1. Field of the Invention
The invention relates generally to optical scanners, and in particular in some embodiments to scanners having dual or multiple working ranges.
Most optical scanners such as bar code scanners are adapted for use at a particular distance, or a range of distances, from an indicia to be scanned. If the user holds the scanner too close to the indicia, or too far away, the indicia and/or the flying spot beam will not be in focus, and decoding will not be possible.
Such scanners may not be particularly convenient in environments where a series of indicia to be read are presented to the scanner at various distances, and where it is difficult or impossible for the user to alter the distance between the scanner and the indicia. To deal with such situations, attempts have been made to expand the acceptable working range of conventional scanners, to give the user as much leeway as possible, and also to provide multi-distance scanners which can operate, for example, at a first working range or at a second working range according to the user""s preference or requirements. One possibility is for the provision of a two-position switch on the scanner, with the scanner operating at a first working distance in a first position of the switch and at a second working distance in a second position. A disadvantage of such scanners is that they require additional moving parts to provide for operation at the two separate working ranges. Such systems are also not xe2x80x9cautomaticxe2x80x9d in the sense that the user has manually to select the correct working range, according to the distance of the current indicia to be read; if the incorrect working range is chosen, a decode will not result.
One of the difficulties that bar code reader designers face when attempting to produce increased working ranges is that the greater the working range, and the greater the range of possible indicia that might be read, the lower tends to be the resultant signal to noise ratio in light that is reflected from the indicia. One approach for dealing with this involves the provision of non-conventional optics, in which the optics associated with either the laser or with the photodetector have two distinct focal points. An example of this is shown in U.S. Pat. No. 5,332,892, which is commonly assigned with the present application. In the device shown in that document, the two focal points are associated with corresponding circuitry to provide two separate channels of data derived from the scanned bar code. The two channels have differing resolutions. As the working angle and density vary, at least one of the resolutions is likely to be appropriate for sensing all or most of the bar coded data, regardless of the distance of the bar code with respect to the scanner and/or the size or density of the code. The scanning beams of bar code readers are typically derived from laser diodes. Such diodes are robust and relatively inexpensive, but they do suffer from the disadvantage that the beam emerging from a laser diode is astigmatic. The astigmatic laser diode can be characterised as having two apparent light sources spaced apart from each other along the optical path. One of the light sources lies in a horizontal plane, appears to be coming from inside the laser diode chip, and has a low angular divergence. The other apparent light source lies in a vertical plane, appears to be coming from a facet of the chip, and has a high angular divergence. The two apparent light sources, which are spaced apart from each other by typically about 20 micrometers, form two beam waists in different planes and in different directions, as measured relative to the planar junction of the chip.
The resultant relatively complex beam profile may need selective shaping before it can efficiently be used in an optical scanner. Some methods of providing such beam shaping are described in our co-pending U.S. patent application Ser. No. 08/268,982, filed Jun. 30, 1994, now U.S. Pat. No. 5,742,038, the teachings of which are incorporated herein by reference.
A simpler option is simply to provide separate long and short range visible laser diodes, as is suggested in our earlier patent U.S. Pat. No. 5,420,411.
Yet a further option is to provide a multi-focus lens for the outgoing laser beam, thereby providing that the outgoing laser beam may be focused on an object in a first predetermined working range and simultaneously on a second object in a second predetermined working range.
Where a scanner has several different working ranges, a further difficulty arises in that reflected light from a bar code at the far working range will tend to be much weaker than light reflected from a bar code in the near working range. This makes automatic gain control of the signal difficult. One approach to this problem is shown in U.S. Pat. No. 5,591,954, to Spencer. Spencer discloses a retro-reflective arrangement in which common segmented mirrors are used both for scanning and for the collection of reflected light. Different segments are coated differently, in an effort to equalise the returned light intensity.
A further common difficulty with bar code scanners, particularly with scanners in which the y dimension of the scanning beam is greater than the x dimension, is that the y dimension can be too tall close to the scanner, so reducing the ability of the scanner to read high-density symbols when the symbol is not accurately aligned. In particular, difficulties can occur when the longer axis of the beam cross-section is not parallel with the bars and spaces in the symbol to be read. This problem can be reduced by decreasing the vertical dimension of the laser aperture, but this can sometimes be unacceptable since it results in the loss of laser power.
It is well known that it is advantageous to focus the laser beam, in a bar code scanner, so that it is taller than it is wide. The tall spot reduces speckle and paper noise by increasing the overall spot area, and helps to filter out small printing defects. The width of the spot, on the other hand, is determined by the performance requirements of the scanner. The width is normally chosen so that it is small enough to resolve the smallest elements in a bar code symbol which the scanner is required to read.
Various means of creating elongate laser spots have been used. One is shown in U.S. Pat. No. 5,440,111 to Eastman. This system orients the laser so that the laser""s astigmatism causes the spot to be narrower in the direction of spot motion (the x direction) than it is in the direction perpendicular to the direction of motion (the y direction). The amount of elipticity of the laser spot, in this system, is dependent on the amount of astigmatism that the laser has, and by the magnification of the focusing system.
It is an object of the present invention at least to alleviate the problems of the prior art.
It is a further object to enhance the performance of scanners having multiple working ranges.
It is yet a further object to provide a scanner with improved reading characteristics when the symbol to be read is presented in an orientation which is not ideal.
It is a further object to reduce the costs of manufacturing a scanner having parts in accurate optical alignment.
According to a first aspect of the present invention there is provided an optical scanner for reading indicia comprising:
(a) a beam generator and scanner for producing a scanning light beam and directing said light beam toward an indicia to be read;
(b) a light detector; and
(c) a collection mirror for receiving reflected light, the collection mirror having at least first and second segments of differing optical properties whereby said first segment reflects toward the detector light received from an indicia at a first working range from the scanner and said second segment reflects toward the detector light received from an indicia at a second, different, working range.
Such an arrangement improves scanner performance via the collection mirror which is composed of two or more separate segments. Each segment preferably has a different optical surface, such as a difference in focal length, surface orientation, aberration or surface definition (for example, paraboloid rather than spherical). At least one segment directs light back to the photodetector from an indicia which is at a first working distance. At least another segment directs light back to the photodetector from an indicia which is at a second working distance. The segments may operate independently, so that one segment operates only at the first working distance and the other operates only at the second working distance. Alternatively, one segment may operate only at the first working distance, while the second returns light from indicia both at the first and the second working distances. Alternative arrangements may be envisaged in which there are more than two segments.
The segments are preferably contiguous with one another, and may be integrally molded. In one embodiment, the segments are adjacent to one another, and in another one segment surrounds the second in a coaxial fashion.
Maximum signal strength may be achieved, and automatic gain control simplified, where both mirror segments collect light from an indicia at a far working distance, but only one of them collects light from an indicia at a near working distance.
In laser bar code scanners, the size of field of view is critical to the scanner performance, especially under strong ambient light conditions. Reducing the field of view angle will typically improve performance, but optical alignment during manufacture becomes more difficult and labor intensive.
According to the present invention, this difficulty is solved by providing the collection mirror with a field of view angle adjustment, allowing accurate alignment of the mirror with the other optical components within the scanner. Preferably, the mirror may be adjusted in angle both in the x and y axis directions. Ideally, x and y axis translation of the mirror is also possible.
An adjustable mirror of this type may be used in conjunction with the optical scanner previously described, or may be used independently.
A collection mirror of this type having both x axis and y axis angular adjustment can align the field of view angle of the receiving optics to the laser beam axis with very high accuracy. Assuming that the x and y angular adjustments are independent of each other, the total system field of view angle may typically be controlled to within about plus or minus 2xc2x0.
According to a further aspect of the present invention there is provided an optical scanner for reading indicia by effecting a scanning motion of a light beam in an x-axis direction across an indicia to be read, said scanner comprising:
(a) a laser for producing a light beam of non-circularly-symmetric cross-section, having an x-axis and a y-axis, beam divergence in the x axis being greater than beam divergence in the y axis;
(b) negative beam-shaping optics in the beam for adjusting the y axis divergence independently of the x axis divergence.
Such an arrangement improves scanner performance, particularly in the reading of bar code symbols which are not accurately aligned along the scanning (x) axis. In this invention the laser has been rotated so that its wider divergence angle is horizontal (in the x axis direction), the beam height (y) at the laser aperture is quite small. The vertical (y) dimension of the aperture can therefore be reduced without sacrificing laser power.
It is also not necessary, in contrast with the prior art such as the above-mentioned Eastman U.S. Pat. No. 5,440,111, to use higher magnification to increase the separation between the x and y waists in the beam. In the present invention, the spacing between the waists is controlled not by magnification, but by the negative optical element in the laser path. Adequate waist separation is possible even with lower magnification systems.
The negative optical element in the laser path is preferably cylindrical or part-cylindrical, thereby allowing independent control of the y-axis dimension of the beam cross-section. Preferably the optical element comprises a part-cylindrical mirror having a longitudinal axis which is horizontal (in the x-axis direction). In the preferred embodiment, the cylindrical element is molded into a plastic fold mirror. This is a standard element in most scanning systems in any event, so the invention adds very little extra cost. There is also no additional loss of optical output power as would occur if an additional lens were to be added to the system.
The invention is particularly although not exclusively useful in connection with index guided lasers, in which astigmatism is typically rather lower than is the case with gain guided lasers. Previously, gain guided lasers have been preferred in this type of application, for the very reason that their astigmatism is typically higher which results in greater separation between the x and y waists of the focused beam. This is important because if the waists were close together, the spot area would be very small near the waist resulting in a noisy signal in this area. Separating the waists helps to avoid this.
The present invention opens up this field to the use of index guided lasers. It further provides simplification, in that adjustment between the x and y waists may now be achieved without change in magnification.