GB2256298A - Barcode reading - Google Patents

Abstract

A bar code reader has a Bessel beam source 50, a scanner 52 for scanning the beam across a bar code 58, a detector 54 for detecting the beam after being scanned across the bar code and a signal processor 56 for producing an output corresponding to the scanned bar code. In an alternative embodiment the bar code is scanned by the combination of a Bessel beam and a Gaussian beam. <IMAGE>

Description

BARCODE READING The present invention relates to Bessel beam sources and apparatus including such sources for use in reading bar codes.

According to the present invention there is provided a bar code reader comprising a Bessel beam source, means for scanning the beam across a bar code, detector means for detecting the beam after being scanned across the bar code and a signal processor for producing from the output of the detector an output corresponding to the scanned bar code.

The reader may further comprise a Gaussian beam source, wherein the scanning means comprises means for scanning the Bessel beam and the Gaussian beam across a bar code, the detector means is for detecting both beams after being scanned across the bar code and the signal processor includes a comparator for providing a difference signal from the scanned Bessel and Gaussian beams.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic cross-sectional view illustrating the geometry of the creation of a Bessel beam; Figure 2 shows a typical cross-section of a Bessel beam; Figures 3(a) and 3(b) show diagrammatically two methods of generating a Bessel beam; Figure 4 shows an improved version of the method of generating a Bessel beam shown in Figure 3(b); Figure 5 shows a diagrammatic view of a basic bar code reader; Figure 6(a) shows the envelope of a Gaussian beam; Figure 6(b) shows the envelope of a Bessel beam; Figure 7 shows the intensity profiles of Gaussian and Bessel beams; Figure 8 shows a bar code cross-section; Figure 9 shows the power received by a detector from a bar code scanned by a Bessel beam;; Figure 10 shows the power received by a detector from a bar code scanned by a Gaussian beam; Figures 11, 12, 13 are similar to Figures 8, 9, 10 but using a 2:1 mark/space ratio.

Figures 14, 15, 16 are similar to Figures 11, 12, 13 but with a different bar code; Figure 17 shows a diagrammatic view of a bar code reader using both a Gaussian and a Bessel beam; Figures 18, 19, 20 show the outputs from the bar codes of Figures 8, 11 and 14 when read with Gaussian and Bessel beams together; Figure 21 shows the reading of bar codes using beams of orthogonal polarisation; Figure 22 shows a further development of the technique shown in Figure 21; Figure 23 shows a Gaussian/Bessel beam scanning technique using a single detector.

The Gaussian intensity profile associated with the TEMoo mode of for instance, a laser resonator has, until recently, been the preferred beam shape in a majority of applications. The finite divergence and almost featureless profile of a Gaussian beam is, however, not always desirable. In some envisaged tasks where high directionality is considered important, a new class of beam, characterised by a Bessel profile, offers distinct advantages. A Bessel beam, as it is commonly known, possesses a non-diverging central peak which after a predetermined distance dissipates rapidly, see Figure 1.

Such a beam of electromagnetic radiation may be in the optical, millimetre wave, microwave or other region of the spectrum.

Frequently the form of radiation used is in the optical spectrum, but not necessarily so.

All fields of electromagnetic radiation, confined by an aperture or area of finite extent will undergo diffraction as they propagate. Generally, the larger the aperture and smoother the intensity profile, the smaller the divergence experienced. However, both these requirements tend to dilute the characteristics of a beam which should exhibit a high concentration of intensity along the axis of propagation.

There is a solution to the wave equation which is both beam-like and non-diffracting. The amplitude profile of such a field is proportional to the zero order Bessel function of the first kind, J0(x). It propagates as a narrow axial beam surrounded by an infinite set of concentric rings whose intensity envelope decays proportionally with radial distance, x, see Figure 2. Each ring contains approximately the same amount of energy as that present in the central beam, hence such a Bessel beam would require an infinite amount of energy to create. In practice a Bessel-like beam would be apertured and contain a finite number of rings. This limitation does not effect the diffraction free property of the central peak (in terms of its radius) and in fact confers the potential advantage of a finite range to the beam.

Considering the generation of Bessel beams with reference to Figure 1, and considering an infinite set of equal amplitude plane waves 10, all travelling at the same angle, 8, to the propagating axis but having different azimuthal angles ranging from 0 to 21t radians. If the plane waves are apertured as shown, at an aperture 12 they will overlap for a certain distance, Zmax. The shaded region 14 beyond Zmax is known as the geometric shadow. If the plane waves are coherent i.e. of the same wavelength and phase, they will interfere in the overlap region 16. Since the angles of overlap remain constant with Z, the profile of the interference pattern generated also remains invariant as the beam propagates (apart from its radial extent which diminishes as Zmax is approached).The axial symmetry of the system results in the interference pattern having an intensity profile which is proportional to the square of the zero order Bessel function and having a central peak 18 and the invariance of this pattern with Z leads to such a beam being given the description of diffraction-free.

Beyond Zmax, the overlap and hence the interference ceases, and the on-axis intensity decays rapidly to zero. The light continues to propagate in the form of a diverging annular ring 20 with the energy associated with the beam rapidly dissipating over an increasingly large area.

The basic properties of a Bessel beam are determined by just three factors; the wavelength of the light \ , the radius of the limiting aperture Rb, and the angle which the plane waves subtend to the Z- axis, 0. From Figure 1, it can be seen that Zmax = Rb/tan 8 from which it can be derived that Zmax = [8/33Rr/ where r is the radius of the central peak.

Thus Zmax can be increased by decreasing either or both the wavelength and overlap angle and increasing the aperture.

It was noted previously that a collection of uniform plane waves all propagating at the same angle to the Z axis is required to generate a Bessel beam. Such a set of conditions can be achieved by employing either a thin conical lens 22, known as an axicon or an annular mask/collimating lens combination, see Figures 3(a) and (b) respectively.

The axicon 22 shown in Figure 3(a) has a monochromatic plane wave 21 incident upon the plane face 24. The waves are refracted by the axicon and emerge from the conical face 26 as a collection of plane waves 28 subtending a common angle to the Z-axis.

As described earlier with reference to Figure 1 a Bessel beam 30 is created in the space in which the beams overlap.

Alternatively, as shown in Figure 3(b), a plane wave 21 is incident upon a mask 32, having an annular aperture 34, which produces a diverging annular ring 36 of spherical waves. The ring is incident upon a lens 38 which produces the collection of plane waves 28 and the Bessel beam 30 as for the arrangement in Figure 3(a).

Due to the blocking of the majority of the incident light the system as shown in Figure 3(b) is extremely inefficient. A considerable improvement in the efficiency of the system by placing an annular lens 40 before the mask 32 as shown in Figure 4. This lens 40 focusses the wave on the aperture 34 minimising the blockage of light. The remaining features are as described with reference to Figure 3(b).

The use of an aperture introduces subtle Fresnel diffraction effects. In particular the on-axis intensity undergoes a chirped modulation along the Z-axis.

A Gaussian beam of the same wavelength and comparable size to the central lobe of a Bessel beam will spread to a size greater than the original aperture, Rb, over a distance of Zmax The aforemost property of a Bessel beam is its unique combination over a defined range of profile invariance with high axial intensity. A Gaussian beam which has been expanded to the same size as the entire Bessel beam will not diffract significantly but its profile will contain little in the way of structural information compared to the Bessel beam and thus offer much less in terms of potential directional accuracy.

Finally, the finite range of the intense on-axis beam, with its abrupt transition into the geometric shadow, offers distinct advantages where safety and covert operation are important.

In many applications, the two main advantages of Bessel beams are their invariance with distance and their finite range.

The first results in an equivalent focussed laser beam of vast depth of field. For example, employing conventional optics a spot size of all 5 m (diameter) in the visible region of the optical spectrum has a depth of field of only +13 pm. A Bessel beam of radius lmm would, however, possess a central lobe of similar dimensions whilst providing a range, (and an equivalent depth of field) of 17mm, a factor of 650 greater.

Bar codes are extensively used to label and identify objects such as consumer products, manufactured components and other items. The bar code itself typically comprises a series of diffusely reflective or retroreflective parallel lines, separated by lines of a lower reflectivity. The number, width and spacing of the reflective lines constitutes a code which is used to identify the object to which the bar code is applied, or to record other information about the object.

One means currently employed for the reading of bar codes is to scan a laser beam in a predefined pattern across the surface of an object which bears a bar code, and to subsequently detect the temporal variation of light which is collected by a detection system.

The signal from the detector may be subject to further electronic processing to recover the bar code information. The spatial intensity profile of the laser beam in previous apparatus is typically Gaussian in shape, or near-Gaussian.

The description covers an apparatus which employs a Bessel beam of electromagnetic radiation in order to achieve an improved performance bar code reading system. The invention utilises the properties of the Bessel beam that its spatial profile is nearly constant over a predefined distance along the axis of the beam, and furthermore, that the spatial profile has a characteristic structure, in the form of its high intensity on-axis peak and its surrounding concentric rings.

In its simplest form, the invention comprises the following (with reference to Figure 5): a source of spatially coherent radiation (such as a laser, but other regions of the electromagnetic spectrum could be accommodated) with Bessel beam generating elements 50; scanning elements 52 or other means for deflecting the Bessel beam in a predetermined scan pattern, a detector of radiation 54 (compatible with the radiation source used) and any associated radiation collection elements such as lenses, and electronic apparatus 56 to process the signal from the detector.

The Bessel beam radiation is caused to scan a bar code 58, and radiation reflected from the code, which is temporally modulated by the action of scanning, is arranged to fall on the detector 54.

The electrical signal from the detector is amplified and fed to signal processing apparatus 56. The processed signal is fed to a display unit or to further apparatus, (not shown) depending on the application of the invention.

The detector unit 54 may share common elements with the scanner unit 52, such that its direction of view is collinear with the axis of the Bessel beam.

The scanner unit 52 may operate in one dimension or two dimensions, or may be omitted if the bar code 58 is caused to move in a known manner to intercept and traverse the Bessel beam.

The scanning of a bar code with a Bessel beam has advantages over the conventional scanning technique based on the scanning of a bar code with a Gaussian beam. For a given range of operation, the Bessel beam can be arranged to have a central lobe which has a significantly smaller radius over the entire range than the maximum diameter of a Gaussian beam. A Bessel beam bar code reader can as a result read smaller bar codes than its Gaussian beam counterpart, allowing more compact and less obtrusive bar codes to be applied to objects. Alternatively, the Bessel beam bar code reader can read bar codes of a given size at a greater distance than its Gaussian beam counterpart, allowing greater flexibility in the use of bar code reader systems.

An example for the near infrared region of the electromagnetic spectrum is given (with reference to Figure 6): A confocal Gaussian beam, as illustrated schematically in Figure 6(a), has a maximum radius w0 over a length Zc, where

and where X is the wavelength of the radiation, all variables being in SI units. For Zc=15m A=850nm, we obtain wO=2.Omm.

A Bessel beam, illustrated schematically in Figure 6(b), has a central lobe of radius rb given by

where Rb is the radius of the aperture of the apparatus producing the Bessel beam and Zb is the maximum propagation distance of its central lobe. For Zb=15m, =850no and Rb=1cm, we obtain rb=0.48mm. Thus, the radius of the Bessel beam central peak is more than four times less that of the equivalent confocal Gaussian beam for the same operating range and source wavelength.

Examples will now be given to compare the reading of a simple miniature bar code with Bessel and Gaussian beams respectively.

Figure 7 shows the intensity profile of cross-sections through a radially Bessel beam truncated to five rings and a Gaussian beam of equal radius, truncated radially at its e'4 intensity point.

The two truncated beams are normalised so that they contain equal power.

Figure 8 shows a diagrammatic cross-section of a simple bar code, having eight bars and unity mark-space ratio. The pitch of the bars is equal to the radius of the first minimum of the Bessel beam profile. The vertical axis denotes reflectance, in arbitrary units.

Figure 9 shows the convolution of the two-dimensional bar code profile and the truncated Bessel beam, as a function of the lateral displacement of the two bar code and Bessel beam patterns.

This corresponds, to within a scaling factor, to the power received by the detector 54 shown schematically in Figure 5 as a function of time as the beam is scanned at a constant linear rate across the bar code 58. Figure 10 shows the corresponding function for a scan of the same bar code 58 with the Gaussian beam function described above.

As can be seen, there is little or no structure in this latter function, whereas with the Bessel beam scanner, there is sufficient structure in the output signal to allow recovery of the original bar code by means of electronic signal processing.

Figure 11 shows a cross-section of a similar two-dimensional bar code with the same pitch as in the function of Figure 8, but with 1:2 mark-space ratio. Figures 12 and 13 show the results of scanning this bar code with the Bessel beam and the Gaussian beam respectively.

Figure 14 shows a cross-section of a further bar code, similar to that of Figure 11, but with one bar missing. Figures 15 and 16 show the results of scanning this bar code with the Bessel beam and the Gaussian beam respectively, and indicate that arbitrary bar codes of suitable dimensions may be readable by this technique.

Another possible embodiment of the invention uses both a Bessel beam and a Gaussian beam to read a bar code 58. The Gaussian and Bessel beams are transmitted coaxially, and the signal due to each type of beam is received on a separate detector. In one version of this apparatus, the beams are generated by separate sources having distinct wavelengths of operation. In the optical regime, the beams are combined using a beamsplitter or a dichroic reflector (Figure 17) and separated with a further dichroic reflector, although other regions of the electromagnetic spectrum could be accommodated.

This difference signal bears a closer resemblance to the original bar code function than either the Bessel beam signal or the Gaussian beam signal individually (facilitating bar code reading), as gross changes in signal produced by reading with a Bessel beam are compensated by the signal produced by reading with the Gaussian beam.

Comparisons of the three bar codes represented in Figures 8, 11 and 14 when read by both Bessel and Gaussian beams simultaneously are shown in Figures 18, 19 and 20. In each of these figures, the upper trace shows the output of each channel (Bessel or Gaussian), and the lower trace shows the difference signal (Bessel Gaussian). It may be seen that the original bar code may be received by suitably thresholding the difference signal.

Other embodiments of the invention cover alternative ways of combining the Bessel beam with the Gaussian beam. The physical arrangement is similar to that shown in Figure 17 except that a Bessel beam generator 70 produces a beam at a wavelength -h 1 and a Gaussian beam generator 72 produces a beam at a wavelength 2s 2. The two beams are incident on a dichroic beam splitter 74 which combines them and directs the combined beam 76 by way of a scanner 78 to scan a bar code 80.

The beam 82 reflected by the bar code 80 is split into its component parts by a second dichroic beam splitter 84, the Bessel component going to a detector 86 and the Gaussian component going to a detector 88. The outputs of the detectors 86, 88 are fed to respective amplifiers 90, 92 and thence to a comparator 94 and the comparator output is sent to a signal processor 96 which typically would produce a data output in some form. The difference signal from the comparator 94 is (signal (Bessel beam) - signal (Gaussian beam)).

Figure 21 shows a technique applicable to the reading of particular types of bar code which preserve the polarisation state of the radiation reflected from them. In the optical regime, a polarising beamsplitter 98 is used to combine Gaussian and Bessel beams of orthogonal polarisation, and a second polarising beamsplitter 100 is positioned before the detectors to separate the two contributions due to the two beams. A further development of this technique is shown in Figure 22, where a single spatially coherent source 102 is used, which is linearly polarised at an angle such that a polarising beamsplitter 104 splits the light into two orthogonally polarised beams, each of which is subsequently shaped by appropriate optics 106, 108 before being recombined with a second polarising beamsplitter 98.An optional half-wave retarder 110 can rotate the polarisation of the incoming radiation, and in doing so vary the proportion of power directed into each output beam. Similar schemes may be devised for operation in other regions of the electromagnetic spectrum.

Figure 23 shows a further embodiment of the invention which obviates the need for two detectors when generating the difference signal between the Bessel beam bar code output and the Gaussian beam bar code output. In this scheme, a Bessel beam and a Gaussian beam are combined using one of the above techniques or as described below.

They are each modulated by some electrical or mechanical means so that they transmit alternately, i.e. their signals are interleaved.

A control unit 112 generates two antiphase signals 114, 116 which are applied to a beam generator/combination module 118 so as to produce alternatively switched Gaussian/Bessel beams which are scanned across a bar code 120 by a scanner 122. The reflected signal from the bar code 120 is received by a detector 124. The detector 124 receives a gating or reference signal 126 from the control unit 112 which separates the signals 128, 129 to correspond with the Gaussian/Bessel beam signals from the module 118. The signals 128, 128 are then applied to a signal subtraction/processing unit 130 which provides a data output.

Claims (4)

1. A bar code reader comprising a Bessel beam source, means for scanning the beam across a bar code, detector means for detecting the beam after being scanned across the bar code and a signal processor for producing from the output of the detector an output corresponding to the scanned bar code.

2. A reader as claimed in Claim 1, further comprising a Gaussian beam source, wherein the scanning means comprises means for scanning the Bessel beam and the Gaussian beam across a bar code, the detector means is for detecting both beams after being scanned across the bar code and the signal processor includes a comparator for providing a difference signal from the scanned Bessel and Gaussian beams.

3. A reader as claimed in Claim 2, wherein the Bessel beam source and the Gaussian beam source are common.

4. A bar code reader, substantially as hereinbefore described, with reference to and as illustrated in the accompanying drawings.