Abstract:
An arrangement for improving performance of a sensor operative for collecting light from a target to generate a data signal in a presence of ambient light, includes a sample and hold circuit for operating the sensor during a sampling time period in which the sensor only collects the ambient light to generate an ambient signal, and during a holding time period in which the sensor collects both the ambient light and the light from the target to generate a composite signal comprised of the ambient signal and the data signal; and a subtracting circuit operative for subtracting the ambient signal from the composite signal to produce the data signal as an output.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to sensors for collecting light from a target in the presence of ambient light and, more particularly, to improving the performance of such sensors, especially as used in electro-optical readers, such as laser scanners for reading indicia, such as bar code symbols, and in laser safety circuits for monitoring the output power of lasers, such as are used in laser scanners and in laser projection displays. 
     2. Description of the Related Art 
     Bar code readers are known in the prior art for reading various symbologies such as Universal Product Code (UPC) bar code symbols appearing on a label, or on the surfaces of an article. The bar code symbol itself is a coded pattern of graphic indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers electro-optically transform the graphic indicia into electrical signals, which are decoded into information, typically descriptive of the article or some characteristic thereof. Such information is conventionally represented in digital form and used as an input to a data processing system for applications in point-of-sale processing, inventory control, and the like. 
     Readers of this general type have been disclosed, for example, in U.S. Pat. No. 5,600,121, and may employ a portable laser scanning device held by a user, which is configured to allow the user to aim the device and, more particularly, a scanning laser light beam, at a targeted symbol to be read. In moving laser beam readers known in the art, the laser light beam is focused by a lens or other optical components along a light path as a beam spot on a target that includes the bar code symbol. The moving-beam reader operates by repetitively scanning the beam spot in a scan pattern across the symbol by means of motion of a scanning component, such as a moving scan mirror placed in the path of the light beam. The scanning component may either sweep the beam spot across the symbol and trace a scan line, or a series of scan lines, or another pattern, across the symbol, or scan a field of view of the reader, or both. 
     Bar code readers also include a sensor or photodetector which detects light reflected or scattered from the symbol. The photodetector or sensor is positioned in the reader in an optical path so that it has a field of view which ensures the capture of a portion of the light which is reflected or scattered off the symbol. The light is detected and converted into an electrical signal. Electronic circuitry and software decode the electrical signal into a digital representation of the data represented by the symbol that has been scanned. For example, the analog electrical signal generated by the photodetector is converted by a digitizer into a pulse width modulated digitized signal, with the widths corresponding to the physical widths of the bars and spaces. Such a digitized signal is then decoded, based on the specific symbology used by the symbol, into a binary representation of the data encoded in the symbol, and subsequently to the information or alphanumeric characters so represented. Such signal processors are disclosed in U.S. Pat. No. 5,734,153. 
     Bar code readers are required to work under variable ambient lighting conditions, including indoor office lighting and outdoor sunlight, both indoor and outdoor lighting ranging from dim to bright. A combination of optical and electrical measures is employed to prevent bright ambient light from overwhelming the reader and preventing a successful decoding and reading of a symbol to be read. Such measures can be optimized for indoor or outdoor lighting at anticipated light intensity levels, but not for both. Thus, performance is sacrificed when a reader is required to work at non-optimized light intensity levels. 
     In a typical retro-reflective or non-retro-reflective reader, collection optics collect the laser light scattered from the symbol and concentrate the collected light onto a photodiode acting as the sensor. The collection optics also unavoidably collects ambient light and concentrates it on the photodiode. The photodiode generates an electrical composite signal proportional to the brightness of the total collected light from the symbol and from the ambient light. The composite signal is comprised of a data signal derived from the symbol and an ambient signal derived from the ambient light. If the ambient signal is high due to very bright ambient light, or if the ambient signal is on the same order of magnitude as the data signal as in the case where the reader is operating at the end of its useful working range, then the reader may not function because the data signal derived from the symbol will be lost. 
     Photodiodes are not only used in collection optics in a laser scanner, but are also used in laser safety monitoring circuits in laser scanners and laser projection display arrangements. As is known, a laser projection display arrangement projects a two-dimensional image in color on a screen based on a pair of scan mirrors which oscillate in mutually orthogonal directions to scan a plurality of differently colored laser beams, for example, red, blue and green, over a raster pattern of scan lines, each scan line having a number of pixels. A controller processes video data from a host, as well as control data with the host in order to form the image by selectively energizing and deenergizing a plurality of lasers that emit the laser beams. 
     In the event that a laser beam exits a device, such as a laser scanner or a laser projection display arrangement, the output power of each exiting beam is monitored by a photodiode and, if the monitored output power exceeds a safety value, then the safety circuit deenergizes the laser that emitted the beam. If the ambient signal is on the same order of magnitude as the monitored output power data signal, then the safety circuit may not function. 
     The prior art has proposed in FIGS. 9-11 of U.S. Pat. No. 5,923,021 preamplifier circuits for processing the output signal from a photodiode. These circuits include components that reduce the resistance of a feedback resistor, thereby reducing gain and worsening the signal-to-noise ratio, or that introduce shot or white noise from bipolar transistors and diodes which again worsen the signal-to-noise ratio. Hence, such preamplifier circuits are unsatisfactory for enabling a reader to perform reliably under all lighting conditions. 
     U.S. Pat. No. 7,128,264 discloses an arrangement for nulling ambient current in a photodiode circuit. However, this arrangement utilizes a low frequency, low pass filtering circuit that requires large bulky capacitors, and undesirably increases the size, weight and assembly cost of the arrangement. 
     SUMMARY OF THE INVENTION 
     One feature of the present invention resides, briefly stated, in an arrangement for, and a method of, improving performance of a sensor operative for collecting light from a target to generate a data signal in a presence of ambient light. The arrangement comprises a sample and hold circuit for operating the sensor during a sampling time period in which the sensor only collects the ambient light to generate an ambient signal, and during a holding time period in which the sensor collects both the ambient light and the light from the target to generate a composite signal comprised of the ambient signal and the data signal; and a subtracting circuit for subtracting the ambient signal from the composite signal to produce the data signal as an output. 
     Preferably, the sensor is a photodiode that generates the composite signal and the ambient signal as electrical current signals, and the subtracting circuit includes a transimpedance amplifier that generates the data signal at the output as an electrical voltage signal. The transimpedance amplifier has a non-inverting input to which the composite signal is applied, and an inverting input to which the ambient signal is applied. 
     The sample and hold circuit includes a switch for switching between the sampling and holding time periods, and a pair of sample and hold transistors connected to the switch and the transimpedance amplifier, one sample and hold transistor being operative for conducting the composite signal to the non-inverting input, and the other sample and hold transistor being operative for conducting the ambient signal to the inverting input. A reference transistor is connected to one of the sample and hold transistors, and a biasing circuit is operative for biasing the reference transistor. The biasing circuit preferably includes a d.c. current sink and a biasing transistor. 
     In accordance with this aspect of the invention, sampling is used to perform the function of low pass filtering and, hence, the prior art need for large bulky capacitors has been eliminated. The entire arrangement can be fully and cost effectively integrated into an application specific integrated circuit with all the benefits of reduced cost, size, weight and complexity, as well as the inherent higher processing speed and higher overall small signal bandwidth. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a non-retro-reflective electro-optical reader in accordance with the prior art; and 
         FIG. 2  is a circuit schematic of an arrangement for improving performance of a photodiode in accordance with this invention for use in the reader of  FIG. 1 . 
     
    
    
     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 which may be recognized or identified either by scanning a light beam and detecting reflected or scattered light as a representation of variations in light reflectivity at various points of the pattern or indicia.  FIG. 1  shows an indicia  15  as one example of a “symbol” to be read. 
       FIG. 1  depicts a handheld laser scanner device  10  for reading symbols. The laser scanner device  10  includes a housing having a barrel portion  11  and a handle  12 . Although the drawing depicts a handheld pistol-shaped housing, the invention may also be implemented in other types of housings such as a desk-top workstation or a stationary scanner. In the illustrated embodiment, the barrel portion  11  of the housing includes an exit port or window  13  through which an outgoing laser light beam  14  passes to impinge on, and scan across, the bar code symbol  15  located at some distance from the housing. 
     The laser beam  14  moves across the symbol  15  to create a scan pattern. Typically, the scanning pattern is one-dimensional or linear, as shown by line  16 . This linear scanning movement of the laser beam  14  is generated by an oscillating scan mirror  17  driven by an oscillating motor  18 . If desired, means may be provided to scan the beam  14  through a two-dimensional scanning pattern, to permit reading of two-dimensional optically encoded symbols. A manually-actuated trigger  19  or similar means permits an operator to initiate the scanning operation when the operator holds and aims the device  10  at the symbol  15 . 
     The scanner device  10  includes a laser source  20 , e.g., a gas laser tube or a semiconductor laser diode, mounted within the housing. The laser source  20  generates the laser beam  14 . A photodetector  21  is positioned within the housing to collect at least a portion of the light reflected and scattered from the bar code symbol  15 . The photodetector  21 , as shown, faces toward the window  13  and has a static, wide field of view characteristic of the non-retro-reflective readers described above. 
     Alternatively, in a retro-reflective reader, a concave portion of the scan mirror  17  may focus collected light on the photodetector  21 , in which case the photodetector faces toward the scan mirror. As the beam  14  sweeps the symbol  15 , the photodetector  21  detects the light reflected and scattered from the symbol  15  and creates an analog electrical signal proportional to the intensity of the collected light. Details of the processing of this analog signal, especially in different ambient lighting conditions, form the basis of this invention and are described in detail below. 
     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 symbol  15 . A decoder (not shown), typically comprising a programmed microprocessor with associated RAM and ROM, decodes the pulse width modulated digital signal according to the specific symbology to derive a binary representation of the data encoded in the symbol, and the alphanumeric characters represented by the symbol. 
     The laser source  20  directs the laser beam through an optical assembly comprising a focusing lens  22  and an aperture stop  23 , to modify and direct the laser beam onto the scan mirror  17 . The mirror  17 , mounted on a vertical shaft and oscillated by the motor drive  18  about a vertical axis, reflects the beam and directs it through the exit port  13  to the symbol  15 . 
     To operate the scanner device  10 , the operator depresses trigger  19  which activates the laser source  20  and the motor  18 . The laser source  20  generates the laser beam which passes through the element  22  and aperture  23  combination. The element  22  and aperture  23  modify the beam to create an intense beam spot of a given size which extends continuously and does not vary substantially over a range  24  of working distances. The element and aperture combination directs the beam onto the rotary mirror  17 , which directs the modified laser beam outwardly from the scanner housing  11  and toward the bar code symbol  15  in a sweeping pattern, i.e., along scan line  16 . The bar code symbol  15 , placed at any point within the working distance  24 , reflects and scatters a portion of the laser light. The photodetector  21 , shown mounted in the scanner housing  11  in a non-retro-reflective position, detects the reflected and 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 mirror  17 . The system circuitry then converts the analog signal to a pulse width modulated digital signal which a microprocessor-based decoder decodes according to the characteristics of the bar code symbology rules. 
     In accordance with this invention, as shown in  FIG. 2 , the photodetector  21 , shown as a photodiode, collects light  25  from the symbol  15 , as well as from ambient light, and generates a composite current signal whose magnitude is proportional to the intensity of the collected light. Thus, the composite signal includes a data current signal I sig  that corresponds to the information encoded in the symbol, as well as a noise current signal I DC  that corresponds to the ambient light. The noise signal is, of course, undesirable and has a d.c. component and slow varying currents such as dark currents. 
     A d.c. current sink  26  and a common gate input transistor  28  are connected to the photodiode  21  and are used to bias a reference transistor  30  with a bias current I bias . A sample and hold (S/H) circuit includes a S/H switch  32  for switching between successive sampling and holding time periods, and a pair of S/H mirror transistors  34 ,  36  connected to opposite sides of the switch. An S/H capacitor  38  is connected between the gate terminals of the mirror transistors  34 ,  36  and their source terminals. 
     A time period is chosen when the laser  20  is off, and only ambient current is generated by the photodiode  21 . During this time period, i.e., the sampling period, the switch  32  is closed, and the two mirror transistors  34 , 36  having equal scaling factors (N=x) make two identical copies of the current (I bias +I DC ) in the reference transistor  30  scaled by the scaling ratio x. During the sampling period, the capacitor  38  is charged to the operating voltage of the mirror transistor  30 . 
     After the sampling period is over, the laser  20  is turned on, and both the ambient current and the data current are generated by the photodiode  21 . During this holding period, the switch  32  is opened. Since the capacitor  38  remembers the operating voltage when the switch was closed, the current in the mirror transistor  36  will flow at the same level as when the switch was open. Thus, the current in the mirror transistor  36  will be the scaled ambient signal [x*(I bias +I DC )], and the current in the mirror transistor  34  will be the scaled composite signal [X*(I bias +I DC +I sig )]. 
     A subtracting circuit for subtracting the ambient signal from the composite signal to produce the data signal as an output includes a differential transimpedance amplifier  40  that generates the data signal at an output  48  as an electrical voltage signal. The transimpedance amplifier  40  has a non-inverting input  42  to which the composite signal is applied, and an inverting input  44  to which the ambient signal is applied. A resistor R f  is connected across the inverting input  44  and the output  48  of the amplifier  40 . The voltage signal at the output  48  can be represented as: R f *[x*(I bias +I DC +I sig )]. By choosing the d.c. bias signal to be large, the overall bandwidth of the circuit of  FIG. 2  is increased. This arrangement increases the signal bandwidth due to the d.c. biasing of the current mirrors. 
     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, as noted above, photodiodes are not only used in collection optics in a laser scanner, but are also used in laser safety monitoring circuits in laser scanners and laser projection display arrangements. As is known, a laser projection display arrangement projects a two-dimensional image in color on a screen based on a pair of scan mirrors which oscillate in mutually orthogonal directions to scan a plurality of differently colored laser beams, for example, red, blue and green, over a raster pattern of scan lines, each scan line having a number of pixels. A controller processes video data from a host, as well as control data with the host in order to form the image by selectively energizing and deenergizing a plurality of lasers that emit the laser beams. 
     In the event that a laser beam exits a device, such as a laser scanner or a laser projection display arrangement, the output power of each exiting beam is monitored by a photodiode and, if the monitored output power exceeds a safety value, then the safety circuit deenergizes the laser that emitted the beam. If the ambient signal is on the same order of magnitude as the monitored output power data signal, then the safety circuit may not function. The circuit of  FIG. 2  is readily integratable into a safety monitoring circuit and, in the case of a color laser projection display having three lasers, is of particular benefit in reducing the size, weight and complexity of the overall arrangement. 
     While the invention has been illustrated and described as embodied in an electro-optical reader with improved performance for the photodiode in ambient light, 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. 
     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. 
     What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.