Patent Application
20070267584
The field of the present disclosure relates generally to optical code readers using Scheimpflug optics. Optical code systems, including for example bar code systems, have come into wide use for marking a great variety of objects for automatic reading. Optical codes are used commercially in many applications, including the identification of retail products at the point of sale, control of inventories, and package identification.
Optical codes include, but are not limited to, a series of light and dark areas of varying widths and heights. The simplest of optical codes are often commonly referred to as one-dimensional (hereinafter 1D), such as the UPC code, and two-dimensional (hereinafter 2D) codes, such as PDF417 and Maxicode. However, other configurations of light and dark areas may also represent optical codes. An example of such a configuration may be symbolic codes, such as a light and dark areas configured in the shape of a lightning bolt to represent electricity. Light and dark areas configured in the shape of alphanumeric text may also be read as an optical code.
Many conventional optical code readers suffer from shallow Depth of Field (DOF). Due to the shallow DOF, optical codes remain in focus over a narrow range of distances.
It will be readily understood that the components of the embodiments as generally described and illustrated in the figures herein could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.
The order of the steps or actions of the methods described in connection with the embodiments disclosed herein may be changed by those skilled in the art. Thus, any order in the figures or detailed description is for illustrative purposes only and is not meant to imply a required order.
An optical code reader detects reflected and/or refracted light from an optical code comprising various optical characters or elements. One method of illuminating the optical code is via a scanning laser beam. In this method a beam of light is swept across the optical code and an optical detector detects the reflected light. Typically, the detector generates an electrical signal having amplitude proportional to the intensity of the collected light.
An alternative method for illuminating an optical code is accomplished by using a uniform light source with the reflected light detected by a one dimensional or two dimensional sensor array, such as a charge-coupled device (CCD) or CMOS image sensor. In such a technique, as with a scanning laser, an electrical signal is generated having an amplitude determined by the intensity of the collected light. In some embodiments using either the scanning laser or imaging technique, the amplitude of the electrical signal has one level for the dark areas of the optical code and a second level for the light areas of the optical code. As the code is read, positive-going and negative-going transitions in the electrical signal occur, signifying transitions between light and dark areas.
By way of example, the magnification of the fan of rays 108 depicted is 0.5×. However, with a fan of rays in the other prime meridian (not shown), the magnification effect is reversed. The lens effect occurs at the first cylindrical surface lens 104 instead of the second 106 and the magnification is greater, such as, for example 2.0×. Thus, the square object 110 has a corresponding rectangular image 102 with a length four times its width.
Alternative anamorphic lens systems may be used as would be apparent to those having skill in the art with the aid of the present disclosure. One exemplary alternative involves using a spherical objective lens combined with a Galilean telescope composed of cylinder lenses. Another alternative anamorphic system may include a Bravais system using cylindrical optics. Yet another alternative is using one or more refracting prisms to achieve an anamorphic effect.
Conventional optical code readers typically have a sensor array aligned generally parallel with its corresponding lens system (i.e., the image plane and lens plane are both parallel to each other and both perpendicular to the optical axis). These conventional readers may be limited in their depth of field (“DOF”), sometimes referred to as the working or reading range. To accurately read objects marked with an optical code, the optical code must typically lie within a particular range of distances before the fixed focal lens distance. The particular range of distances before the fixed focal lens distance is known as the DOF. Thus, unless the entirety of the optical code lies within the shallow DOF of a conventional optical code reader, most of the optical code image produced on the image sensor array may be out of focus and may not be accurately read.
The DOF of an optical code reading system varies as a function of, among other variables, focal distance and aperture setting. Conventional optical code readers typically suffer from a shallow DOF. This shallow DOF is due to the low levels of reflected light available to read an optical code, particularly in ambient light CCD optical code readers. Since low levels of light are available, the optical code reader system requires the use of large aperture settings. This large aperture setting in turn results in a shallow DOF. While a conventional optical code reader may accurately read an optical code at the exact focal distance of the system, slight variations from this focal distance (i.e., outside the DOF) will result in out-of-focus and sometimes unsuccessful reading of the optical code.
One method used to partially counteract this shortcoming is to raise the f-number of the optical system. Unfortunately, when the f-number is increased the corresponding aperture size decreases. As a result, the amount of light passed through the optical system also decreases. This decreased light is particularly evident in an imaging-type optical code reader. The reduced available light level requires that the time for integration of the optical code image on the sensor must be increased, or extra illumination must be provided on the optical code, or both. If longer integration time is used, the sensitivity to image blur due to optical code image motion may be increased. If extra illumination is required, then the cost, complexity, and power requirements of such a system may also be increased.
In the embodiment depicted in
The image sensor array 214 includes a pattern of horizontal raster lines 216. The image sensor array 214 produces a projected image 218 in object space. The object space is the space in which a physical object, such as an optical code can be read. The image space is the space in which an image of a physical object, such as an optical code, is produced by the lens system 200.
Instead of visualizing the image of an object produced in image space,
The projected image 218 of the image sensor array 214 is oriented to read optical code labels, such as 1D optical codes, having optical code elements oriented in a substantially vertical direction (i.e., perpendicular to the horizontal raster lines). In one embodiment, the anamorphic lens system 200 increases the magnification of the projected image 218 of the sensor array 214 in a horizontal direction, i.e., in a direction parallel to a scan line direction of the optical code reader or in a plane parallel to the pattern of horizontal raster lines 216. The path of the reading spot created on an object by a moving illumination beam is referred to as a scan line. Typically, an individual scan line extends across and substantially perpendicular to the bar elements for an optical code to be successfully read.
Because the anamorphic lens system 200 expands the projected image 218 horizontally (compared to the horizontal dimension of the sensor array 214), the imaging system 212 is capable of reading an entire code that is placed closer to the reader and also provides for a sufficient resolution of an optical code that is read further away from the reader.
In an alternative embodiment, the magnification of the projected image 218 may be decreased horizontally (relative to vertical magnification). For example, if the pixel spacing of the image sensor array 214 results in insufficient pixel density to accurately read the narrow elements of an optical code at the furthest desired reading distance, the magnification in the horizontal direction may be decreased relative to vertical magnification. However, if it is desirable to read optical codes at a close range, the magnification in the horizontal direction may be increased relative to the vertical magnification, such that the raster line may traverse the entire code. The appropriate value of magnification in the horizontal direction may depend on the number of pixels available on a raster line and the pixel spacing of the image sensor array 214.
When an optical code (not shown) intersects the object plane 320, the line of intersection formed between the optical code plane and the object plane 320 will be in focus on the image sensor array 314, provided the optical code intersects within the DOF. The DOF is the distance between the inner DOF limit 328 and the outer DOF limit 330 along the object plane 320 as measured along the optical axis. This DOF is not dependent upon the aperture size, and thus the aperture may be fully opened allowing maximum image brightness.
Alternatively, the lens plane 324 may be tilted relative to the sensor array plane 322, and, once again, in accordance with the Scheimpflug principle, the object plane 320, image sensor array plane 322, and lens plane 324 will intersect at the Scheimpflug point 326.
According to the embodiment of
As can be seen from
Rectangle 442 represents the projection of the image sensor array 414 through the lens system 400 onto the object plane 420. The projection 442 of the image sensor 414 is rectangular because the magnification in the horizontal axis 444 is greater than the vertical axis 446 through the anamorphic lens system 400. The row of photodetectors 438 of the sensor array 414 have corresponding projected raster lines 448 in the rectangular sensor array projection 442. An optical code 436 will be in focus on the line of photodetectors 438 when it intersects the corresponding projected raster line 448, as shown.
In order to utilize the most depth-of-field, the optical code 436 may be oriented as shown, generally normal to the optical axis 440. When a 1D optical code 436 in the position shown is imaged, the sharpest region of focus on the sensor array 414 will be centered around the row of photodetectors 438 that corresponds to the line of intersection 448 between the optical code plane 420 and the projection of the image sensor array 442. Because there may be some finite depth-of-field inherent in the lens system 400, there will typically be several rows of detectors in focus above and below the specific row conjugate to the line of intersection 448 between the optical code 436 and the projection of the sensor 442. However, there may be gradually increasing amounts of defocus further above or below the row of photodetectors 438 conjugate to the line of intersection 448.
If the inherent depth-of-field of the lens system 400 is sufficient, there may be enough photodetector rows 438 in focus in order to image a “stacked” or two-dimensional optical code. However, producing a focused image of only a portion of the 2D code may not be sufficient to fully read the optical code.
In a Scheimpflug system using conventional, circularly symmetrical optics, the focal length of the lens is related to the Scheimpflug angle α of the image sensor array 414 required to achieve a particular range of raster line focal distances. The focal length of the lens is also related to the length of the raster lines in the object space. However, an anamorphic lens system 400 provides an additional degree of freedom in the system design, allowing the imager Scheimpflug angle α to be optimized separately from the choice of raster line length versus distance (corresponding to the imager angle of view in the axis parallel to the raster lines).
Inputs for an imaging system 412 design may include the near object distance limit, the far object distance limit, and the minimum required reading line length at the near distance. The designer of the imaging system 412 may choose from the available image sensors, having some particular dimensions, and determine the position of the sensor and lens, and the focal length of the lens. The lens focal length in one axis determines the location of the near and far reading limits relative to the lens 400 and image sensor array 414. The lens focal length in the perpendicular axis determines the reading line length versus distance (a function of the field angle in a plane parallel to the raster lines). By allowing these two focal lengths to differ, as in an anamorphic system 400, the designer has more flexibility to meet what could otherwise be contradictory design goals.
For the image sensor array 514 to be oriented to read an optical code, such as a bar code symbol, the video lines 538 are positioned in a direction substantially perpendicular to the direction of the bars in the optical code. Considering, as an example, a simple 1D bar code, information is encoded as a series of vertically oriented bars of varying widths. Each bar of varying width represents a piece of encoded data. In order to read all of the data encoded on the optical code label, sufficient video lines, such as line 538, collect data across the entire horizontal axis of the optical code label, either all in one line, or in sufficiently usable pieces.
The amount of perpendicular alignment of the raster lines 538 with the optical code depends on the vertical extent of the optical code's edges and the size of the sections that may be successfully “stitched” or merged together by the signal processing or decoding system. Put another way, the amount of orientation manipulation of the raster pattern depends on the actual dimensions of the optical code label and the stitching capabilities of the system. For example, an “oversquare” optical code label (i.e., an optical code label that has a height dimension slightly greater than the width dimension of the smallest usable piece, which is often half of the entire label) may be rotated up to 45 degrees from its vertical alignment and still be accurately read by a horizontal raster pattern. An oversquare optical code label oriented in a direction rotated up to 45 degrees from vertical may still permit at least one horizontal video line 538 of the raster pattern to register a complete cross section of the optical code (i.e., corner-to-corner) usable piece.
On the other hand, truncated optical code labels may be used to conserve space. Truncated optical code labels are labels that are shorter in their vertical bar dimension than their horizontal dimension. Use of a truncated optical code often requires a greater degree of proper orientation with the optical code reader. As a truncated optical code label is rotated beyond a predetermined angle, horizontal video lines 538 are no longer able to produce complete cross sectional images of the truncated optical code label. As truncated optical code labels become shorter, the angle of rotation permitted for proper orientation is reduced.
As shown in
An anamorphic lens system used in conjunction with the Scheimpflug condition provides adequate resolution for bar code elements that are positioned a distance away from the code reader. In some embodiments, 1.5 pixels corresponding to each element of the bar code may be sufficient to provide adequate resolution.
Additionally, when the optical code is positioned close to the reader, pixel density does not typically pose a problem since there are usually plenty of pixels per each optical code element. However, sometimes the entire length of the optical code cannot all fit onto the sensor array 514. Since magnification about the optical axis varies in an anamorphic lens system, an anamorphic system helps to fit the horizontal dimension of an image of the entire optical code on the sensor array 514 when the optical code is positioned close to the reader.
These projected images 618 and 619 represent the region in object space where an object (not shown) will produce a well-focused image and also represent the relative orientation of an optical code which may be positioned at and still be accurately read. As additional raster patterns are added to the optical code reader system 612, the probability that the orientation of an added raster pattern is substantially perpendicular to the orientation of the optical code increases.
According to the embodiment depicted in
This construction allows for a compact scan zone, which may be easier for an operator to use. Thus, an object (positioned in object space) marked with an optical code label with either a substantially vertical or horizontal orientation positioned within the scan zone will likely produce a well-focused, fully-read image of the optical code label on the image sensors 614, 615 in image space.
Referring still to
In an embodiment having two image sensor arrays, the method 770 includes arranging at step 772 the image planes of the first and second sensor arrays so that they are perpendicular to each other. The step of arranging the image planes to be perpendicular with each other may increase the likelihood of successfully reading a randomly positioned optical code. The step 772 of arranging the image planes may also include positioning the projected images of the sensor arrays so that at least one sensor array's raster lines are positioned substantially orthogonal to the optical code.
The method 770 for reading an optical code may further include the reader receiving at step 774 an image of an optical code produced when light is reflected off of the optical code when illuminated. The step 774 of receiving the optical code image may comprise capturing the reflected image by the reader and introduction of the optical code image to the optical train. The optical code image may be split at step 776 using a beam splitter or other suitable method of splitting as would be apparent to those having skill in the art with the aid of the present disclosure.
A portion of the reflected optical image may be transmitted through the partially transmissive beam splitter and directed at step 780 to a first anamorphic lens system. The first anamorphic lens system may then focus at step 782 the optical code image onto the first sensor array. The first sensor array may be tilted with respect to the anamorphic lens system in accordance with the Scheimpflug principle.
According to the method 770 described in conjunction with
In alternative embodiments, the optical code image may not be split via a beam splitter but directed at step 780 solely toward the first lens system and focused at step 782 on the first sensor array. These alternative embodiments may not necessarily include a second sensor array and accompanying beam splitter.
While specific embodiments of various optical imaging systems and related methods have been illustrated and described, it is to be understood that the invention claimed hereinafter is not limited to the precise configuration and components disclosed. Various modifications, changes, and variations apparent to those of skill in the art with the aid of the present disclosure may be made in the arrangement, operation, and details of the methods and systems disclosed.
Furthermore, the methods disclosed herein comprise one or more steps or actions for performing the described method. The method steps and/or actions may be interchanged with one another. In other words, unless a specific order of steps or actions is required for proper operation of the embodiment, the order and/or use of specific steps and/or actions may be modified without departing from the scope of the invention as claimed.
