Enhanced monitoring of laser output power in electro-optical readers

Laser power monitoring arrangements interrupt power to a laser used in electro-optical readers upon detection of operating conditions not conforming to preestablished standards. An auxiliary light detector independently monitors the output power of a laser beam, and directly receives a portion of the laser beam reflected from an optical assembly operative for optically modifying the laser beam.

BACKGROUND OF THE INVENTION

Optical codes or dataforms are patterns made up of image areas having different light-reflective or light-emissive properties, which are typically assembled in accordance with a priori rules. The optical properties and patterns of codes are selected to distinguish them in appearance from the background environments in which they are used. Electro-optical readers identify or extract data from codes and are used in both fixed or portable installations in many diverse environments such as in stores for check-out services, in manufacturing locations for work flow and inventory control, and in transport vehicles for tracking package handling. The code is used as a rapid, generalized means of data entry.

Many conventional readers are designed to read one-dimensional bar code symbols. The bar code symbol is a pattern of variable-width rectangular bars separated by fixed or variable width spaces. The bars and spaces have different light-reflecting characteristics. One example of a one-dimensional bar code symbol is the UPC/EAN code used to identify, for example, product inventory. An example of a two-dimensional or stacked bar code symbol is the PDF417 barcode, which is disclosed in U.S. Pat. No. 5,635,697.

Many conventional readers are handheld and generate one or more moving beams of laser light from a reading laser. The beams sweep one or more scan lines across a symbol that is located anywhere in a range of working distances from a reader. The reader obtains a continuous analog waveform corresponding to the light reflected or scattered from the symbol. The reader then decodes the waveform to extract information from the symbol. A reader of this general type is disclosed, for example, in U.S. Pat. No. 4,251,798. A reader for detecting and decoding one- and two-dimensional symbols is disclosed in U.S. Pat. No. 5,561,283.

Symbols can also be read by employing solid-state imagers in imaging readers, also often deployed in handheld housings. For example, an imager, akin to that used in a digital camera, may have a one- or two-dimensional array of cells or pixel sensors that correspond to image elements or pixels in a field of view of the imager. Such an imager may be a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, and associated circuits for producing electronic signals corresponding to the one- or two-dimensional array of pixel information over the field of view.

Although generally satisfactory for its intended purpose, the use of an imaging reader is often frustrating, because an operator cannot tell whether the imager, or the handheld housing in which the imager is mounted, is aimed directly at the target symbol, which can be located anywhere within a range of working distances from the reader. Contrary to moving laser beam readers in which an operator can see the visible laser beam as at least one scan line on the symbol, the imager is a passive unit and provides no visual feedback to the operator to advise where the imager is aimed. To alleviate such problems, the prior art proposed, for example, in U.S. Pat. No. 6,060,722 an aiming light pattern generator in an imaging reader, for generating and projecting an aiming light pattern from a light source, such as an aiming laser, on the symbol prior to reading.

Reading performance is a function of many factors, one of which is power output of the reading laser. Reading performance in moving laser beam readers is enhanced when the reading laser power output is increased. Yet, stringent safety standards dictate the maximum power output of the reading laser. Also, moving laser beam reader malfunction such as failure of the reading laser must be reliably monitored.

Aiming performance is also a function of many factors, one of which is also power output of the laser. Aiming performance in imaging readers is enhanced when the aiming laser power output is increased. More particularly, the visibility of the aiming light pattern is more pronounced, for both indoor and outdoor lighting environments, as the aiming laser power output is increased. Yet, stringent safety standards also dictate the maximum power output of the aiming laser. Also, imaging reader malfunction such as failure of the aiming laser must be reliably monitored.

SUMMARY OF THE INVENTION

One feature of the present invention resides, briefly stated, in a laser power monitoring arrangement in electro-optical readers for, and methods of, reading indicia, such as bar code symbols, by generating a laser beam with a laser, preferably a laser diode, having an output power, by optically modifying the laser beam with an optical assembly spaced from the laser along an optical axis, by detecting the output power of the laser beam with a light detector, preferably an auxiliary photodiode, spaced transversely of the optical axis between the laser and the optical assembly, and by directly receiving a portion of the laser beam reflected from the optical assembly with the light detector.

The direct reception of the portion of the laser beam enhances detection and monitoring of the output power of the laser. Signal reception at the detector is stronger and more consistent than heretofore. In the case of a moving laser beam reader where the laser is employed for reading the symbol, and where the detector is employed in a control circuit for detecting reading laser malfunction, the enhanced signal reception at the detector enables the reading laser to be configured to emit more power to more closely approach established maximum safety standards, because of the increased safety and reliability afforded by the control circuit. More reading laser output power signifies better reading performance, for example, the range of working distances in which the symbol can be read is extended.

In the case of an imaging reader where the laser is employed for aiming at the symbol prior to reading, and where the detector is employed in a control circuit for controlling the output power of the aiming laser, the enhanced signal reception at the detector enables the aiming laser to be configured to emit more power to more closely approach established maximum safety standards. More aiming laser output power signifies better reading performance, for example, the visibility of an aiming light pattern projected by the aiming laser is more pronounced.

In a preferred embodiment, the optical assembly includes a focusing lens for focusing the laser beam. The laser diode and the focusing lens are mounted axially adjacent each other along the optical axis. The optical assembly also includes a holder for the focusing lens. The holder preferably has walls bounding an aperture stop. The holder has a chamfered surface operative for directly reflecting a part of the laser beam incident on the chamfered surface directly to the auxiliary photodiode. The chamfered surface may be on the walls bounding the aperture stop. Alternatively or in addition, the focusing lens has inclined lens surfaces operative for directly reflecting a part of the laser beam incident on the lens surfaces directly to the auxiliary photodiode.

In a preferred embodiment, the auxiliary photodiode is surface-mounted on a printed circuit board at a location between the laser and the optical assembly. An enclosure surrounds the auxiliary photodiode to prevent stray light from entering the auxiliary photodiode.

The laser power monitoring method of reading indicia according to this invention is performed by generating a laser beam with a laser having an output power, by optically modifying the laser beam with an optical assembly spaced from the laser along an optical axis, by detecting the output power of the laser beam with a light detector spaced transversely of the optical axis between the laser and the optical assembly, and by directly receiving a portion of the laser beam reflected from the optical assembly.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, the term “symbol” broadly encompasses not only symbol patterns composed of alternating bars and spaces of various widths as commonly referred to as bar code symbols, but also other one- or two-dimensional graphic patterns, as well as alphanumeric characters. In general, the term “symbol” may apply to any type of pattern or indicia that may be recognized or identified as a representation of variations in light reflectivity at various points of the pattern or indicia.FIG. 1shows an indicium15as one example of a “symbol” to be read. The term “target” broadly encompasses any one- or two-dimensional symbol, as well as any other object to be imaged, including non-symbols, documents, people, etc.

FIG. 1depicts a known handheld moving laser beam reader10for reading symbols. The reader10includes a housing having a barrel portion11and a handle12. Although the drawing depicts a handheld pistol-shaped housing, the invention may also be implemented in other types of housings such as a desktop workstation or a stationary reader. In the illustrated embodiment, the barrel portion11of the housing includes an exit port or window13through which an outgoing laser light beam14passes to impinge on, and sweep across, the bar code symbol15located in a range24of working distances from the housing.

The laser beam14moves across the symbol15to create a scan pattern. Typically, the scanning pattern is one-dimensional or linear, as shown by line16. This linear scanning movement of the laser beam14is generated by an oscillating scan mirror17driven by an oscillating motor18. If desired, means may be provided to scan the beam14through a two-dimensional scanning pattern, to permit reading of two-dimensional optically encoded symbols. A manually-actuated trigger19or the like permit an operator to initiate the scanning operation when the operator holds and aims the reader10at the symbol15.

The reader10includes a reading laser20mounted within the housing. The reading laser20generates the laser beam14. A photodetector21is positioned within the housing to collect at least a portion of the light reflected and/or scattered from the symbol15. The photodetector21, as shown, faces toward the window13and has a static, wide field of view characteristic of a non-retro-reflective reader. Alternatively, in a retro-reflective reader, a convex portion of the scan mirror17may focus collected light on the photodetector21, in which case, the photodetector faces toward the scan mirror. As the beam14sweeps the symbol15, the photodetector21detects the light reflected and/or scattered from the symbol15and creates an analog electrical signal proportional to the intensity of the collected light.

A digitizer (not shown) typically converts the analog signal into a pulse width modulated digital signal, with the pulse widths and/or spacings corresponding to the physical widths of the bars and spaces of the scanned symbol15. A decoder (not shown), typically comprising a programmed microprocessor with associated RAM and ROM, decodes the pulse width modulated digital signal according to a specific symbology to derive a binary representation of the data encoded in the symbol, and the alphanumeric characters represented by the symbol.

The reading laser20directs the laser beam through an optical assembly comprising a focusing lens22and preferably an aperture stop23, to optically modify and direct the laser beam onto the scan mirror17. The mirror17, mounted on a vertical shaft and oscillated by the motor drive18about a vertical axis, reflects the beam and directs it through the exit port13to the symbol15.

To operate the reader10, the operator depresses trigger19, which activates the reading laser20and the motor18. The reading laser20generates the laser beam that passes through the lens22and the aperture23. The lens22and the aperture23modify the beam to create an intense beam spot of a given size that extends continuously and does not vary substantially over the range24of working distances. The lens and the aperture direct the beam onto the mirror17, which directs the modified laser beam outwardly from the scanner housing11and toward the symbol15in a sweeping pattern, i.e., along scan line16. The symbol15, placed at any point within the working distance range24and substantially normal to the laser beam14, reflects and/or scatters a portion of the laser light. The photodetector21, shown mounted in the scanner housing11in a non-retro-reflective position, detects the reflected and/or scattered light and converts the received light into an analog electrical signal. The photodetector could also be mounted in a retro-reflective position facing the scan mirror17. The system circuitry then converts the analog signal to a pulse width modulated digital signal that a microprocessor-based decoder decodes according to rules of the symbology of the type of symbol being read.

As shown in a known imaging reader depicted inFIG. 2, an imager42having an array of cells or pixel sensors, each having one or more photosensors, is operative for converting return light from a target into electrical signals corresponding to a two-dimensional array of pixel information for processing into an image. A suitable imager is disclosed in U.S. Pat. No. 5,965,875. In operation, a field of view44is imaged by the imager42. More particularly, the return light from a target (not illustrated) in the field of view passes through an imaging lens46that captures and focuses the light onto a receiving surface of the imager.

In order to aim the imager42at the target prior to reading, an aiming system is provided. The aiming system includes an aiming laser48, an optical component50, and preferably an aperture stop52. The aiming laser48generates a laser beam that diverges until it passes through the aperture stop52, in which the beam is optically modified to have a predetermined cross-section. Thereupon, the laser beam passes through the optical component20, in which the beam is focused, collimated, and optically modified to generate a visible, aiming light pattern54comprising mutually perpendicular aiming lines on the target, thereby assisting an operator in aiming the imager42at the target. Other aiming light patterns are contemplated.

The optical component50may be a diffractive optical element (DOE), a holographic element, or a Fresnel element, which generates a light interference pattern useful for framing the field of view. It is also known to use non-interferometric optical components to project an aiming line as described in U.S. Pat. No. 6,069,748, which disclosed the use of a toroidal lens to project a single aiming line. U.S. Pat. No. 7,182,260 disclosed the use of a refractive optical element (ROE) having a plurality of refractive structures to generate an aiming light pattern on a symbol for framing the field of view of an imager.

FIG. 3depicts a laser60that is intended to represent either the reading laser20ofFIG. 1or the aiming laser48ofFIG. 2. The laser60includes a housing, a laser diode25in the housing, and a monitor photodiode26in the housing and operative for monitoring the output power of the diode25. The monitor photodiode26is positioned behind a rear facet of the laser diode25to monitor the output power level. The monitor photodiode26is part of a feedback control circuit operative for maintaining the laser output power constant. The feedback circuit includes a comparator27having a reference voltage applied to a positive input of the comparator through a voltage divider comprised of resistors28,29. The monitor photodiode26is connected to a negative input of the comparator via a resistive network including resistors30,31. The output of the comparator27is conducted through a resistor32and capacitor34to a gate G of a field effect transistor (FET)33. The drain output of the FET33is connected to the laser diode25. The source output of the device33is connected to ground through a current sense resistor35.

The control circuit ofFIG. 3is conventional in that the monitor photodiode26detects changes in output power of the laser beam emitted by the laser diode25and sends a feedback signal to the comparator27for driving the FET33to allow more or less current to pass through the current sense resistor35and, in turn, through the laser diode25. The greater this current, the greater the laser output power, and vice versa.

A current sense comparator36has one input connected to the current sense resistor35to monitor the current flowing therethrough, and another input connected to a reference voltage that corresponds to the maximum current allowable through the resistor35. The output of the comparator36is connected to an OR gate37which, in turn, is connected to a latch38and a switch39, which is connected between a power supply40and the laser diode25. If the comparator36senses that the current passing through the resistor35exceeds a maximum preestablished value, then an output control signal is conducted to the gate37and, in turn, to the latch38for opening the switch39to remove the power source40from energizing the laser diode25.

In further accordance withFIG. 3, a window comparator41is connected to the resistor32and monitors the voltage being applied to the gate G of the FET33. A maximum gate voltage and a minimum gate voltage are also applied to the window comparator41. The comparator41is, in turn, connected to the OR gate37. If the comparator41senses that the gate voltage being applied to the gate G is greater than the preestablished maximum gate voltage, or is less than the preestablished minimum gate voltage, then a signal is sent to the OR gate37to operate the latch38and open the switch39, thereby deenergizing the laser diode. Thus, power is removed from the laser diode25in the event of malfunction or failure of the monitor photodiode26, the FET33, the comparator27, the laser diode25, or any circuit connection.

More specifically, theFIG. 3circuit removes the power source40from the laser60after detecting an out-of-range condition in the error amplifier27that controls the output power of the laser. This circuit will remove power from the laser60under at least one of the following conditions: a failure of the device33in the output of the laser drive causes excess current to flow through the laser, thereby causing the laser output to exceed the factory set limit; the monitor diode26connection is lost due to a device33failure or a circuit connection failure; or, the laser fails and the laser drive current significantly increases as resistor35is used to sense a high current drive condition. Advantageously, a timer could be added to theFIG. 3circuit to remove power only when a malfunction persists for a predetermined time.

In addition to the monitor photodiode26, an auxiliary detector62, preferably a photodiode, is used to independently monitor the output power of the laser60, using the same control circuit depicted inFIG. 3. As shown in the embodiments ofFIGS. 4 and 6, the auxiliary detector62is not mounted within the housing of the laser60, but is mounted outside the housing. The monitor photodiode26is not shown inFIGS. 4 and 6.

As shown inFIGS. 4 and 6, the laser60, preferably a laser diode, emits a laser beam having an output power, and an optical assembly is axially spaced from the laser60along an optical axis64, for optically modifying the laser beam. The optical assembly includes a focusing lens66for focusing the laser beam, a holder68for holding the focusing lens66, and optionally, an aperture stop bounded by walls70of the holder68.

The auxiliary detector62is spaced transversely of, and away from, the optical axis64between the laser60and the optical assembly, for detecting the output power of the laser beam. The auxiliary detector62is surface-mounted on a printed circuit board72lying in a plane generally parallel to the optical axis64. An enclosure74surrounds the auxiliary detector62to prevent stray light from entering the auxiliary detector62. The auxiliary detector62is operative for directly receiving a portion of the laser beam reflected from the optical assembly.

As shown inFIG. 4, the holder68has a chamfered surface76operative for directly reflecting a part of the laser beam incident on the chamfered surface76directly to the auxiliary detector62. The chamfered surface76may be on the walls70bounding the aperture stop. Alternatively or in addition, as shown inFIG. 6, the focusing lens66has inclined lens surfaces78operative for directly reflecting a part of the laser beam incident on the lens surfaces78directly to the auxiliary detector62.

In the embodiments ofFIGS. 4 and 6, the additional light detected by the auxiliary detector62, is monitored and processed by the control circuit ofFIG. 3to generate a control signal used to open the switch39when the laser output power does not meet a preestablished value. This feature promotes safety in the use of a moving beam reader and an imaging reader in each of which a laser is used.

The direct reception of the portion of the laser beam enhances detection and monitoring of the output power of the laser60. Signal reception at the detector62is stronger and more consistent than heretofore. In the case of a moving laser beam reader where the laser20is employed for reading the symbol, and where the detector is employed in a control circuit for detecting reading laser malfunction, the enhanced signal reception at the detector enables the reading laser20to be configured to emit more power to more closely approach established maximum safety standards, because of the increased safety and reliability afforded by the control circuit. More reading laser output power signifies better reading performance, for example, the range of working distances in which the symbol can be read is extended.

In the case of an imaging reader where the laser48is employed for aiming at the symbol prior to reading, and where the detector is employed in a control circuit for controlling the output power of the aiming laser, the enhanced signal reception at the detector62enables the aiming laser48to be configured to emit more power to more closely approach established maximum safety standards. More aiming laser output power signifies better reading performance, for example, the visibility of an aiming light pattern projected by the aiming laser48is more pronounced.

To further insure and boost signal reception at the detector62, the cross-section80of the laser beam is oriented such that its longer dimension D, as shown inFIG. 5, extends in a direction generally perpendicular to the optical axis64and to the board72. The laser beam diverges away from the laser60at different angles along mutually orthogonal axes, thereby imparting an elongated or oval cross-section80to the laser beam. By orienting the longer dimension D to extend in a direction generally perpendicular to the board72, the laser beam will affirmatively be incident on the chamfered surfaces76or the inclined lens surfaces78for reflection and/or scattering therefrom. The chamfered surfaces76and the inclined lens surfaces78are outside the aperture and do not reduce the usable portion of the laser beam.

Although described in connection with moving beam readers and imaging readers, this invention is also useful in laser projection displays and, in general, any system in which a laser is used.