Multifocal holographic scanning system

A holographic scanning system for scanning bar code indicia is disclosed in which the light beam of a laser is directed to a first set of holograms located on a single rotating disc in which each hologram will generate an individual scan beam having a slightly different focal length and direction angle from that of the other holograms. The generated scanning beams are directed on a target area through which passes a label or object bearing a bar code indicia. Each of the scan beams is projected in an overlapping relationship on the target area, thereby providing an enhanced depth of focus enabling a more effective reading operation. The light reflected from the bar code indicia is picked up by a second set of holograms mounted on the rotating disc and focused at a point at which is located an optical detector for use in reading the bar code.

BACKGROUND OF THE INVENTION 
In present-day merchandising point-of-sale operations, data pertaining to 
the purchase of a merchandised item is obtained by reading data encoded 
indicia such as a bar code printed on the merchandised item. In order to 
standardize the bar codes used in various point-of-sale readout systems, 
the grocery industry has adopted a uniform product code (UPC) which is in 
the form of a bar code. Various reading systems have been constructed to 
read this bar code, including hand-held wands which are moved across the 
bar code and stationary optical reader systems normally located within the 
check-out counter and in which the bar code is read when a purchased 
merchandise item is moved across a window constituting the scanning area 
of the counter, which movement is a part of the process of loading the 
item in a baggage cart. 
Various scanning systems presently available utilize a rotating mirror for 
generating a scan pattern used for reading the bar codes. Other systems 
utilize holograms for generating a scanning pattern on a target area 
through which the object bearing a bar code indicia passes. The light 
reflected from the bar code indicia is used in reading the bar code. An 
example of this type of scanning system may be found in U.S. Pat. No. 
4,224,509, issued in the name of C. C. K. Cheng, which is assigned to the 
assignee of the present application. In this patent, a rotating disc 
supports a plurality of holograms, each of which produces a scanning line 
on a target area off-set to the lines generated by the other holograms, 
thereby producing a wide scanning area through which the object bearing a 
UPC coded label passes. Another example of a holographic scanning system 
is found in U.S. Pat. No. 4,113,343, in which a moving hologram generates 
a locus of points from a stationary light beam on a target area with the 
same hologram collecting the light being reflected from the document at 
each of the locus points and focuses this energy onto a stationary 
detector to provide electrical signals corresponding to the information 
scanned on a document. 
In all of the prior art patents, the projected scan beams are all focused 
in a single plane which is located in the plane of movement of the object 
bearing the coded label. Since many mechandise items support the coded 
label at various angles to the focal plane of the scanning hologram, 
complex and expensive optical reflecting systems have been developed to 
transmit the scanning beam at different angles to cover all possible 
orientations of the label to ensure a valid scan operation. This 
requirement limits the operating efficiency of such scanning systems, 
while increasing their cost. It is therefore an object of this invention 
to provide an improved scanning system for projecting a multiple-line scan 
pattern in a bar code reader which provides an enhanced depth of focus at 
the target area and thus allowing the reading of a label on an object to 
take place irrespective of the orientation and position of the label. It 
is a further object of this invention to provide a scanning system which 
is high in reading efficiency, while low in cost. 
SUMMARY OF THE INVENTION 
In order to fulfill these objectives, a scanning system is provided which 
comprises optical means for directing the multiple beams of a laser 
through a plurality of holograms mounted on a rotating disc, each hologram 
having a different focal point, thereby projecting the laser beam along a 
predetermined path across a scan area located in the path of movement of a 
bar code indicia. Each of the holograms will project its focused scan beam 
in an overlapping relationship with the other scan beams to produce an 
enhanced depth of focus of the beams on the label bearing object. A second 
set of holograms located on the rotating member collects the reflected 
scanning beams from the object and focuses the beams on an optical 
detector which generates electrical signals corresponding to the 
modulation of the scanning beams received for use by a processor in 
reading the coded label.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
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 holograms 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 train waveform. The deflected 
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 
reflected 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 operation 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 holograms 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 originating 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 illuminated 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 .alpha. (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 destructive interference 
between the two components takes place at other points. Furthermore, the 
relative amplitude 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 all 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 
illuminating 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.sub.N (FIG. 2) which 
varies with each scanning hologram 42, 44 and 46. A wide range of R.sub.N 
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 
photodetector 34 (FIG. 1). Hence, the photodetector 34 would need a 
greater sensitivity to ambient electronic noise. By selecting a narrow 
range of R.sub.N values there is permitted 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 and 555 ft/sec. 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 
inclusive 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 
scanning 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.sub.f characterized as the depth of focus 
in which the minimum spot size W.sub.0 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 .alpha. 
allowing the holograms to focus a reconstructed 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 
deflected 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 a scanning arc 64 against the 
mirror 63C which is reflected against the mirrors 63A-63F inclusive, 
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 following 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 
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 directed 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 scanning and 
collection holograms 42-52 inclusive (FIG. 3). From the mirror 84 the 
light beam segment 78 is reflected through a spatial filter/microscope 
objective assembly 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 collection holograms. 
As best seen from FIG. 7, th 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 second 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 objective/spatial filter assembly 104 which generates an 
expanding light beam 108. The light beam 108 is 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 assembly 87. 
With the variable position mirror 84 and the spatial filter/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 rotational 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 hologram 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 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-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 
inclusive (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 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 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 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. 
Although the preferred embodiment only of the present invention has been 
described herein, it is not intended that the invention be restricted 
thereto, but that it be limited only by the true spirit and scope of the 
appended claims.