Arrangement for and method of generating uniform distributed illumination pattern for imaging reader

A substantially uniform distributed illumination pattern of light is generated on and along a symbol to be read by image capture. A solid-state imager is mounted on a tilted printed circuit board in a tilted handle of an ergonomic reader. An imaging lens assembly captures return light over a field of view from the symbol along an imaging axis, and projects the captured return light onto the imager. An illumination light source is mounted on the board for emitting illumination light at an acute angle of inclination relative to the imaging axis. An optical component includes a first lens portion with a polynomial incident surface for intercepting, bending and aligning the emitted illumination light to generate the pattern in a scan direction along the symbol, and a second lens portion with a toroidal or cylindrical aspherical surface for collimating the aligned illumination light to generate the pattern in a transverse direction.

DESCRIPTION OF THE RELATED ART

Solid-state imaging systems or imaging readers have been used, in both handheld and/or hands-free modes of operation, to electro-optically read targets, such as one-dimensional bar code symbols, particularly of the Universal Product Code (UPC) symbology having a row of bars and spaces spaced apart along a scan direction, as well as two-dimensional symbols, such as the Code 49 symbology having a plurality of vertically stacked rows of bar and space patterns in a single symbol, as described in U.S. Pat. No. 4,794,239.

The handheld imaging reader includes a housing having a handle held by an operator, and an imaging module supported by the housing and aimed by the operator at the symbol during reading. The imaging module includes a solid-state imager with a sensor array of photocells or light sensors, which correspond to image elements or pixels in a field of view of the imager, and an imaging lens assembly for capturing return light scattered and/or reflected from the symbol being imaged along an imaging axis, and for projecting the return light onto the sensor array to initiate capture of an image of the symbol. Such an imager may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device and associated circuits for producing and processing electronic signals corresponding to a one- or two-dimensional array of pixel data over the field of view.

It is therefore known to use the imager for capturing a monochrome image of the symbol as, for example, disclosed in U.S. Pat. No. 5,703,349. It is also known to use the imager with multiple buried channels for capturing a full color image of the symbol 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.

In order to increase the amount of the return light captured by the sensor array, especially in dimly lit environments and/or at far range reading, the imaging module generally also includes an illuminating light assembly for illuminating the symbol with illumination light for reflection and scattering therefrom. When the sensor array is one-dimensional, i.e., linear, or is two-dimensional with an anamorphic field of view, the illumination light preferably is distributed along a short height, distributed illumination pattern, also termed an illuminating or scan line, that extends lengthwise along the symbol. The distributed illumination pattern is typically generated by using a single light source, e.g., a light emitting diode (LED) sized in the millimeter range and a single cylindrical lens.

Although generally satisfactory for its intended purpose, the use of the single LED and the single cylindrical lens has been problematic, because the distributed illumination pattern typically has a height taller than that desired, does not have sharp edges, is dominated by optical aberrations, and is nonuniform in intensity since the light intensity is brightest along an optical axis on which the LED is centered, and then falls off away from the axis, especially at opposite end regions of the distributed illumination pattern. Also, the coupling efficiency between the LED and the cylindrical lens has been poor. Adding an aperture stop between the LED and the cylindrical lens will improve the sharpness (i.e., shorten the height) of the distributed illumination pattern, but at the cost of a poorer coupling efficiency and a dimmer distributed illumination pattern that, of course, degrades reading performance.

For a brighter distributed illumination pattern, a pair of spaced-apart LEDs and a pair of cylindrical lenses could be employed. However, this further increases cost, introduces more optical aberrations, and further reduces coupling efficiency. Also, the illumination light emitted by the pair of LEDs overlap at a central region of the distributed illumination pattern, thereby creating a bright, “hot” spot and abrupt light intensity transitions in the distributed illumination pattern, all of which can cause reading performance to deteriorate.

The known imaging systems are located up close near a reader housing window through which the illumination light and the return light pass. Hence, the field of view of the imaging lens assembly is relatively wide in order to reliably read symbols located in the near range of working distances relative to the window. This, in turn, reduces resolution at the far range of working distances relative to the window and also spreads the illumination over a wider area, thereby reducing its intensity and again decreasing reading performance.

For good ergonomics, the handle of the housing is advantageously rearwardly tilted, for example, by about fifteen degrees relative to the vertical. The illuminating light assembly may advantageously be mounted on a printed circuit board (PCB) mounted in the tilted handle and, therefore, also tilted relative to the vertical. The illumination light emitted by the LEDs on-board the tilted PCB, therefore, needs to be redirected and aligned with the generally horizontal imaging axis of the imaging lens. Known imaging readers insure such alignment by configuring the handle and the PCB therein to be strictly vertical, but this results in a housing with a poor ergonomic design that increases operator fatigue and discomfort and decreases productivity.

SUMMARY OF THE INVENTION

One feature of the present invention resides, briefly stated, in an arrangement for generating a substantially uniform distributed illumination pattern of light on and along a symbol to be read by image capture. The arrangement includes an imaging system having a solid-state imager with an array of image sensors, such as a CCD or a CMOS, and an imaging lens assembly for capturing return light over a field of view from the symbol along an imaging axis, and for projecting the captured return light onto the array. The array is one-dimensional, i.e., linear, or is two-dimensional with an anamorphic field of view. The field of view of the imaging system is generally perpendicular to the imaging axis and generally matches the distributed illumination pattern of light on and along the symbol. The imaging lens assembly preferably includes a plurality of imaging lenses, advantageously a doublet or a Cooke triplet, spaced apart along the imaging axis, or in close proximity with one another.

The arrangement further includes an illuminating light assembly having an illumination light source for emitting illumination light at an acute angle of inclination relative to the imaging axis, and an optical component including a first lens portion for intercepting, bending and aligning the emitted illumination light to generate the substantially uniform distributed illumination pattern of light along the symbol in a scan direction generally perpendicular to the imaging axis, and a second lens portion for collimating the aligned illumination light in a transverse direction generally perpendicular to the scan direction to generate the substantially uniform distributed illumination pattern of light on the symbol. Advantageously, the optical component can comprise a lower half of a full size lens symmetrical about an optical axis that is offset from the imaging axis.

The light source includes at least one light emitting diode (LED) and, preferably, a plurality of LEDs, such as a pair of LEDs spaced apart along a scan direction lengthwise of the symbol. An aperture stop is positioned between each LED and the optical component, preferably in close proximity to the LED, for limiting the vertical extent or height of the emitted illumination light incident on the optical component and, in turn, the vertical height of the distributed illumination pattern along the transverse direction. The LEDs and the array are preferably surface mounted on a printed circuit board (PCB) tilted at the acute angle of inclination relative to the imaging axis. In the preferred embodiment, the tilted PCB is mounted within a tilted handle of an ergonomic imaging reader for electro-optically reading the symbol by image capture. The reader has a window through which the return light and the distributed illumination pattern of light pass. The window may be tilted relative to the imaging axis to avoid reflections of the emitted illumination light from reaching the imaging lens assembly. The imaging lens assembly is located remotely from the window, for example, over forty millimeters away.

The emitted illumination light from each LED overlap in a central zone of the distributed illumination pattern. Hence, to reduce light intensity in the central zone, the first lens portion is configured with an incident polynomial surface, also operative for optically modifying the illumination light to lie generally along a straight line along the scan direction. The second lens portion is configured with an exit toroidal or cylindrical aspherical surface for projecting the illumination light of limited vertical height passing through the aperture stop towards the symbol, and for collimating the aligned illumination light on the symbol. The optical component may be a unitary lens extending along the scan direction between the LEDs, or a pair of discrete lenses, one for each LED, each lens being configured with the first and second lens portions.

In accordance with this invention, the optical component forms the distributed illumination pattern on and along the symbol with a uniform intensity not dominated by optical aberrations or abrupt intensity transitions. The coupling efficiency between the light source and the optical component is much improved, thereby increasing light throughput, enhancing reading performance, and improving visibility of the distributed illumination pattern. Reader ergonomics is enhanced.

Another feature of the present invention resides in a method of generating the substantially uniform distributed illumination pattern of light on and along the symbol to be read by image capture. The method is performed by capturing return light over a field of view from the symbol along an imaging axis, projecting the captured return light onto a solid-state imager, emitting illumination light at an acute angle of inclination relative to the imaging axis, intercepting, bending and aligning the emitted illumination light with a first lens portion of an optical component, and collimating the aligned illumination light with a second lens portion of the optical component to generate the substantially uniform distributed illumination pattern of light on and along the symbol.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference numeral30inFIG. 1generally identifies an ergonomic imaging reader configured as a gun-shaped housing having an upper barrel or body32and a lower handle28tilted rearwardly away from the body32at an angle of inclination, for example, fifteen degrees, relative to the vertical. A window26is located adjacent the front or nose of the body32and is preferably also tilted at an angle of inclination, for example, fifteen degrees, as best shown inFIG. 5, relative to the vertical. The imaging reader30is held in an operator's hand and used in a handheld mode in which a trigger34is manually depressed to initiate imaging of target indicia, especially one-dimensional symbols, to be read in a range of working distances relative to the window26. Housings of other configurations can also be employed.

As schematically shown inFIG. 2, an imaging system or module includes an imager24mounted on a printed circuit board (PCB)22in the reader30. The PCB22is mounted within the tilted handle28and, as best shown inFIG. 5, is also tilted at an angle of inclination, for example, fifteen degrees, relative to the vertical. The imager24is a solid-state device, for example, a CCD or a CMOS imager having a one-dimensional array of addressable image sensors or pixels arranged in a single, linear row, or a two-dimensional array of such sensors arranged in mutually orthogonal rows and columns, preferably with an anamorphic field of view, and operative for detecting return light captured by an imaging lens assembly20along an imaging axis46through the window26. The return light is scattered and/or reflected from a target or symbol38over the field of view. The field of view is generally perpendicular to the imaging axis46.

The imaging lens assembly20is part of the imaging system and is operative for focusing the return light onto the array of image sensors to enable the symbol38to be read. Details of the imaging lens assembly20, as best seen inFIG. 6, are described below. The symbol38may be located anywhere in a working range of distances between a close-in working distance (WD1) and a far-out working distance (WD2). In a preferred embodiment, WD1is about one-half inch from the window26, and WD2is about thirty inches from the window26. The imaging lens assembly20is located remotely from the window26, for example, over forty millimeters away.

An illuminating light assembly is also mounted in the imaging reader and includes an illumination light source, e.g., at least one light emitting diode (LED), and preferably a plurality of LEDs, such as a pair of LEDs10,12, and an optical component configured to generate a substantially uniform distributed illumination pattern of light on and along the symbol38to be read by image capture. At least part of the scattered and/or reflected return light is derived from the illumination pattern of light on and along the symbol38. The optical component can comprise a lower half of a full size lens (shown in dashed lines inFIG. 5) that is symmetrical about an optical axis56. The optical component can be a unitary lens as shown inFIG. 7, or a pair of lenses16,18as shown inFIG. 8. Details of the illuminating light assembly, as best seen in theFIGS. 3-5, are described below. The window26is tilted to avoid reflections of the illumination light from the LEDs10,12from reaching the imaging lens assembly20.

As shown inFIG. 2, the imager24and the LEDs10,12are operatively connected to a controller or microprocessor36operative for controlling the operation of these components. A memory14is connected and accessible to the controller36. Preferably, the microprocessor is the same as the one used for processing the return light from target symbols38and for decoding the captured target images.

In operation, the microprocessor36sends a command signal to energize the LEDs10,12for a short exposure time period, say 500 microseconds or less, and energizes and exposes the imager24to collect the return light, e.g., illumination light and/or ambient light, from the target symbol38only during said exposure time period. A typical array needs about 18-33 milliseconds to acquire the entire target image and operates at a frame rate of about 30-60 frames per second.

Turning now toFIGS. 3-5, both LEDs10,12and the imager24are each surface mounted on the tilted PCB22. Surface mounting eliminates the use of ribbon cables and separate PCBs and connectors. The LEDs10,12are mounted at a higher elevation than, and at opposite sides of, the imager24. The LEDs10,12are spaced apart along a scan direction lengthwise of the symbol38. When energized, each LED10,12downwardly emits a beam of illumination light substantially towards the optical component16,18at the acute angle of inclination relative to the generally horizontal imaging axis46. An aperture stop40is positioned on the optical axis56in front of each LED, preferably in close proximity to the respective LED, for limiting the vertical extent or height of the emitted illumination light beam incident on the optical component16,18. The LEDs10,12are contained in a casing42overlaid by a canopy44. The optical component16,18is supported at the front of the casing42.

The optical component16,18images each aperture stop40and includes a first lens portion48for intercepting, bending and aligning the emitted illumination light beams to generate the substantially uniform distributed illumination pattern of light along the scan direction that is generally perpendicular to the imaging axis46, and a second lens portion50for vertically collimating the aligned illumination light beams along a transverse direction generally perpendicular to the scan direction. The emitted illumination light beams from the LEDs overlap in a central zone of the distributed illumination pattern. The first lens portion48is configured with an incident polynomial surface52for reducing the light intensity of the overlapping beams in the central zone, and for shaping the distributed illumination pattern as a generally straight line along the scan direction. The second lens portion50is configured with an exit toroidal or cylindrical aspherical surface54for projecting the illumination light of limited vertical height passing through the aperture stop40towards the symbol38and for collimating the aligned illumination light on and along the symbol38. The optical component16,18can be a unitary lens (seeFIG. 7) extending along the scan direction between the LEDs10,12. Alternatively, the optical component comprises a pair of lenses16,18(seeFIG. 8), one for each LED, each lens16,18separately having the first and second lens portions48,50and separately configured with the incident polynomial surface52and the exit toroidal or cylindrical surface54. Each polynomial surface52creates an asymmetrical light intensity pattern with less intense light at the central zone; hence, when the less intense light of both beams overlap at the central zone, they tend to match the intensity of the light at opposite end zones of the distribution pattern throughout the entire range of working distances.

As shown inFIG. 6, the imaging lens assembly20includes a plurality of imaging lenses58,60,62, preferably configured as a doublet or a Cooke triplet, spaced apart along the imaging axis46, or in close proximity with one another, and mounted in a holder64. The center lens60has a negative power. The triplet compensates for optical aberrations.

Returning toFIG. 5, the center of the aperture stop40lies on, or is substantially parallel and close to, the optical axis56so that the outgoing emitted illumination light rays exiting the optical component16,18are substantially parallel to the optical axis56. A central axis70is defined as the geometric center of the optical component16,18and is either collinear with, or substantially parallel and close to, the imaging axis46. The central axis70is also substantially parallel to, and vertically offset from, the optical axis56. In some embodiments, it may be desirable that the center of the aperture stop40be decentered or tilted with respect to the optical axis56so that the outgoing illumination light rays are directed at an angle of inclination relative to the central axis70.

The arrangement of this invention wastes less illumination than prior art arrangements and better matches the illumination field of view to the field of view of the imaging system. This invention enables an imager of less resolution to be employed, but without sacrificing readability at far-out working distances.

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. For example, the optical component could be replaced by a lens and an optical wedge.

While the invention has been illustrated and described as an arrangement or module for, and a method of, generating a substantially uniform distributed illumination pattern of light on and along a symbol to be read by image capture by an imaging reader, 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.