GB1595487A - Light scanning apparatus - Google Patents

Description

(54) LIGHT SCANNING APPARATUS (71) We, FUJITSU LIMITED, a Company organized and existing under the laws of Japan of 1015, Kamikodanaka, Nakahara-ku, Kawasaki, Japan, do hereby declare the invention for which we pray that a Patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: The present invention relates to a light scanning apparatus which scans an object with a converged light beam radiated from a laser beam source by passing the light beam through a hologram.

The light scanning apparatus according to the present invention can be applied to any system wherein a light spot is to be scanned across a surface. However, the following explanation refers to a light scanning apparatus which is employed to read data from articles at a point-of-sale. Such a system will hereinafter be called a P.O.S. monitoring system. In a commodity market, it is important to have an account of the goods in-stock, to be able to calculate the volume of sales and to classify the goods which have been sold. If the goods to be traded are small in quantity and few in varieties it is easy, then, to maintain an inventory of which goods are in stock, to calculate the volume of sales, and to classify the goods which have been sold. However, it is not so easy to carry out the above procedures when dealing with an enormous quantity and a large variety of goods.In recent years, the above-mentioned POS monitoring system has been proposed. By utilizing the POS monitoring system, a large quantity and variety of goods can be monitored by using a computer, and accordingly, calculation and classification of the sales and supply of goods can be performed automatically by virtue of the computer. In this POS monitoring system, wherein goods are monitored by the computer, information concerning each of the goods is marked directly on the goods in advance. Such information is marked on an outer surface of each of the goods by attaching a so-called bar-code label thereto. The bar-code is usually arranged by using UPC (Universal Product Code) symbols. The information on the bar-code with regard to each of the goods, can be read and detected by the aid of a light scanning apparatus and a photo-sensor which is connected to an input of the computer.The light scanning apparatus provides a coherent light beam such as a laser beam which scans the bar-code while the photo-sensor receives the coherent light beam reflected by the bar-code label. The intensity of the reflected coherent light beam subsequently changes in accordance with the bar-code. The computer reads the information in accordance with changes in the intensity of the reflected coherent light beam provided from the photo-sensor which are in the form of a train of electric pulses.

One example of a suitable light scanning apparatus has already been disclosed in Figure 10 of our British Patent No. 1,556,174. This example will be described hereinafter. In this example, it is required to increase the length of the scanning line on the object without increasing the length of the hologram. It is already known that in order to increase the length of the scanning line without increasing the length of the hologram, 1) the surface to be scanned must be positioned beyond the focal plane of the hologram.

2) a small aperture laser beam, as opposed to the large aperture plane wave front laser beam and the spherical wave front laser beam used for recording the hologram, must be employed as the reconstruction wave, 3the narrow laser beam must be comprised of a spherical wave front laser beam, and; 4) the centre of the spherical wave front of the reconstruction beam must be located further from the hologram than the centre of the spherical wave front of the spherical wave front laser beam used for recording the hologram. Thus, the length of the scanning line can be easily increased without increasing the length of the hologram by utilizing a reproduction laser beam which satisfies the requirements stated in the above items 1), 2), 3) and 4).

However, the reconstructed beam according to the above requisites 1), 2), 3) and 4) suffers from a defect in that the image plane, which is defined by the locus of the focal points of the reconstructed beam when the reconstructing beam and the hologram are moved relative to each other is a curved surface and not a flat plane. The image surface exhibits so-called field curvature. It should be noted that the image surface must be a flat plane in order to read the bar-code correctly. If image field curvature is present, the bar-code cannot be read correctly at all due to the fact that the size of the light spot produced by the reconstructed beam on the bar-code label varies with scan angle and becomes relatively large at the extremities of the scan.It should be noted that the bar-code is comprised of many bars arranged in parallel to each other and also that the width of each is very narrow, for example, 0.3 mm. Accordingly, when the size of the light spot is relatively large, each bar of the bar-code cannot be illuminated sequentially one by one. For example, two or more bars of the bar-code may be illuminated by the reconstructed beam at the same time.

As mentioned above, although the reconstructed beam produced in accordance with the above items 1), 2), 3) and 4) is useful for increasing the length of the scanning line without increasing the length of the hologram, this beam is not useful from the viewpoint of obtaining a high-quality POS monitoring system for reading a bar-code on the bar-code label.

Therefore, is it an object of the present invention to provide a light scanning apparatus which is useful for use in for example, an effective POS monitoring system for reading the bar-code on a bar-code label, wherein the above mentioned problems are obviated or mitigated.

According to the present invention there is provided a light scanning apparatus comprising a disc having a plurality of holograms disposed in succession on a circle concentric with the disc a laser beam source radiating a laser beam, said laser beam acting as a reconsturction wave, and a motive means for moving said laser beam and said disc relative to each other so that the recontructing wave is directed sequentially onto said holograms, whereby each of said holograms causes the reconstructed beam to converge to a focal point in a scanning plane at which an object to be scanned may be placed, the focal point being moved in a scanning direction as the hologram moves relative to the recontruction wave, characterised in that said reconstructing laser beam has a spherical wave front, and that at least one of said holograms is recorded by a process in which both a plane wave front laser beam and a spherical wave front laser beam are incident a photosensitive layer coated on a transparent plate, the arrangement being such that the direction of propogation of said plane wave front laser beam intersects the photosensitive surface at a predetermined angle of incidence offset from 90".

The construction and operation of the present invention will become more apparent and better understood from the following detailed description in conjunction with the accompanying drawings, in which: Figure 1 is an enlarged plane view showing a conventional bar-code label; Figure 2 is an enlarged plane view of a conventioanl bar-code label and some possible scanning lines, in which any one of the scanning lines traverses the entire length of the bar-code; Figure 3 is an enlarged plane view of a conventional bar-code label and some possible scanning lines, in which none of the scanning lines traverses the entire length of the bar-code; Figure 4 schematically shows one example of a conventional two-dimensional scanning pattern exhibiting a right-crossing mode; ; Figures 5A and 5B, respectively, are explanatory perspective views of a hologram and a laser beam which illuminates the hologram and a reconstructed beam which radiates therefrom; Figure 6 is a pictorial view showing a prior art light scanning apparatus. This Figure corresponds to Figure 10 of our British Patent No. 1556174.

Figure 7A is a plan view showing the conventional method for constructing the hologram used in the prior art in the apparatus of Figure 6; Figure 7B is a plan view showing a method for constructing a hologram used in a light scanning apparatus according to the present invention Figure 8 is a plan view of a hologram made by the process corresponding to Figure 7A; Figure 9 is an illustrative view showing the optical properties of a conventional hologram made in accordance with Figure 7A; Figure 10 illustrates how image field curvature is produced; Figures 11 through 14 are respective views illustrating the occurrence of image field curvature.

Figure 15 is a table showing the different shapes of light spots in relation to varying diffraction angles; Figure 16 is a graph showing the profiles of the curvatures of the imaging fields; Figure 17 is a graph showing the profiles of the curvatures of the imaging fields with respect to each respective offset angle P, according to the present invention; Figure 18 is a graph of the curves utilized for determining the offset angle P from the deflection multiplying factor M and the central diffraction angle p (x), according to the present invention; Figure 19 is a graph showing an improvement of the profiles of the curvatures of the imaging fields shown in Figure 17 with respect to each respective offset angle P, according to the present invention Figure 20 schematically illustrates a sectional and front view of the POS terminal device of the prior art;; Figure 21 schematically illustrates a sectional and front view of the POS terminal device comprised of a light scanning apparatus according to the present invention and; Figure 22 is a graph utilized for explaining the steps of determining the offset angle P from a rotating angle Q shown in Figure 20, according to the present invention.

Figure 1 shows an enlarged plane view of a bar-code lable, a respective one of which is attached to the outer surface of each of the goods to be scanned (not shown). In Figure 1, numeral 11 represents a bar-code label. Numeral 12 is an example of a bar-code. The bar-code 12 is usually printed on the surface of the bar-code lable 11. The bar-code 12 uses a series of block bars in accordance with the UPC code to represent data, such as maker, category and date of production of each of the goods (not shown) to which the bar-code label 11 is attached. The information with regard to each of the goods is read and processed by a computer (not shown), wherein the reading is automatically done by a combination of a light scanning apparatus and a photo-sensor, which are not shown in Figure 1.The light scanning apparatus provides a coherent light beam (not shown), such as a laser beam, which scans the bar-code 12 traversely while the photo-sensor receives the coherent light beam reflected by the bar-code 12. The intensity of the reflected coherent light beam changes in accordance with the arrangement of the bar-code 12.

The coherent light beam provided from the light scanning apparatus, scans the bar-code by traversing the entire bar-code, such as is shown in Figure 2. In Figure 2, lines 21a, 21b and 21c denote possible scanning lines. Any one of the scanning lines 21a, 21b or 21c may be used to scan the bar-code 12, and will traverse the entire length of the bar-code.

However, if the bar-code lable is located at some particular angle with respect to the light scaning beam, the coherent light beam may not be able to scan the entire bar-code. That is, if the bar-code label is located at some particular angles such those shown in Figure 3, then none of the scanning lines 21a, 21b or 21c will be able to scan the entire length of the bar-code 12. Under such conditions it is impossible for a computer to read information without producing any errors. Therefore, since each of the scanning lines 21a, 21b and 21c is one-dimensional, it is difficult for a computer to read information without producing any errors, especially when the bar-code label is orientated at some particular angle with respect to the scanning line.

Accordingly, to prevent the above error-producing problem, two-dimensional scanning is required. When two-dimensional scanning of the bar-code is utilized, such as is shown in Figure 4, it is then assured that the computer will be able to read information without producing errors even though the bar-code label is orientated at any angle. Figure 4 shows a two-dimensional scanning pattern which exhibits a right-crossing mode.

The principle of the light-scanning method employed in a light-scanning apparatus according to the present invention is illustrated in Figures SA and SB. In Figure SA. a coherent light beam 51 i.e., a laser beam, is projected onto a point 52 of a hologram 53. The laser beam 51 is then transmitted through the hologram 53 so that it reaches a surface 54 to be scanned. The hologram 53 is so made that when it is shifted parallel to the arrow 55 perpendicular to the direction of the fixed laser beam 51, the light spot 56 traverses the scanned surface 54 parallel to the arrow 57, thereby obtaining one scanning line 58.Since the hologram 53 has a diffraction grating provided with a radial gradient of the spatial frequency and the frequency therefore changes gradually toward a high frequency in accordance with the movement of the light spot 56 from the center of the hologram 53 and with the movement to the periphery thereof, the hologram 53 acts as a conventional convex lens. The light spot 56 is formed by diffraction of the laser beam 51. In Figure SB, if the hologram 53 is alternately shifted parallel to the arrow 55 and parallel to the arrow 59, light spots 56' and 56" each scan the image suface 54 in a horizontal direction parallel to the arrow 57, and thereby scanning lines 58' and 58" are alternately obtained.

Figure 6, is a pictorial view showing a light scanning apparatus, employing conventional holograms and corresponding to Figure 10 of the British Patent No. 1556174. In Figure 6, both first holograms 61-1, 61-2 to 61-4 and second holograms 62-1 62-2 to 62-12 are secured into a circular disk 63. These holograms are arranged along one circular locus defined by a central point 0. A laser beam 64 is reflected toward the circular disk 63 by a mirror 65. The laser beam 64 radiated by a laser source 77 is then transmitted through a convex lens 66 to fixedly illuminate an area 67 on the circular disk 63.When the circular disk 63 is rotated in a direction along arrow 60 by an electric motor 68, each of the holograms 61-1, 61-2 provides a scanning laser beam 69 which illuminates a prism 70, while each of the holograms 62-1, 62-2 ... provides a scanning laser beam 71 which illuminates a Dove prism 72. This is because, each of the holograms 61-1, 61-2 . is previously designed so that the scanning laser beam which is transmitted therethrough may be directed to the prism 70 by diffraction, and also each of the holograms 62-1, 62-2 ... is previously designed so that the scanning laser beam which is transmitted therethrough may also be directed to the Dove prism 72 by diffraction. The scanning laser beam 69 which is transmitted through the prism 70, forms one of the corresponding scanning lines 73 which extend from left to right in Figure 6.The scanning beam 71 which is transmitted through both the Dove prism 72 and a prism 74, forms one of the corresponding scanning lines 75 which extend from top to bottom on the image surface in Figure 6. It should be recognized that the direction of the deflection of the scanning laser beam 71 can easily be adjusted by the Dove prism 72 at any angle of from 0 through 360".

In Figure 6, numeral 76 represents a part of a POS terminal device. The device 76 has a transparent window 78 for scanning the bar-code label shown as 11 in Figure 1. When each of goods (not shown in Figure 6) is located above the window 78 and when the bar-code label which is attached to the outer surface of each of the goods faces towards the window 78, the scanning laser beams 79 and 80 scan the bar-code (shown as 12 in Figure 1) with a scanning pattern having a right-crossing mode.

Each of the scanning laser beams 79 and 80 is reflected by the bar-code label, and the reflected scanning laser beam 82 is collected by a condenser lens 83, if necessary, for illuminating a photo-sensor 84 comprised of a photomultiplier tube. The intensity of the reflected scanning laser beam 82 changes in accordance with the bar-code, and the changes of the intensity are converted to a train of electric pulses by the photo-sensor 84. This train of electric pulses from the photo-sensor 84 is demodulated by a demodulator 85. The output signals are then transmitted to a central processing unit (CPU) (not shown in Figure 6).

Figure 7A is a plan view showing the conventional method for constructing a hologram.

In Figure 7A, numeral 90 denotes a transparent glass plate with a photosensitive layer coated thereon. A hologram to be produced is formed in the photosensitive layer. A laser source (not shown) provides a laser beam, and the laser beam is separated into two laser beams 91 and 92 by means of a semi reflector (not shown). The first laser beam 91 is a plane wave, i.e., the so-called reference wave. The second laser beam 92 is a spherical wave.

Interference fringes are produced on the photosensitive layer of the plate 90. The interference fringes form a hologram. The symbol AO, indicates a predetermined deflection angle between the plane wave 91 and the spherical wave 92. The symbol AO' indicates an angle of intersection between the plate 90 and the plane wave 91, and it should be noted that the angle of intersection AO' is a right angle (90").

Figure 8 is a plan view of a hologram made by the process corresponding to Figure 7A. In Figure 8, an area A corresponds to a plan view of the hologram seen from the direction of the arrow 8 in Figure 7A. In Figure 8, a hologram 93 has interference fringes therein which form a plurality of concentric circles. However, only ten concentric circles are shown in Figures 8. This hologram 93 is essentially a fresnel zone plate. A hologram 93 is divided into a plurality of hologram pieces 94-1 through 94-4 (not shown to scale) which are distributed on the circular disk 63 (Figure 6) as holograms 61-1 through 61-4 (Figure 6), respectively.

Another hologram 93 is divided into a plurality of hologram pieces 95-1 through 95-12 (not shown to scale) which are distributed on the circular disk 63 (Figure 6) as holograms 62-1 through 62-12 (Figure 6), respectively.

Returning to Figure 6, the sum total length of all the scanning lines 73 and 75 added end to end is generally quite long, for example about 3 m. The sum total length of the holograms 61-1 through 61-4 and 62-1 through 62-9 added end to end also corresponds to about 3 m.

Therefore, the diameter of the circular disk 63 will be almost the same as equivalent to 1 m (3/n m). As a result of these measurements, the POS terminal device, a part of which is referenced by numeral 76, becomes very large in size.

Next, the conventional method for decreasing the total length of each of the holograms 61-1 through 61-4 and 62-1 through 62-12 (Figure 6), without decreasing the total length of the scanning lines 73 and 75, is described with reference to Figure 9. Figure 9 is an illustrative view showing an optical chart produced by a conventional hologram.

In Figure 9, the hologram 93 moves on and along a flat plane 97 defined by the coordinates X-Y in Figure 9. Along and on a focal plane 97' defined by the coordinates X'-Y', a focal point S' moves in accordance the movement of the hologram 93. This focal point is obtained by imaging a collimated laser beam 98 through the hologram 93. Along and on an image plane 97" defined by the coordinates X"-Y", a light spot S" moves in accordance with the movement of the hologram 93. Each intersection point of the coordinates X-Y, X'-Y' and X"-Y" is positioned in the Z-axis.

In Figure 9, the movement of the focal point S' on the focal plane 97' is proportional to the movement of the center 0 of the hologram 93 on the flat plane 97. Accordingly, the coordinates (x, y) for defining the center 0 are the same as the coordinates (x', y') for defining the focal point S'. In this case, the coordinates of the light spot S" are defined as (Mx, My), wherein the number M is greater than 1 (M > 1) because the image plane 97" is located far from the focal plane 97' with respect to the flat plane 97. M is known as the deflection multiplying factor and is derived from the following equation (1): M = ft (1) fH where e is a distance between the planes 97 and 97", and fH is a distance between the planes 97 and 97'. Thus, the amount of deviation of the scanning light spot S" is increased to M times the amount of deviation of the moving center 0.Accordingly, in Figures 6, the length of each hologram may be decreased without decreasing the length of the corresponding scanning line. However, in Figure 9, since the laser beam 98 is a plane wave according to conventional prior art practice, the imaging light spot S" on the image plane 97" is a spot whose size is directly related to (e - fH). Therefore, a high resolving power for reading a bar-code cannot be obtained. In this respect, it is desirable to increase the imaging distance fH in order to form a focal point on the image plane 97". An increase of the imaging distance may be achieved by using a laser beam 98 which is a spherical wave and not a plane wave.

Thus, a focal point can be provided on the image plane 97". In this case, the following equation (2) is obtained.

1 1 1 (2) a e fH where a is the distance between a spherical wave source 99 and the plane 97. Thereby, the focal point of a coherent spherical wave 98 can be provided on the image plane 97" by locating the source at the appropriate distance a behind plane 97. A disadvantage is brought about by the fact that the locus of focal points form a curved image surface. The field curvature of the image is shown in Figure 10. Figure 10 illustrates how the field curvature is produced. Accordingly, when the hologram 93 and the spherical wave source 99 move relative to each other, the focal points will fall on a curved imaging field 100.The reason why the imaging field is not flat but is curved is that the reproduction wave 99 (Figure 10) is not identical with either of the hologram forming laser beams 91 and 92 (Figure 7A).

Generally, a laser beam functioning as a reproduction wave has characteristics matched to one of the beams used to generate the hologram. A plane wave laser beam and a spherical wave reference wave laser beam were both used for contructing the hologram. but the character and arrangement of the "source" of the spherical wave from the hologram plane was not the same as that of the reconstructing wave 99.

The occurrence of the field curvature of the image will be explained in more detail.

Figure 11 is the first of four illustrations which describe the occurrence of the field curvature of the image. In Figure 11, interference fringes 102 are formed in a recording medium 103 comprised of a photosensitive layer (refer to description of the photosensitive layer with regard to Figure 7A) by illuminating the recording medium 103 with two different coherent waves 104 and 105. The coherent plane waves 104 and 105 are directed to the photosensitive layer 103 at incidence angles 0 and 02, respectively, with respect to a normal L of the recording medum 103. The interference fringes 102 are formed parallel to each other at an angle ( 20l)/2 (see Figure 11).The diffraction grating pitch thereof (see dG in Figure 11) is derived from the following equation (3), in which the thickness of the recording medium 103 is not taken into consideration because the thickness is very small:

where, A is a wave length of both coherent waves 104 and 105.

Figure 12 is a second illustration which describes the occurrence of the field curvature of the image. In Figure 12, the reference numeral 113 schematically indicates a hologram produced in the recording medium 103 (Figure 11). The symbol a represents the angle of incidence of a coherent reproduction wave 114, and the sybol p represents the diffraction angle of a recontructed imaging wave 115. The relation between the angles a and ss is defined by the following equation (4):

In the above equation (4), when the angles a and ss are located on the same plane, that is, above the normal L (Figure 11) or below the normal L (Figure 11), these angles a and p will have the same positive or negative signs, respectively.From the above equations (3) and (4), the following equation (5) is derived: ss = sin-l (sin0, + sin02 - sina) (5) From the above equation (5), the diffraction angle ss is determined, after the angles 0, and 2 (Figure 11) and the angle a (Figure 12) have all been determined.

In Figure 13, which is the third view describing the field curvature of the image, when a spherical wave 121 and a plane wave 122 are directed onto a recording medium (not shown in Figure 13 but please refer to the member 103 in Figure 11) located on a plane defined by the coordinates X-Y, an interference between the waves 121 and 122 occurs on the X-Y coordinates and a diffraction grating is obtained.The diffraction rating pitches dc (x+Ax), dG (x) and d6 (x-Ax) occurring at points x+#x, x and x-Ax on the ordinate X, respectively, are defined by the following equation (6):

where 6(x+Ax), 0(x) and 0(x-Ax) are angles of incidence of the spherical wave (121) from a spherical wave source 123 directed onto the points x +Ex, x and x -Ax, in Figure 13, respectively. The angles of invidence 0(x+Ex), 0(x) and O(x-Ax) are defined, in Figure 13, by the following equation (7):

where fH is the distance between the spherical wave source 123 and the plane defined by the coordinates X-Y.

In the fourth illustration, Figure 14, a hologram is located between x+Ex and x-Ax on a plane defined by the coordinates X-Y. when a spherical wave source 131 is located at a position (x, a) as shown in Figure 14, incidence angles at the points x+nx, x and x-Ax become +a, 0 and -a, respectively. Furthermore, diffraction angles of reconstructed imaging wave 132 at the points x+Ax, x and x-Ax become p(xtAx), ss(x) and p(x-Ax), respectively. The ss(x) is called the central diffraction angle. The diffraction angles ss(x+Ex), ss(x) and p(x-Ax) are derived from the above equations (4), (6) and (7) and are defined by the following equation (8):

The reconstructed waves 132 converge at a point 133. The symbol e represents the distance between the plane defined by the coordinates X-Y and a plane which is parallel thereto, in which latter plane the point 133 exists. The distance e is called the imaging distance.The value of the imaging distance e can be derived by referring to the methods described with reference to Figures 13 and 14 and to the above equation (8), and is defined by the following equation (9): 2a tan α tan B (x+Ax) - tan p (x-Llix) where a is the distance between the spherical wave source 131 and the plane defined by the coordinates X-Y, and A x may be defined as: A x = a tan a In the above equation (9), when the value of A x is a fixed value and the value of x is gradually increased, in other words, when the central diffraction angle p (x) is gradually increased, the imaging distance e is extended greatly in accordance with the increase of the value of p (x).Due to the extension of the imaging distance e according to the increase of the value of ss (x), the curvature of the imaging field 100 (Figure 10) is produced. It is noted that the curvature of the imaging field cannot be created when the deflection multiplying factor M (See Figure 9) equals 1 (M=1).

As is apparent from the above description regarding the curvature of the imaging field, as p (x) increases further the imaging distance e becomes longer. The central diffraction angle may also be defined as an angle between the direction of the imaging wave and the direction of a zero order wave. The zero order wave is a wave which passes undeflected straight through a hologram.

Returning to Figure 6, the central diffraction angle, that is the aforesaid p (x), of the scanning laser beam 71 is quite large when compared to that of the scanning laser beam 69.

Accordingly, it is inevitable for the scanning lines 75 scanned by the scanning laser beam 71 radiated by being passed through the holograms 62-1 through 62-12 to be subject to a defective field curvature of the image. Therefore, a light spot focused on the bar-code label increases in size, such as that shown in column I at Figure 15. Column I shows light spots (write areas) imaged onto the bar-code label at diffraction angles of 16 , 20 , 24", 28 and 32 of the scanning laser beam 71 with respect to the zero order wave of each of the holograms 62-1 through 62-12, according to the prior art light-scanning apparatus. It is easily understood that the light spots shown in column I do not have a high resolving power for reading the bar-code.Accordingly, the bar-code cannot be read without danger of errors being produced.

Figure 16 is a graph showing different profiles of image field curvatures. In this graph, the ordinate indicates the imaging distance e in millimeters and the abscissa indicates the diffraction angle ss (x) in degrees. The curves Cl, C2, C3, C4, C8 and C11 are profiles of the image field curvatures when the deflection multiplying factors M are 1, 2, 3, 4, 8 and 10, respectively. When the factor M equals 1, no image field curvature is created. However. the sum total length of all the scanning lines 73 (Figures 6) is the same as the total length K (Figure 6) of the holograms 61-1 through 61-4 (Figure 6). Also, the total length of all the scanning lines 75 is the same as the total length K (Figure 6) of the holograms 62-1 through 62-12 (Figure 6).Accordingly, if the total length of the scanning lines 73 and 75 is 3 m, the diameter of the disk 63 (Figure 6) would be about 1 m. In order to decrease the diameter of the disk 63 (Figure 6) to, for example, one fourth of the diameter, while retaining the original scan path dimensions, the deflection multiplying factor M must be selected to be 4 (M=4) and the focal length fH of the hologram must be reduced to 1/4 of the previous value.

When M equals 4, the curve C4 in Figure 16 is obtained. In the case of the curve C4, when the diffraction angle ss (x) is within a range of 0 +10 , the variation of the value A t of the imaging distance e is not so large. Thus, the resolving power of each of the scanning beams, that is the scanning lines 75 (Figures 6), is relatively high.Each of the scanning lines 73 is scanned by the scanning laser beam 69 (Figure 6) radiated from the corresponding holograms with the diffraction angles p (x) being within a range of 001100. Contrary to this, when the diffraction angle p (x) is within a range of 2001100, the variation of the value A et of the imaging distance e is extremely large. Thus, the resolving power of each of the scanning lines 75 (Figure 6) is considerably low. Each of the scanning lines 75 is scanned by the scanning laser beam 71 (Figure 6) radiated from the corresponding holograms with the diffraction angles ss (x) being within a range of 2001100.

The holograms used in the light scanning apparatus according to the present invention will be described hereinafter. The hologram based on the present invention is constructed by applying both a plane wave and spherical wave onto the photosensitive layer. The plane wave is applied onto the photosensitive layer with a predetermined offset angle P0 therebetween, which offset angle is not equal to 90". Figure 7B is a plan view which illustrates a method for producing a hologram employed in a light scanning apparatus according to the present invention. In Figure 7B, the numerals 90, 91 and 92 indicate the same members as those shown in Figure 7A.The difference between the view of Figure 7A and the view of Figure 7B is that the transparent glass plate 90 intersects the plane wave laser beam 91 not at a right angle (see A 0' (=90 ) in Figure 7A) but at a predetermined offset angle P. A hologram which is produced in the photoresistive layer (not shown) coated on the plate 90 in Figure 7B is very useful for producing uniform imaging distances with respect to any of the given central diffraction angles p (x).

The effectiveness of the hologram produced by the process shown in Figure 7B will be clarified by referring to Figure 17. Figure 17 is a graph showing the different profiles of the field curvatures of the images. In this graph, the ordinate indicates the imaging distance e in millimeters and the abscissa indicates the central diffraction angle p (x) in degrees. It should be noted that, although this graph pertains only to the curve C4 in Figure 16, graphs similar to the graph in Figure 17 can be obtained for curves C2, C3, C8 and C1(1 in Figure 16.

In the graph of Figure 17, the curves C4, p5, plo, p,5 and p20 are obtained by using holograms produced by the process shown in Figure 7B with the offset angles P being equal to 00, 50, 15 and 20 , respectively. The hologram produced with an offset angle P of 0 is a conventional hologram. Accordingly, the curve C4 drawn by the solid curve in Figure 17 is a conventional curve which is exactly the same as the curve C4 shown in Figure 16. In Figure 17, when the diffraction angle p (x) changes within a range of 10 < p (x) < 30 , that is, 2001100, the least variation A(' of the imaging distance e is obtained by the curve p,5.The diffraction angles ss (x+ A x), ss (x) and ss (x- A x) are defined by the following equation (8)', which is a modification of the above equation (8):

The imaging distance e can be obtained by substituting the expressions ss (x+ Ax) and 3 (x- Ax) in equation (8)' for the expressions ss (x+ Ax) and ss (x- Ax) in the above equation (9).Accordingly, it is easily recognized from this graph that the most appropriate offset angle P is 15 when the deflection multiplying factor M is set to be 4 and the central diffraction angle ss (x) is set to be 20". As previously mentioned, the variation aet (refer to ael in Figure 16) of the imaging distance 6 is extremely large, and according defective light spots such as those shown in column I of Figure 15 are illuminated on the bar-code label.

Contrary to this, in the present invention, the variation aet (refer to net in Figure 17) of the imaging distance e is very small. Therefore, since grossly defective light spots are prevented from occurring, and small light spots such as those shown in column II of Figure 15 are projected onto the bar-code label, a considerably higher resolving power of the scanning beam for reading the bar-code can be obtained.

In practice, both the deflection multiplying factor M and the central diffraction angle ss (x1 of each hologram are determined at the beginning of the design process for manufacturing the POS terminal device (Figure 6). Accordingly, both the factor M and the angle ss (x) are determined first and the angle P may be determined thereafter.

Consequently, it is convenient to prepare a reference curve for determining the angle P from the determined M and ss (x). One example of the above-mentioned reference curve is shown in Figure 18. In the graph of Figure 18, the ordinate indicates the optimum offset angle P in degrees and the abscissa indicates the deflection amplifying factor M.The curves p25, p20, p15, p10, p5 and ss0 are plotted, respectively, at the central diffraction angles p (x) of 25 , 20 , 15 , 10 , 5 and 0". By using the above reference curve, the optimum offset angle P can be instantly determined for any desired factor M and at any desired angle p (x).For example, the factor M is determined to be 4 and the angle p (x) is determined to be 20 , the optimum angle P may be decided from a point X on the curve 20. The point X provides the optimum offset angle P, that is 15". The offset angle 15 provides, when M=4 and p (x)=20", the least variation aet of the imaging distance e within a range of 20 +10 (refer to ae relating to the curve P15 in Figure 17).

The reason why the offset angle P is ettective for reducing the magnitude of the variation ne of the imaging distance e at any diffraction angle p (x), is not completely clear, but such condition may be due to the following reasoning. Generally, the following equation is obtained; 1 1 1 Y f where X is the distance between the hologram and the reproduction spherical wave source, Y is the imaging distance defined by any single location on the hologram, f is the focal distance defined by any single location on the hologram. In the above equation, the focal distance f gradually increases in accordance with the increase of the distance from the center of the hologram to each of the respective points thereon.Therefore, the imaging distance Y gradually increases in accordance with the increase of the distance from the center of the hologram to each of the respective points thereon. Contrary to the above, in the present invention, it is considered that the focal distance f does not vary in accordance with the increase of the distance from the center of the hologram to each of the respective points thereon due to the aforementioned offset angle P. Thereby, the imaging distance Y also does not vary in accordance with the aforesaid increase. In other words, in the above equation, if f is almost constant, then Y is also almost constant because X is a fixed value.

Through various kinds of experiments regarding the offset angle P, the applicant made the following discovery. The profiles shown in Figure 17 can be improved by suitably determining a particular relation between the wave length kl of the laser beam used for producing a hologram and the wave length 2 of the laser beam used as the reproduction wave. Such improvements of the profiles due to the above discovery will be clarified by referring to Figure 19. In the graph of Figure 19, the curves C4, p5,, pl" and p15, are improvements of the curves C4, p5, Pia and p15 shown in Figure 17, respectively.As is apparent from the graph of Figure 19 in comparison with that of Figure 17, the area of each profile of the curvatures along which the imaging distance e varies by a small amount is relatively wide with respect to the respective diffraction angles ss (x) when compared to the corresponding area of each profile of the curvatures shown in the graph of Figure 17. The curves in the graph of Figure 17 are plotted by using a conventional wave length ratio, that is, 2/ = 1. Contrary to the above, the curves in the graph of Figure 19 are plotted by using a given wave length ratio, wherein 2/, equals not 1 but, for example, 1.3.Thus, if the wave length ratio /i is selected to be over 1, the profile of the imaging distance can be made constant with respect to a relatively wide range of diffraction angles ss (x). For example, the laser beam for producing a hologram is an Ar laser, the wave length of which being 4880A (Xi =4880A). While, the laser beam for producing a reproduction wave is He-Ne laser, the wave length of which is 6328A (k2=6328 ).

In a POS monitoring terminal device, it is convenient for an operator to have a light scanning apparatus which can scan a bar-code label which is attached either to the bottoms or to the sides of goods. Therefore, the scanning laser beam is usually directed onto goods in such a direction that it can scan either the sides or the bottoms of goods. Figure 20 schematically illustrates a sectional and front view of the POS terminal device of the prior art. In Figure 20, a laser beam scanning device 201 radiates a scanning laser beam 202. The reference numerals 76 and 78 represent, respectively, a part of the POS monitoring terminal device and a transparent window. These members 76 and 78 are also shown in Figure 6. Goods 203 each displaying a bar-code label are manually moved by the operator above and along the transparent window 78 and in a direction indicated by the arrow 200.

The bar-code label 11 may be attached to the sides of goods as shown in Figure 20 or to the bottoms of goods (not shown). In order to scan either the bar-code label attached to the sides of goods or the bar-code label attached to the bottoms of goods, the scanning laser beam 202 is applied onto a mirror 204. The reflected scanning laser beam 202', which is slanting at about 45" with respect to the window 78, is used to illuminate the goods 203.

In Figure 20, both the hatched zone B and crosshatched zone A represent the area within which the bar-code can be read correctly. In other words, the imaging distance of the scanning laser beam 202 is restricted to the zones A and B. However, although the zone B is a zone in which the bar-code can be effectively read, it is not used for reading the bar-code because the zone B is located under the window 78 and inside the POS terminal device.

Accordingly, if the zone B were to be located above the window 78 and outside the POS terminal device, such as the crosshatched zone C shown in Figure 21, then zone B could be effectively utilized for reading the bar-code.

Figure 21 schematically illustrates a sectional and front view of the POS terminal device which comprises a light-scanning apparatus according to the present invention. In Figure 21, a laser beam scanner 211 comprising a light-scanning apparatus according to the present invention radiates a scanning laser beam 212. A reflected scanning laser beam 212' is obtained by means of the mirror 204. As seen in this Figure, the imaging distance is restricted to the zone C, and the bar-code can correctly be read at any location inside such zone C. A change of the zone from the zones A and B (Figure 20) to the zone C (Figure 21) can easily be achieved by properly selecting one of the curves shown in the graph of Figure 17 or Figure 19 and determining the optimum offset angle P therefrom.Such change of the zone is achieved by shifting a boundary of the zone, represented by a line 220 (also a line 221) in Figure 20, by rotating angle Q (Figure 20). In order to achieve the change under the conditions of, for example, M=4, p (x)=20" and -A2/kl = 1 the curves in the graph of Figure 22 are utilized. Figure 22 is similar to Figure 17. In Figure 22, a straight vertical line 223 is constructed first. The line 223 intersects a predetermined central diffraction angle p (x). In this case the p (x) is set to be 20 . Secondly, tangent lines 224, 225, 226 and 227 are drawn, respectively, through points where the curves P20, P15, P10 and pS respectively intersect the vertical line 223. Thirdly, a tangent line having a tangent angle Q' which is proportional to the rotating angle Q is selected. Specifically, the tangent line 226 is selected.The tangent line 226 is a line which is drawn through point where the line 223 intersects with the curve Pia As a result, a hologram having the characteristic curve Pia which is most suitable for achieving the change of the zones can be produced. The zone C in Figure 21 can be obtained by using the light-scanning apparatus (211) comprised of the hologram which has the charateristic curve p() and which is produced by the process illustrated in Figure 7B, wherein the offset angle P is set to be 10 .

As mentioned above, the light-scanning apparatus according to the present invention is useful in for example, a POS monitoring system having a high resolving power for reading the bar-code of the bar-code label.

WHAT WE CLAIM IS: 1. A light scanning apparatus comprising a disc having a plurality of holograms disposed in succession on a circle concentric with the disc, a laser beam sorce radiating a laser beam, said laser beam acting as a reconstructing wave and a motive means for moving said laser beam and said disc relative to each other so that the reconstructing wave is directed sequentially onto said holograms whereby each of said holograms causes the reconstructed beam to converge to a focal point in a scanning plane at which an object to be scanned may be placed, the focal point being moved in a scanning direction as the hologram moves relative to the reconstructing wave, characterised in that said reconstructing laser beam has a spherical wave front, and that at least one of said holograms is recorded by a process in which both a plane wave front laser beam and a spherical wave front laser beam are incident a photosensitive layer coated on a transparent plate, the arrangement being such that the direction of propogation of said plane wave front laser beam intersects the photosensitive surface at a predetermined angle of incidence offset from 90".

2. A light scanning apparatus according to claim 1, wherein said offset angle is determined in accordance with a desired central diffraction angle which defines the centre of the scanning motion.

3. A light scanning apparatus according to claim 2, wherein the wave length of said reconstructing laser beam is equal to or longer than the wave length of the laser light used for recording said hologram.

4. A light scanning apparatus according to claim 1, wherein the position is an area in which said object to be scanned may be correctly scanned may be adjusted by selecting various offset angles.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. scanning laser beam 202 is restricted to the zones A and B. However, although the zone B is a zone in which the bar-code can be effectively read, it is not used for reading the bar-code because the zone B is located under the window 78 and inside the POS terminal device. Accordingly, if the zone B were to be located above the window 78 and outside the POS terminal device, such as the crosshatched zone C shown in Figure 21, then zone B could be effectively utilized for reading the bar-code. Figure 21 schematically illustrates a sectional and front view of the POS terminal device which comprises a light-scanning apparatus according to the present invention. In Figure 21, a laser beam scanner 211 comprising a light-scanning apparatus according to the present invention radiates a scanning laser beam 212. A reflected scanning laser beam 212' is obtained by means of the mirror 204. As seen in this Figure, the imaging distance is restricted to the zone C, and the bar-code can correctly be read at any location inside such zone C. A change of the zone from the zones A and B (Figure 20) to the zone C (Figure 21) can easily be achieved by properly selecting one of the curves shown in the graph of Figure 17 or Figure 19 and determining the optimum offset angle P therefrom.Such change of the zone is achieved by shifting a boundary of the zone, represented by a line 220 (also a line 221) in Figure 20, by rotating angle Q (Figure 20). In order to achieve the change under the conditions of, for example, M=4, p (x)=20" and -A2/kl = 1 the curves in the graph of Figure 22 are utilized. Figure 22 is similar to Figure 17. In Figure 22, a straight vertical line 223 is constructed first. The line 223 intersects a predetermined central diffraction angle p (x). In this case the p (x) is set to be 20 . Secondly, tangent lines 224, 225, 226 and 227 are drawn, respectively, through points where the curves P20, P15, P10 and pS respectively intersect the vertical line 223. Thirdly, a tangent line having a tangent angle Q' which is proportional to the rotating angle Q is selected. Specifically, the tangent line 226 is selected.The tangent line 226 is a line which is drawn through point where the line 223 intersects with the curve Pia As a result, a hologram having the characteristic curve Pia which is most suitable for achieving the change of the zones can be produced. The zone C in Figure 21 can be obtained by using the light-scanning apparatus (211) comprised of the hologram which has the charateristic curve p() and which is produced by the process illustrated in Figure 7B, wherein the offset angle P is set to be 10 . As mentioned above, the light-scanning apparatus according to the present invention is useful in for example, a POS monitoring system having a high resolving power for reading the bar-code of the bar-code label. WHAT WE CLAIM IS:

1. A light scanning apparatus comprising a disc having a plurality of holograms disposed in succession on a circle concentric with the disc, a laser beam sorce radiating a laser beam, said laser beam acting as a reconstructing wave and a motive means for moving said laser beam and said disc relative to each other so that the reconstructing wave is directed sequentially onto said holograms whereby each of said holograms causes the reconstructed beam to converge to a focal point in a scanning plane at which an object to be scanned may be placed, the focal point being moved in a scanning direction as the hologram moves relative to the reconstructing wave, characterised in that said reconstructing laser beam has a spherical wave front, and that at least one of said holograms is recorded by a process in which both a plane wave front laser beam and a spherical wave front laser beam are incident a photosensitive layer coated on a transparent plate, the arrangement being such that the direction of propogation of said plane wave front laser beam intersects the photosensitive surface at a predetermined angle of incidence offset from 90".

2. A light scanning apparatus according to claim 1, wherein said offset angle is determined in accordance with a desired central diffraction angle which defines the centre of the scanning motion.

3. A light scanning apparatus according to claim 2, wherein the wave length of said reconstructing laser beam is equal to or longer than the wave length of the laser light used for recording said hologram.

4. A light scanning apparatus according to claim 1, wherein the position is an area in which said object to be scanned may be correctly scanned may be adjusted by selecting various offset angles.

5. A light scanning apparatus substantially as hereinbefore described with reference to

the accompanying drawings.