Abstract:
An apparatus for measuring a concentration of a light-absorbing substance in blood is disclosed. A light emitter emits light beams to irradiate a living tissue, each of the light beams being associated with one wavelength which is absorbed by the blood. A first instrument measures first intensities of the light beams, which are to be incident on the living tissue. A second instrument measures second intensities of the light beams, which are transmitted through the living tissue. A first calculator calculates an attenuation variation ratio, which is a ratio of attenuation variations of the respective light beams due to variation of a volume of the blood caused by pulsation, based on the second intensities of the light beams. A second calculator calculates the concentration based on the first intensities, the second intensities, and the attenuation variation ratio.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an improvement in an apparatus for measuring a concentration of a light-absorbing substance in blood which employs pulse photometry as its operating principle: such as a pulse oximeter or an apparatus for measuring a pulse dye-dilution curve.  
           [0002]    Pulse photometry goes beyond a pulse oximeter and is currently employed as a pulse dye dilution method. This method is made commercially practical as an apparatus for measuring a cardiac output, a circulating blood volume, a blood plasma disappearance rate of indocyanine green (ICG), and ICG clearance by administering a dye called ICG into blood and determining the concentration of the ICG in blood. This method is described in detail in the following references: Takehiko Iijima, et al. Cardiac output and circulating blood volume analysis by pulse dye-densitometry. J Clin Monit 1997; 13: 81-89; Takasuke Imai, et al. Measurement of cardiac output by pulse dye-densitometry using indocyanine green. Anesthesiology 1997; 87: 816-822; and Takasuke Imai, et al. Measurement of blood concentration of indocyanine green by pulse dye-densitometry-Comparison with the conventional spectrophotometric method. J Clin Monit 1998; 14: 477-484.  
           [0003]    Further, the pulse dye dilution method is also applied to measurement of the concentration of abnormal hemoglobin, such as carboxyhemoglobin or methemoglobin, the concentration of hemoglobin, or the glucose level (see e.g., Japanese Patent Publication No. 3-71135B corresponding to U.S. Pat. No. 5,127,406 and Japanese Patent Publication No. 2002-228579A corresponding to U.S. Pat. No. 6,415,236).  
           [0004]    Conventionally, for instance, when the concentration of a certain substance in blood is measured through use of two light beams having different wavelengths, the ratio Φ12 between variation in the attenuation of one wavelength and that in the attenuation of the other wavelength, the variation stemming from pulsation of blood, is determined. The concentration of the substance is calculated on the basis of the phenomenon that a certain constant relationship exists between Φ12 and the concentration of the substance (see e.g., Japanese Patent Publication No. 53-26437B). Specifically, the concentration of the substance is expressed as:  
             C=F (Φ12),  
           [0005]    where C denotes the concentration of a substance in blood and F denotes a function representing a constant relationship.  
           [0006]    In general, when “n” light beams having “n” different wavelengths are used, there are used, at most, “n−1” of attenuation variation ratios Φ of the respective wavelengths. For instance, if the light beams have three wavelengths, the concentration of a substance is expressed as:  
             C=F (Φ12,Φ13)  
           [0007]    through use of a ratio Φ12 between variation in attenuation of a first wavelength and that in the attenuation of a second wavelength and a ratio Φ13 between variation in attenuation of the first wavelength and variation in attenuation of a third wavelength.  
           [0008]    In the case of a pulse oximeter, the concentration C of a substance in blood is expressed as oxygen saturation in arterial blood SpO 2  (a ratio of oxyhemoglobin concentrations to hemoglobin concentrations; that is, O 2 Hb/Hb). In the case of pulse dye-dilution curve measurement instrument, the concentration C of a substance in blood is expressed as a ratio of dye concentrations Cd to hemoglobin concentrations Hb; that is, a ratio of Cd/Hb.  
           [0009]    However, according to such a measurement method, an approximately constant relationship exists between the concentration of a substance and the attenuation variation ratio. However, the relationship involves an individual difference. Even in the case of a single individual, the relationship varies according to a time point when measurement is performed or a measurement location, and variations are responsible for an error in measurement. For instance, in the case of a pulse oximeter, a calculated value varies by about 1% as a result of changing an attached probe from one finger to another finger or raising/lowering a hand, provided that an actual oxygen saturation in arterial blood SpO 2  is constant. The following are conceivable as leading causes of the measurement error.  
           [0010]    (1) Since blood has a light scattering nature, an attenuation derived from scattering varies depending on the thickness of blood.  
           [0011]    (2) Two light beams are present; that is, a light beam passing through blood and another light beam not passing through blood.  
           [0012]    When the concentration C of a light-absorbing substance in blood is determined through use of pulse photometry in the previously-described manner, a function taking, as a variable, only the attenuation variation ratio Φ has hitherto been used. Therefore, no consideration has been given to the dependence of an attenuation derived from scattering on the thickness of blood (not a thickness corresponding to a change but the overall thickness of blood). Further, there exist a light beam passing through blood and another light beam not passing through blood (i.e., a light beam passing through only a living tissue other than blood). Hence, no consideration has been given to the light beam not passing through blood, which in turn causes an error.  
         SUMMARY OF THE INVENTION  
         [0013]    It is therefore an object of the invention to provide an apparatus capable of accurately measuring a concentration of a light-absorbing substance in blood through measurement based on pulse photometry.  
           [0014]    According to the present invention, when the concentration of a light-absorbing substance in blood C is calculated through use of a function F, variables of the function F are taken as the attenuation variation ratio Φ and DC components of attenuations Adc (hereinafter, referred as DC attenuations Adc), with regard to the entire thickness of blood and the thickness of a living tissue other than blood. Therefore, the concentration C is expressed by the following function F.  
             C=F (Φ,  Adc )  
           [0015]    Here, when measurement is performed by light beams of “n” wavelengths, “n−1” for Φ and “n” for Adc are used at most. For instance, when measurement is performed through use of three wavelengths, the function F is expressed as follows through use of DC attenuations Adc 1 , Adc 2 , Adc 3  for respective wavelengths.  
             C=F (Φ12, Φ13,  Adc   1 ,  Adc   2 ,  Adc   3 )  
           [0016]    The DC attenuation Adc is expressed as follows through use of an incident light intensity li and a transmitted light intensity lt.  
             Adc=log ( li/lt )= log li−log lt    
           [0017]    Here, lt denotes the intensity of the light having passed through a living body, which can be measured consecutively. In contrast, the intensity of incident light li must be ascertained in advance through measurement. A method for ascertaining li is described in Japanese Patent Publication No. 5-212016A corresponding to U.S. Pat. No. 5,385,143. According to this method, a phantom (i.e., a sample member simulating a living body) having a known light-absorbing characteristic is sandwiched by a probe, and the intensity of light having passed through the phantom is measured, thereby determining the intensity of incident light.  
           [0018]    At wavelengths of 660 nm, 805 nm, and 940 nm, hemoglobin absorbs light but water essentially does not absorb light. Therefore, when a living tissue is exposed to light beams having these wavelengths, DC attenuations are primarily relevant to the quantity of blood flowing through a location to be measured. At a wavelength 1300 nm, a small quantity of light is absorbed by hemoglobin and a large quantity of light is absorbed by water. The DC attenuations are primarily relevant to the thickness of a living tissue (i.e., the quantity of water content). Therefore, the accuracy of measurement of concentration of a light-absorbing substance in blood can be enhanced, by measuring the DC attenuations at these wavelengths, substituting results of measurement as variations into formulae, and correcting errors attributable to error factors (1) and (2).  
           [0019]    Further, according to the invention, in view of the fact that a constant relationship exists between the DC attenuations and the DC transmitted light intensities (DC components of the transmitted light intensities), the concentration of a light-absorbing substance in blood C is expressed by the following equation through use of the function F 1  using variables as Φ and DC, in consideration of the DC transmitted light intensities.  
             C=F   1 (Φ D, DC )  
           [0020]    Here, when measurement is performed through use of “n” kinds of light beams having “n” kinds of wavelengths, “n−1” for Φ and “n” for DC are used at most. For instance, when measurement is performed through use of three wavelengths, the function F 1  is expressed as follows through use of DC transmitted light intensities DC 1 , DC 2 , DC 3  for respective wavelengths.  
             C=F   1 (Φ12, Φ13,  DC   1 ,  DC   2 ,  DC   3 )  
           [0021]    As mentioned above, even when there is used the function using, as variables, the attenuation variation ratio Φ and the DC transmitted light intensities DC, the accuracy of measurement of concentration of a light-absorbing substance in blood can be enhanced similarly.  
           [0022]    Specifically, in order to achieve the above object, according to the invention, there is provided an apparatus for measuring a concentration of a light-absorbing substance in blood, comprising:  
           [0023]    a light emitter, which emits light beams to irradiate a living tissue, each of the light beams being associated with one wavelength which is absorbed by the blood;  
           [0024]    a first instrument, which measures first intensities of the light beams, which are to be incident on the living tissue;  
           [0025]    a second instrument, which measures second intensities of the light beams, which are transmitted through the living tissue;  
           [0026]    a first calculator, which calculates an attenuation variation ratio, which is a ratio of attenuation variations of the respective light beams due to variation of a volume of the blood caused by pulsation, based on the second intensities of the light beams; and  
           [0027]    a second calculator, which calculates the concentration based on the first intensities, the second intensities, and the attenuation variation ratio.  
           [0028]    Preferably, the second calculator calculates DC components of attenuations of the light beams, based on the first intensities and the second intensities. The second calculator obtains the concentration, based on the DC components and the attenuation variation ratio.  
           [0029]    Here, it is further preferable that the second calculator calculates a DC attenuation ratio which is a ratio of the DC components. The second calculator obtains the concentration, based on the DC attenuation ratio and the attenuation variation ratio.  
           [0030]    Alternatively, it is preferable that the second calculator calculates DC components of intensities of the light beams transmitted through the living tissue, based on the first intensities and the second intensities. The second calculator obtains the concentration, based on the DC components and the attenuation variation ratio.  
           [0031]    Here, it is further preferable that the second calculator calculates a DC transmission ratio which is a ratio of the DC components. The second calculator obtains the concentration, based on the DC transmission ratio and the attenuation variation ratio.  
           [0032]    Preferably, the light emitter comprises light emitting elements, and a controller which controls a current value or a voltage value supplied to the light emitting elements. The second calculator corrects the first intensities in accordance with the current value or the voltage value.  
           [0033]    Preferably, the second instrument measures third intensities of the light beams which are transmitted through a phantom placed between the light emitter and the second instrument. The first instrument obtains the first intensities based on the third intensities.  
           [0034]    Here, it is further preferable that the first instrument comprises a sensor which senses whether the phantom is placed between the light emitter and the second instrument. The first instrument begins calculation to obtain the first intensities when the sensor senses that the phantom is placed between the light emitter and the second instrument.  
           [0035]    According to the invention, there is also provided an apparatus for measuring a concentration of a light-absorbing substance in blood, comprising:  
           [0036]    a light emitter, which emits (n) kinds of light beams to irradiate a living tissue, each of the light beams being associated with one wavelength which is absorbed by the blood;  
           [0037]    a first instrument, which measures (n) kinds of first intensities of the light beams, which are to be incident on the living tissue;  
           [0038]    a second instrument, which measures (n) kinds of second intensities of the light beams, which are transmitted through the living tissue;  
           [0039]    a first calculator, which calculates, at most, (n−1) kinds of attenuation variation ratios, which is a ratio of attenuation variations of the respective light beams due to variation of a volume of the blood caused by pulsation, based on the second intensities of the light beams; and  
           [0040]    a second calculator, which calculates, at most, (n) kinds of DC components of attenuations of the light beams, based on the first intensities and the second intensities, and obtains the concentration based on the DC components and the attenuation variation ratio,  
           [0041]    wherein (n) is an integer which is three or more.  
           [0042]    According to the invention, there is also provided an apparatus for measuring a concentration of a light-absorbing substance in blood, comprising:  
           [0043]    a light emitter, which emits (n) kinds of light beams to irradiate a living tissue, each of the light beams being associated with one wavelength which is absorbed by the blood;  
           [0044]    a first instrument, which measures (n) kinds of first intensities of the light beams, which are to be incident on the living tissue;  
           [0045]    a second instrument, which measures (n) kinds of second intensities of the light beams, which are transmitted through the living tissue;  
           [0046]    a first calculator, which calculates, at most, (n−1) kinds of attenuation variation ratios, which is a ratio of attenuation variations of the respective light beams due to variation of a volume of the blood caused by pulsation, based on the second intensities of the light beams; and  
           [0047]    a second calculator, which calculates, at most, (n) kinds of DC components of intensities of the light beams transmitted through the living tissue, based on the first intensities and the second intensities, and obtains the concentration based on the DC components and the attenuation variation ratio,  
           [0048]    wherein (n) is an integer which is three or more.  
           [0049]    In the above apparatuses, it is preferable that the light emitter emits a light beam having a wavelength which is absorbed by a living tissue other than the blood.  
           [0050]    According to the above configurations, the concentration of a light-absorbing substance in blood can be measured accurately on the basis of the principle of pulse photometry. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0051]    The above object and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein:  
         [0052]    [0052]FIG. 1 is a flow chart for explaining a processing flow of an apparatus according to a first embodiment of the invention;  
         [0053]    [0053]FIG. 2 is a block diagram of the apparatus of the first embodiment;  
         [0054]    [0054]FIGS. 3A and 3B are schematic views for explaining a relationship between the intensity of incident light and the intensity of transmitted light obtained when a living body is exposed to light;  
         [0055]    [0055]FIGS. 4A to  4 C are views for explaining how to obtain DC components of transmitted light intensities;  
         [0056]    [0056]FIGS. 5A and 5B are views for explaining the advantageous effect obtained by the apparatus of the first embodiment;  
         [0057]    [0057]FIG. 6 is a block diagram of an apparatus according to a second embodiment of the invention;  
         [0058]    [0058]FIG. 7 is a flow chart for explaining a processing flow of the apparatus of the second embodiment;  
         [0059]    [0059]FIGS. 8A and 8B are views for explaining the advantageous effect obtained by the apparatus of the second embodiment;  
         [0060]    [0060]FIG. 9 is a flow chart for explaining a processing flow of an apparatus of the third embodiment;  
         [0061]    [0061]FIGS. 10A and 10B are views for explaining the advantageous effect obtained by the apparatus of the third embodiment;  
         [0062]    [0062]FIG. 11 is a schematic view showing a first modified example of the apparatus of the first embodiment; and  
         [0063]    [0063]FIG. 12 is a schematic view showing a second modified example of the apparatus of the first embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0064]    A first embodiment of the invention will now be described. A first embodiment is directed to an apparatus for measuring a hemoglobin concentration.  
         [0065]    [0065]FIG. 2 is a block diagram showing the overall configuration of the apparatus of the embodiment. A light emitter  1  comprises: LEDs  2 ,  3  for generating light beams having two different wavelengths; and a driver  4  for driving the LEDs  2 ,  3 . The wavelength of a light beam originating from the LED  2  is taken as a first wavelength, and the wavelength of a light beam originating from the LED  3  is taken as a second wavelength. In this apparatus, the first wavelength is 1300 nm, and the second wavelength is 805 nm.  
         [0066]    A light receiver  5  comprises: a photodiode  6  disposed opposite the LEDs  2 ,  3 ; a current/voltage converter  7  for converting an output electric current from the photodiode  6  into a voltage signal; and an amplifier  8 .  
         [0067]    A multiplexer  9  is a circuit for distributing a signal sent from the amplifier  8  to a filter  10  or a filter  11 . The filters  10 ,  11  are circuits for eliminating noise from signals corresponding to intensities of transmitted light beams having respective wavelengths. Output timings of the output signals are controlled by a multiplexer  12 , and the signals are then delivered to an A/D converter  13 . The A/D converter  13  is a circuit for converting the signal output from the multiplexer  12  into a digital signal.  
         [0068]    A CPU  14  is a circuit for controlling the driver  4 , the multiplexer  9 , and the multiplexer  12  and performing operation on the basis of a signal output from the A/D converter  13 .  
         [0069]    Memory  15  stores a program for use with processing to be performed by the CPU  14  and data output from the CPU  14 .  
         [0070]    A display  16  displays the data output from the CPU  14 , and a control panel  17  is equipped with a plurality of switches (including a calibration switch and a measurement switch, which will be described later) and a plurality of keys, and outputs to the CPU  14  a signal corresponding to an operator&#39;s operation.  
         [0071]    A probe of this apparatus to be attached to a living body is equipped with the LEDs  2 ,  3  and the photodiode  6 . A living body (e.g., a finger tip or an ear flap)  30  is nipped between the LEDs  2 ,  3  and the photodiode  6 . Next, operation of the apparatus will be described by reference to FIG. 1.  
         [0072]    In step  1 A, the intensity of the light incident on a living body is measured. Specifically, the intensity of light radiated from the LEDs  2 ,  3  of the probe onto the living body is determined. In the embodiment, the intensity of incident light is determined through use of a phantom  30 A having a known light-absorbing characteristic. For instance, a milky white acrylic plate is suitable as the phantom  30 A.  
         [0073]    First, the operator places the phantom  30 A at a predetermined position between the LEDs  2 ,  3  of the probe and the photodiode  6  and instructs the CPU  14  to start measuring the intensity of the incident light by operating the calibration switch of the control panel  17 . As a result, the LEDs  2 ,  3  generate light beams having respective wavelengths, and the light beams reach the photodiode  6  after having passed through the phantom  30 A and are converted into electric signals. The signals are processed in subsequent stages by the current/voltage converter  7 , the amplifier  8 , the multiplexer  9 , the filters  10 ,  11 , the multiplexer  12 , and the A/D converter  13 . The signals then reach the CPU  14 , and the signals are stored in the memory  15  as transmitted light intensities ltcal 1 , ltcal 2  of respective wavelengths. The CPU  14  performs calculating operation by substituting the thus-measured ltcal 1 , ltcal 2  into following Equations (1) and (2), thereby determining the incident light intensities lical 1 , lical 2  with respect to the phantom  30 A.  
           lical   1 = ltcal   1 · exp ( Af   1 )   (1)  
           lical   2 = ltcal   2 · exp ( Af   2 )   (2)  
         [0074]    In the equations, Af 1 , Af 2  denote known attenuations of the phantom  30 A which are achieved at the respective wavelengths and stored in the memory  15  in advance. Computation results are also stored in the memory  15 . The computation results lical 1 , lical 2  are written into a predetermined area in the memory  15 . If the values lical 1 , lical 2  that have been measured last time are available, the values are rewritten. The most current values lical 1 , lical 2  are used for calculating operation to be performed in step  3 A, which will be described later. Therefore, step  1 A is for calibrating the intensity of incident light lical  
         [0075]    Computation of the intensity of incident light lical is performed when the operator attaches the probe to the phantom  30 A and presses the calibration switch. However, as shown in FIG. 11, any kind of a sensor (optical, mechanical or magnetic)  220  may be provided in a probe  200  and a phantom  30 A and arranged such that, when the probe  200  is attached to the phantom  30 A, the sensor  220  detects the attaching action and the CPU  14  starts processing such as that mentioned previously, to thereby calculate the intensity of incident light lical.  
         [0076]    Alternatively, as shown in FIG. 12, the phantom  30 A per se may be formed into a holder which holds a probe  200 . The foregoing sensor  220  may be provided on such a holder. When the probe  200  has been housed in the holder (as a matter of course, a portion of the phantom  30 A is nipped between the LEDs  2 ,  3  and the photodiode  6 ), the sensor  220  may detect the holding action, whereupon the CPU  14  may calculate the intensity of incident light lical.  
         [0077]    When having been used over a long period of time, the light-emitting element, such as an LED, undergoes a drop in emission intensity. Emission intensity is also changed by stains on the surface of the probe. Hence, difficulty is encountered in continuous use of the emission intensity of the probe that has been set at the time of shipment. A better arrangement is to calibrate the intensity of incident light lical immediately before measurement.  
         [0078]    In step  2 A, the probe is attached to the living body  30 , and variations in the attenuations at the respective wavelengths caused by pulsation of blood are measured, and the ratio Φ between the variations in the attenuations is determined.  
         [0079]    Processing pertaining to this step is started by the operator operating the measurement switch of the control panel  17 . At this time, the CPU  14  determines the ratio Φ12 between variation ΔA 1  in an attenuation A 1  of the first wavelength and variation ΔA 2  in an attenuation A 2  of the second wavelength on the basis of the signal indicating the transmitted light intensities lt 1 , lt 2  output from the A/D converter  13 . Specifically, following Equation (3) is calculated, and a result of computation is stored in the memory  15 .  
         Φ12=Δ A   1 /Δ A   2 =( AC   1 / DC   1 )/( AC   2 / DC   2 )   (3)  
         [0080]    Here, DC 1  denotes a DC component of the pulsating transmitted light intensity lt 1 , and DC 2  denotes a DC component of the pulsating transmitted light intensity lt 2 . Both DC 1  and DC 2  are called DC transmitted light intensities. Moreover, AC 1  denotes an AC component of the pulsating transmitted light intensity lt 1 , and AC 2  denotes an AC component of the pulsating transmitted light intensity lt 2 . Both lt 1  and lt 2  are called AC transmitted light intensities.  
         [0081]    Equation (3) is obtained in the following manner. As shown in FIG. 3A, a living body can be considered to be constituted of arterial blood, venous blood, and tissues other than blood. When the living body is exposed to light having the incident light intensity li, light having the transmitted light intensity lt is measured. At this time, an attenuation A caused by the living body is expressed by following Equation (4) on the basis of the Lambert-Beer law.  
           A=log ( li/lt )= log li−log lt    (4)  
         [0082]    Next, as shown in FIG. 3B, given that a variation, which would be caused by a change in the thickness of a layer of arterial blood, is taken as Δlt, a corresponding variation in the intensity of transmitted light is expressed as lt-Δlt. At this time, a variation ΔA is added to the attenuation caused by the living body and expressed by following Equation (5).  
           A+ΔA=log[li/ ( lt-Δlt )]= log li−log ( lt−Δlt )   (5)  
         [0083]    A is deleted from Equations (4) and (5), thereby determining ΔA. At this time, ΔA is expressed as follows by an equation which does not use the incident light intensity li.  
         Δ A=log lt−log ( lt−Δlt )= log[lt/ ( lt−Δlt )]  (6)  
         [0084]    Equation (6) is transformed as follows:  
         Δ A=log{ 1/[1−( Δlt/lt )]}  (7)  
         [0085]    Here, (Δlt/lt) assumes a value which is considerably smaller than 1 (because the variation Δlt of the transmitted light intensity lt derived from pulsation of an arterial blood layer of a living body is considerably smaller than the transmitted light intensity lt). Equation (7) can be made approximate by following Equation (8).  
         Δ A=Δlt/lt    (8)  
         [0086]    Therefore, following Equation (9) can be obtained from a definition equation of Φ12 and Equation (8).  
         Φ12=Δ A   1 /Δ A   2 =(Δ lt   1 / lt   1 )/(Δ lt   2 / lt   2 )   (9)  
         [0087]    As a result, logarithmic operation becomes obviated. Equation (9) is considered to be obtained when the transmitted light intensities are changed by Δlt 1 , Δlt 2  with reference to lt 1 , lt 2 .  
         [0088]    As shown in FIG. 4A, under the assumptions that a peak value of the pulsating transmitted light intensity lt is taken as a DC transmitted light intensity DC and that a difference between the peak value and a bottom value (i.e., a maximum variation) is taken as an AC transmitted light intensity AC, the transmitted light intensity is considered to have changed by AC with reference to DC. Therefore, there is attained Δlt/lt=AC/DC, and Equation (9) is transformed as follows.  
         Φ12=Δ A   1 /Δ A   2 =( AC   1 / DC   1 )/( AC   2 / DC   2 )  
         [0089]    In short, Equation (3) is obtained.  
         [0090]    Here, the peak value of the transmitted light intensity is taken as the DC transmitted light intensity DC. However, AC is considerably smaller than DC. As shown in FIG. 4B, even when the DC transmitted light intensity DC is taken as the bottom value of the pulsating transmitted light intensity lt, Equation (3) stands. As shown in FIG. 4C, even when the DC transmitted light intensity DC is taken as a mean value between the peak value and the bottom value (i.e., an intermediate value between the peak value and the bottom value), Equation (3) stands.  
         [0091]    As mentioned above, the DC transmitted light intensity DC may be any value located between the peak value and the bottom value. Accordingly, the transmitted light intensity lt achieved at a time point where AC transmitted light intensity is achieved or at immediate before or after that time point may be used as it is.  
         [0092]    In this step, computation of Φ12 is performed per each wave of the pulsating transmitted light intensities lt 1 , lt 2 ; that is, per each heartbeat.  
         [0093]    In subsequent step  3 A, the DC attenuations Adc 1 , Adc 2  of the respective wavelengths are determined. Here, the CPU  14  calculates the DC attenuations Adc 1 , Adc 2  of the respective wavelengths by substituting, into following Equations (10) and (11), the incident light intensities lical 1 , lical 2  determined in step  1 A and the DC transmitted light intensities DC 1 , DC 2  determined in step  2 A.  
           Adc   1 = log ( lical   1 / DC   1 )= log lical   1 − log DC   1    (10)  
           Adc   2 = log ( lical   2 / DC   2 )= log lical   2 − log DC   2    (11)  
         [0094]    When the apparatus has a circuit configuration which does not cause any change in the current flowing through the light-emitting elements (LEDs  2 ,  3 ), the incident light intensities lical 1 , lical 2  determined in step  1 A are used in their unmodified forms.  
         [0095]    However, in view of the fact that the living tissue involves an individual difference, a difference may arise between the electric currents Ccal 1 , Ccal 2  flowing to the light-emitting elements when the incident light intensity measurement (i.e., at the time of calibration) is performed through use of the phantom  30 A, and electric currents Cmeas 1 , Cmeas 2  flowing to the light-emitting elements when an attenuation of the living body is measured (i.e., at the time of measurement). In a case where the apparatus is provided with a circuit configuration for performing adjustment so as to achieve an optimum transmitted light intensity by changing the electric current of the light-emitting element, the CPU  14  employs, as the intensity of incident light, the values liA 1 , liA 2  corrected according to the electric current flowing through the light-emitting elements. liA 1  and liA 2  are expressed as follows.  
           liA   1 = lcal   1 · Cmeas   1 / Ccal   1    (12)  
           liA   2 = lcal   2 · Cmeas   2 / Ccal   2    (13)  
         [0096]    In this step, computation of the DC attenuations Adc 1 , Adc 2  is performed per each wave of the pulsating transmitted light intensities lt 1 , lt 2 ; that is, per each heartbeat.  
         [0097]    In step  4 A, the CPU  14  calculates the concentration of hemoglobin Hbdc through use of following Equation (14), which employs, as variables, the Φ12 determined in step  2 A and the DC attenuations Adc 1 , Adc 2  determined in step  3 A.  
           Hbdc=a   1 ·Φ12+ b   1 + c   1 · Adc   2 / Adc   1    (14)  
         [0098]    Coefficients a 1 , b 1 , c 1  of Equation (14) are values which have been determined beforehand by the method of least squares so as to minimize a difference between the hemoglobin concentration Hbdc determined by calculating a certain population (e.g., data pertaining to ten selected persons) through use of Equation (14) and the accurate concentration of hemoglobin Hbs measured through blood sampling and the cyanmethemoglobin method.  
         [0099]    Here, in order to exhibit the advantageous effect of the apparatus, the concentration of hemoglobin calculated through use of only the attenuation variation ratio Φ12 of the pulse wave is compared with the concentration of hemoglobin determined by blood sampling. Further, the concentration of hemoglobin calculated by the apparatus through use of Φ12 and the DC attenuations Adc 1 , Adc 2  is compared with the concentration of hemoglobin determined by blood sampling.  
         [0100]    When only the attenuation variation ratio Φ12 is used, the concentration of hemoglobin is calculated by Equation (15).  
           Hbp=a   2 ·Φ12+ b   2    (15)  
         [0101]    Coefficients a 2 , b 2  employed in this case are also determined in advance on the basis of the data pertaining to the population by the method of least squares. FIG. 5A shows a correlation between the hemoglobin concentration Hbp calculated through use of Equation (15) and the hemoglobin concentration Hbs measured by blood sampling.  
         [0102]    [0102]FIG. 5B shows a correlation between the hemoglobin concentration Hbdc determined by the apparatus of the embodiment based on Equation (14) through use of the DC attenuations and the hemoglobin concentration Hbs measured by blood sampling. As is evident from a comparison between the drawings, an improvement is achieved in a match between the hemoglobin concentration determined by the apparatus and that determined by blood sampling, by substituting the DC attenuations into the formula.  
         [0103]    The following equation may be employed in lieu of Equation (14).  
           Hbdc=a   3 ·Φ12+ b   3 + c   3 · Adc   2 + d   3 · Adc   1    (16)  
         [0104]    Coefficients a 3  to d 3  of this equation have been determined beforehand in the same manner as mentioned previously.  
         [0105]    A constant reverse correlation exists between the DC attenuations Adc 1 , Adc 2  and the DC transmitted light intensities DC 1 , DC 2 . Hence, there is yielded the same effect as that achieved by performing calculating operation through use of the DC transmitted light intensities in their unmodified forms without use of the DC attenuations. In this case, the DC transmitted light intensity is proportional to the intensity of the light incident on the probe. Therefore, the incident light intensities lical 1 , lical 2  must be corrected, while being normalized, by a specific current value flowing into the light-emitting elements (the LEDs  2 ,  3 ). The corrected DC transmitted light intensities ltcomp 1 , ltcomp 2  for the respective wavelengths are calculated by following Equations (17) and (18).  
           ltcomp   1 = DC   1 ( lical   1 / lstd   1 )( Cmeas   1 / Ccal   1 )   (17)  
           ltcomp   2 = DC   2 ( lical   2 / lstd   2 )( Cmeas   2 / Ccal   2 )   (18)  
         [0106]    where, DC 1 , DC 2  denote measured DC transmitted light intensities; lical 1 , lical 2  denote incident light intensities calculated at the time of calibration; lstd 1 , lstd 2  denote standard incident light intensities; Cmeas 1 , Cmeas 2  denote current values flowing into light-emitting elements obtained when a living body is measured; and Ccal 1 , Ccal 2  denote current values flowing into the light-emitting elements obtained at the time of calibration of the incident light intensities.  
         [0107]    Therefore, following Equations (19) and (20) may be employed in place of Equations (14) and (16).  
           Hbdc=a   4 ·Φ12+ b   4 + c   4 · ltcomp   1 / ltcomp   2    (19)  
           Hbdc=a   5 ·Φ12+ b   5 + c   5 · ltcomp   1 + d   5 · ltcomp   2    (20)  
         [0108]    Here, coefficients a 4  to c 4  in Equation (19) and coefficients a 5  to d 5  in Equation (20) are determined in advance by the same method as that used for determining the coefficients of Equation (14).  
         [0109]    Even in this step, computation of Hbdc is performed per each wave of the pulsating transmitted light intensities lt 1 , lt 2 ; that is, per each heartbeat. The CPU  14  stores the thus-determined Hbdc into the memory  15  and displays the same on the display  16 .  
         [0110]    According to the apparatus of the embodiment, the concentration of hemoglobin is calculated through use of the DC attenuation or DC transmitted light intensity achieved at the first wavelength 1300 nm at which light is absorbed by a living tissue other than blood, and the DC attenuation or DC transmitted light intensity achieved at the second wavelength 805 nm at which light is absorbed by blood. As a result, consideration has been given to the blood flowing through an area to be measured and the thickness of the entire living tissue formed from a tissue other than blood, whereby the concentration of hemoglobin can be measured accurately.  
         [0111]    In the embodiment, the first wavelength can be made to red light (having a wavelength of, e.g., 660 nm), and the second wavelength can be made to infrared light (having a wavelength of, e.g., 940 nm), thereby determining the concentration of oxyhemoglobin. Thus, the apparatus can be applied to measurement of oxygen saturation in arterial blood.  
         [0112]    A second embodiment of the invention will now be described. The second embodiment is directed to an apparatus for a carboxyhemoglobin concentration.  
         [0113]    [0113]FIG. 6 is a block diagram showing the overall configuration of the apparatus of the embodiment. A light emitter  100  comprises: LEDs  20   a,    20   b,  and  20   c  for generating light beams of three different wavelengths; and a driver  40  for driving the LEDs  20   a,    20   b,  and  20   c.    
         [0114]    The wavelength of a light beam originating from the LED  20   a  is taken as a first wavelength; the wavelength of a light beam originating from the LED  20   b  is taken as a second wavelength; and the wavelength of a light beam originating from the LED  20   c  is taken as a third wavelength. In this apparatus, the first wavelength is 940 nm; the second wavelength is 660 nm; and the third wavelength is 620 nm.  
         [0115]    A light receiver  50  comprises: a photodiode  60  disposed opposite the LEDs  20   a,    20   b,  and  20   c;  a current/voltage converter  70  for converting an output electric current from the photodiode  60  into a voltage signal; and an amplifier  80 .  
         [0116]    A multiplexer  90  is a circuit for distributing a signal sent from the amplifier  80  to one of among three filters; that is, a filter  10   a,  a filter  10   b,  and a filter  10   c.  The filters  10   a,    10   b,  and  10   c  are circuits for eliminating noise from signals corresponding to intensities of transmitted light beams having respective wavelengths. Output timings of the output signals are controlled by a multiplexer  120 , and the signals are then delivered to an A/D converter  130 . The A/D converter  130  is a circuit for converting the signal output from the multiplexer  120  into a digital signal.  
         [0117]    A CPU  140  is a circuit which outputs control signals to the driver  40 , the multiplexer  90 , and the multiplexer  120 , to thus control the same and which performs operation on the basis of a signal output from the A/D converter  130 .  
         [0118]    Memory  150  stores a program for use with processing to be performed by the CPU  140  and data output from the CPU  140 .  
         [0119]    A display  160  displays the data output from the CPU  140 , and a control panel  170  is equipped with a plurality of switches (including a calibration switch and a measurement switch, which will be described later) and a plurality of keys, and outputs a signal corresponding to an operator&#39;s operation to the CPU  140 . A probe of this apparatus is equipped with the LEDs  20   a,    20   b,  and  20   c  and the photodiode  60 . The living body  30  is nipped between the LEDs  20   a,    20   b,  and  20   c  and the photodiode  60 . Next, operation of the apparatus will be described by reference to FIG. 7.  
         [0120]    In step  1 B, the intensity of the light incident on the living body  30  is measured. Specifically, the intensity of light radiated from the three LEDs  20   a,    20   b,  and  20   c  of the probe onto the living body  30  is determined. As in the step  1 A to be performed by the apparatus of the first embodiment, the operator places the phantom  30 A at a predetermined position between the LEDs  20   a,    20   b,  and  20   c  of the probe and the photodiode  60  and instructs the CPU  140  to start measuring the intensity of the incident light by operating the calibration switch. The CPU  140  performs calculating operation of the following equations by substituting the light intensities ltcal 1 , ltcal 2 , and ltcal 3  of the three wavelengths of the light beams having passed through the phantom  30 A, thereby determining incident light intensities lical 1 , lical 2 , and lical 3  of three wavelengths and storing them into the memory  150 .  
           lical   1 = ltcal   1 · exp ( Af   1 )  
           lical   2 = ltcal   2 · exp ( Af   2 )  
           lical   3 = ltcal   3 · exp ( Af   3 )  
         [0121]    Here, Af 1 , Af 2 , and Af 3  denote known attenuations of the phantom  30 A achieved at the respective wavelengths and are stored in the memory  150  in advance.  
         [0122]    In step  2 B, the operator attaches the probe to the living body  30 , thereby actuating the measurement switch. As a result, the CPU  140  measures variations in the attenuations at the respective wavelengths caused by pulsation of blood of the living body  30  and the ratio Φ between the variations in the attenuations.  
         [0123]    In this step, the CPU  140  determines the DC transmitted light intensities and AC transmitted light intensities of the respective wavelengths in the same manner as in the case of step  2 A of the first embodiment and determines attenuation variation ratios of the respective wavelengths through use of the intensities. Here, the light beams of three wavelengths are used, and hence, Φ12 and Φ13 are determined by calculating following Equations (21) and (22), and results of the computation are stored in the memory  150 .  
         Φ12=Δ A   1 /Δ A   2 =( AC   1 / DC   1 )/( AC   2 / DC   2 )   (21)  
         Φ13=Δ A   1 /Δ A   3 =( AC   1 / DC   1 )/( AC   3 / DC   3 )   (22)  
         [0124]    Here, Φ12 denotes a ratio between variation ΔA 1  in an attenuation A 1  of the first wavelength and variation ΔA 2  in an attenuation A 2  of the second wavelength; Φ13 denotes a ratio between variation ΔA 1  in the attenuation Al of the first wavelength and variation ΔA 3  in an attenuation A 3  of the third wavelength; DC 1 , DC 2 , DC 3  respectively denote DC transmitted light intensities obtained at transmitted light intensities lt 1 , lt 2 , and lt 3  of the first through third wavelengths; and AC 1 , AC 2 , AC 3  respectively denote AC transmitted light intensities obtained at the transmitted light intensities lt 1 , lt 2 , and lt 3  of the first through third wavelengths.  
         [0125]    In this step, computation of Φ12, Φ13 is performed per each wave of the pulsating transmitted light intensities lt 1 , lt 2 , lt 3 ; that is, per each heartbeat.  
         [0126]    In subsequent step  3 B, the DC attenuations Adc 1 , Adc 2 , Adc 3  of the respective wavelengths are determined. Here, the CPU  140  calculates the DC attenuations Adc 1 , Adc 2  of the respective wavelengths by substituting, into following Equations (23) to (25), the incident light intensities lical 1 , lical 2 , and lical 3  determined in step  1 B and the DC transmitted light intensities DC 1 , DC 2 , and DC 3  determined in step  2 B.  
           Adc   1 = log ( lical   1 / DC   1 )= log lical   1 − log DC   1    (23)  
           Adc   2 = log ( lical   2 / DC   2 )= log lical   2 − log DC   2    (24)  
           Adc   3 = log ( lical   3 / DC   3 )= log lical   3 − log DC   3    (25)  
         [0127]    In a case where the apparatus has a circuit configuration which does not cause any change in the current flowing through the light-emitting elements (LEDs  20   a,    20   b,  and  20   c ), the incident light intensities lical 1 , lical 2 , lical 3  determined in step  1 B are used in their unmodified forms.  
         [0128]    However, in view of the fact that the living tissue involves an individual difference, a difference may arise between the electric currents Ccal 1 , Ccal 2 , Ccal 3  flowing to the light-emitting elements when the incident light intensity measurement is performed through use of the phantom  30 A (i.e., at the time of calibration), and electric currents Cmeas 1 , Cmeas 2 , Cmeas 3  flowing to the light-emitting elements when the attenuation of the living body is measured (i.e., at the time of measurement). In a case where the apparatus has a circuit configuration for performing adjustment so as to achieve an optimum transmitted light intensity by changing the electric current of the light-emitting element, the incident light intensities liA 1 , liA 2 , liA 3  employ values which have been corrected in accordance with the electric current values flowing through the light-emitting elements. Here, liA 1 , liA 2  and liA 3  are expressed by following Equations.  
           liA   1 = lcal   1 · Cmeas   1 / Ccal   1    (26)  
           liA   2 = lcal   2 · Cmeas   2 / Ccal   2    (27)  
           liA   3 = lcal   3 · Cmeas   3 / Ccal   3    (28)  
         [0129]    Even in this step, computation of the DC attenuations Adc 1 , Adc 2 , Adc 3  is performed per each wave of the pulsating transmitted light intensities lt 1 , lt 2 , lt 3 ; that is, per each heartbeat.  
         [0130]    In subsequent step  4 B, the CPU  140  calculates a concentration of carboxyhemoglobin COHbdc through use of following Equation (29), which employs, as variables, Φ12 and Φ13 determined in step  2 B and the DC attenuations Adc 1 , Adc 2 , Adc 3  determined in step  3 B.  
           COHbdc=a   6 ·Φ12+ b   6 ·Φ13+ c   6 + d   6 · Adc   2 / Adc   1 + e   6 · Adc   3 / Adc   1    (29)  
         [0131]    Coefficients a 6 , b 6 , c 6 , d 6 , and e 6  of Equation (29) are values which have been determined beforehand by the method of least squares so as to minimize a difference between the carboxyhemoglobin concentration COHbdc determined by calculating a certain population (e.g., data pertaining to ten selected persons) through use of Equation (29) and the accurate concentration of carboxyhemoglobin COHbs measured through blood sampling.  
         [0132]    Here, in order to exhibit the advantageous effect of the apparatus, the concentration of carboxyhemoglobin calculated through use of only the attenuation variation ratios Φ12 and Φ13 of the pulse wave is compared with the concentration of carboxyhemoglobin determined by blood sampling. Further, the concentration of carboxyhemoglobin calculated by the apparatus through use of Φ12, Φ13 and the DC attenuations Adc 1 , Adc 2 , Adc 3  is compared with the concentration of hemoglobin determined by blood sampling.  
         [0133]    When only the attenuation variation ratios Φ12, Φ13 are used, the concentration of carboxyhemoglobin is calculated by Equation (30).  
           COHbp=a   7 ·Φ12+ b   7 ·Φ13+ c   7    (30)  
         [0134]    Coefficients a 7 , b 7 , c 7  employed in this case are also determined in advance on the basis of the data pertaining to the population in the same manner as that mentioned previously. FIG. 8A shows a correlation between the carboxyhemoglobin concentration COHbp calculated through use of Equation (30) and the carboxyhemoglobin concentration COHbs measured by blood sampling.  
         [0135]    [0135]FIG. 8B shows a correlation between the carboxyhemoglobin concentration COHbc determined by the apparatus based on Equation (29) through use of the DC attenuations and the carboxyhemoglobin concentration COHbs measured by blood sampling.  
         [0136]    As is evident from a comparison between the drawings, an improvement is achieved in a match between the carboxyhemoglobin concentration determined by the apparatus and that determined by blood sampling, by taking the DC attenuations into Equations.  
         [0137]    The following equation may be employed in lieu of Equation (29).  
           COHbdc=a   8 ·Φ12+ b   8 ·Φ13+ c   8 + d   8 · Adc   1 + e   8 · Adc   2 + f   8 · Adc   3    (31)  
         [0138]    Coefficients a 8 , b 8 , c 8 , d 8 , e 8 , f 8  of this equation have been determined beforehand in the same manner as mentioned previously.  
         [0139]    A constant reverse correlation exists between the DC attenuations Adc 1 , Adc 2 , Adc 3  and the DC transmitted light intensities DC 1 , DC 2 , DC 3 . Hence, there is yielded the same effect as that achieved by performing calculating operation through use of the DC transmitted light intensities in their unmodified forms without computation of the attenuations. In this case, the DC transmitted light intensity is proportional to the intensity of the light incident on the probe. Therefore, the incident light intensities must be corrected, while being normalized, by a specific current value flowing into the light-emitting elements (the LEDs  20   a,    20   b,  and  20   c ). The corrected DC transmitted light intensities ltcomp 1 , ltcomp 2 , ltcom 3  for the respective wavelengths are calculated by following Equations (32) to (34).  
           ltcomp   1 = DC   1 ( lical   1 / lstd   1 )( Cmeas   1 / Ccal   1 )   (32)  
           ltcomp   2 = DC   2 ( lical   2 / lstd   2 )( Cmeas   2 / Ccal   2 )   (33)  
           ltcomp   3 = DC   3 ( lical   3 / lstd   3 )( Cmeas   3 / Ccal   3 )   (34)  
         [0140]    Here, DC 1 , DC 2 , DC respectively denote measured DC transmitted light intensities; lical 1 , lical 2 , lical 3  respectively denote incident light intensities calculated at the time of calibration; lstd 1 , lstd 2 , lstd 3  respectively denote standard incident light intensities; Cmeas 1 , Cmeas 2 , Cmeas 3  respectively denote current values flowing into light-emitting elements obtained when measurement is performed on a living body; and Ccal 1 , Ccal 2 , Ccal 3  respectively denote current values flowing into the light-emitting elements obtained at the time of calibration of the incident light intensities.  
         [0141]    Therefore, following Equations (35) and (36) may be employed in place of Equations (29) and (31).  
           COHbdc=a   9 ·Φ12+ b   9 ·Φ13+ c   9 + d   9 · ltcomp   1 / ltcomp   2 + e   9 · ltcom   1 / ltcomp   3    (35)  
           COHbdc=a   10 ·Φ12+ b   10 ·Φ13+ c   10 + d   10   ltcomp   1 + e   10 · ltcomp   2 + f   10 · ltcomp   3    (36)  
         [0142]    Here, coefficients a 9  to e 9  in Equation (35) and coefficients a 10  to f 10  in Equation (36) are determined in advance in the same manner as mentioned previously and through use of data pertaining to a population analogous to those mentioned previously.  
         [0143]    Even in this step, computation of COHbdc is performed per each wave of the pulsating transmitted light intensities lt 1 , lt 2 , lt 3 ; that is, per each heartbeat. The CPU  140  stores the thus-determined COHbdc into the memory  150  and displays the same on the display  160 .  
         [0144]    According to the apparatus of the embodiment, the concentration of carboxyhemoglobin is calculated through use of the DC attenuation or DC transmitted light intensity achieved at the first wavelength 940 nm at which light is absorbed by blood, the DC attenuation or DC transmitted light intensity achieved at the second wavelength 660 nm at which light is absorbed by blood, and the DC attenuation or DC transmitted light intensity achieved at the third wavelength 620 nm at which light is absorbed by blood. As a result, consideration has been given to the thickness of the entire blood layer located at an area to be measured, whereby the concentration of carboxyhemoglogin can be measured accurately.  
         [0145]    A third embodiment of the invention will now be described. The third embodiment is directed to an apparatus for measuring a dye concentration. This apparatus employs light beams having three wavelengths as in the case of the second embodiment, the entire configuration of the apparatus is the same as shown in FIG. 6, and the repetitive explanations will be omitted.  
         [0146]    However, as shown in FIG. 9, a program of processing to be performed by the CPU  140  differs from that described in the second embodiment. Moreover, the first wavelength assumes a value of 940 nm; the second wavelength assumes a value of 660 nm; and the third wavelength assumes a value of 805 nm. Operation of the apparatus will be described hereinbelow by reference to FIG. 9.  
         [0147]    In step  1 C, the intensity of the light incident on the living body  30  is measured. As in the case of step  1 B to be performed by the apparatus of the second embodiment, the incident light intensities lical 1 , lical 2 , lical 3  of the three light beams having different wavelengths are determined through use of the phantom  30 A, and the thus-determined incident light intensities are stored in the memory  150 .  
         [0148]    In next step  2 C, after having attached the probe to the living body  30 , the operator infuses a dye into the blood of the living body  30 . For instance, indocyanine green is used as a dye. A ratio between changes in the attenuations of the respective wavelengths is determined in the same manner as in step  2 B of the second embodiment. Specifically, Φ12 and Φ13 are determined by Equations (21) and (22), and results of computation are stored in the memory  150 .  
         [0149]    In this step, computation of Φ12, Φ13 is performed per each wave of the pulsating transmitted light intensities lt 1 , lt 2 , lt 3 ; that is, per each heartbeat.  
         [0150]    In subsequent step  3 C, the DC attenuations Adc 1 , Adc 2 , Adc 3  of the respective wavelengths are determined. Here, the CPU  140  calculates the DC attenuations Adc 1 , Adc 2  of the respective wavelengths by substituting, into following Equations (23) to (25) in the same manner as in the step  3 B of the second embodiment.  
         [0151]    When the apparatus has a circuit configuration which does not cause any change in the current flowing through the light-emitting elements (LEDs  20   a,    20   b,  and  20   c ), the incident light intensities lical 1 , lical 2 , lical 3  determined in step  1 C are used as they are, as previously mentioned.  
         [0152]    However, in view of the fact that the living tissue involves an individual difference, a difference may arise between the electric currents Ccal 1 , Ccal 2 , Ccal 3  flowing to the light-emitting elements when the incident light intensity measurement is performed through use of the phantom  30 A (i.e., at the time of calibration), and electric currents Cmeas 1 , Cmeas 2 , Cmeas 3  flowing to the light-emitting elements when the attenuation of the living body is measured (i.e., at the time of measurement). In a case where the apparatus has a circuit configuration for performing adjustment so as to achieve an optimum transmitted light intensity by changing the electric current of the light-emitting element, the incident light intensities liA 1 , liA 2 , liA 3  employ values which have been corrected in accordance with the electric current values flowing through the light-emitting elements. Here, liA 1 , liA 2  and liA 3  are expressed by Equations (26) through (28).  
         [0153]    Even in this step, computation of the DC attenuations Adc 1 , Adc 2 , Adc 3  is performed per each wave of the pulsating transmitted light intensities lt 1 , lt 2 , lt 3 ; that is, per each heartbeat.  
         [0154]    In subsequent step  4 C, the CPU  140  calculates the concentration of a dye Cddc through use of following Equation (37), which employs, as variables, Φ12 and Φ13 determined in step  2 C and the DC attenuations Adc 1 , Adc 2 , Adc 3  determined in step  3 C.  
           Cddc=a   1 ·Φ12+ b   11 ·Φ13+ c   11 + d   11 · Adc   2 / Adc   1 + e   11 · Adc   3 / Adc   1    (37)  
         [0155]    Coefficients a 11  to e 11  of Equation (37) are values which have been determined beforehand by the method of least squares so as to minimize a difference between the concentration of a dye Cddc determined by calculating a certain population (e.g., data pertaining to ten selected persons) through use of Equation (37) and the accurate concentration of a dye Cds measured through blood sampling.  
         [0156]    Here, in order to exhibit the advantageous effect of the present apparatus, the concentration of dye calculated through use of only the attenuation variation ratios Φ12, Φ13 of the pulse wave and the concentration of dye calculated through use of the attenuation variation ratios Φ12, Φ13 and the DC attenuations Adc 1 , Adc 2 , Adc 3  of the present apparatus are compared with the concentration of dye determined by blood sampling.  
         [0157]    When only the attenuation variation ratios Φ12, Φ13 are used, the concentration of a dye is calculated by Equation (38).  
           Cdp=a   12 ·Φ12+ b   12 ·Φ13+ c   12    (38)  
         [0158]    Coefficients a 12 , b 12 , c 12  employed in this case are also determined in advance on the basis of the data pertaining to the population in the same manner as that mentioned previously. FIG. 10A shows a correlation between the dye concentration Cdp calculated through use of Equation (38) and the dye concentration Cds measured by blood sampling. FIG. 10B shows a correlation between the dye concentration Cddc determined by the apparatus through Equation (37) and the dye concentration Cds measured by blood sampling.  
         [0159]    As is evident from a comparison between the drawings, improvement is achieved in a match between the dye concentration determined by the apparatus and that determined by blood sampling, by taking the DC attenuations into Equations.  
         [0160]    Moreover, following Equation (39) may be employed in lieu of Equation (37).  
           Cddc=a   13 ·Φ12+ b   13 ·Φ13+ c   13 + d   13 · Adc   1 + e   13 · Adc   2 + f   13 · Adc   3    (39)  
         [0161]    Coefficients a 13  to f 13  of this equation have been determined beforehand in the same manner as mentioned previously.  
         [0162]    A constant reverse correlation exists between the DC attenuations Adc 1 , Adc 2 , Adc 3  and the DC transmitted light intensities DC 1 , DC 2 , DC 3 . Hence, there is yielded the same effect as that achieved by performing calculating operation through use of the DC transmitted light intensities in their unmodified forms without computation of the attenuations. In this case, the DC transmitted light intensity is proportional to the intensity of the light incident on the probe. Therefore, the incident light intensities must be corrected, while being normalized, by a specific current value flowing into the light-emitting elements (the LEDs  20   a,    20   b,  and  20   c ). The corrected DC transmitted light intensities ltcomp for the respective wavelengths are calculated by following Equations (40) through (42).  
           ltcomp   1 = DC   1 ( lical   1 / lstd   1 )( Cmeas   1 / Ccal   1 )   (40)  
           ltcomp   2 = DC   2 ( lical   2 / lstd   2 )( Cmeas   2 / Ccal   2 )   (41)  
           ltcomp   3 = DC   3 ( lical   3 / lstd   3 )( Cmeas   3 / Ccal   3 )   (42)  
         [0163]    Here, DC 1 , DC 2 , DC respectively denote measured DC transmitted light intensities; lical 1 , lical 2 , lical 3  respectively denote incident light intensities calculated at the time of calibration; lstd 1 , lstd 2 , lstd 3  respectively denote standard incident light intensities; Cmeas 1 , Cmeas 2 , Cmeas 3  respectively denote current values flowing into light-emitting elements obtained when measurement is performed on a living body; and Ccal 1 , Ccal 2 , Ccal 3  respectively denote current values flowing into the light-emitting elements obtained at the time of calibration of the incident light intensities.  
         [0164]    Therefore, following Equations (43) and (44) may be employed in place of Equations (37) and (39).  
           Cddc=a   14 ·Φ12+ b   14 ·Φ13+ c   14 + d   14 · ltcomp   1 / ltcomp   2 + e   14 · ltcom   1 / ltcomp   3    (43)  
           Cddc=a   15 ·Φ12+ b   15 ·Φ13+ c   15 + d   15 · ltcomp   1 + e   15 · ltcomp   2 + f   15 · ltcomp   3    (44)  
         [0165]    Here, coefficients a 14  to e 14  in Equation (43) and coefficients a 15  to f 15  in Equation (44) are determined in advance in the same manner as mentioned previously and through use of data pertaining to a population analogous to those mentioned previously.  
         [0166]    Even in this step, computation of Cddc is performed per each wave of the pulsating transmitted light intensities lt 1 , lt 2 , lt 3 ; that is, per each heartbeat. The CPU  140  stores the thus-determined Cddc into the memory  150  and displays the same on the display  160 .  
         [0167]    According to the apparatus of the embodiment, the concentration of a dye is calculated through use of the DC attenuation or DC transmitted light intensity achieved at the first wavelength 940 nm at which light is absorbed by blood, the DC attenuation or DC transmitted light intensity achieved at the second wavelength 805 nm at which light is absorbed by blood, and the DC attenuation or DC transmitted light intensity achieved at the third wavelength 660 nm at which light is absorbed by blood. As a result, consideration has been given to the thickness of the entire blood layer located at an area to be measured, whereby the concentration of dye can be measured accurately.  
         [0168]    The above descriptions have described the cases where the intensity of the light irradiated onto the living body is changed by controlling the electric current values flowing through the light-emitting elements. However, the incident light intensities and the DC transmitted light intensities, which are to be measured, may be corrected in accordance with the value of a voltage by controlling the voltage applied to the light-emitting elements to thereby change the intensity of light beams of the light-emitting elements.