Patent Application
20090084856
Flat bed laser readers, also known as horizontal slot scanners, have been used to electro-optically read one-dimensional bar code symbols, particularly of the Universal Product Code (UPC) type, at a point-of-transaction workstation in supermarkets, warehouse clubs, department stores, and other kinds of retailers for many years. As exemplified by U.S. Pat. No. 5,059,779; No. 5,124,539 and No. 5,200,599, a single, horizontal window is set flush with, and built into, a horizontal countertop of the workstation. Products to be purchased bear an identifying symbol and are typically slid across the horizontal window through which a multitude of scan lines is projected in a generally upwards direction. When at least one of the scan lines sweeps over a symbol associated with a product, the symbol is processed and read.
The multitude of scan lines is generated by a scan pattern generator which includes a laser for emitting a laser beam at a mirrored component mounted on a shaft for rotation by a motor about an axis. A plurality of stationary mirrors is arranged about the axis. As the mirrored component turns, the laser beam is successively reflected onto the stationary mirrors for reflection therefrom through the horizontal window as a scan pattern of the scan lines.
It is also known to provide a point-of-transaction workstation not only with a generally horizontal window, but also with a generally upright window that faces an operator at the workstation. The generally upright window is oriented generally perpendicularly to the horizontal window, or is slightly rearwardly or forwardly inclined. The laser scan pattern generator within this dual window or bi-optical workstation also projects the multitude of scan lines in a generally outward direction through the upright window toward the operator. The generator for the upright window can be the same as or different from the generator for the horizontal window. The operator slides the products past either window from right to left, or from left to right, in a “swipe” mode. Alternatively, the operator merely presents the symbol on the product to a central region of either window in a “presentation” mode. The choice depends on operator preference or on the layout of the workstation.
Sometimes, the upright window is not built into the workstation as a permanent installation. Instead, a vertical slot scanner is configured as a portable reader that is placed on the countertop of an existing horizontal slot scanner in a hands-free mode of operation. In the frequent event that large, heavy, or bulky products, that cannot easily be brought to the reader, have symbols that are required to be read, then the operator may also manually grasp the portable reader and lift it off, and remove it from, the countertop for reading the symbols in a handheld mode of operation.
Each product must be oriented by the operator with the symbol facing away from the operator and generally towards either window of the bi-optical workstation. Hence, the operator cannot see exactly where the symbol is during scanning. In typical “blind-aiming” usage, it is not uncommon for the operator to repeatedly swipe or present a single symbol several times before the symbol is successfully read, thereby slowing down transaction processing and reducing productivity.
The blind-aiming of the symbol is made more difficult because the position and orientation of the symbol are variable. The symbol may be located low or high, or right to left, on the product, or anywhere in between, or on any of six sides of a box-shaped product. The symbol may be oriented in a “picket fence” orientation in which the elongated parallel bars of the one-dimensional UPC symbol are vertical, or in a “ladder” orientation in which the symbol bars are horizontal, or at any orientation angle in between.
In such an environment, it is important that the laser scan lines located at, and projected from, either window provide a full coverage scan zone which extends down as close as possible to the countertop, and as high as possible above the countertop, and as wide as possible across the width of the countertop. The scan patterns projected into space in front of the windows grow rapidly in order to cover areas on products that are positioned not on the windows, but several inches therefrom. The scan zone must include scan lines oriented to read symbols positioned in any possible way across the entire volume of the scan zone.
As advantageous as these laser-based, point-of-transaction workstations are in processing transactions involving products associated with one-dimensional symbols each having a row of bars and spaces spaced apart along one direction, the workstations cannot process stacked symbols, such as Code 49 which introduced the concept of vertically stacking a plurality of rows of bar and space patterns in a single symbol, as described in U.S. Pat. No. 4,794,239, or PDF417 which increased the amount of data that could be represented or stored on a given amount of surface area, as described in U.S. Pat. No. 5,304,786, or two-dimensional symbols.
Both one- and two-dimensional symbols, as well as stacked symbols, can also be read by employing solid-state imagers which have a one- or two-dimensional array of pixels, cells or photosensors that correspond to image elements or pixels in a field of view (FOV) of the imager. Such an imager may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, as well as imaging optics and associated circuits for producing electronic signals corresponding to the one- or two-dimensional array of pixel information over the field of view.
It is therefore known to use a solid-state imager for capturing a monochrome image of a symbol as, for example, disclosed in U.S. Pat. No. 5,703,349. It is also known to use a solid-state imager with multiple buried channels for capturing a full color image of a target as, for example, disclosed in U.S. Pat. No. 4,613,895. It is common to provide a two-dimensional CCD with a 640×480 resolution commonly found in VGA monitors, although other resolution sizes are possible.
It is also known to install the solid-state imager, analogous to that conventionally used in a consumer digital camera, in a bi-optical, point-of-transaction workstation, as disclosed in U.S. Pat. No. 7,191,947 in which the dual use of both the solid-state imager and the laser scan pattern generator in the same workstation is disclosed. It is possible to replace all of the laser scan pattern generators with solid-state imagers in order to improve reliability and to enable the reading of two-dimensional and stacked symbols, as well as other targets.
It is desirable to have the FOV of the imager large at a near working distance or a close proximity to either window of the workstation so that the FOV covers the entire symbol. At further working distances, it is preferred to have the FOV diverge slowly. As advantageous as the use of imagers are, the range of working distances or ability to read symbols by the imager is limited by the imaging optics and by the number of pixels in, or resolution of, the imager. A ratio between the smallest imaged bar/space area, or module, of the symbol to the number of pixels on which the bar/space area is projected at the imager, that is, the number of pixels covered by the bar/space area, is known as “pixels per module” or PPM. When the FOV diverges rapidly, the PPM decreases rapidly too, which, in some cases, limits the capability of the imager to decode and read symbols at far working distances.
In cases when the symbol is tilted in a particular direction at a substantial imaging angle, for example, 45 degrees, to the imager, such as in the bi-optical workstation described above, the PPM is further reduced as a function of the cosine of the imaging angle due to the projection effect. Also, the tilted symbol appears to look denser than it actually is. It is desirable to increase the PPM in that particular direction to compensate for the loss of the PPM, the reduced resolution, the apparent increased density, and the decreased capability to decode and read such tilted symbols.
One feature of this invention resides, briefly stated, in a reader for, and a method of, electro-optically reading indicia, comprising a housing and at least one solid-state imager, and preferably a plurality of solid-state imagers, within the housing. Each imager is operative for capturing light from the indicia along an optical axis over a field of view. Each imager preferably comprises a two-dimensional, charge coupled device (CCD) array, preferably, but not necessarily, of submegapixel size, e.g., 752 pixels wide×480 pixels high, in order to reduce the costs of the imagers, as compared to supermegapixel arrays. Each imager includes an illuminator for illuminating the indicia with illumination light from illumination light sources, e.g., light emitting diodes (LEDs). A controller is operative for controlling each illuminator to illuminate the indicia, for controlling each imager to capture the illumination light returning from the indicia over an exposure time period to produce electrical signals indicative of the indicia being read, and for processing the electrical signals to read the indicia. Each illuminator is only operative during the exposure time period. Each imager is controlled to capture the light from the indicia during different exposure time periods to avoid mutual interference among the illuminators.
In accordance with one aspect of this invention, an optical converter is provided in the housing, preferably in conjunction with an imaging lens, for optically modifying and asymmetrically magnifying the field of view of at least one of the imagers along mutually orthogonal directions generally perpendicular to the optical axis of the one imager, and for increasing resolution of the one imager along one of said directions. Thus, if the magnification is reduced along the one direction, then there will be more pixels in a given area, a higher PPM, and, as a result, the resolution of the imager will be increased along the one direction. The optical converter with asymmetrical magnification improves the resolution of the imager, especially in those cases where the indicia being read is imaged at a steep, oblique angle. Asymmetrical magnification compensates for any loss in imager resolution due to the projection effect caused by such a steep, oblique angle.
In a preferred embodiment, the optical converter comprises non-rotationally symmetrical optics for optically modifying the field of view differently along said mutually orthogonal directions. The non-rotationally symmetrical optics advantageously include at least one of a toroidal lens, a cylindrical lens, an aspherical toroidal lens, a toroidal mirror, a cylindrical mirror, and an aspherical toroidal mirror. If a pair of lenses is employed, then the lenses are preferably provided with respective positive and negative optical powers. At least one of the lenses is preferably made of a plastic material that filters visible light in a predetermined frequency range, e.g., red light, and corrects for chromatic aberration of visible light in a different frequency range, e.g., blue and green light.
In a preferred embodiment, the housing has one window located in a generally horizontal plane, and another window located in a generally upright plane that intersects the generally horizontal plane, thereby comprising a bi-optical workstation. The imagers capture the light from the indicia through at least one of the windows, and preferably each imager has its own window. The optical axes of multiple imagers intersect, as do their fields of view. Indicia can thus be reliably read when located anywhere at a respective window, as well as within a range of working distances therefrom, and can even be tilted at steep angles relative to the windows.
In accordance with another feature of this invention, the method of electro-optically reading indicia is performed by capturing light from the indicia with a solid-state imager along an optical axis over a field of view; controlling the imager to produce an electrical signal indicative of the indicia being read, and processing the electrical signal to read the indicia; and optically modifying and asymmetrically magnifying the field of view along mutually orthogonal directions generally perpendicular to the optical axis, and increasing resolution of the imager along one of said directions.
The method includes focusing the light from the indicia onto the imager with an imaging lens. The field of view is magnified differently along said mutually orthogonal directions, for example, by using such non-rotationally symmetrical optics as at least one of a toroidal lens, a cylindrical lens, an aspherical toroidal lens, a toroidal mirror, a cylindrical mirror, and an aspherical toroidal mirror.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
As schematically shown in
In use, an operator 24, such as a person working at a supermarket checkout counter, processes a product 26 bearing a UPC symbol 28 thereon, past the windows 12, 16 by swiping the product across a respective window in the abovementioned swipe mode, or by presenting the product at the respective window in the abovementioned presentation mode. The symbol 28 may located on any of the top, bottom, right, left, front and rear, sides of the product, and at least one, if not more, of the imagers 30 will capture the illumination light reflected, scattered, or otherwise returning from the symbol through one or both windows. The imagers are preferably looking through the windows at the symbol that is oriented at around 45 degrees so that the imagers can each see a side of the product that is generally perpendicular to, as well as generally parallel to, a respective window.
As also schematically shown in
In operation, the microprocessor 34 sends successive command signals to the illuminators 32 to pulse the LEDs for a short time period of 100 microseconds or less, and successively energizes the imagers 30 to collect light from a target only during said time period, also known as the exposure time period. By acquiring a target image during this brief time period, the image of the target is not excessively blurred even in the presence of relative motion between the imagers and the target.
There are several different types of targets that have particular utility for the enhancement of the operation of the workstation. The target may be a personal check, a credit card, or a debit card presented by a customer for payment of the products being purchased. The operator need only swipe or present these payment targets at one of the windows for image capture.
The target may also be a signature, a driver's license, or the consumer himself or herself. Capturing an image of the driver's license is particularly useful since many licenses are encoded with two-dimensional indicia bearing age information, which is useful in validating a customer's age and the customer's ability to purchase age-related products, such as alcoholic beverages or tobacco products.
The target may be the operator himself or herself, which is used for video surveillance for security purposes. Thus, it can be determined if the operator is actually scanning the products, or passing them around the window in an effort to bypass the window and not charge the customer in a criminal practice known in retailing as “sweethearting”.
The target may, of course, be a two-dimensional symbol whose use is becoming more widespread, especially in manufacturing environments and in package delivery. Sometimes, the target includes various lengths of truncated symbols of the type frequently found on frequent shopper cards, coupons, loyalty cards, in which case the area imagers can read these additional symbols.
The energization of the imagers 30 can be manual and initiated by the operator. For example, the operator can depress a button, or a foot pedal, at the workstation. The energization can also be automatic such that the imagers operate in a continuous image acquisition mode, which is the desired mode for video surveillance of the operator, as well as for decoding two-dimensional symbols. In the preferred embodiment, all the imagers will be continuously sequentially energized for scanning symbols until such time as there has been a period of inactivity that exceeds a pre-program time interval. For example, if no symbols have been scanned for ten minutes, then after this time period has elapsed, the reader enters a power-savings mode in which one or more of the imagers will be omitted from sequential energization. Alternatively, illumination levels may be reduced or turned off. At least one imager will remain active for periodically capturing images. If the active imager detects anything changing within its field of view, this will indicate to the operator that a product bearing a symbol is moving into the field of view, and illumination and image capture will resume to provide high performance scanning.
As previously stated,
Each array should have a global shutter so that the captured images will not be disturbed by motion of the indicia relative to the window(s) during the exposure time period. The indicia can be presented or swiped at speeds up to around 100 inches per second across any part of either window. For an imager to be able to read an indicium that is moving rapidly, the indicium must be brightly illuminated by the illuminator 32 so that a short exposure time can be used. Bright illumination light shining out of either window can be annoying or uncomfortable to the operator, so the illumination light sources must not be directly viewable by the operator, or by a consumer standing nearby. A rolling or a mechanical shutter could also be employed.
An aspect ratio of the field of view of an imager is normally the same as the aspect ratio of the pixel array. For example, if the array is 752×480 pixels, then the aspect ratio of the field of view is 752/480, or 1.56:1. Another unique and important aspect of this invention is that an optical converter is provided in the housing, for optically modifying and asymmetrically magnifying the field of view of the imager along mutually orthogonal directions generally perpendicular to an optical axis of the imager, to increase resolution of the imager along one of said directions. The optical converter includes non-rotationally symmetrical optical elements, as shown in
In the prior art, an imager is normally associated with an optical system, such as a convex imaging lens, having a rotationally symmetrical magnification around the optical axis. In other words, the magnification along a horizontal axis is the same, or nearly the same due to optical distortion by the imaging lens, as along a vertical axis, as considered in a plane perpendicular to the optical axis. Magnification may vary with field angle, and an imaged symbol may appear with slightly different sizes if imaged on the optical axis versus off the optical axis.
By contrast, an optical system with asymmetrical magnification, as shown in
The first lens 40 is an aspherical cylindrical lens with a negative optical power along the horizontal axis, and a zero optical power along the vertical axis. The second lens 38 is an aspherical cylindrical lens with a positive optical power along the horizontal axis, and a zero optical power along the vertical axis. Since the optical powers of both lenses 38 and 40 are zero along the vertical axis, the converter does not modify the vertical magnification. However, since the optical powers of both lenses 38 and 40 are not equal to zero along the horizontal axis and do not cancel each other out, the magnification of the horizontal FOV is changed. In other applications, the optical powers of the lenses can be reversed.
The lens 38 and 40 can be molded out of plastic, especially a red plastic to filter the red light only. This corrects the problem with inherited chromatic aberration of green and blue light. In other embodiments, the toroidal lenses 38, 40 can be integrated directly into the imaging lens 36 for a more compact design and cost reduction purpose.
In another preferred embodiment, as shown in
The optical power of lens 42 and the optical power of the mirror 44 can be optimized in such a way as to yield a desired horizontal and vertical magnification. Lens 42 can be molded out of plastic. This system exhibits less chromatic aberration relatively to the two cylindrical lens design of
It will be understood that each of the elements described above, or two or more together, also may find a useful application in other types of constructions differing from the types described above.
While the invention has been illustrated and described as embodied in a point-of transaction workstation for electro-optically reading indicia by using imagers with asymmetrical magnification, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. For example, the magnification along the horizontal and vertical axes for a single imager can be reversed. The magnifications for plural imagers along one axis can be the same or different.
Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.
