981 and 985) is positioned further back from the detector plane 380 (as shown m Figs. 3A - 3C) so that distant targets 993 (FIG. 12B) will focus onto a wide area of the detector plane 980 covering both the center detector 985 and the side detector 981, and near targets 993 (FIG. 12A) will focus onto a small region - i.e., on the center detector 985. While the principle is similar to the embodiment shown FIGS. 3A - 3C, the effect is reversed, m that when targets are distant the side detector 981 will receive a greater proportion of the collected light, whereas when targets are near the center detector 985 will receive a greater proportion of the collected light. Between the two, the embodiment illustrated m FIGS. 3A - 3C is generally more preferred than that shown m FIGS. 12A and 12B, because typically it is preferred to have the most collected light when at the far-field distance.

Another embodiment is illustrated m FIG. 13, which depicts a rangefmder using the same principles as described with respect to FIGS. 3A - 3C, but with additional photodetectors. In the embodiment shown FIG. 13, an array of photodetectors 1005a - 1005d ( this example, four photodetectors, but workable with any plural number n of photodetectors) are aligned along a detector plane 1016. A first photodetector 1005a, referred to as the center photodetector, is positioned so as to be aligned with the optical axis 1017 of the optical system. The other photodetectors 1005b, 1005c and 1005d, referred to as side photodetectors, are sequentially lined up next to the center photodetector 1005a, with photodetector 1005b being immediately adjacent to the center photodetector 1005a, the next photodetector 1005c being immediately adjacent to the second photodetector 1005b, and the next photodetector 1005d being immediately adjacent to the third photodetector 1005c (the term immediately adjacent not necessarily meaning physically touching, but close enough to provide meaningful information for rangefinding purposes) .

When a target 1013 is at or near the maximum range, all or almost all of the collected light is directed to the center photodetector 1005a, whereas when the target 1013 is at or near the closest range, the light is distributed among the different photodetectors 1005a - 1005d (or, if the optional mirror/light-block g element 1012 is utilized as m FIGS. 3A - 3C, light is shadowed from the center photodetector (s) and only present on the side photodetectors) . Similar to the embodiment of FIGS. 3A - 3C, as the target 1013 moves closer to the lens 1015, the spot spread increases, causing light to gradually to reach one or more of the side detectors 1005b, 1005c and 1005d depending upon how close the target 1013 is. Extending the principles described with respect to FIGS. 3A - 3C and FIG. 5, the relative amount of collected light at each of the photodetectors 1005a - 1005d can be compared to arrive at an estimate of the target distance.

As with the embodiment of FIGS. 3A - 3C, when incorporated an optical scanner, the center photodetector 1005a, possibly m conjunction with one or more of the side photodetectors 1005b - 1005d, may also be used as the photodetector for scanning purposes, thus advantageously combining a rangefinding and scanning function m the same photodetector elements, and reducing the total number of parts needed for the device.

FIGS. 9A, 9B, and 9C are diagrams of alternative embodiments using a collection mirror as the focusing system m place of the lens 315 used m the embodiment depicted m FIGS. 3A-3C. FIG. 9D is a diagram of a collection mirror

975 with an insert 976 as may be used m the embodiments

(specifically, as collection mirror 902, 922 or 942) of

FIGS. 9A, 9B and 9C. The collection mirror 975 may be plastic molded and reflection-coated. FIG. 9A is an optical schematic diagram for a rangefmder using a right-angle collection system, wherein a collection mirror 902 collects and focuses light upon a pair of photodetectors 901, 905, one of which (905) is centrally positioned along the optical axis and the second of which (901) is adjacent thereto, similar to the configuration FIGS. 3A - 3C. Insert 903, which may be a tilted piece or portion of collection mirror 902, directs light from the laser 908 along an outgoing beampath 907 towards the target 913. In the embodiment depicted m FIG. 9A, the insert 903 acts as the opaque object to block the incoming light. Operation of the FIG. 9A embodiment is analogous to the operation of the FIG. 3A - 3C embodiment: (a) for far field targets, the mirror 902 will focus all of the light from the target 913 onto the center photodetector 905, and the side photodetector 901 will not receive any light; (b) for mid- field targets, the mirror 902 will focus the light from the target 313 onto a focal point beyond the detector plane, so both the center photodetector 905 and the side photodetector 901 will receive light; and (c) for near field targets, the mirror 902 will focus all of the light from the target 313 out to infinity, so the shadow of the collection mirror insert 903 will prevent any light from reaching the center photodetector 905, and only the side photodetector 901 will receive light.

FIGS. 9B and 9C collectively illustrate, from a side view and top view, respectively, a rangefinder using a tilted collection plane system, again with a collection mirror 922 having an insert 923 which acts as a shadowing

(i.e., light-blocking) element. In the embodiment of FIGS.

9B and 9C, the detectors 921, 925 are positioned at an angle from the collection mirror 922 so as to received light focused by the collection mirror 922. As with the FIG. 9A embodiment, one photodetector 925 is centrally positioned along the optical axis of the collection mirror 922, while the second photodetector 921 is positioned adjacent thereto, in a manner similar to the photodetector configuration shown in FIGS. 3A-3C. Insert 923, which may be a tilted piece or portion of collection mirror 922, directs light from the laser 928 along an outgoing beampath 927 towards the target 933. The insert 923 also may act as the opaque, light- blocking element which shadows the centrally positioned photodetector 925. Operation of the embodiment of FIGS. 9B - 9C is analogous to the operation of the FIG. 3A - 3C embodiment and the FIG. 9A embodiment. The rangefinder systems of FIGS. 9A - 9C may also be implemented, similar to the embodiments shown m FIGS. 3A -

3C, without an insert 903 or 923 to provide an outgoing beampath and to shadow the centrally positioned photo- detector 905 or 925.

Incorporating a rangefinder into a barcode scanner accordance with the preferred embodiments advantageously facilitates compensation for a number of variables that affect the barcode scanning process. Preferably, this compensation is performed by adjusting the system parameters before each barcode is read, based on the range signal received from the rangefinder 140. This predictive compensation is possible because the scanning spot will usually traverse at least some small amount of non-barcode information before actually reaching the barcode on the target, which provides the opportunity to determine the range before scanning of the barcode begins. Adjusting the system parameters to their desired values before entering the barcode improves the odds of reading any given barcode m a single scanning pass.

A first parameter that may be adjusted based on the range signal from the rangefinder is the amplifier gam. The amount of light power collected by the photodetector is predicted for each individual barcode based on the range signal, and the gam of the amplifier is adjusted to compensate for variations m the collected power. This adjustment avoids readings where the gam is set too low

(which would result m a poor signal to noise ratio) , as well as readings where the gam is too set high (which would result saturation of the detecting electronics) . For example, when the range signal received from the rangefinder 140 indicates that the target is far away, the predicted WO 01/29576 3 _χ PCT/USOO/27344

collected power will be small, since the collected power is inversely proportional to the square of the distance to the target (as shown m FIG. 1A) . To compensate for this low power, the gam of the gam amplifier 103 may be increased m proportion to the square of the detected range. The specifics of implementing an amplifier with a gain that is controlled by an input signal are conventional the field of amplifier design.

Controlling the amplifier gam based on range provides superior performance compared to conventional systems that control the amplifier using automatic gam control (AGC) . One reason for this improvement is that conventional AGC sets the amplifier gam based on characteristics of previous reading attempts, instead of the upcoming reading attempt. But when the upcoming barcode is not similar to the previous barcodes, a reading may be missed, which would require a rescan. In contrast, by using the above-described configuration, the amplifier ga is set based on characteristics of the upcoming barcode, which improves the first-pass read rate of the scanner.

Moreover, the AGC based ga control may not be suited for use m multi-beam barcode scanning systems, because the distances between the scanner and successive targets vary widely (as compared to handheld scanners) , and because successive scans for any given target follow different optical paths, which will usually have different target ranges .

A second parameter that may be adjusted based on the range signal is light-source brightness, which may be increased for distant targets and decreased for near targets. Returning to FIG. 2, brightness control may be implemented using the laser driver 120 to supply a larger current to the laser diode 121 when a brighter beam is desired (i.e., when the range signal received from the rangefinder 140 indicates that the target is far away) . The brightness selected by the laser driver 120 can be set to vary m discrete steps or, alternatively, m a continuous range. This technique may be used alone or m combination with the adjustments to the gam of the amplifier 103 described above. Of course, when brightness control is combined with gam amplifier control, suitable adjustments to the gam control should be made to ensure that an over- compensation for distance does not occur. For example, to compensate for the received signal being inversely proportional to the square of the range, both the brightness of the laser diode 121 and the gam of the amplifier 103 may be increased direct proportion to the range. Details of implementing a current source with an output that is controlled by an applied signal may be accomplished according to any of a number of techniques well known m the art . A third set of parameters that may be adjusted based on the range signal are the noise thresholds . Because the level of speckle noise varies with range, barcode scanners accordance with a preferred embodiment can adjust their noise thresholds before reaching a given barcode to reject speckle noise without sacrificing sensitivity for detecting low contrast or distant barcodes. This adjustment can be accomplished, for example, by setting a higher first derivative threshold m the noise reduction circuit 105 when the detected range is near the laser waist (where larger speckle noise is expected) , and setting a lower first derivative threshold for distant targets (where smaller speckle noise is expected) . Implementing a signal processor with controllable noise thresholds is considered well withm the purview of persons skilled the art.

A fourth parameter that may be adjusted based on the range signal is the equalization function, which is used to compensate for blurring. This adjustment can be accomplished by dividing the scanning space into a set of ranges, and pre-computing the equalization function to be used m each of those ranges. For example, m an exemplary embodiment wherein the scanning space is between 6 and 12 inches (15 and 30 cm) , the scanning space can be divided into six one-inch (2.5 cm) subranges. Then, six equalization functions (one for each of the six subranges) are pre-computed based on the expected spot speed and size for targets withm that range. Each of these six equalization functions is stored, preferably m a nonvolatile memory such as ROM, EPROM, or EEPROM. Immediately before a given barcode is scanned, the rangefinder measures the distance to the target barcode. A particular equalization function is then selected based on which of the subranges corresponds to the measured range. When the barcode is actually scanned, the selected equalization function is used to deblur the signals received by the photodetectors .

Preferably, the selected equalization function is performed m a digital signal processor (DSP) . Of course, depending on the optical characteristics of the scanning system and the required accuracy, the scanning space may be divided into a smaller or larger number of subranges. Optionally, the subranges may be selected so that they are not all equally spaced (e.g., a scanning space of 6-12 inches (15-30 cm) may be divided into three subranges of 6-7 inches (15-18 cm) , 7-9 inches (18-23 cm) , and 9-12 inches (23-30 cm)) . Alternatively, the DSP may compute an equalization function based on the range and known characteristics of the optical system, without dividing the scanning space into subranges. As yet another alternative, a set of individual filters arranged parallel may be implemented m hardware, and the output from the desired filter can be selected by a multiplexer based on the range signal. Range based equalization can then be used to deblur the laser spot optimally and yield a substantial increase m depth of field.

A fifth parameter that may be adjusted based on the range signal is scanning speed. In most conventional barcode scanners, the motor driver always operates at the same speed. As a result, the scanning spot moves more rapidly across targets that are farther away from the scanner than targets that are nearer. In a preferred embodiment, the range signal may be used to slow down the motor driver for distant barcodes, so that more light energy can be provided to the relevant portion of the target. This adjustment can be accomplished by using a motor driver withm the deflector 130 to control the speed of the deflector motor m accordance with the range signal received from the rangefinder 140. Details of implementing motor control systems for varying speed of the deflector motor may be accomplished according to any of a number of techniques well known m the art . In the case of an oscillating dithering mechanism for deflecting the beam, spot speed may be controlled so as to be relatively constant by varying the amount of arc m the oscillatory member. In such a compensation scheme, the dithering mechanism is adjusted according to measured target range so as to provide less dither arc at long target ranges, and more dither arc at short target ranges, thereby controlling the spot speed (i.e., keeping it constant) while allowing the oscillation frequency to be kept the same (which is advantageous in resonant dithering systems) . Because the blurring function depends on the spot speed, the equalization function should preferably be adjusted to compensate for changes in spot speed whenever the spot speed is controlled. This adjustment can be accomplished, for example, by storing a different set of equalization functions for each expected spot speed.

Preferred embodiments of the rangefinder described above are particularly advantageous for use in flying spot scanners, because most of the required hardware is already present in the scanner. In particular, the laser diode, the output-beam-directing mirror, the collection optics, and at least one of the photodetectors (i.e., the center photodetector) are available to the rangefinder for shared use. In cases when the sum and quotient can be computed in an existing controller, the only additional hardware might be the circuitry for removing the effects of the ambient light from the side photodetector. Optical scanners constructed according to various preferred embodiments as described herein may be dramatically smaller, lighter, cheaper, more reliable, and less power- consumptive than, for example, the radar-based rangefinding systems of the prior art .

While the rangefinders in accordance with the preferred embodiments are described above in the context of barcode scanners, they may be used in other applications as well. For example, rangefinders in accordance with the preferred embodiments may be used in other types of optical readers such as optical character recognition (OCR) systems, in WO 01/29576 _{3 g} PCT/USOO/27344

cameras, security systems, spatial -measurement devices (e.g., for measuring object dimensions), and in numerous other applications.

Moreover, while the present invention has been explained in the context of the preferred embodiments described above, it is to be understood that various changes may be made to those embodiments, and various equivalents may be substituted, without departing from the spirit or scope of the invention, as will be apparent to those persons skilled in the art.

Claims

What is claimed is:

1. An apparatus for determining a distance to a target, comprising: a light collection system; a plurality of photodetectors, said photodetectors positioned so as to receive light collected by said light collection system m amounts depending upon a spot spread resulting from collecting light reflected from the target; and a range signal generator connected to said photodetectors, said range signal generator outputtmg a range signal having a value dependent upon a relative amount of collected light at more than one of said photodetectors.

2. The apparatus of claim 1, wherein the number of said photodetectors is two.

3. The apparatus of claim 2, wherein a first one of said two photodetectors is positioned along an optical axis of said light collection system, and wherein a second one of said two photodetectors is positionally offset from said first photodetector along a detector plane.

4. The apparatus of claim 3 , wherein said range signal comprises a ratio of an amount of light collected at one of said two photodetectors with respect to an amount of light collected at both of said two photodetectors.

5. The apparatus of claim 3 , further comprising a light blocking element positioned along said optical axis between said first photodetector and the target.

6. The apparatus of claim 5, further comprising means for generating a light beam and sweeping said light beam across said target, wherein said light blocking element comprises a folding mirror positioned in a path of said light beam.

7. The apparatus of claim 1, wherein said light collection system comprises a lens.

8. The apparatus of claim 1, wherein said light collection system comprises a light collection mirror.

9. The apparatus of claim 1, further comprising a synchronous detection circuit interposed between said photodetectors and said range signal generator, said synchronous detection circuit outputting a signal for each photodetector indicative of an amount of collected light absent the effect of ambient light.

10. The apparatus of claim 1, wherein said range signal output is normalized so as to be unaffected by changes in target reflectivity.

11. A method for determining a distance to a target, comprising the steps of : collecting light energy from a target and focusing it on a detector plane; measuring variations m the collected light energy across a portion of the detector plane; and determining a target range based upon said variations m collected energy.

12. The method of claim 11, further comprising the steps of generating a beam of light, and sweeping said beam of light across said target.

13. The method of claim 11, wherein: said step of collecting light energy from a target and focusing it on a detector plane comprises the step of focusing said collected light energy onto one or both of a pair of photodetectors, depending upon said target range; and said step of measuring variations the collected light energy across said portion of the detector plane comprises the steps of measuring an amount of collected light at each of said photodetectors.

14. The method of claim 13 , wherein said step of determining said target range based upon said variations m collected energy comprises the step of comparing the amount of collected light at one of said two photodetectors with the total amount of collected light at both photodetectors.

15. The method of claim 14, wherein one of said two photodetectors is aligned with an optical axis of light collection optics used m said step of collecting light energy from said target and focusing it on said detector plane, and wherein the other of said two photodetectors is WO 01/29576 _{4 Q} PCT/USOO/27344

positionally offset from said first photodetector along said detector plane.

16. The method of claim 15, further comprising the step of shadowing the photodetector aligned with the optical axis using a light-blocking element.

17. The method of claim 14, further comprising the step of outputtmg a range signal normalized so as to be unaffected by target reflectivity.

18. The method of claim 13, wherein said step of measuring said amount of collected light at each of said photodetectors comprises the step of measuring collected light at each of said photodetectors m a synchronous pattern with modulation of an outgoing laser beam so as to eliminate ambient light effects.

20. The method of claim 11, further comprising the step of compensating one or more processes m an optical scanner depending upon said target range.

21. A rangefinder for determining a distance to a target, the rangefinder comprising: a light source that projects light along an outgoing light path onto the target ; a light gathering system arranged to focus light reflected from targets onto a detector plane; a first photodetector positioned on the detector plane and along an optical axis of the light gathering system so as to detect light focused on said first photodetector by the light gathering system, and generating a first photodetector output signal thereby; a second photodetector positioned on the detector plane but offset from said first photodetector so as to detect light focused on said second photodetector by the light gathering system, and generating a second photodetector output signal thereby; and a range measurement circuit connected to said first photodetector output signal and said second photodetector output signal, said range measurement circuit outputting a range signal having a value dependent upon a relative amount of light detected at said first photodetector and at said second photodetector.

22. The rangefinder of claim 21, further comprising a light-blocking element positioned so as to shadow said first photodetector to a varying degree dependent upon a target distance .

23. The rangefinder of claim 22, wherein, for targets in near field, said light-blocking element blocks substantially all light from reaching said first photodetector, and wherein, for targets in far field, said light-blocking element blocks minimal light from reaching said first photodetector.

24. The rangefinder of claim 23, wherein said light gathering system comprises a lens having an insert and a collection region, said light-blockmg element positioned so as to block, to a varying degree dependent upon target distance, light from being focused by insert on said first photodetector .

25. The rangefinder of claim 21, wherein said light gathering system comprises a light collection mirror.

26. The rangefinder of claim 21, wherein the light source comprises a laser.

27. The rangefinder of claim 21, wherein said range signal comprises a normalized output signal generated by said range measurement circuit by computing a ratio of one of said first photodetector signal and said second photodetector signal with the sum of said first photodetector signal and said second photodetector signal .

28. The rangefinder of claim 21, further comprising a synchronous detection circuit interposed between said first and second photodetectors and said range measurement circuit .

29. The rangefinder of claim 28, wherein said synchronous detection circuit comprises: a modulation circuit for modulating the light source on and off m response to a control signal; a first sampling circuit for sampling signal values from sa d first photodetector output signal when the light source is on and when the light source is off; a second sampling circuit for sampling signal values from said second photodetector output signal when the light source is on and when the light source is off; means for calculating a difference m the signal values from said first photodetector output signal when the light source is on and when the light source is off, and thereby outputtmg a signal indicative of an amount of light detected by said first photodetector absent ambient light effects; and means for calculating a difference m the signal values from said second photodetector output signal when the light source is on and when the light source is off, and thereby outputtmg a signal indicative of an amount of light detected by said second photodetector absent ambient light effects .

30. An optical reader for reading information encoded a target, said optical reader comprising: a light source that projects light along an outgoing light path onto the target; a deflector causing the light beam emitted by the light source to move; a plurality of photodetectors; a light gathering system arranged to focus light reflected from targets onto said photodetectors; and a range measurer for determining a target distance m response to variations m light detected by said photodetectors .

31. The optical reader of claim 30, further comprising an edge detector connected to an output of at least one of said photodetectors .

32. The optical reader of claim 30, further comprising a decoder for determining the information encoded m said target .

33. The optical reader of claim 30, further comprising a signal processor including an amplifier that amplifies an output signal from at least one said photodetectors.

34. The optical reader of claim 33, wherein the signal processor compensates for said target distance determined by said range measurer.

35. The optical reader of claim 33, wherein the signal processor compensates for said target distance to a given target prior to reading the given target .

36. The optical reader of claim 33, wherein said amplifier comprises circuitry for adjusting a signal ga m response to said target distance determined by said range measurer .

37. The optical reader of claim 33, wherein said signal processor comprises a noise reduction circuit that compensates for said target distance determined by said range measurer by increasing a noise threshold at target distances where high speckle noise is expected.

38. The optical reader of claim 33, wherein said signal processor comprises equalization circuitry, said signal processor compensating for said target distance determined by said range measurer by selecting an equalization function matched to said target distance.

39. A method for determining a distance to a target comprising the steps of: projecting light along an outgoing light path onto the target; focusing light reflected from targets onto a plurality of photodetectors, said photodetectors positionally offset from one another; shadowing, to a degree dependent upon a target range, at least one of said photodetectors from light reflected by the target; measuring an amount of collected light at each of said photodetectors; and calculating a distance to said target based upon a relative amount of light collected by said photodetectors.

40. The method of claim 39, wherein the step of measuring said amount of collected light at each of said photodetectors further comprises the step of removing effects of ambient light from signals output from said photodetectors .

41. The method of claim 39, further comprising the step of processing and decoding a signal output from at least one of said photodetectors to determine a value of information encoded m the target.

42. The method of claim 41, further comprising the step of compensating for the distance to the target prior to reading a given target .

43. The method of claim 41, wherein said step of processing and decoding said signal output from at least one of said photodetectors comprises a step of amplifying said signal output from at least one of said photodetectors, said method further comprising a step of compensating for the distance to the target by adjusting an amplification gam m response to changes m the distance to the target.

44. The method of claim 41, wherein said step of processing and decoding said signal output from at least one of said photodetectors comprises a step of reducing noise m said signal output from at least one of said photodetectors, said method further comprising a step of compensating for the distance to the target by adjusting a noise threshold m response to changes the distance to the target .

45. The method of claim 45, wherein said noise threshold is increased at ranges where high speckle noise is expected .

46. The method of claim 41, wherein said step of processing and decoding said signal output from at least one of said photodetectors comprises a step of processing said signal output from at least one of said photodetectors using an equalization filter, said method further comprising a step of compensating for the distance to the target by selecting an equalization function for said equalization filter matching the distance to the target .