Abstract:
A non-invasive emitter-photodiode sensor which is able to provide a data-stream corresponding to the actual wavelength of light emitted thereby allowing calibration of the sensor signal processing equipment and resulting in accurate measurements over a wider variation in emitter wavelength ranges.

Description:
CROSS-REFERENCE To RELATED APPLICATIONS  
       [0001]     This application claims the benefit of Provisional Patent Application Ser. No. 60/225,021 filed on Aug. 11, 2000 and entitled SELF CALIBRATING NON-INVASIVE BLOOD COMPONENT SENSOR. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field  
         [0003]     This invention relates generally to non-invasive sensing devices, and in particularly to calibrating these non-invasive sensing devices.  
         [0004]     2. Related Art  
         [0005]     Coherent light sources are utilized in a broad range of applications in many distinct fields of technology including the consumer, industrial, medical, defense and scientific fields. In the medical field an emitter-receiver pair of coherent light sources in form of light-emitting diodes (LEDs) are often utilized in medical sensing devices to obtain accurate non-invasive measurements. An example application of such a medical sensing device may include a blood constituent monitoring system and/or a non-invasive oximeter that may be utilized to monitor arterial oxygen saturation.  
         [0006]     In non-invasive oximetry, coherent light having a known specific wavelength is typically transmitted from an emitter LED through a target, such as biological tissue carrying blood, to a photodetector. The photodetector receives and measures a portion of transmitted coherent light that is neither absorbed nor reflected from the blood in the biological tissue in order to determine the oxygen saturation (SP02) within the blood. Similarly, an example of an industrial application may include a non-invasive sensor system having a coherent light of a known specific wavelength transmitted from a coherent light source (such as an LED emitter) through a target, such as a fluid or material, to photodetector.  
         [0007]     Unfortunately, these types of non-invasive sensor systems utilizing a coherent light source require accurate prior knowledge of the wavelength of the coherent light source in order to determine the amount of coherent light that is absorbed or reflected through the target. One way of having the prior knowledge of the wavelength is to select coherent light source emitters that have wavelengths within a certain range of tolerance. As such, attempts at determining the wavelength have included a binning process of selecting LEDs within the required nominal wavelength specifications.  
         [0008]     However, it is appreciated by those skilled in the art and familiar with the production of emitter-photodiode sensing devices that there is a need to be able to select from a wider variation of emitter output wavelengths in reducing the production costs and defect rates of the sensing devices. As an example, typical production techniques require selection of an emitter within 2 nm of a target wavelength, which may lead to rejection of 40-60% of the component emitters. Moreover, an additional problem is that a selected emitter, which was within the target wavelength at time of production, will typically degrade over time, vary with temperature, and the drive circuit may become unstable and cause a wavelength shift.  
         [0009]     Attempts to solve the wavelength shift problem have included systems that correlate the wavelength shift to a change in drive circuit current. The change in drive circuit current drives the LED to a specific wavelength. Typically, these systems include a scheme for determining the wavelength shift of the photodiodes via a series of filters, diffusers and a plurality of photodetectors. Unfortunately, this approach is too complex and expensive for practical manufacturing techniques.  
         [0010]     Therefore, there is a need for a non-invasive sensor system that is capable of measuring the wavelength of a light source without requiring prior knowledge of the wavelength of the light source and is not complex or expensive to manufacture.  
       SUMMARY  
       [0011]     This invention is a self-calibrating sensor system “SCSS” capable of determining the actual wavelength of light emitted from a light source resulting in accurate measurements over a wide variation of wavelength ranges. In an example operation, the SCSS is capable of receiving incident light radiation from the at least one light source at a sensor probe and producing a calibrated signal corresponding to the received incident light radiation at the sensor probe.  
         [0012]     As an example implementation of the SCSS architecture, the SCSS may include a sensor probe receiving incident light radiation from at least one light source and a calibration circuit in signal communication with the sensor probe. The calibration circuit may produce a calibrated signal corresponding to the received incident light radiation at the sensor probe. The sensor probe may include a wavelength sensor. The wavelength sensor may include a first diode configured to receive short wavelengths from the incident light radiation and produce a first photocurrent signal and a second diode configured to receive long wavelengths from the incident light radiation and produce a second photocurrent signal. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0013]     The invention may be better understood with reference to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the several views.  
         [0014]      FIG. 1  illustrates a block diagram of an example implementation of a self-calibrating sensor system (SCSS).  
         [0015]      FIG. 2  illustrates a block diagram of an example implementation of the probe block of the SCSS shown  FIG. 1 .  
         [0016]      FIG. 3  illustrates a cross-sectional view of an example implementation of the probe shown in  FIG. 2 .  
         [0017]      FIG. 4A  illustrates a cross-sectional view of another example implementation of the probe shown in  FIG. 2 .  
         [0018]      FIG. 4B  illustrates a cross-sectional view of example reflective implementation of the probe shown in  FIG. 2 .  
         [0019]      FIG. 5  is a top view of an example implementation of the probe shown in  FIG. 4 .  
         [0020]      FIG. 6  is a cross-sectional view of the probe implementation of  FIG. 5 .  
         [0021]      FIG. 7  illustrates an example implementation of the probe block shown in  FIG. 2  utilizing photodiodes.  
         [0022]      FIG. 8  illustrates a cross-sectional view of an example implementation of the wavelength sensor block shown in  FIG. 7  utilizing a double diffusion photodiode.  
         [0023]      FIG. 9  is a graph of the response curve of the wavelength sensor shown in  FIG. 8 .  
         [0024]      FIG. 10  is schematic diagram depicting an exemplary implementation of the calibration circuit block shown in  FIG. 1 .  
         [0025]      FIG. 11  is schematic diagram depicting another exemplary implementation of the calibration circuit block shown in  FIG. 1 .  
         [0026]      FIG. 12  is a flow chart illustrating the process performed by the SCSS shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]      FIG. 1  illustrates a block diagram of a self-calibrating sensor system (SCSS)  100 . The SCSS  100  may include a probe  102 , a calibration circuit  104 , a controller  106 , software  108  located on in memory (not shown) and optional lookup table (“LUT”)  110 . The probe  102  is in signal communication, via signal path  112 , to the calibration circuit  104 . The calibration circuit  106  may be a divider and/or comparator circuit.  
         [0028]     The calibration circuit  104  is in signal communication to the controller  106  and an external output device (not shown) via signal paths  114  and  116 , respectively. The controller  106  is in signal communication to software  108  and optional LUT  110  via signal paths  118  and  120 , respectively.  
         [0029]     The controller  106  may be any general-purpose processor such as an Intel XXX86, Motorola 68XXX or PowerPC, DEC Alpha or other equivalent processor. Alternatively, a specific circuit or oriented device may selectively be utilized as the controller  106 . Additionally, the controller  106  may also be integrated into a signal semiconductor chip such as an Application Specific Integrated Chip (ASIC) or Reduced Instruction Set Computer (RISC), or may be implemented via a Digital Signal Processor (DSP) chip. Examples of a specific circuit or oriented device for the controller  106  may also be a mixed sionac ASIC.  
         [0030]     The software  108  may be resident in memory (not shown) located either internally or externally to the controller  106 . The software  108  includes both logic enabling the controller  106  to operate and also logic for self-calibrating the SCSS  100 .  
         [0031]     An example of the external output device may be an oximeter such as a NPB40 manufactured by Nellcor of Pleasanton, Calif., a 9840 Series pulse oximeter manufactured by Nonin Medical, Inc. of Plymouth, Minn., or an equivalent device.  
         [0032]      FIG. 2  shows an example implementation of probe  102 . Probe  102  may include a probe light source  200  and wavelength sensor  202 . Probe light source  200  may include a first light source  204  and second light source  206 . First light source  204  and second light source  206  may be implemented utilizing light-emitting diodes (LEDs). As an example implmentation in oximeter application, first light source  204  may be an LED emitting light radiation at a wavelength of approximately 660 nm and second light source  206  may be an LED emitting light radiation at a wavelength of approximately 880 nm. Wavelength sensor  202  may be implemented utilizing a double diffusion photodiode. It is appreciated by those of skill in the art that probe light source  200  may also include multiple light sources in the order of three or more.  
         [0033]     In  FIG. 3 , a cross-sectional view of an example implementation of the probe  300  is shown. In this example, probe  300  may be a medical device such as a transmissive blood oxygen saturation and pulse rate sensor. However, it would be appreciated by one skilled in the art that probe  300  may also be a reflective sensor. Additionally, probe  300  may also be utilized for measuring other blood constituents including, but not limited to, oxyhemoglobin, bilirubin, carboxy-hemoglobin, and glucose. Probe  300  may include a rigid casing  302  having a cavity  304  and casing butt  306 , first light source  204 , second light source  206  and wavelength sensor  202 . Probe  300  is connected to calibration circuit  104 ,  FIG. 1 , via signal path  112 . A material  308 ,  FIG. 3 , such as a finger may be inserted into the cavity  304 .  
         [0034]     As an example, first light source  204  and second light source  206  may be two LED emitters that produce light radiation at a wavelength of approximately 660 nm and 880 nm, respectively. Wavelength sensor  202  is supported within the rigid casing  302  opposite first light source  204  and second light source  206 . First light source  204  and second light source  206  and wavelength sensor  202  may be in signal communication with a control cable (not shown). The control cable is in signal communication with an oximeter (not shown) via signal path  112 . The oximeter determines the oxygen saturation of the blood in the material  308  (in this example a finger) by measuring and processing the amount of incident light radiation reaching wavelength sensor  202  from a pulse of light radiation from first light source  204 .  
         [0035]     In operation, the SCSS  100 ,  FIG. 1 , performs a self-calibration procedure prior to measuring any of the properties of the material  308 ,  FIG. 3 . This self-calibration procedure includes emitting a pulse of light radiation from the first light source  204  that is received as incident light radiation by wavelength sensor  202  prior to inserting material  308  into the cavity  304 . The oximeter utilizes the measured incident light radiation received by wavelength sensor  202  to determine the operating wavelength of the first light source  204 . Once the operating wavelength of the first light source  203  is known, the SCSS  100 ,  FIG. 1 , is utilized in combination with the oximeter to accurately determine blood oxygen saturation of the material  308 .  
         [0036]     The self-calibration procedure is beneficial because it is appreciated by those skilled in the art that light radiation output by first light source  204  of 660 nm in this example implementation is in the red spectral region. It is the absorption of this red light radiation that the oximeter utilizes to determine the oxygen saturation of the blood. As such, a relatively small variation in operating wavelength may results in inaccurate readings at the oximeter. As an example, without the self-calibration procedure, if the light radiation output by first light source  204  varied in excess of ±2 nm from an operating wavelength required by the oximeter, the results would be inaccurate.  
         [0037]      FIG. 4A  illustrates a cross-sectional view of another example implementation of probe  400 . In this example, probe  400  may include a rigid or flexible casing  402  having a cavity  404 , first light source  204 , second light source  206  and wavelength sensor  202 . Similar to the previous example implementation, probe  400  is connected to calibration circuit  104 ,  FIG. 1 , via signal path  112 , however, probe  400 ,  FIG. 4A , does not have a cavity butt. A material  406  may be inserted into the cavity  404 .  
         [0038]     Similar to the previous example, first light source  204  and second light source  206  may be two LED emitters that produce light radiation at different wavelengths. Wavelength sensor  202  is supported within the rigid casing  402  opposite first light source  204  and second light source  206 . First light source  204  and second light source  206  and wavelength sensor  202  may be in signal communication with a control cable (not shown). The control cable is in signal communication with a measuring device (not shown) via signal path  112 . The measuring device determines the properties in the material  406  by measuring and processing the amount of incident light radiation reaching wavelength sensor  202  from a pulse of light radiation from first light source  204 .  
         [0039]     As an industrial example, the material  406  may be a fluid, liquid or solid material that exhibits optical transmissive characteristics that may be measured and utilized to determine the properties of the material. An example implementation would include measuring the properties of the material for process or quality control purposes.  
         [0040]     Again in operation, the SCSS  100 ,  FIG. 1 , performs a self-calibration procedure prior to measuring any of the properties of the material  406 ,  FIG. 4A . This self-calibration procedure includes emitting a pulse of light radiation from the first light source  204  that is received as incident light radiation by wavelength sensor  202  prior to inserting material  406  into the cavity  404 . The measuring device utilizes the measured incident light radiation received by wavelength sensor  202  to determine the operating wavelength of the first light source  204 . Once the operating wavelength of the first light source  204  is known, the SCSS  100 ,  FIG. 1 , is utilized in combination with the measuring device to accurately determine the properties of the material  406 .  
         [0041]      FIG. 4B  illustrates a cross-sectional view of an example reflective implementation of probe  408 . In this example, probe  408  may include a rigid or flexible casing  410  having a cavity  412 , first light source  204 , second light source  206  and wavelength sensor  202 . Similar to the previous example implementation, probe  408  is connected to calibration circuit  104 ,  FIG. 1 , via signal path  112 , however, probe  408 ,  FIG. 4B , does not have a cavity butt. A material  412  may be inserted into the cavity  412 .  
         [0042]     Similar to the previous example, first light source  204  and second light source  206  may be two LED emitters that produce light radiation at different wavelengths. However, in this example, wavelength sensor  202  is supported within the rigid casing  410  adjacent to first light source  204  and second light source  206 . First light source  204  and second light source  206  and wavelength sensor  202  may be in signal communication with a control cable (not shown). The control cable is in signal communication with a measuring device (not shown) via signal path  112 . The measuring device determines the properties in the material  412  by measuring and processing the amount of incident light radiation reflected by material  412  and reaching wavelength sensor  202  from a pulse of light radiation from first light source  204 .  
         [0043]     Again, as an industrial example, the material  412  may be a fluid, liquid or solid material that exhibits optical transmissive characteristics that may be measured and utilized to determine the properties of the material. An example implementation would include measuring the properties of the material for process or quality control purposes.  
         [0044]     Again in operation, the SCSS  100 ,  FIG. 1 , performs a self-calibration procedure prior to measuring any of the properties of the material  412 ,  FIG. 4B . This self-calibration procedure includes emitting a pulse of light radiation from the first light source  204  that is reflected by flexible casing  410  and later received as incident light radiation by wavelength sensor  202  prior to inserting material  412  into the cavity  410 . The measuring device utilizes the measured incident light radiation received by wavelength sensor  202  to determine the operating wavelength of the first light source  204 . Once the operating wavelength of the first light source  204  is known, the SCSS  100 ,  FIG. 1 , is utilized in combination with the measuring device to accurately determine the properties of the material  412 .  
         [0045]     It is appreciated by of skill in the art that it is possible to generate signals from the wavelength sensor  202 ,  FIG. 2  during operation of the light sources  204  and  206  through the medium (i.e., material  308 ,  FIG. 3, 406 ,  FIG. 4A , and/or  414 ,  FIG. 4B ) being inspected. It is also possible to generate the same signals using light reflected off the medium. Therefore, it is not necessary to couple the light sources  204 ,  FIGS. 2 and 206  directly to the wavelength sensor  202  as long as the medium either transmits or reflects enough light to generate processable signals from the wavelength sensor  202 .  
         [0046]     In  FIG. 5 , a top view of an example medical implementation of probe  500  having a flexible casing (i.e., flexible strip)  502  is shown. Probe  500  may include first light source  204 , second light source  206  and wavelength sensor  202 . In this example implementation, probe  500  is a blood oxygen saturation and pulse rate sensor that utilizes the flexible strip  502  to attach to a material, such as a body part (not shown). The probe  500  is connected to an oximeter (not shown) via signal path  112 . The flexible strip  502  may be wrapped around the body part and affixed to itself via an attachment strip (such as an adhesive strip)  504 . Example body parts would include a finger, toe, ear-lobe, arm, leg or other similar parts.  
         [0047]     As an example, first light source  204  and second light source  206  may be two LED emitters that produce light radiation at a wavelength of approximately 660 nm and 880 nm, respectively. Wavelength sensor  202  is supported within the flexible strip  502  and placed opposite first light source  204  and second light source  206  when the flexible strip  502  is wrapped around a body part. First light source  204  and second light source  206  and wavelength sensor  202  may be in signal communication with a control cable (not shown). The control cable is in signal communication with an oximeter (not shown) via signal path  112 . The oximeter determines the oxygen saturation of the blood in the body part by measuring and processing the amount of incident light radiation reaching wavelength sensor  202  from a pulse of light radiation from first light source  204 .  
         [0048]     As before, in operation, the SCSS  100 ,  FIG. 1 , performs a self-calibration procedure prior to measuring any of the properties of the body part. This self-calibration procedure includes, prior to wrapping flexible strip  502  around the body part, bending the flexible strip  502  so that the first light source  204  and second light source  206  are opposite in special orientation to wavelength sensor  202  and then emitting a pulse of light radiation from the first light source  204  that is received as incident light radiation by wavelength sensor  202 . The oximeter utilizes the measured incident light radiation received by wavelength sensor  202  to determine the operating wavelength of the first light source  204 . Once the operating wavelength of the first light source  204  is known, placed around a body part and the wavelength sensor  202  measures the incident light radiation emitted by the first light source  204  and passing through the blood flowing within the body part. The SCSS  100 ,  FIG. 1 , is then utilized in combination with the oximeter to accurately determine blood oxygen saturation of the body part. In  FIG. 6 , a cross-sectional view of the probe  500  is shown in a wrap type position.  
         [0049]     In  FIG. 7 , an example implementation of the probe  700  is shown utilizing photodiodes. Similar to  FIG. 2 , Probe  700 ,  FIG. 7 , includes probe light source  702  and wavelength sensor  704 . Probe light source  702  includes first light source  706  and second light source  708 . First light source  706  may include LED  710  and second light source may include LED  712 . Wavelength sensor  704  is a double diffusion photodiode.  
         [0050]     As an example of operation, LED  710  and LED  712  may have their cathodes grounded in common at signal path  714  and may emit light radiation  716  at wavelengths 660 nm and 880 nm, respectively, when a voltage is applied at anodes  718  and  720 , respectively. The emitted light radiation  716  is incident on material  722 . A part of the emitted light radiation  716  is transmitted through material  722  and is received as incident light radiation  724  by wavelength sensor  704 . As before, in order to properly measure the properties of the material  722  from the received incident light radiation  724 , the SCSS  100 ,  FIG. 1  performs a self-calibration procedure.  
         [0051]     The SCSS  100 ,  FIG. 1 , performs a self-calibration procedure prior to measuring any of the properties of the material  722 . This self-calibration procedure includes emitting a pulse of light radiation  716  from LED  710  that is received as incident light radiation  724  by wavelength sensor  704  prior to inserting material  722  between the probe light source  702  and wavelength sensor  704 . The oximeter utilizes the measured incident light radiation  724  received by wavelength sensor  704  to determine the operating wavelength of LED  710 . Once the operating wavelength of LED  710  is known, the SCSS  100 ,  FIG. 1 , is utilized in combination with the oximeter to accurately determine blood oxygen saturation of the material  722 .  
         [0052]      FIG. 8  illustrates a cross-sectional view of the wavelength sensor  704  receiving incident light radiation  724  utilizing a double diffusion photodiode (also known as a double junction photodiode). Photodiodes with double diffusion are typically utilized to accurately measure the centroid wavelength of light sources such as LEDs  710  and  712 . Double diffusion photodiodes are processed with two junctions, one on the top surface and one on the back surface of a semiconductor photodiode (such as a Si-photodiode), each junction typically exhibits a different and well-defined spectral response. As result, by measuring the quotient of signals generated by the two junctions, the centroid wavelength of any given monochromatic light source may be determined.  
         [0053]     The wavelength sensor  704  has two p-n junctions constructed vertically on a common silicon substrate. The wavelength sensor  704  includes a first anode  800 , common cathode  802 , first diode  804  (also known as an upper diode), second diode  806  (also known as a lower diode), second anode  808 , and a thin active region  810 . The first anode  800  is positioned on the top surface above the common cathode  802  forming the first diode  804 . The thickness of the first diode  804  is chosen so that the energy of the shortest wavelength being measured from the incident light radiation  724  is absorbed entirely therein. The second diode  806  is formed between the common cathode  802  and the second anode  808  that placed on the bottom surface with the thin active region  810  between the common cathode  802  and the second anode  808 . The thickness of the thin active region  810  is selected to allow for absorption of substantially all of the longest measured wavelength of incident light radiation  724 .  
         [0054]      FIG. 9  illustrates a typical plot  900  of the spectral response of the wavelength sensor  704 ,  FIG. 8 . The plot  900 ,  FIG. 9 , has a vertical axes  902  representing relative response, in percentage, of the wavelength sensor  704 ,  FIG. 8 , and a horizontal axis  904 ,  FIG. 9 , representing the wavelength of the incident light radiation  724 ,  FIG. 8 . The plot  900 ,  FIG. 9 , shows two response curves  906  and  908  representing the relative response versus wavelength for the first diode  804 ,  FIG. 8 , and the second diode  806 , respectively.  
         [0055]     As an example of operation of the wavelength sensor  704 , the first diode  804  may have an enhanced blue response and the second diode  806  may have an enhanced red response. In this example, the absorbed radiation of the incident light radiation  724  between the red and blue responses (such as between 450 and 900 nm) generates two photocurrent signals proportional to the wavelength of the incident light radiation  724 . The quotient of these photocurrent signals is independent of the light level up to the saturation point of the wavelength sensor  704 . Utilizing this example, the wavelength of either monochromatic incident light radiation  724  or the spectral density peak of polychromatic incident light radiation  724  may be determined. An example of the wavelength sensor  704  may be a PSS WS-7.56 wavelength sensor produced by Pacific Silicon Sensor, Inc. of Westlake Village, Calif.  
         [0056]     In  FIG. 10 , a schematic diagram depicting an exemplary implementation of the calibration circuit  1000  is shown. The calibration circuit  1000  is in signal communication with the probe  1002  and controller  106 ,  FIG. 1 , via signal paths  112  and  114 , respectively. The calibration circuit  1000  may include a pair of amplifiers  1004  and  1006  (such as log amplifiers) in signal communication with first anode  800 ,  FIG. 8  and second anode  808  of wavelength sensor  1008 ,  FIG. 10 , and a differential amplifier  1010 , via signal paths  1010 ,  1012  and  1014 , respectively. The differential amplifier  1008  is in signal communication with the controller  106 ,  FIG. 1 , via signal path  112 .  
         [0057]     In operation, the wavelength sensor  1004  produces two photocurrent signals from the two junctions (i.e., photodiodes  804  and  806 ) in the double diffusion photodiode. Each junction in the wavelength sensor  1004  exhibits a different and well-defined spectral response, which is know to the controller  106 ,  FIG. 1 , and the magnitude of these two resulting photocurrent signals are proportional to the wavelength of the measured incident light radiation  724 , which corresponds to one of the light sources (either 204 or 206,  FIG. 2 ) in probe  1002 ,  FIG. 10 . The photocurrent signals are amplified by amplifiers  1004  and  1006  via signal paths  1010  and  1012 , respectively, and input into the differential amplifier  1008  via signal path  1018  and  1020 . If the amplified photocurrent signals  1018  and  1020  are approximately equal the corresponding differential output signal  1022  of the differential amplifier  1008  is almost equal to zero. Once the differential output signal  1022  is almost equal to zero the wavelength of the incident light radiation is determined and the SCSS  100 ,  FIG. 1 , is calibrated.  
         [0058]     When the amplified photocurrent signals  1018  and  1020  are not approximately equal the corresponding differential output signal  1022  will vary according to the difference in magnitude value between the amplified photocurrent signals  1018  and  1020 . The differential output signal  1022  is the utilized as a reference by the controller  106 ,  FIG. 1 . The controller  106  determines the wavelength of the incident light radiation  724  by knowing the spectral response of the photodiodes  804  and  806 ,  FIG. 8 . The controller  106  either determines the wavelength of the incident light radiation  724  utilizing software  108  or other hardware (not shown) located in the SCSS  100 . The software  108  may include logic that allows the controller  106  to calculate the wavelength values in real-time from the measure values received from the wavelength sensor  1004 .  
         [0059]     Alternatively, the controller  106  may determine the wavelength of the incident light radiation  724  utilizing the lookup (“LUT”) table  110 . The LUT  110  may be resident in memory (not shown) resident either internally or externally to the controller  106 . The LUT  110  includes a tabulation of known spectral response in voltage versus wavelength for each photodiode  804  and  806 ,  FIG. 8 . Once the controller  106  measures the differential output signal  1022 ,  FIG. 10 , the software  108 ,  FIG. 1 , compares the value of the differential output signal  1022 ,  FIG. 10 , against values stored in the LUT  110 ,  FIG. 1 , and then retrieves a corresponding wavelength value. The controller  106  then utilizes the retrieved wavelength wave to self-calibrate the SCSS  100 .  
         [0060]     Besides self-calibration, the SCSS  100  is also capable of temperature compensating for variation in the wavelength of the incident light radiation  724  due to temperature variations. The SCSS  100  may compensate for temperature variations by the same process utilized to self-calibrate.  
         [0061]      FIG. 11  is another exemplary implementation of the SCSS  100 ,  FIG. 1 , with the calibration circuit  1100 ,  FIG. 11 , in an oximeter device  1102 . The oximeter device  1102  is in signal communication with probe  1104 , via signal path  112 , and includes calibration circuit  1100 , controller  1106 , first driver  1108  and second driver  1110 . The probe  1104  includes wavelength sensor  1112  and probe light source  1114  having first light source  1116  and second light source  1118 .  
         [0062]     In operation, the first driver  1108  drives the first light source  1116  and the second driver  1110  drives the second light source  1118 . First light source  1116  and the second light source  1118  may individually produce light radiation which is incident of the wavelength sensor  1112 . The wavelength sensor  1112  produces two photocurrent signals from the two junctions (i.e., photodiodes  804  and  806 ) in the double diffusion photodiode. Again, each junction in the wavelength sensor  1112  exhibits a different and well-defined spectral response, which is know to the controller  1106  and the magnitude of these two resulting photocurrent signals are proportional to the wavelength of the measured incident light radiation, which corresponds to one of the light sources (either 1116 or 1118) in probe  1104 . The photocurrent signals  1120  and  1122  processed and input into the differential amplifier  1224 . If the photocurrent signals  1120  and  1122  are approximately equal the corresponding differential output signal  1126  of the differential amplifier  1124  is almost equal to zero. Once the differential output signal  1126  is almost equal to zero the wavelength of the incident light radiation is determined and the SCSS  100 ,  FIG. 1 , is calibrated.  
         [0063]     When the photocurrent signals  1120  and  1122  are not approximately equal the corresponding differential output signal  1126  will vary according to the difference in magnitude value between the photocurrent signals  1120  and  1122 . The differential output signal  1126  is the utilized as a reference by the controller  1106 . The controller  1106  determines the wavelength of the incident light radiation by knowing the spectral response of the photodiodes  804  and  806 . The controller  1106  either determines the wavelength of the incident light radiation utilizing software  108 ,  FIG. 1 , or other hardware (not shown) located in the SCSS  100 . The software  108  may include logic that allows the controller  1106 ,  FIG. 11 , to calculate the wavelength values in real-time from the measure values received from the wavelength sensor  1112 .  
         [0064]     Alternatively, the controller  1106  may determine the wavelength of the incident light radiation utilizing the lookup LUT  110 ,  FIG. 1 . The LUT  110  may be resident in memory (not shown) resident either internally or externally to the controller  1106 ,  FIG. 11 . The LUT  110 ,  FIG. 1 , includes the tabulation of known spectral response in voltage versus wavelength for each photodiode  804  and  806 . Once the controller  1106  measures the differential output signal  1126 ,  FIG. 11 , the software  108 ,  FIG. 1 , compares the value of the differential output signal  1126 ,  FIG. 11 , against values stored in the LUT  110 ,  FIG. 1 , and then retrieves a corresponding wavelength value. The controller  1106 ,  FIG. 11 , then utilizes the retrieved wavelength wave to self-calibrate the SCSS  100 ,  FIG. 1 .  
         [0065]      FIG. 12  illustrates the process performed by the SCSS  100 ,  FIG. 1 . The process begins in step  1200 ,  FIG. 12 . In step  1202 , the wavelength sensor  202 ,  FIG. 2 , receives incident light radiation  200  from the probe light source  200 . Within the wavelength sensor  202 , the first diode  804 ,  FIG. 8 , receives short wavelengths from the incident light radiation  200 , in step  1204 ,  FIG. 12 , and the second diode  806 ,  FIG. 8 , receives long wavelengths from the incident light radiation  200  in step  1206 ,  FIG. 12 . In step  1208 , the first diode  804  produces a first photocurrent signal  1010 ,  FIG. 10 , in response to receiving short wavelengths from the incident light radiation  200  and the second diode  806  produces a second photocurrent signal  1012 ,  FIG. 10 , in response to receiving short wavelengths from the incident light radiation  200  in step  1210 ,  FIG. 12 . Finally, in step  1212 , the calibration circuit  104  and/or controller  106 ,  FIG. 1 , determine the wavelength of the incident light radiation  200  by comparing the first photocurrent signal  1010  to the second photocurrent signal  1012 . The process then ends in step  1214 .  
         [0066]     The SCSS  100  may be selectively implemented in software, hardware, or a combination of hardware and software. For example, the elements of the SCSS  100  may be implemented in software  108  stored in a memory (not shown) located in a controller  106 . The controller  106  may be in signal communication with a DSP or ASIC chip via communication link  112  (which may selectively be a system bus). The software  108  configures and drives the DSP or ASIC chip and performs the steps illustrated in  FIG. 12 .  
         [0067]     The software  108  comprises an ordered listing of executable instructions for implementing logical functions. The software  108  may be embodied in any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that may selectively fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. In the context of this document, a “computer-readable medium” is any means that may contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer readable medium may be for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer-readable medium would include the following: an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a RAM (electronic), a read-only memory (ROM) (electronic), an erasable programmable read-only memory (EPROM or Flash memory) (electronic), an optical fiber (optical), and a portable compact disc read-only memory (CDROM) (optical). Note that the computer-readable medium may even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via for instance optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory.  
         [0068]     While various implementations of the application have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of the invention. Accordingly, the invention is not to be restricted except in light of the attached claims and their equivalents.