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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an oximeter and, more particularly, to an
apparatus for measuring the degree of oxygen saturation of arterial blood
(hereinafter also referred to as SaO.sub.2) a non-invasive manner.
2. Description of the Prior Art
The oximeter of non-invasive type generally has a source of a light from
which a light is directed to a body member of the subject, such as a
finger or ear lobe. The light passes through the body member and is then
detected by the detector. The amount of light absorbed by the body member
as light is transmitted through it is a function of the attenuation which
is dependent on the amount of oxygenated hemoglobin in the arterial blood
in the body member. Accordingly, by measuring the transmitted light,
oxygen saturation (SaO.sub.2) in the arterial blood can be determined.
According to the prior art oximeter of the above described type, the active
motion of the body member, such as a movement of the body member or an
irregular pulse of blood pressure causes an undesirable change in the
light amount measured by the detector, thus resulting in undesirable noise
signal in the detected signal. Since it is impossible to stop the active
motion of the body member, a means for detecting the active motion is
provided. When the detecting means detects the active motion, the measured
result is ignored. Here, the problem is to provide such a detecting means
which has a high accuracy in detecting the active motion in the body
member.
According to a first prior art oximeter, the active motion is detected by
the detection of a sudden and great change in the amplitude of the pulse
signal. In this case, the active motion with a sudden change can be
detected, but the active motion that changes gradually and incessantly can
not be detected.
According to a second prior art oximeter, the active motion is detected by
the detection of a difference in the step-up time, or step-down time,
between the pulse signal under active motion and pulse signal under a
steady condition. However, because of wide variations between patients in
such a step-up time or step-down time, and even in the same patient due to
the environmental change, the accuracy of the detected result is very
poor.
According to a third prior art oximeter, the active motion is detected by
the detection of change in the pulse rate. This oximeter, however, can not
detect the active motion when the active motion occurs periodically.
An improved oximeter, which is invented by one of the present invention,
Kenji HAMAGURI, and is assigned to the same assignee, is disclosed in
Japanese Patent Laid-open Publication (Tokkaisho) No. 55-120858. According
to this publication, the light passed through the body member is detected
at three different wavelengths. Using the detected amount of light Ea1,
Ea2 and Ea3 at three different wavelengths, the amount of absorption Eb1,
Eb2 and Eb3 by the blood in the body member is calculated for each of the
three wavelengths. Then, differences Ec1 and Ec2 are obtained through the
calculations:
Ec1=Eb1-Eb3
and
Ec2=Eb2-Eb3.
Then, using the obtained differences Ec1 and Ec2, the degree of oxygen
saturation of arterial blood (SaO.sub.2) is obtained.
Even in this improved oximeter, since the oxygen saturation (SaO.sub.2) is
calculated using the signal which has been already influenced by the
active motion of the body, the influence of the active motion can not be
completely removed from the result obtained from the improved oximeter. In
the case where the action motion is great, the obtained result will be
very poor in reliability.
Also, according to the prior art of noninvasive type, a digital display
device and/or an analog current meter are employed for the indication of
oxygen saturation (SaO.sub.2) and pulse rate in the instantaneous values
or in the averaged values. By the continuous watch on the patient's oxygen
saturation using the oximeter of noninvasive type, the sudden condition
change of the patient can be catched easily. However, with the digital
display device or analog current meter itself, it is difficult to know the
gradual condition change of the patient.
Furthermore, according to the prior art oximeter, the indication through
the digital display device and/or the analog current meter is effected
after a certain period of time, but when it is detected that the measured
result does not have a sufficient accuracy, the indication is skipped.
When the skipping takes place for a number of times frequently, the
indication on the display is effected irregularly.
Accordingly, because of the disadvantages mentioned above, it has been
difficult to obtain accurate and trustworthy results from the oximeter of
the prior art.
SUMMARY OF THE INVENTION
The present invention has been developed with a view to substantially
solving the above described disadvantages and has for its essential object
to provide an improved oximeter of non-invasive type which can provide a
degree of oxygen saturation of arterial blood (SaO.sub.2) with a high
accuracy.
It is also an essential object of the present invention to provide an
oximeter of the above described type which can present the obtained data
in various fashions to utilize the data in every possible way.
In accomplishing these and other objects, an oximeter according to the
present invention comprises a light source for projecting light to a body
member to be measured, a light responsive circuit for receiving the light
which has transmitted through said body member and for generating at least
first, second and third signals at three different wavelengths, and a
calculator for calculating at least a first SaO.sub.2 data using first and
second signals and second a SaO.sub.2 data using first and third signals.
Then, it is detected whether or not a difference between the first and
second SaO.sub.2 data is within a predetermined level. When the difference
is within the predetermined level, it is assumed that the first and/or
second SaO.sub.2 data are valid, but if not, they are assumed as invalid.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and features of the present invention will become
apparent from the following description taken in conjunction with
preferred embodiments thereof with reference to the accompanying drawings,
throughout where like parts are designated by like reference numerals, and
in which:
FIG. 1a is a block diagram of an oximeter according to a first embodiment
of the present invention;
FIG. 1b is a graph showing waveforms obtained from major points in the
circuit of FIG. 1a;
FIG. 2 is a diagrammatic view of a light source used in the circuit of FIG.
1a;
FIG. 3 is a block diagram showing a detail of circuits from photoelectric
cells to logarithmic amplifiers;
FIG. 4 is a circuit diagram showing a detail of logarithmic amplifiers and
the associated parts thereto;
FIG. 5 is a circuit diagram showing an example of a standard full-wave
rectifier;
FIG. 6 is a graph showing a pulsating signal and a rectified signal, both
carried on unwanted offset voltage;
FIG. 7 is a circuit diagram showing an improved full-wave rectifier;
FIG. 8 is a graph showing a relationship between a difference
.vertline.SaO.sub.2 (1)-SaO.sub.2 (2).vertline. and true SaO.sub.2 (2);
FIG. 9 is a plan view of a control panel;
FIGS. 10a-10e taken together as shown in FIG. 10 show a flow chart for the
control operation of calculation carried out in the oximeter of the first
embodiment;
FIG. 11 is a flow chart showing an operation for setting upper and lower
limits for SaO.sub.2 (2) and pulse rate;
FIG. 12 is a graph showing a relationship between calculated SaO.sub.2 and
a square of amplitude ratio;
FIG. 13 is a block diagram of an oximeter according to a second embodiment
of the present invention; and
FIG. 14 is a flow chart for the control operation of calculation carried
out in the oximeter of FIG. 13.
THEORY OF SaO.sub.2 MEASURING ACCORDING TO THE INVENTION
Before the description proceeds to the preferred embodiments, the theory of
measuring the degree of oxygen saturation of arterial blood (SaO.sub.2) is
described.
When light transmits through the body member, it is absorbed and scattered
by blood, muscle, and other members constituting the body member. Thus the
transmitted light is attenuated. Since the arterial blood is pulsating,
its volume at a place where the light transmits changes periodically.
Thus, the degree of attenuation of the transmitted light also changes
periodically. The intensity Iw of the light, having a wavelength w,
transmitted through the body member can be given as follows:
Iw=Iow.times.Ftw.times.Fvw.times.f(Qw)e.sup.-g(Qw)(d+.DELTA.d),
wherein Iow is an intensity of light directed to the body member; Ftw is a
transmittance of a bloodless body member; Fvw is a transmittance of a body
member with venous blood; Qw is an absorption coefficient of light having
the wavelength w with arterial blood; f(Qw) and g(Qw) are functions with a
variable Qw; d+.DELTA.d is a thickness of the body member; and .DELTA.d is
the change in the thickness of the body member that takes place
periodically.
A DC component Yw in the logarithmically compressed values of Iw can be
given:
Yw=-g(Qw).DELTA.d.
Since g(Qw) is approximately proportional to the square root of Qw, the
above equation may be written:
Yw.sup.2 =kwQw(.DELTA.d).sup.2,
wherein kw is a constant determined dependently on the wavelength w. The
absorption coefficient Qw can be given:
##EQU1##
in which
Ct=CHbo.sub.2 +CHb
and
S=CHbo.sub.2 /Ct=CHbo.sub.2 /(CHbo.sub.2 +CHb),
and wherein CHbo.sub.2 and CHb are the amounts of oxyhemoglobin and
deoxyhemoglobin, respectively, in a unit volume; and EwHbo.sub.2 and EwHb
are absorption coefficients of the light with the wavelength w in
oxyhemoglobin and deoxyhemoglobin, respectively. Accordingly, Yw.sup.2 can
be given as follows:
Yw.sup.2 =kwCt[S(EwHbo.sub.2 -EwHb)+EwHb](.DELTA.d).sup.2.
By obtaining two different DC components Yw1 and Yw2 at two different
wavelengths w1 and w2, it is possible to calculate the ratio S in a manner
described below.
Since
(Yw1).sup.2 =kw1Ct[S(Ew1Hbo.sub.2 -Ew1Hb)+Ew1Hb](.DELTA.d).sup.2
and
(Yw2).sup.2 =kw2Ct[S(Ew2Hbo.sub.2 -Ew2Hb)+Ew2Hb](.DELTA.d).sup.2,
we obtain
##EQU2##
By selecting a certain light having a wavelength w1 which satisfies an
equation:
Ew1Hbo.sub.2 =Ew1Hb,
the above equation can be simplified as:
##EQU3##
Since the degree of oxygen saturation of arterial blood (SaO.sub.2) can be
defined as
SaO.sub.2 =S.times.100(%),
the above equation may be written as:
##EQU4##
wherein A and B are amounts which can be obtained from the optical
characteristics of the blood. When A and B are fixed, SaO.sub.2 is in
relation to Yw2/Yw1. Therefore, a term "SaO.sub.2 data" represents not
only A.times.(Yw2/Yw1).sup.2 +B, but also (Yw2/Yw1).sup.2 and Yw2/Yw1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1a, a body member F, such as a finger, is steadily held
between a light source 1, such as a halogen lamp, and a light receiver 2.
Light receiver 2 is sensitive to light at three different wavelengths,
which are, e.g., 805, 710 and 650 nanometers. It is to be noted that these
figures are given merely as an example, and, therefore, any other figures
can be used. Thus, light receiver 2 generates three different signals
representing the intensity of light at three different wavelengths. The
generated signals are applied to logarithmic circuits 5, 6 and 7,
respectively. According to a preferred embodiment, light receiver 2
includes three photoelectric cells 36, 37 and 38, shown in FIG. 3, which
are coupled to I/V (current-to-voltage) converters 39, 40 and 41,
respectively. Thus, converters 39, 40 and 41 generate voltage signals
representing the light intensity at wavelengths 805, 710 and 650
nanometers, respectively. These signals produced from I/V converters 39,
40 and 41 are substantially equal to the signals produced from light
receiver 2, and are applied to logarithmic amplifiers 42, 43 and 44
provided in logarithmic circuits 5, 6 and 7, respectively. The signals
produced from light receiver 2 are composite signal having both AC and DC
components, as shown in FIG. 1b, first row. As indicated therein, each
signal is carried on a different DC level y and has a different amplitude
x of fluctuation. The fluctuation is caused by the heart pulsation of the
body.
The difference in the DC level is caused by the difference of the light
intensity at three different wavelengths. Therefore, it is necessary to
compensate the voltage signals obtained from I/V converters 39, 40 and 41
in such a way that the signals are obtained in the same condition, so that
three signals have the same DC level. To this end, the light from light
source 1 is also applied to a monitor receiver 3 which monitors the
intensity of the light at three different wavelengths, which are the same
as those mentioned above.
According to the preferred embodiment, as shown in FIG. 3, monitor receiver
3 includes one photoelectric cell 34, which is coupled to an I/V converter
and further to three AD/DC converters 45, 46 and 47. As will be described
in detail later, AC/DC converters 45, 46 and 47 generate voltage signals
indicating the intensity of direct light from light source 1 at three
different wavelengths. The signals from AD/DC converters 45, 46 and 47 are
applied to logarithmic amplifiers 42, 43 and 44 to change the gain
therein. Thus, the voltage signals obtained from I/V converters 39, 40 and
41 can be evaluated under the same condition such that the light intensity
at three different wavelengths from light source 1 are the same.
Accordingly, the outputs from logarithmic amplifiers 42, 43 and 44, that
is, outputs from logarithmic circuits 5, 6 and 7 are carried on the same
DC level, and the signal on the DC level is identical to x/y, as indicated
in FIG. 1b, second row.
Referring back to FIG. 1a, the output of logarithmic circuit 5 is applied
to a low pass filter 8 for passing unwanted low frequency, such as 3 Hz,
noise to a differential amplifier 11. Differential amplifier 11 also
receives the signal directly from logarithmic circuit 5. Accordingly,
differential amplifier 11 generates only the AC component amplified by a
certain gain, as indicated in FIG. 1b, third row. The output of
differential amplifier 11 is connected to a full-wave rectifier 14 which
rectifies the output signal from differential amplifier 11, as indicated
in FIG. 1b, fourth row, and integrates the rectified signal for a
predetermined period of time, such as 1 second, repeatedly. The output of
full-wave rectifier 14, such as shown in FIG. 1b, last row, is applied to
a multiplexer 18.
In a similar manner, the output of logarithmic circuit 6 is connected
through a low pass filter 9, a differential amplifier 12 and an full-wave
rectifier 15 to multiplexer 18. Also, the output of logarithmic circuit 7
is connected through a low pass filter 10, a differential amplifier 13 and
an full-wave rectifier 16 to multiplexer 18. The output of logarithmic
circuit 7 is further connected to a light amount detector 17 which carries
out a certain calculation to detect the amount of light received by light
receiver 2. The output of light amount detector 17 is applied to
multiplexer 18 for use in detecting whether or not the amount light
received by receiver 2 is above a predetermined level sufficient for the
SaO.sub.2 detection.
Multiplexer 18 selects one of the outputs from circuits 14, 15, 16 and 17
and sequentially transmits the selected output to an A/D
(analog-to-digital) converter 20, which is defined by a double integrator
19a and a comparator 19b. Thus, the analog signal obtained from
multiplexer 18 is changed to a digital signal, which is applied to a CPU
(central processing unit) 26. CPU 26 also receives the signal from
differential amplifier 11 through a wave-shaping circuit 21 so as to count
the pulse rate of the patient.
The output of differential amplifier 11 is also applied to an ampere meter
control 22 which changes the received signal to a fashion capable of
driving an ampere meter 29, and to a recorder control 23 which changes the
received signal to a fashion capable of operating a recorder (not shown).
CPU 26 is coupled with various devices, such as: an alarm device 30 for
producing an alarm sound when an obtained signal fails to fall within a
selected range; a range setting device 24 which has a number of switches
for setting upper and lower limits of the range and switches for
controlling the generation of alarm sound; a display device 25 for
displaying various data, such as measured SaO.sub.2 and pulse rate;
printer 27 for making a hard copy of the information shown on display
device 25; and a digital output device 28 capable of being connected to an
external device, such as a printer (not shown). CPU 26 also produces
various control signals for controlling the sequence of operation of the
circuits 14, 15, 16, 18 and 20, and devices 24, 25 and 30. It also carries
out various control steps, which will be described later in connection
with FIGS. 10a-10e, for obtaining SaO.sub.2 and pulse rate.
According to the first embodiment, the oximeter further includes a power
source 31 for supplying power to the circuits and devices shown in FIG.
1a.
According to the preferred embodiment, the light source 1, shown as a
halogen lamp, emits light which is directed partially to a photoelectric
cell provided in monitor receiver 3, and partially directed through an
optical path defined, e.g., by an optical fiber (not shown) to a light
measuring portion. At the light measuring portion, the light is separated
into a spectrum for obtaining the three different wavelength lights, which
are directed to three different photoelectric cells 36, 37 and 38. Each of
the photoelectric cells 36, 37 and 38 and the one provided in monitor
receiver 3 produces a current signal which is in relation to the intensity
of received light. The current signal produced from monitor receiver 3 is
converted to a voltage signal in a voltage adjuster 4, and the voltage
signal is applied to each of logarithmic circuits 5, 6 and 7.
Logarithmic circuit 5 receives the voltage signal from voltage adjuster 4
and also the voltage signal from light receiver 2 and produces a
logarithmically compressed voltage signal carrying information about a
particular wavelength light (such as light having a wavelength of 805
nanometers). Similarly logarithmic circuit 6 receives the voltage signal
from voltage adjuster 4 and also the voltage signal from light receiver 2
and produces a logarithmically compressed voltage signal carrying
information about a particular wavelength light (such as light having a
wavelength of 710 nanometers). Furthermore, logarithmic circuit 7 receives
the voltage signal from voltage adjuster 4 and also the voltage signal
from light receiver 2 and produces a logarithmically compressed voltage
signal carrying information about a particular wavelength light (such as
light having a wavelength of 650 nonometers).
Each of the logarithmic circuits 5, 6 and 7 includes, as will be described
later, a correction circuit which will eliminate the noise signal caused
by the undesirable fluctuation in the light emitted from the light source.
The outputs of logarithmic circuits 5, 6 and 7 are connected,
respectively, to low pass filters 8, 9 and 10 and also to differential
amplifiers 11, 12 and 13. The outputs of low pass filters 8, 9 and 10 are
also connected to differential amplifiers 11, 12 and 13, respectively.
Each differential amplifier calculates a difference between the outputs
from the logarithmic circuit and the low pass filter and amplifies the
obtained difference. Therefore, each differential amplifier produces a
photoelectric volume pulsating signal obtained at a particular wavelength
light. The pulsation in this signal is caused by the volume change of
venous blood in the measuring portion and, therefore, the pulsating signal
contains information about absorption coefficient of venous blood.
Differential amplifiers 11, 12 and 13 are connected, respectively, to
full-wave rectifiers 14, 15 and 16, each of which is defined by a
half-wave rectifier and an integrator. In each full-wave rectifier, the
output signal from the differential amplifier are full-wave rectified and,
thereafter, under the control of CPU 26, the rectified signal is
integrated for a given time, such as 0.9 second, and thereafter, it is
temporarily held for a period of time. The temporarily held signals from
full-wave rectifiers 14, 15 and 16 and a signal from light amount detector
17 are selected by analog multiplexer 18 and are sequentially transmitted
to double integrator 19a of A/D converter 20. Which signal multiplexer
selects is controlled by CPU26.
Double integrator 19a, comparator 19b and a counter provided in CPU 26
define a double integration type A/D converter. Thus, the signals
sequentially applied to double integrator 19a are converted to digital
signals. After A/D conversion of each signal, the integrators in full-wave
rectifiers 14, 15 and 16 are reset to discharge the integrated signal.
Accordingly, each of the integrators in full-wave rectifiers 14, 15 and 16
repeats the operations of integration, hold and discharge in a
predetermined frequency, and the A/D conversion is carried out in relation
to such operations. Similarly, A/D conversion of output signal from light
amount detector 17 is carried out repeatedly with a predetermined
frequency. Each of the converted digital signals from full-wave rectifiers
14, 15 and 16 is subtracted by an offset voltage of the corresponding
full-wave rectifier, which is previously converted to a digital signal and
stored, thereby obtaining three amplitude signals which are in relation to
the amplitude of the corresponding pulsating signal. Using the first
amplitude signal, which is based on the 805 nanometer wavelength light,
and the second amplitude signal, which is based on the 710 nanometer
wavelength light, a first SaO.sub.2 signal (hereinafter indicated as
SaO.sub.2 (1)) is obtained. Similarly, using the first amplitude signal
and the third amplitude signal, which is based on the 650 nanometer
wavelength light, a second SaO.sub.2 signal (hereinafter indicated as
SaO.sub.2 (2)) is obtained. The obtained SaO.sub.2 (1) and SaO.sub.2 (2)
are stored for a period of time. As will be described in detail later,
SaO.sub.2 (1) will be used for the detection of any noise signal caused by
the undesirable movement of the body member. The obtained SaO.sub.2 (1)
and SaO.sub.2 (2) are calculated and renewed after every predetermined
period of time. As to SaO.sub.2 (2), the newly obtained SaO.sub.2 (2) and
those obtained in the several previous operations are used for obtaining
an average SaO.sub.2 (2) which will be displayed through display device
25. Detailed steps for obtaining the average SaO.sub.2 (2) will be
described later.
The pulsating signal produced from differential amplifier 11 is applied to
wave-shaping circuit 21 in which the pulsating signal is compared with a
predetermined threshold level thereby changing the pulsating signal to a
binary pulse signal which takes either a HIGH level or LOW level. The
binary signal is applied to CPU 26 which detects the step up and step down
of the pulse for calculating the pulse repetition rate, thereby
calculation the pulse rate of the patient, i.e., the number of heart beats
per one minute. Similar to SaO.sub.2 (1) and SaO.sub.2 (2), the pulse rate
is calculated repeatedly after predetermined periods of time and is
displayed through a display device 25. A detail of calculation of pulse
rate will be described later.
Ampere meter control 22 changes the pulsating signal obtained from
differential amplifier 11 to a signal appropriate for driving an ampere
meter 29 which indicates whether or not the pulsating signal is in a
normal condition. Recorder control 23 changes the pulsating signal
obtained from differential amplifier 11 to a signal appropriate to be
applied to a recorder (not shown). Upon depression of a switch provided on
display device 25, the information displayed on display device 25 can be
immediately transferred to printer 27 for producing a hard copy of data,
such as average SaO.sub.2 and pulse rate, for graph recording.
The signals representing the average SaO.sub.2 and pulse rate can be
produced in a digital form through output device 28. In the case where the
obtained SaO.sub.2 and pulse rate exceeds the upper limit or falls below
the lower limit, it is possible that the patient is in a dangerous
condition. In such a case, an alarm is required to enable a prompt action
to the patient. According to the present invention, it is possible to set
the upper and lower limits of each of SaO.sub.2 and pulse rate by
operating various switches provided on range setting device 24. The set
values are indicated on display device 25.
The alarm may be produced by way of sound from alarm device 30 and/or
visual indication on display device 25. An operator can select whether or
not to produce the alarm sound by a suitable switch means provided on
alarm device 30. Furthermore, it is possible to change the mode of alarm
device 30 such that it may be prohibited from generating a sound alarm
even when the obtained result exceeds the upper limit and/or falls below
the lower limit.
Light amount detector 17 is a circuit which changes the style of the output
signal from logarithmic circuit 7 in a fashion applicable for the A/D
conversion input signal. The signal produced from the light amount
detector 17 is converted to a digital signal after every predetermined
period of time, and the converted digital signal is used for determining
whether or not the amount of light from light source 1 is at a reasonable
level. In the case where the light from light source 1 is very strong, the
operation of the photoelectric cells and logarithmic circuits 5, 6 and 7
saturates, thereby deteriorating the accuracy of the calculated result.
Also, when the body member is offset the light path between light source 1
and photoelectric cells, the received light amount becomes great, thereby
disabling the calculation of SaO.sub.2. A similar problem arises when the
light is very weak. In this case, the signals produced from the
photoelectric cells and logarithmic circuits 5, 6 and 7 are so poor that
it is impossible to carry out the SaO.sub.2 measurement with a reasonable
accuracy. Therefore, in order to watch and detect whether or not the
received light is at a reasonable level, the output signal from light
amount detector 17 is converted to a digital form and is examined whether
or not it is within a predetermined range.
Referring to FIG. 2, the description is directed to the way how the noise
signal, caused by the undesirable fluctuation in the light amount, is
eliminated. The light from light source 1, such as from a halogen lamp 32,
is directed to an end face of optical fiber 33. A reference number
designated a photoelectric cell mounted in a hood 34a for monitoring the
halogen light. Since photoelectric cell 34 and the end face of optical
fiber 33 are positioned adjacent each other, the same type of light and
approximately the same amount of light will be directed to both places.
Thus, the spectral characteristics will be the same and the amount of
light will be proportional between the light received by the cell 34 and
by the end face of fiber 33. According to the embodiment shown in FIG. 2,
only one photoelectric cell 34 is shown, but it is possible to provide a
plurality of such cells. The light directed to the end face of optical
fiber 33 is further directed to a measuring portion where the body member
should be located. The light which has transmitted through the body member
is directed to light receiver 2, at which the light is separated to
spectrum for obtaining lights at three different wavelengths. The
separated lights are directed to three photoelectric cells 36, 37 and 38.
Referring to FIG. 3, a circuit for receiving signals from photoelectric
cells 34, 36, 37 and 38 and for producing signals having no influence of
light fluctuation and carrying only the information about the attenuation
of light caused only by the body member is shown. Photoelectric cell 34 of
monitor circuit 3 is connected to I/V converter 35. Similarly,
Photoelectric cells 36, 37 and 38 of light receiver 2 are connected to I/V
converters 39, 40 and 41, respectively. Each I/V converter is provided for
converting the current signal from the photoelectric cell to a voltage
signal. Each of logarithmic amplifiers 42, 43 and 44 has two inputs A and
B and calculates a ratio of A to B and logarithmically compresses the
obtained ratio. A further detail of the circuit is given below.
The light separated at light receiver 2 and having a particular wavelength,
such as 805 nanometers, impinges on photoelectric cell 36. Thus, the
spectral characteristics of the light received by photoelectric cell 36
differs from that received by photoelectric cell 34 in the monitor
receiver. Accordingly, the degree of change in the light amount, caused by
the light intensity change of light source 1, at the photoelectric cell 36
is not the same as that at the photoelectric cell 34. Thus, the noise
signals produced by the light intensity change in light source 1 can not
be eliminated by merely taking a ratio between outputs from I/V converts
39 and 35 and logarithmically compressing the obtained ratio. In order to
eliminate the noise signals produced by the light intensity change in
light source 1 by the steps of taking a ratio between outputs from I/V
converts 39 and 35 and logarithmically compressing the obtained ratio, it
is necessary to match the spectrum distribution of the light directed to
photoelectric cell 34 and that to photoelectric cell 36. To this end, one
way is to provide a spectral divider, such as a prism, in front of
photoelectric cell 34. However, this method requires three spectral
dividers for monitoring lights at three different wavelengths. According
to the present invention, no spectral divider is provided to photoelectric
cell 34, but the signal obtained from cell 34 is electrically processed.
There is a certain relationship between a ratio of change in the light
amount, caused by the light intensity change of light source 1, to the
intensity of light impinging on photoelectric cell 34 and a ratio of
change in the light amount, caused by the light intensity change of light
source 1, to the intensity of light impinging on photoelectric cell 36.
More specifically, there is a certain relationship between a ratio of AC
component, caused by the light intensity change of light source 1, to DC
component in the output voltage from I/V converter 35 and a ratio of AC
component, caused by the light intensity change of light source 1, to DC
component in the output voltage from I/V converter 39. According to the
present invention, the ratio of AC component, caused by the light
intensity change of light source 1, to DC component in the output voltage
from I/V converter 35 is so adjusted such that these ratios have the same
amount.
The same adjustment as described above is made for each of the two other
lights having different wavelengths. These adjustments are accomplished by
AC/DC converters 45, 46 and 47 shown in FIG. 3. Thus, a ratio of AC
component, caused by the light intensity change of light source 1, to DC
component in the output voltages from AC/DC converter 45 is fixed
substantially the same as a ratio of AC component, caused by the light
intensity change of light source 1, to DC component in the output voltages
from I/V converter 39. Similarly, a ratio of AC component, caused by the
light intensity change of light source 1, to DC component in the output
voltages from AC/DC converter 46 is fixed substantially the same as a
ratio of AC component, caused by the light intensity change of light
source 1, to DC component in the output voltages from I/V converter 40,
and a ratio of AC component, caused by the light intensity change of light
source 1, to DC component in the output voltages from AC/DC converter 47
is fixed substantially the same as a ratio of AC component, caused by the
light intensity change of light source 1, to DC component in the output
voltages from I/V converter 41.
In other words, AC/DC converter 45 produces a signal representing a ratio
of AC component to DC component with respect to the light directly
measured from light source 1 and having a wavelength of 805 nanometers.
Other AC/DC converters 46 and 47 produce similar signals, but they are
based on lights having wavelength of 710 nanometers and 650 nanometers.
Accordingly, logarithmic amplifiers 42, 43 and 44 produce signals having no
noise signal caused by the light intensity change of light source 1.
Referring to FIG. 4, a circuit diagram of circuits 39-44 is shown. In the
circuit shown in FIG. 4, the output currents from photoelectric cells 36,
37 and 38 are applied directly to a logarithmic compression circuits.
Logarithmic amplifier 42 has a transistor 48 which has its collector
connected to the cathode side of photoelectric cell 36. Therefore, the
collector current of transistor 48 is substantially the same as
photocurrent I1 can be given by a product of a light intensity Iow of
light at the wavelength 805 nanometers from light source 1 and the
transmittance, or reflectance, of the body member. The light intensity Iow
is a composite signal of DC component Iowdc and AC component Iown
representing the noise signal caused by the light intensity change of
light source 1, and therefore, the following equation may be obtained:
Iow=(Iowdc+Iown)F=Iowdc(1+Nw)F,
wherein
##EQU5##
and F is a constant determined by the sensitivity of the photoelectric
cell and transmittance of the body member. Since a photocurrent Ir from
photoelectric cell 34 is proportional to the light intensity Io of light
from light source 1 in every wavelength, and since Io has a DC component
and AC component Ion caused by the light intensity change of light source
1, the following equation may be obtained:
Ir=K1(Iodc+Ion)=K1.times.Iodc(1+N),
wherein
##EQU6##
and K1 is a constant. The output voltage Vr produced from I/V converter
35, which is coupled to photoelectric cell 34, can be given as follows:
Vr=Ro.times.K1.times.Iodc(1+N).
Thus, a current that flows through a resistor R1 may be written as follows:
##EQU7##
and through a resistor R2, a current that has no AC component flows, which
may be written as follows:
##EQU8##
wherein R is equal to a value of a parallel connection of resistors R2 and
R5. Thus,
##EQU9##
Accordingly, the collector current of transistor 51 can be written as
follows:
##EQU10##
Therefore, logarithmic amplifier 42 produces an output voltage Vout which
can be written as:
##EQU11##
In the above equation, the first term | | |
