Abstract:
This invention is directed to a method and apparatus for determining the level of ambient light impinging on a selected number of pixels in an imaging array where each pixel includes a photodiode. The ambient light may be determined by resetting the pixels in the array and by detecting current flow through the photodiodes in a selected number of the pixels as they are being reset. Alternately, the ambient light may be determined by resetting a selected number of the pixels in the array and by detecting current flow through the photodiodes in the selected number of the pixels as they are being reset. The photodiodes are reset by applying a reverse bias voltage across them and the current flow is detected by measuring the current flow through a resistance in parallel to the selected photodiodes. The selected number of pixels may be divided into one or more groups each having at least one pixel, and the pixels in each group may be arranged in specific patterns within the array. The array may be laid out in rows and columns, and the groups may be located in predetermined rows or columns. When only a selected number of pixels are reset and these pixels are divided into groups, the groups may be sequentially reset to permit differentiation between the groups.

Description:
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/177,496 filed on Jan. 21, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of image scanning devices and in particular to determining ambient light intensity for image scanning devices. 
     BACKGROUND OF THE INVENTION 
     In the imaging industry, scanners are expected to operate effectively under a wide range of ambient light. A number of solutions have been developed wherein ambient light is measured in order to control the scanning system. The sensing of ambient light may be done by an ambient light detection circuit which is separate from the imaging array, or ambient light can be detected through the use of the imaging array itself. The ambient light measurement is then used either to adjust the exposure time for the imaging array/lens, to set the gain of the image signal or to control the brightness of a light source. 
     U.S. Pat. No. 4,970,379 which issued on Nov. 13, 1990 to Danstrom discloses exposure/illumination control for a bar code scanner consisting of a controllable light source and an optical sensor that is independent of the scanner array. The optical sensor converts the light reflecting from the object to be scanned into an electrical signal representative of the ambient light. This signal is coupled to a comparator, which determines the illumination required by the scanner array, and then adjusts the power to the controllable light source accordingly. A major drawback of this method is that during low light conditions the light source will be driven by the comparator to generate bright illumination, which consumes a large amount of power. In a hand held device this is extremely detrimental, as most hand held devices have a self-contained power supply. 
     Other systems use the imaging array itself to determine ambient light levels which is then used to control exposure time. U.S. Pat. No. 4,471,228 which issued on Sep. 11, 1984 to Nishizawa et al., describes an image sensor consisting of non-destructive readout-type image cells, the sensor uses the array of image cells as both photo-detector cells for the measurement of ambient light and as image capturing cells for imaging an object. The imaging array is exposed to the object and an ambient light measurement run is made through previously selected imaging cells. The added value of the selected imaging cells is compared to a reference value to determine the exposure level required. The selected imaging cells are then erased, and an image scan of the object is performed with a controlled exposure time. 
     The shortcoming of this method is that it consists of too many steps. The multi-step process of using the array to measure ambient light and then forcing the array to be reset before the image is scanned, slows the process down. Additionally, the extra step requires an extra expenditure of power, which is a severe detriment in a hand-held device. 
     Another method using the imaging array is U.S. Pat. No. 4,338,514, which issued on Jul. 6, 1982 to Bixby, discloses a further method of controlling exposure time by operating a mechanical shutter in response to radiant energy impinging on the sensor array. The semiconductor array substrate current is monitored during the exposure of the imaging array to produce an integrated signal that is proportional to the exposure level of the array. The signal is compared to a threshold voltage and when it exceeds a threshold value the shutter is closed. 
     There are drawbacks to this method in that it requires additional processing steps in order to create an apparatus to monitor the substrate current. Specifically, the apparatus requires the addition of a layer of conductive material between the non-conductive base-plate and the semi-conductive substrate. While this type of process is typical in some CCD imagers, it would be a costly additional fabrication step in a CMOS imager. 
     A further system in which exposure time is adjusted is described in U.S. Pat. No. 5,986,705 which issued on Nov. 16, 1999 to Shibuya et al. A video camera is described having an image sensing device, an exposure adjustment apparatus which controls the gain of the amplifier to adjust the scanned output signal and further controls a drive pulse generator to control the exposure time of the sensing device. In one embodiment, the video camera controls exposure by capturing an image with the image sensing device, amplifying the output signal which is driven externally as well as being fed back into the exposure adjustment apparatus where the signal is compared to a reference. When the comparison indicates that the image is either overexposed, underexposed or without need of adjustment, control signals are sent to the drive pulse generator to adjust exposure time and to the amplifier to adjust the gain of the amplifier. 
     This method has several disadvantages, its iterative style of exposure control is only advantageous for a video camera. Controlling only exposure time and signal gain is limiting in terms of the range of light intensity under which the device would remain useful. Still cameras, bar-code readers and the like, would not find such a method useful as it would require additional circuitry to filter out the overexposed and underexposed images. Low-light conditions would be difficult for the device to image as it has no control over an external light source. 
     While each of the measurement methods has its merits, the measurement methods are inherently limited by either the addition of; extra circuitry, increasing cost and size; fabrication steps, increasing cost; time, slowing the overall performance of the imaging circuit. 
     Therefore, there is a need for an ambient light detector that is integrated with a scanning device without adding costly additional circuitry and that provides reliable ambient light detection without undue interference with the image capture process. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a method and apparatus for determining the level of ambient light impinging on a pixel having a photodiode. The method comprises resetting the photodiode in the pixel and at the same time detecting the current flow through the photodiode as an indication of the ambient light level. The photodiode is reset by applying a reverse bias voltage across it and the current flow is detected by measuring the current flow through a resistance in parallel to the photodiode. 
     In accordance with another aspect, this invention is directed to a method and apparatus for determining the level of ambient light impinging on a selected number of pixels in an imaging array where each pixel includes a photodiode. The ambient light may be determined by resetting the pixels in the array and by detecting current flow through the photodiodes in a selected number of the pixels as they are being reset. Alternately, the ambient light may be determined by resetting a selected number of the pixels in the array and by detecting current flow through the photodiodes in the selected number of the pixels as they are being reset. 
     The selected number of pixels may be divided into one or more groups each having at least one pixel, and the pixels in each group may be arranged in specific patterns within the array. The array may be laid out in rows and columns, and the groups may be located in predetermined rows or columns. When only a selected number of pixels are reset and these pixels are divided into groups, the groups may be sequentially reset to permit differentiation between the groups. 
     In accordance with another aspect of this invention, an apparatus determines ambient light on an imaging array of light sensitive pixels where each has a photodiode and photodiode reset switch adapted to apply a predetermined reset voltage across the photodiode and further has one or more power rails each connected to one or more of the pixels for supplying power to them. The apparatus comprises current monitoring circuitry that measures current flow in the photodiodes of selected pixels as the photodiodes are being reset to provide an output signal representative of the ambient light. 
     With regard to a further aspect of this invention, the current monitoring circuitry and the imaging array may be integrated on the same die. 
     The current monitoring circuitry may further include one or more current monitors each connected to at least one of the power rails for monitoring the current flow in the photodiodes connected to the power rails and an analog-to-digital converter coupled to each of the current monitors to provide a digital output signal representative of the ambient light. The current monitor may be a current-to-voltage converter connected to a power rail through a resistance. The current-to-voltage converter may include an op-amp having an inverting input terminal coupled to the resistance, a non-inverting input terminal adapted to be coupled to a reference voltage and an output terminal, the output terminal being coupled to the inverting input terminal through a further resistance. 
     In accordance with an aspect of this invention, in an imaging array where the pixels are positioned in rows and columns, the power rails are may each be connected to a different group of the pixels located in a rows or a column. The power rails for the selected pixels may all be adapted to be connected to the same power supply directly. Alternately, the power rails may each be adapted to be connected to a power supply through a diode or they may each be adapted to be connected to a separate power supply. 
     With regard to a further aspect of this invention, the apparatus may further include a control for the pixel reset switches that will reset individual groups of pixels sequentially to allow the current in the reset photodiodes to be monitored individually and sequentially. 
     In accordance with another aspect, this invention may be integrated into a system for controlling the output signal during image capture of an object by an imager where the imager includes an imaging array of light sensitive pixels each having a photodiode and photodiode reset means adapted to apply a predetermined reset voltage across the photodiode, and one or more power rails each connected to one or more pixels on a die. 
     Other aspects and advantages of the invention, as well as the structure and operation of various embodiments of the invention, will become apparent to those ordinarily skilled in the art upon review of the following description of the invention in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the accompanying drawings, wherein: 
     FIG. 1 schematically illustrates a voltage supply coupled to an array of light sensitive pixels as well as a pixel circuit that may be used in the present invention; 
     FIG. 2 a  illustrates a block diagram of the present invention; 
     FIG. 2 b  illustrates a block diagram of the present invention having a single current monitor; 
     FIG. 2 c  illustrates a block diagram of the present invention having multiple voltage supplies; 
     FIG. 2 d  illustrates a block diagram of the present invention having isolated power rails; 
     FIG. 3 a  illustrates a block diagram of the ambient light detector; 
     FIG. 3 b  illustrates a circuit that may be used as a current monitor in the ambient light detector; 
     FIGS. 4 a,    4   b,    4   c  and  4   d  illustrate various possible layouts for the imaging array for the detection of ambient light in specific areas of an array. 
     FIG. 5 illustrates a block diagram of an exposure control system with which the ambient light detector in accordance with the present invention may be used; and 
     FIG. 6 illustrates the face of an image scanner. 
    
    
     DETAILED DESCRIPTION 
     CMOS image sensors are comprised of an array of light sensitive pixels integrated on a die. In operation, after the pixels have been reset, the signal generated by each pixel is proportional to the amount of charge collected by the pixel during an exposure or integration period. However, during the reset process for each pixel, a leakage current flows through the reset transistor and the photodiode in each pixel. The current flowing through the reverse biased photodiode is proportional to the level of photons impinging on the photodiode at that time. The level of ambient light present when an image is being captured by the image array can greatly influence the quality of the captured image; this is particularly important when the captured image is being used for image recognition in instances such as bar-code reading. The level of light present when an image is being captured may also influence the amount of amplification that the image signals require as they are being processed for image recognition. 
     With reference to FIG. 1, an array  101  of light sensitive pixels  102  which are normally laid out in rows and columns, are powered by a stable voltage supply  103  providing an output voltage V dd . One type of active pixel  102  that may be used in conjunction with the present invention is illustrated, however, the invention may be carried out in conjunction with other types of active pixels. Some pixels may use photodiodes such as p−n photodiodes, p−i−n photodiodes and Schottky photodiodes. The active pixel  102  illustrated consists of a reset transistor  108 , a source-follower transistor  109 , a p−n photodiode  107 , and a row-selection transistor  110 . In order to reset each pixel  102 , a positive voltage signal V R  is applied to the gate of the reset transistor  108  through reset line  106  turning the transistor  108  ON in order to apply the voltage V dd  across the photodiode  107 . 
     The pixels  102  are coupled to the voltage supply  103  through the array power rail  104  and a row power rails  105 . As each pixel  102  is being reset, a current will flow through the pixel  102  as a pixel leakage current I PL . The pixel leakage Current I PL  flowing through the reverse-biased p−n photodiode  107  is proportional to the level of photons impinging on the photodiode  107 . In effect, the intensity of light hitting the photodiode  107  can be measured by monitoring the pixel leakage current I PL . The total current flowing through a row power rail  105  for all of the pixels  102  in that row during their reset is current I RP , and the total current flowing through the power rail  104  for all of the pixels  102  in the array  101  during their reset is current I AP . The, ambient light level impinging on the array  101  may be determined by measuring the current flowing through a selected number of pixels  102  while they are being reset. 
     An embodiment of the present invention for monitoring the pixel leakage current is illustrated in FIG. 2 a  wherein an ambient light detection circuit  220  is connected to the imaging array  201  in which the pixels  202  are laid out in rows and columns. It is preferred to have the ambient light detection circuit  220  integrated on the same die as the imaging array  201 , however this is not essential for the proper operation of the present invention. The voltage supply  203  provides power to the imaging array  201  through the main power rail  204  which is coupled to the row power rails  205 . Further, a number of selected individual row power rails  205  are each coupled to the ambient light detection circuit  220 . The ambient light detection circuit  220  detects the individual currents flowing through each of the selected row rails  205 , and outputs a signal P OUT  representative the currents flowing. The output signal P OUT  is a function of the currents I RP  flowing to the pixels  202  in each of the selected row rails  205  and therefore can be used as a representation of the level of ambient light impinging on the pixels  202  in the array. 
     The ambient light detection circuit  220  shown in FIG. 3 a  illustrates one form that it may take to monitor currents flowing in one or more groups of pixels  202  in array  201 . A number of inputs  221  for individual connection to a selected number of row rails  205 , are each connected to a current monitor  211  through a small resistance  212  of value R RM . Care must be taken to assure that the inputs  221  are isolated from one another such that the monitors  211  will only monitor the current in the row rails  205  to which they are connected. The current monitor  211  detects the current flowing through the small resistance  212 , and outputs an analog signal representative of that current flow to an analog to digital converter  213 . The analog to digital converter  213  transforms the analog signal into a digital signal P consisting of a number of bits. Analog to digital converters are well known to those skilled in the art, and hence shall not be described further here. The outputs P from the various analog to digital converters  213  are fed to a combiner  222  which may either combine all of the P signals into a single digital output signal P OUT  or which may sequence the P signals into a string of digital outputs as signal P OUT  representing the currents in the row rails  205  that had been selected. 
     FIG. 3 b  illustrates one form that the current monitor  211  may take. It consists basically of a current-to-voltage converter  313 . The current I RM  flowing through the small resistance  212  is equal to the total amount of current I RT  flowing through the row power rail  205  less the total leakage currents I RP  flowing into the pixels  202  connected to that particular row rail  205 . The voltage at the inverting input  315  to the op-amp  314  is approximately equivalent to the reference voltage V REF  applied to the non-inverting input  316  to the op-amp  314 . This is possible by what is commonly known as a virtual ground between the inverting input  315  and non-inverting input  316  of the op-amp  314 . Due to the infinite impedance of the op-amp  314  all of the current I RM  is forced to flow through the large resistance  317  of value R L . This leads to an output voltage level V OUT  represented by the following equation: 
     
       
           V   OUT   =V   REF −( I   RM   *R   L ) 
       
     
     This establishes an output voltage level V OUT  on output terminal  318  that is a function of the current I RM  flowing through the small resistor  212 , which is a function of the total leakage current I RP  flowing through the pixels  202  in that particular row  205 , which is a function of the amount of light impinging on the pixels  202  in that particular row  205 . In effect the output voltage V OUT  is directly proportional to the intensity of ambient light impinging on the pixels  202  in the selected row of the image sensor array  201 . V OUT  on the output  318  is then applied to the analog to digital converter  213 . 
     Alternate arrangements for monitoring the pixel leakage current(s) I PL  of anywhere from one to all of the pixels  202  are also possible. For instance, as illustrated in FIG. 2 b,  a single current monitor  211  may be coupled to the array rail  204  so as to measure the total leakage current for all pixels  202  in the array  201  as they are being simultaneously reset. Referring to FIG. 1 for detail, the apparatus in this embodiment may be operated such that the reset transistors  108  for the pixels  102  is controlled to reset sequentially one or more pixels in selected rows or columns or groups of pixels  202  as illustrated in FIG. 2 b.  The resulting output signal V OUT  will consist of sequential digital outputs representing ambient light from the different parts of the array which can be combined to provide an output signal representative of the ambient light on the array. 
     It is usually preferred to measure the ambient light on the array  201  while the imaging scanner is operating normally where the pixels  202  in the array  201  are reset simultaneously during the resetting process; in this manner, the scanning process is interfered with the least. FIG. 2 c  is an embodiment of the present invention where selected rows  205  of pixels  202  to be monitored by the ambient light detection circuit  220  are individually connected to separate voltage sources  203 . As illustrated row rails  205   a,    205   b,    205   c  and  205   d  are connected to voltage supplies  203   a,    203   b,    203   c  and  203   d  respectively. The remaining row rails  205  are connected to a further voltage supply  203 . The ambient light detection circuit  220  includes a current monitor  211   a,    211   b,    211   c  and  211   d  and associated circuitry as described with respect to FIG. 3 a  for monitoring the current individually on each of the row rails  205   a,    205   b,    205   c  and  205   d  respectively. In this way, all of the pixels  202  in the entire array  201  can be reset simultaneously and at the same time the currents in row rails  205   a,    205   b,    205   c  and  205   d  can be monitored. 
     A further preferred embodiment is illustrated in FIG. 2 d  where the row rails  205   a,    205   b,    205   c  and  205   d  are all connected to the same voltage supply  203  however through diodes  223   a,    223   b,    223   c  and  223   d  respectively. The diodes  223   a,    223   b,    223   c  and  223   d  allow the row rails  205   a,    205   b,    205   c  and  205   d  to be monitored individually while avoiding interference by currents in the remaining row rails  205 . 
     As was described with regard to FIG. 2 a,  all pixels  202  in an array  201  are normally reset simultaneously, though this need not be the case to implement the present invention. When it is desired to monitor the ambient light in a certain predetermined pattern on the imaging array  201 , it is necessary to measure the leakage currents I PL  flowing through the pixels  202  that are contiguous with that pattern. In such a circumstance, only the pixels  202  which are contiguous with that pattern can be reset at one point in time, allowing for the simple measurement of the leakage current to all pixels  202  that are being reset. 
     In addition, though all pixels  202  are shown as being connected to the voltage supply  203  through the row rail  205 , other arrangements are possible. Examples of some such patterns are illustrated in FIGS. 4 a  to  4   d  which each show pixels  402  being laid out in an array  401  of 15 rows by 15 columns. 
     If it is desired to measure the ambient light level on the array  401  using only the center rows and columns for instance rows 7 to 9 and columns 7 to 9, then the voltage supply to these rows and columns must be isolated from the remaining rows and columns in order to monitor the leakage currents while the pixels  402  are being reset. FIG. 4 a,  illustrates one such configuration wherein the pixels  402  in column 7 are connected to a power rail  421 , the pixels  402  in column 8 are connected to a power rail  422  and the pixels  402  in column 9 are connected to a power rail  423 . In addition, the pixels  402  in row 7 that are in columns 1 to 6 are connected to a rail  424 , the pixels  402  in row 8 that are in columns 1 to 6 are connected to a rail  425 , the pixels  402  in row 9 that are in columns 1 to 6 are connected to a rail  426 , the pixels  402  in row 7 that are in columns 10 to 15 are connected to a rail  427 , the pixels  402  in row 8 that are in columns 10 to 15 are connected to a rail  428 , the pixels  402  in row 9 that are in columns 10 to 15 are connected to a rail  429 . In addition, rails  424  and  427  may be connected together, rails  425  and  428  may be connected together, and rails  426  and  429  may be connected together. Such a configuration would allow a current monitor to be connected to each of the rails  421  to  426  in order to measure the leakage currents in the pixels  402  in rows 7 to 9 and columns 7 to 9 which results in a measurement of the ambient light falling in a cross pattern on the array  401 . 
     Similar results would be achieved if the power rails for the pixels  402  in rows 7 to 9 carried across the entire array  401  while the power rails for the pixels  402  in columns 7 to 9 were interrupted for rows 7 to 9, as illustrated in FIG. 4 b.  In addition, the configurations in FIGS. 4 a  and  4   b  would allow ambient light measurements to be taken for the four corners of the array  401  if the leakage currents were measured on alternate power rails  431  to  450 . The pixels  402  are connected to these power rails in the following manner: the pixels  402  in column 1 that are in rows 1 to 6 are connected to a rail  431  and the pixels  402  that are in rows 10 to 15 are connected to rail  441 , the pixels  402  in column 2 that are in rows 1 to 6 are connected to a rail  432  and the pixels  402  that are in rows 10 to 15 are connected to rail  442 , the pixels  402  in column 3 that are in rows 1 to 6 are connected to a rail  433  and the pixels  402  that are in rows 10 to 15 are connected to rail  443 , the pixels  402  in column 4 that are in rows 1 to 6 are connected to a rail  434  and the pixels  402  that are in rows 10 to 15 are connected to rail  444 , and the pixels  402  in column 5 that are in rows 1 to 6 are connected to a rail  435  and the pixels  402  that are in rows 10 to 15 are connected to rail  445 . Similarly, the pixels  402  in column 11 that are in rows 1 to 6 are connected to a rail  436  and the pixels  402  that are in rows 10 to 15 are connected to rail  446 , the pixels  402  in column  12  that are in rows 1 to 6 are connected to a rail  437  and the pixels  402  that are in rows 10 to 15 are connected to rail  447 , the pixels  402  in column 13 that are in rows 1 to 6 are connected to a rail  438  and the pixels  402  that are in rows 10 to 15 are connected to rail  448 , the pixels  402  in column 14 that are in rows 1 to 6 are connected to a rail  439  and the pixels  402  that are in rows 10 to 15 are connected to rail  449 , and the pixels  402  in column 15 that are in rows 1 to 6 are connected to a rail  440  and the pixels  402  that are in rows 10 to 15 are connected to rail  450 . By monitoring the leakage currents in power rails  331  to  350 , the ambient light level at the four corners of the array  401  may be determined. 
     FIG. 4 c  illustrates an array  401  having a configuration wherein all of the pixels  402  in each column 1 to 15 are connected to a different power rail  430 . This configuration allows for the selection of particular columns, rather than rows as illustrated in FIG. 2 a,  to measure the ambient light on the array  401 . 
     FIG. 4 d  illustrates yet another configuration wherein power rails  451  are connected to the row 1 to 8 pixels  420  in columns 6 to 10 and the power rails  453  are connected to the row 9 to 15 pixels  420  in columns 6 to 10 the power rails  451  and  453  extend across only half the imaging array  401 . Each of the power rails  452  is connected to the column 1 to 5 pixels  420  for each of the rows 1 to 15, while each of the power rails  454  is connected to the column 11 to 15 pixels  420  for each of the rows 1 to 15 providing versatility in monitoring the leakage currents. 
     FIG. 5 illustrates the use of the present invention in an exposure controlled imager  500  as described in co-pending U.S. patent application Ser. No. 09/1689,368 filed on Oct. 12, 2000 which is incorporated herein by reference. The imaging circuit  501  which is located on a wafer or die and which is represented by broken lines, normally includes an imaging array  502 , wordline drivers  503  and wordlines  504 , bitline readers  505  and bitlines  506 , an integration timer  507 , and a signal amplifier  509 . The bitline readers  505  are connected to the signal amplifier  508  which amplifies the bitline reader  505  signals to produce the image output data. Further, light detector circuits  520  are also located on the die  501  adjacent to the imaging array  502 . 
     The imaging circuitry  501  on the die may further include an averaging circuit  510 , a look-up table and signal driver  511  and an illumination source control  512 . The signal driver  511  includes output lines  513  to  515  respectively for signals to control the signal amplifier  508 , the integration timer  507  and the illumination control  512 . The illumination control  512  is adapted to control an illumination source  513  may not necessarily be located on the imaging circuitry die  501 . 
     Once the look-up table and signal driver  511  determines the proper values for the illumination source control signal, the integration time control signal, and the gain control signal, these signals are fed to illumination source controller  512 , the integration timer  507  and the signal amplifier  508  respectively to adjust the brightness of the light source  513 , the exposure time of the imaging array  502  and the gain of the amplifier  508 , respectively. 
     The look-up table and signal driver  511  may consist of a microcontroller device such as the Strong-Arm SA-1110 and a read only memory programmed with data defining particular imaging needs in terms of light intensity, integration time, and signal gain in response to a measured level of ambient light. The sort of data contained therein would depend on the type of application the device was to be used for; for example a bar code reader would try to rely mostly on alterations of the integration time as this would be the power conscious method of ambient light adjustment. 
     The light source  513  may consist of any type of conventional light source that can be controlled in intensity. However, a particularly advantageous arrangement is illustrated in FIG. 6, which schematically illustrates the face of a scanner  600 . The imaging circuit  601  is located at the center of the scanner face  602 . One or more LED light sources  603  are positioned about the imaging circuit  601  to provide further lighting if required. In operation, the one or more LED&#39;s  603  may each be controlled by a separate line in order to turn each LED  603  OFF or ON as desired. For example, if an object or target is close to the scanner face  602 , only one or two LED&#39;s might be turned ON; with the target a little further away, such as five or six inches, possibly three or four LED&#39;s  603  could be turned ON. Alternatively, the light source controller  512  could control the driving current to each LED  603 , and increase or decrease the illumination from each LED  603  as required. 
     While the invention has been described according to what is presently considered to be the most practical and preferred embodiments, it must be understood that the invention is not limited to the disclosed embodiments. Those ordinarily skilled in the art will understand that various modifications and equivalent structures and functions may be made without departing from the spirit and scope of the invention as defined in the claims. Therefore, the invention as defined in the claims must be accorded the broadest possible interpretation so as to encompass all such modifications and equivalent structures and functions.