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CCD SCANNER HAVING IMPROVED
SPECULAR REFLECTION
DISCRIMINATION
This application is a continuation of U.S. patent appli- 5 cation Ser. No. 08/790,956 Jan. 29,1997, now U.S. Pat. No. 5,942,762.
BACKGROUND OF THE INVENTION
1. Field of the Invention 10 The present invention relates generally to optical scanning
systems. More particularly, this invention relates to a system and method capable of detecting coded symbologies in the presence of specular reflection. 15
2. Description of Related Art
Coded symbologies are being used in an increasingly diverse array of applications. The ability to track a large amount of items quickly and efficiently has led coded symbologies to be used in applications such as retail 20 checkout, warehousing, inventory control and document tracking. As the volume of items tracked by coded symbologies has increased, the need for optical scanners which operate at high speeds has likewise increased.
Various optical scanning systems have been developed for 25 reading and decoding coded symbologies. Scanning systems include optical laser scanners and optical charge-coupled device (CCD) scanners. Optical laser scanners generally employ a laser diode, a multifaceted polygonal mirror, focusing optics and a detector. The scanning rate of an 30 optical laser scanner is limited by the number of facets on the mirror and the available motor speed.
CCD scanners may incorporate a non-laser light source and a CCD light detecting means, such as a CCD linear sensor. A portion of the light which is reflected from the coded symbology is detected by the CCD linear sensor and converted into an electrical signal which is the basis for a digital image of the coded symbology that has been scanned. The digital image is then processed and decoded according to the specific type of coded symbology.
One disadvantage with current CCD scanners is that they are susceptible to specular reflection which saturates areas of the CCD linear sensor and prohibits the detection of a portion of the optically coded information. This is particu- 4J larly a problem when the coded symbology is printed under a highly reflective surface, such as a plastic coating.
Specular reflection is only a problem at a single angle, known as the "critical angle", between the light source, the reflective surface and the CCD linear sensor. Current meth- 50 ods of coping with specular reflection include placing separate scanners at different angles with respect to the surface. However, providing duplicate CCD scanners for this purpose is extremely expensive. Techniques involving light polarizers have also been used. However, due to the light 55 losses introduced by the materials used to make light polarizers, they are extremely inefficient.
Accordingly, there exists a need for an efficient and inexpensive scanning system with the speed of a CCD scanner that can accurately read and decode coded symbolo- 60 gies in the presence of specular reflection.
SUMMARY OF THE INVENTION
The present invention utilizes two CCD linear sensors and a bandpass means to improve the ability of an optical 65 scanner to discriminate against specular reflection. A coded symbology is illuminated by a noncoherent light source and
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light reflected from the coded symbology travels along a first path and strikes the front face of the bandpass means. The bandpass means, functioning as a notch filter, transmits a select bandwidth of light while reflecting all other light onto a first CCD linear sensor. Simultaneously, light reflected from the bar code symbol travels along a second path, at a different angle with respect to the plane of the coded symbology than the first path, and is reflected from a mirror onto the back face of the bandpass means. The bandpass means transmits the select bandwidth of light onto a second CCD linear sensor and reflects all other light. The CCD linear sensors each have a notch filter which permits the detection of only a select bandwidth. Since specular reflection is only experienced at a single angle with respect to the plane of the coded symbology, and each CCD linear sensor detects an image at a different angle with respect to the plane of the coded symbology; a complete image of the coded symbology is obtained by one or both of the CCD linear sensors, or can be reconstructed by combining information obtained from both CCD linear sensors.
Accordingly, it is an object of the invention to provide a CCD scanner which can read and decode coded symbologies in the presence of specular reflection.
Other objects and advantages will become apparent to those skilled in the art after reading the detailed description of a presently preferred embodiment.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a coded symbology scanning system made in accordance with the present invention;
FIG. 2A is a diagram showing the spectrum of light;
FIG. 2B is a more detailed diagram of the CCD detectors;
FIG. 3 illustrates the method of using valid information from two views and selectively combining the information;
FIG. 4 is a block diagram of the coded symbology logic unit;
FIG. 5 is a flow diagram of the method of the present invention; and
FIG. 6 is an alternative embodiment of the present invention.
DETAILED DESCRIPTION OF THE
PREFERRED EMBODIMENT
The preferred embodiment will be described with reference to the drawing figures wherein like numerals represent like elements throughout. Referring to FIG. 1, a coded symbology scanning system 10 made in accordance with the present invention is shown. The coded symbology scanning system 10 is able to scan any type of coded symbology. However, for simplicity, reference hereinafter will be made to a particular type of coded symbology, i.e. a bar code symbol. The scanning system 10 includes a non-coherent light source 12, a bandpass means 14, a planar mirror 22, focusing optics 17, two CCD linear sensors 16A and 16B, two filters 19A and 19B, a logic unit 32 and an output means 34.
The light source 12 facilitates detection of a subject bar code symbol 18 by illuminating the bar code symbol 18 located on a package 8 or other object. Preferably, the package 8 is supported by a moving conveyor belt 7. The planar mirror 22 and the bandpass means 14 are aligned such that light reflected from the bar code symbol 18 along a first path 20A strikes the front of the bandpass means 14, while light traveling along a second path 20B reflects off the planar mirror 22 and strikes the rear of the bandpass means art 14.
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It should be recognized by those skilled in the art that FIG. 1 is illustrative only and is not drawn to scale. For example, the angle 9A between the light source 12 and the bar code symbol 18 is typically 77°. The angle QB between the first path 20A and the second path 20B is approximately 3-5°. 5 However, it should be recognized by those skilled in the art that these angles are approximate and may vary widely depending upon the specific application and the mounting of the system 10 in relation to the bar code 18.
The bandpass means 14 permits light of predetermined 1° wavelengths around r\A, striking either its front or rear surface, to pass through the bandpass means 14, and reflects the remainder of the light spectrum. The spectrum of light T|20A traveling along the first path 20Astrikes the front of the bandpass means 14. Light having wavelengths around :5 passes through the bandpass means 14, while the remainder of the spectrum of light T|20A-r|A± is reflected toward the CCD detectors 16A, 16B. The spectrum of light r\20B traveling along the second path 20B is reflected off the planar mirror 22 and strikes the back of the bandpass means 20 14. Light having wavelengths around T|A passes through the bandpass means 14 toward the CCD detectors 16A, 16B, while the remainder of the light spectrum r\20B-r\A± is reflected off the back of the bandpass means 14.
It should be appreciated that the bandpass means 14 may 25 function as a filter wherein the bandpass means 14 transmits a small bandwidth of light while reflecting the remainder of the light spectrum. Alternatively, the bandpass means 14 may function as a mirror, wherein the bandpass means 14 reflects a small bandwidth of light while transmitting the 30 remainder of the light spectrum. Preferably a mirrored dichroic filter is used.
The composite spectrum r\s of light which reaches the focusing optics 17 comprises predetermined wavelengths 3J around T|A from the second path 20B and the remainder of the spectrum T|20a-t|a± from the first path 20A. The composite spectrum r\s passes through the focusing optics 17, through the filters 19A, 19B and onto the CCD linear array detectors 16A, 16B. Both filters 19A, 19B permit the respec- 4Q tive detector 16A, 16B to detect non-overlapping bands of light.
Referring to FIG. 2, the second CCD detector 16B is filtered to detect light having wavelengths around r^. The first CCD detector 16A is filtered to permit the detection of 45 light around a different wavelength r\B. For example, the bandpass means 14 may be calibrated to transmit light around wavelength 1X4 of 650 NM±. The second CCD detector 16B is filtered to detect light around the wavelength T|A of 650 NM± originating from the second path 20B. The 50 first CCD detector 16B is filtered to detect light around wavelength r\B which originates from first path 20A, for example 600 NM±. Accordingly, the detectors 16A, 16B will detect two separate images of the bar code symbol 18.
Although the detectors 16A, 16B have been referred to as 55 separate CCD linear sensors, preferably they comprise two of the three channels commonly found in a color CCD line scan sensor. In this embodiment, the color filters are preferably replaced with the appropriate notch filters 19A, 19B. Those of skill in the art should realize that the bandwidth 60 transmitted by each notch filter 19A, 19B, including tolerances, should not overlap with the other notch filter 19A, 19B. Additionally, the notch filters 19A, 19B need not be of equal bandwidth. One notch filter 19A may have a narrow bandwidth of 590-610 NM±, and the other notch 65 filter 19B may have a wide bandwidth of 625-675 NM±. Additionally, although two notch filters 19A, 19B may be
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employed, it is also possible to use one notch filter 19A, wherein the other filter 19B transmits all other wavelengths of light except for the bandwidth transmitted by the notch filter 19A. In all of these examples, the tolerances of the filters 19A, 19B should be kept in mind to avoid any overlap.
It should be apparent to those skilled in the art that the bandpass means 14 and the filters 19A, 19B over the CCD detectors 16A, 16B may be calibrated to detect any wavelength of light that is suitable for the desired application. The above values are illustrative only and should not be viewed as a limitation of the invention.
The light detected by the second CCD detector 16B comprises light from the second path 20B having wavelengths around T|A. The light detected by the first CCD detector 16A comprises light from the first path 20A having wavelengths around r\B. By definition, specular reflections only occur at a "critical angle". Once specular reflection occurs, this angle is defined and will be present only in one of the optical paths. Therefore, the other path will have useful information. If specular reflection "washes out" the view of the bar code symbol 18 at any point along the first path 20A, specular reflection will not be present in the second path 20B at the same point since the angle of the bar code symbol 18 with respect to the second path 20B is different than the angle with respect to the first path 20A.
Referring to FIG. 2B, since the lengths of the two paths 20A, 20B are different, the detectors 16A, 16B must be selectively placed to account for this difference. In FIG. 1, path 20A is shorter than path 20B. Preferably, the detectors 16A, 16B are mounted upon a common substrate which is rotated upon a center line CL to position the first detector 16A further from the focusing optics 17 than the second detector 16B.
Each of the CCD detectors 16A, 16B produces an electrical signal which corresponds to the detected light. Using the images 30A, 30B, 30C in FIG. 3 as a visual example of the reconstruction process, comparison of images 30A and 30B shows that image 30A has portions of specular reflection distortion, while image 30B also has portions of specular reflection distortion. However, the non-distorted areas of the images 30A, 30B can be used to form the complete image 30C. Although the images 30A, 30B, 30C are illustrated as area images, the preferred embodiment of the present invention detects and combines multiple line scans which make up the area images. It is clearly within the scope of the present invention to utilize detectors which detect either line or area scans.
Processing of the data from CCD detectors 16A, 16B to construct a complete bar code symbol 18 will be explained with reference to FIG. 4. The data from the CCD detectors 16A, 16B is output and analyzed by the logic unit 32. Depending upon the amount of specular reflection, data from one or both of the CCD detectors 16A, 16B may comprise a complete image of the bar code symbol 18. In that case, the complete image is used for further decoding in accordance with the specific type of symbology. If specular reflection is detected by the logic unit 32 in the data output from the first CCD detector 16A the logic unit 32 replaces the data with the data from the second CCD detector 16B.
Referring to FIG. 4, the logic unit 32 comprises two buffers 70A, 70B, a selector 72 and an arbitration unit 74. The logic unit 32 receives the data, containing bar code information, from the CCD detectors 16A, 16B. The information coming from the CCD detectors 16A, 16B is selectively buffered depending upon the height of the package 8 upon which the bar code 18 is affixed. Referring back to
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