WO1982002104A1

WO1982002104A1 - Holographic scanning system

Info

Publication number: WO1982002104A1
Application number: PCT/US1981/001622
Authority: WO
Inventors: Corp Ncr; Brian A Gorin; James A Hardy
Original assignee: Ncr Co
Priority date: 1980-12-12
Filing date: 1981-12-03
Publication date: 1982-06-24
Also published as: DE66608T1; EP0066608B1; DE3176711D1; EP0066608A1; US4333006A; EP0066608A4; CA1156862A; JPS57502142A

Abstract

In a holographic scanning system suitable for scanning bar code indicia, a light beam (54) provided by a laser source (20) is directed to a first set of holograms (42, 44, 46) located on a rotating disc (26), each hologram generating an individual scan beam having a slightly different focal length and direction angle from that of the other holograms. The scan beams are projected such that the depth of focus portions (D<uf>u) of the projected beams overlap so as to provide an enhanced depth of focus for the scanning operation, thereby alleviating the problem of precisely positioning the object to be scanned, and enabling a more effective scanning operation. The light reflected from the bar code is collected by a second set of holograms (48, 50, 52) on the scanning disc arranged to focus the received light at a collection point (62) on an optical detector.

Description

HO OGRAPHIC SCANNING SYSTEM

^'' Technical Field

% This invention relates to optical scanning apparatus of the kind including light beam generating 5 means adapted to generate a light beam, a rotatable member provided with a plurality of first holograms adapted to provide reconstructed light beams arranged to produce a scan pattern at a scanning location, and detection means adapted to detect light provided from 10 said scanning location in response to said scan pattern.

The invention also relates to a method of optically scanning a record member.

Background Art

Optical scanning apparatus of the kind

15 specified is known from U.S. Patent Specification No.

4,224,509. According to this U.S. patent specification bar code indicia on merchandise items are scanned by utilizing a rotatable disc carrying a plurality of holograms to provide a scan pattern of intersecting

20 lines located in the plane of movement of the bar code indicia provided on the merchandise items. This known apparatus can provide acceptable scanning despite vari¬ ations in the orientation of the bar code indicia. However, the known apparatus has the disadvantage that

25 the operating efficiency thereof is unduly limited in practice by the requirement that the bar code indicia be located substantially in the plane of the scan pat¬ tern.

Disclosure of the Invention 30 ₍ It is an object of the present invention to provide optical scanning apparatus of the kind specified wherein the aforementioned disadvantage is alleviated.

Therefore, according to the present invention there is provided optical scanning apparatus of the kind specified, characterized in that said holograms have different focal lengths such that there are pro¬ vided depth of focus portions of the reconstructed light beams which overlap each other, and in that said ro- tatable member is provided with a plurality of second holograms respectively^'associated with said first holo¬ grams and arranged to collect light provided from said scanning location in response to said scan pattern and to direct the collected light towards a collection point associated with said detection means.

According to another, aspect of the invention, there is provided a method of optically scanning a record member, including the steps of generating_^ a light beam, moving a plurality of first holograms through said light beam thereby providing reconstructed light beams, utilizing said reconstructed light beams to provide a scan pattern for scanning said record medium and detecting the light reflected from said record member in response to said scan pattern, char- acterized by the steps of providing said reconstructed light beams to have different focal lengths whereby depth of focus portions of the reconstructed light beams overlap each other, and moving a second set of holograms so as to collect light provided from said scanning location in response to said scan pattern, and directing the collected light towards a collection point.

It will be appreciated that in optical scan¬ ning apparatus according to the invention, since the depth of focus portions of the reconstructed light beams overlap, a wide focus range for the reconstructed light beams is provided thereby reducing the criticality of the positioning of the member being scanned. A fur¬ ther advantage is that the second set of holograms on the rotatable member provides the capability of achieving a low noise collection of reflected light. Still another advantage is that the apparatus is compact in construction and low in cost. Brief Description of the Drawings

One embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of the holographic scanning system of the present invention showing projection lines of the scanning beams generated • by the holograms located in the rotating disc;

Fig. 2 is a schematic representation of the holographic system for reading the UPC bar code;

Fig. 3 is a front view of the rotating disc showing the location of the scanning and collection holograms;

Fig. 4 is a front view of a mirror system used to generate a scanning pattern;

Fig. 5 is an illustration of a scan pattern generated by the scanning apparatus of Figs. 3 and 4;

Fig. 6 is a top view of a schematic represen¬ tation of the method for constructing the holograms located on the rotating disc;

Fig. 7 is a cross-sectional view of the rota¬ ting disc showing details of the construction of the holograms;

Fig. 8 is a block diagram of the present system for reading the UPC bar code;

Fig. 9 is a schematic representation of the scanning beam of light showing the location of the depth of focus portions of the beams.

Best Mode for Carrying Out the Invention Referring now to Fig. 1, there is shown a schematic representation of a UPC holographic scanning system employing the present invention which includes a 1 milliwatt helium-neon laser 20 which directs a light beam to a pair of optical lenses 22, 24, which in turn expands and/or otherwise shapes the light beam incident on a rotating disc 26 driven by a motor 28. As will, be described more fully ^"hereinafter, the disc 26 has located thereon a. plurality of hologram^'s which project the shaped light beam at a mirror 30 which deflects the light beam at a target area through which a UPC label 32 passes. As is well-known in the art, the UPC bar code comprises a plurality of light and dark bars which, when scanned by the scanning apparatus, will generate a specific pulse tra.n waveform. The reconstructed light beam, upon scanning the bar code label 32, is scattered from the bar code surface and part of this scattered light is directed back towards the disc 26, wherein a second set of collection holograms located on the disc 26 direct the collected light beams at a mirror 32 from which they are projected at a photodetector 34. The photodetector 34 generates electrical signals in response to detecting the changing intensity level in the reflec¬ ted light beams. These signals are transmitted to an analog electronic unit 36 which decodes the electrical signals and transmits the decoded data to a digital processor 38. The processor 38 will determine if a valid read has occurred and will notify a utilizing device such as terminal 40 that such a valid read opera¬ tion has occurred.

Referring now to Figs. 2 and 3, there is shown details of the construction of the rotating disc 26

(Fig. 1). As best seen in Fig. 3, the disc 26 comprises a glass plate on which is formed a plurality of scanning holograms 42, 44 and 46 positioned around the peripheral edge of the disc 26 and a plurality of collection holo- grams 48, 50 and 52 which coact with an associated scanning hologram to collect the diffusely reflected light beams from the UPC label 32. As is well-known in the art, a hologram is a recording of all the information in a wave front of light obtained from an object which is illuminated with spatially-coherent monochromatic light, rather than an image of the object obtained in ordinary photography. The term "monochromatic" light, as used herein, means light composed substantially of a single wave length, while "spatially-coherent" light, as used herein, means light emanating actually or apparently from a point source. The hologram consists of the recording of the interference fringes in the wave front covering a given area in a plane resulting from the interference between a first component of light obtained directly from a spatially-coherent monochromatic orig¬ inating light source, which first component is directed to the given area in the plane at a predetermined angle with respect thereto, and a second component of light obtained from the object to be recorded which is illum¬ inated by a light originating from the same light source simultaneously with the first component, the second component being directed at least in part to the given area in the plane of an angle (Fig. 2) other than the aforesaid predetermined angle.

These interference fringes result from the fact that the difference in path length and hence the difference in phase, between the first or reference component of spatially-coherent monochromatic light and the second or information component of spatially- coherent monochromatic light varies from point to point. Therefore, constructive interference between the two components takes place at certain points, and des¬ tructive interference between the two components takes place at other points. Furthermore, the relative ampli¬ tude of the second or information component of such light varies from point to point. This causes a variation in the contrasts of the resulting interference fringes. In this manner, the recorded interference fringes form a pattern which defines both the amplitude and the phase of the second or information components as modulations in the contrast and spacing of the recorded interference fringes. This recorded pattern, which is called a hologram, contains ail the information that can be carried by light waves transmitted through, reflected or scattered from an object. A replica of the wavefront which comprises the second or information component may be constructed by illuminating a hologram with a source of spatially- coherent monochromatic light. In this case, the hologram diffracts light impinging thereon to form two sets of first order diffracted waves, each of which is a replica of the wave that issued from the original object. One of these two sets, the one projected back to the illum¬ inating source, produces a virtual image of the original object, while the other of these two sets produces a real image of the object through the use of a lens. The virtual image is in all respects like the original object and, if the original object was three-dimensional, the reconstructed virtual image shows depth and gives rise to parallax effects between near and far objects in the same manner as did the original dimension object. The real image, however, is pseudo^-scopic, that is, its curvature is reversed with respect^' to the original object, convex regions appearing to be concave, and vice-versa.

As illustrated in Figs. 2 and 3, upon the laser 20 (Fig. 2) propagating a light beam 54 at one of a number of scanning holograms located along the outer perimeter of the disc 26, the light beam 54 will be deflected by the holograms in a 120 degree arc at an angle which varies slightly with each hologram. The scanning arc will have a radius R^ (Fig. 2) which varies with each scanning hologram 42, 44 and 46. A wide range of R.- values for the scanning holograms would result in a proportionately wide range of scan beams and therefore a wide range of tangential spot velocities which in turn would require a wide electronic bandwidth for the photo¬ detector 34 (Fig. 1). Hence, the photodetector 34 would need a greater sensitivity to ambient electronic noise. 3y selecting a narrow range of R.. values there is permit¬ ted a reduction in cost for the detector electronics while permitting enhanced electronic noise rejection. The tangential spot velocities for the scanning holograms 42-46 inclusive (Fig. 3) are located between 534 ft/sec (162.8 metres/sec.) and 555 ft/sec (169.2 metres/sec) to produce spot velocities which vary by less than one per cent. The deflected light beam 56 will be focused at a point adjacent a target area represented by the line 58 in Fig. 2. Each of the scanning holograms 42-46 inclu¬ sive is constructed to have a different focal length which results in each of the deflected light beams 56 overlapping each other adjacent the plane 58 to produce an enhanced depth of focus formed by each of the scan¬ ning beams deflected by the holograms 42-46 inclusive. As shown in Fig. 9, the scan beam 56 projected by each of the scanner holograms 42-46 inclusive includes a portion D- characterized as the depth of focus in which the minimum spot size occurs at the center of the depth of focus portion. The scanning holograms 42-46 inclusive (Fig. 3) will project their light beams at the plane 58 (Figs. 2 and 9) at which a UPC label 32 or the like is located. As shown in Fig. 9, this scanning action results in the depth of focus portions of each of the scanning beams 56 overlapping each other adjacent the plane 58 through which a bar coded label passes. Providing each of the scanning holograms 42-46 inclusive with a different depth of focus enables the scanning system to scan a bar coded label which may be orientated on the article passing through the target area at an angle with the plane 58, thereby increasing the rate of success of the scanning operations. As shown in Fig. 3, the rotating disc 26 further includes a plurality of wedge-type collection holograms 48, 50 and 52, each of which is constructed so that the diverging point source characteristics of the reference wave of each of the collection holograms corresponds directly with the scan beam point source of its associated scanning hologram. This arrangement is illustrated in Fig. 2, wherein the reflected diffused wavefront 60 is directed at. the collection holograms at the angle allowing the holograms to focus a recon¬ structed scanning beam at a point 62 at which is located a stationary optical detector allowing the detector to ■ generate the proper signals used in reading the bar coded label 32 (Fig. 1).

Referring now to Fig. 4, there is shown a front view of a mirror system corresponding to the mirror 30 (Fig. 1) which may be used to redirect the deflec- ted scanning beams generated by the scanning holograms located in the rotating disc 26 at the target area. As .^' shown in Fig. 4, the deflected scanning beam 56 (Fig. 2) ^• will produce parallel scanning arcs 64 against the mirror 63C which are reflected against the mirrors 63A-63F in- . elusive, thereby producing the crossed hatch scanning pattern generally indicated by the numeral 65 (Fig. 5). Each of the scan lines shown in Fig. 5 which constitute the scan pattern is composed of scanning beams generated by more than one of the scanning holograms. The follow- ing Table illustrates the mirror sequence for generating^' the scan line as numbered in Fig. 5 for each of the scanning holograms 42, 44 and 46 as identified in Fig. 3.

MIRROR SCAN LINE

HOLOGRAM SEQUENCE NUMBER COMMENT

1 (42) B,E 1.1 Vertical, Left

C,E 1.2 Horizontal, Left

C,F 1.3 Center

C,D 1.4 Horizontal, Right

A,D 1.5 Vertical, Right

2 (44) B,E 2.1 Vertical, Left

C,E 2.2 Horizontal, Left

- ^{' """}.-- . ^C'^F 2.3 Center

C,D 2.4 Horizontal, Right

A,D 2.5 Vertical, Right

3 (46) A,E 3.1 Vertical, Left

C,E 3.2 Horizontal, Left MIRROR SCAN LINE

HOLOGRAM SEQUENCE NUMBER COMMENT C,6 3.3 Center C,D 3.4 Horizontal, Right A,D 3.5 Vertical, Right

Referring now to Fig. 6, there is shown a schematic representation of a top view of the method for constructing the rotating disc 26 of Fig. 1. The output beam of a 50-80 milliwatt helium-neon laser 66 is direc- ted toward a mirror system comprising mirrors 68, 70 and 72 which deflect the light beam into a variable beam splitter 74 through a shutter 76 with the beam splitter 74 splitting the beam into two segments 78 and 80. The beam segment 78 is reflected from a mirror 82 to a variable-position mirror 84 which is orientated in one of three positions as shown, wherein each position is associated with the fabrication of one set of the scan¬ ning and collection holograms 42-52 inclusive (Fig. 3). From the mirror 84 the light beam segment 78 is reflec- ^• ted through a spatial filter/microscope objective assem¬ bly 86 which provides a translating off-axis point source with the corresponding position of the variable position mirror 84. The resulting diverging wave front functioning as the object beam will be centered on a rotating disc holder assembly 87 mounted for rotation and supporting the rotating disc 26 (Figs. 2 and 3) in a position for the fabrication of the scanning and collec¬ tion holograms.

As best seen from Fig. 7, the rotating disc 26 at the time of fabricating the holograms 42-52 inclusive comprises a glass substrate 88, the face of which is coated with a silver-halide emulsion 90 on which is positioned a movable wedge-shape exposure mask 92 for exposing portions of the silver-halide emulsion to form the interference pattern which constitutes the scanning and collection holograms. Sandwiched between the glass substrate 88 and a se^'cond glass substrate 94 are layers of an anti-halation backing 96, a low viscosity index matching fluid 98, and processed silver grains 100.

Referring again to Fig. 6, it is seen that the light beam segment 80 projected by the splitter 74 will be reflected by a mirror 102 toward a microscopic ob¬ jective/spatial filter assembly 104 which generates an expanding light beam 108. The light beam 108 i's directed to a lens element 110 which projects a collimated beam 114 to a second lens element 112 which in turn converges the collimated beam 114 toward the disc holder assembly 87.

In the operation of the method disclosed in Fig. 6, the rotating disc 26 with a properly-dimensioned collection hologram exposure mask 92 (Fig. 7) located on the face of the disc 26 is mounted in the holder assem¬ bly 87. With the variable position mirror 84 and the spatial ilter/microscope objective assembly 86 adjusted to a first location, the shutter 76 is activated to expose the face of the disc 26 in which the mask 92 permits only one pie-shaped wedge area of the loaded silver-halide disc to be exposed for each angular rota¬ tional position of the disc 26. The mask 92 is then covered completely and the holder assembly 87 rotated to the next position for exposure of the second collection hologram. The variable position mirror 84 and the objective assembly 86 are readjusted to locate the focal point at a distance which differs with that of the other collection holograms. After the third collection holo- gram has been exposed, the lens member 112 is removed resulting in the collimated beam 114 being directed at the disc 26.

At this time the mask 92 associated with the collection holograms 48-52 inclusive is replaced with a mask for exposing the scanning hologram 42-46 portion of the rotating disc 26. The sequence of adjusting the position of the variable position mirror 84 together with the objective assembly 86' and the holder assembly 87 is, in the manner described above, repeated, which allows the scanning holograms 42-46 inclusive (Fig. 3) to be fabricated. The exposed rotating disc 26 may be 5 processed in any conventional manner as is well-known in the art.

Referring now to Fig. 8, there is shown in block form a flowchart of the operation of the scanning system disclosed in Fig. 1. Operation of the helium- 10 neon laser 20 (block 116) results in the projection of a scanning beam to the beam forming optics, an example of which may comprise the expansion and collimation lenses 22, 24 (block 118) from which the collimated scanning beam will impinge on the scanning holograms 42-46 inclu- 15 sive (Fig. 3) of the rotating disc 26 (block 120), each^' of which deflects the scanning beam to the pattern focusing mirror 30 (block 122) which in turn directs the scanning beam at the target area where the beam will scan the bar coded label 32 (block 124) in a scan ^'20 pattern (Fig. 5) as determined by the arrangement of the mirrors 63A-F inclusive (Fig. 4). The scanning beam is reflected from the label 32 as a modulated diffusely- reflected optical signal (block 126) which is deflected by the pattern folding mirror 30 (block 128) towards the 25 collection holograms in the rotating disc 26 (block

130). The collection holograms 48-52 inclusive focus the collected light beams (block 132) through a filter (block 134) at a point occupied by the photodiode 34 (block 136) which converts the received light beams into 30 electrical signals, which signals are then processed (block 138) and then decoded (block 140). If a valid read operation has occurred, a display on the scanner 40 is energized (block 142) indicating such a condition to the operator.

C PI

Claims

CLAIMS :

1. Optical scanning apparatus, including light beam generating means (20) adapted to generate a light beam (54), a rotatable member (26) provided with a plurality of first holograms (42, 44, 46) adapted to provide reconstructed light beams arranged to produce a scan pattern (65) at a scanning location, and detec¬ tion means (34) adapted to detect light provided from said scanning location in response to said scan pattern (65), characterized in that said holograms (42, 44, 46) have different focal lengths such that there are pro¬ vided depth of focus portions (D_f) of the reconstructed light beams which overlap each other,^" and in that said rotatable member is provided with a plurality of second holograms (48, 50, 52) respectively associated with said first holograms (42, 44, 46) and arranged to col¬ lect light provided from said scanning location in response to said scan pattern (65) and to direct the collected light towards a collection point (62) asso¬ ciated with said detection means (34).

2. Optical scanning apparatus according to claim 1, characterized in that each of said first holo¬ grams (42, 44, 46) is constructed from a reference beam and an object beam and each of said second holograms (48, 50, 52) is constructed in association with the respective associated one of said first holograms (42, 44, 46), each of said second holograms (48, 50, 52) being constructed such as to focus on said collection point (62) a converging light beam corresponding to the reference beam of its associated first hologram (42,

44, 46) and reconstructed from the light provided from said scanning location.

3. Optical scanning apparatus according to claim 2, characterized in that said rotatable member 3. ( concluded ) includes a disc (26) having said first holograms (42, 44, 46) provided thereon adjacent the periphery thereof and said second holograms (48, 50, 52) provided thereon adjacent the respective associated first holograms (42, 44, 46).

4. Optical scanning apparatus according to claim 3, characterized in that said first holograms (42, 44, 46) are constructed to provide reconstructed light beams at respectively different angles with res¬ pect to the generated light beam (54) incident on said first holograms (42, 44, 46).

5. Optical scanning apparatus according to claim 4, characterized in that said reconstructed light beams are projected by said first holograms (42, 44, 46) so as to provide parallel scanning paths (64), there being provided a directing system (63A-63F) adapted in response to the provision of said parallel scanning paths (64) to provide said scan pattern (65).

6. Optical scanning apparatus according to claim 4 or 5, characterized in that the spot velocities produced at said scanning location in response to the respective reconstructed light beams vary by less than one per cent.

7. A method of optically scanning a record member (32), including the steps of generating a light beam (54), moving a plurality of first holograms (42, 44, 46) through said light beam thereby providing recon¬ structed light beams, utilizing said reconstructed light beams to provide a scan pattern (65) for scanning said record medium and detecting the light reflected from said record member (32) in response to said scan pat¬ tern (65), characterized by the steps of providing said 7 . ( concluded ) reconstructed light beams to have different focal lengths whereby depth of focus portions (D-) of the reconstructed light beams overlap each other, and moving a second set of holograms (48, 50, 52) so as to collect light pro¬ vided from said scanning location in response to said scan pattern (65), and directing the collected light towards a collection point (62).