(12) INTERNATIONAL APPLICATION P UBLISHED UNDE R THE PATENT COOPE RATION TREATY (PCT)
`
`(19) World Intellectual Property Organization
`International Bureau
`
`I IIIII 11111111 II 1111111111111111111111111 111111111111111111111111111111111111111111 111111111111
`
`(43) International Publication Date
`1 November 2007 (01.11.2007)
`
`PCT
`
`(10) International Publication Number
`WO 2007/122375 A2
`
`(51) Interna tion al Patent C lassifica tion:
`A6JB 5/14!i5 (2006.0 1)
`A6JB 5108 (2006.0 1)
`M JB 51024 (2006.01)
`
`(74) Agent: C HA RlG, Raymond; Eri e Potter Clarkson LLP,
`Park View Hoose, 58 The Ropewalk, Nottingham NG L
`5DD (GB).
`
`(21) Interna tion al App ti<.'3tion N um ber :
`PCT/GB2007/001355
`
`(22) internation al F iling Da le:
`
`11 April 2007 ( 11.04.2007)
`
`(25) F ili ng Lan guage:
`
`(26) Pubti<.'Btion Language:
`
`English
`
`English
`
`(30) Priority Da ta:
`0607270.6
`
`ll April2006 (11.04.2006) GB
`
`(71) Applicant (for alL designated States except US): T HE
`UNIVERSITY O F NOTTINGHAM [GB/GB]; Univer(cid:173)
`sity Park, Nottingham NG7 2RD (GB).
`
`(72) In ventors; a nd
`(75) In ventors/Applicant<; (for US only): C ROWE, J ohn
`[GB/GB]; School of Electrical and Electronic Engineer(cid:173)
`ing, The University of Nottingham, University Park,
`Nottingham NG7 2RD (GB). G R UBB, Ma rk [GB/GB];
`School of Electrical and Electronic Engineeri ng, The
`University of Nottingham, University Park, Nottingham
`NG7 2RD (GB). BAYES-GILL, Barrie [GB/GB]; School
`of Electrical and Electronic Engineering, The University
`of Nottingham, University Park, Nottingham NG7 2RD
`(GB). M lLES, Nicolas [GB/GB]; School of Electrical and
`Electronic Engineering, The University of Nottingham,
`University Park, Nottingham NG7 2RD (GB).
`
`--
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`-------------
`
`(81) Designated States (unless otherwise indicated, for every
`kind of national protection available): AE, AG, AL, AM,
`AT, AU, AZ, BA, BB, BG, BH, BR, BW, BY, BZ, CA, CH,
`CN, CO, CR, CU, CZ, DE, DK, DM, DZ, EC, EE, EG, ES,
`Fl, GB, GD, GE, GH, GM, GT, I IN, HR, l-TU, TD, U.., IN,
`IS, Jl>, KE, KG, KM, KN, KP, KR, KZ, LA, LC, LK, LR,
`LS, LT, LU, LY, MA, MD, MG, MK, MN, MW, MX, MY,
`MZ, NA, NG, Nl, NO, NZ, OM, PG, PH, PL, PT, RO, RS,
`RU, SC, SD, SE, SG, SK, SL, SM, SV, SY, TJ, TM, TN,
`TR, Tf, TZ, UA, UG, US, UZ, VC, VN, ZA, ZM, ZW.
`
`(84) Designated States (unless othenvise indicated. for every
`kind of regional protection available): AR1PO (BW, GH,
`GM, KE, LS, MW, MZ, NA, SD, SL, SZ, TZ, UG, ZM,
`ZW), Eurasian (AM, AZ, BY, KG, KZ, MD, RU , 11, TM),
`European (AT, BE, BG, C H, CY, CZ, DE, DK, EE, ES, Fl,
`FR, GB, GR, 1-fiJ, IE, IS, IT, LT, LU, LV, MC, MT, NL, PL,
`PT, RO, SE, SI, SK, TR), 0/\PI (BI~ BJ, CF, CG, CI, CM,
`GA, GN, GQ, GW, ML, MR, NE, SN, TD, TG).
`
`Publish ed:
`without intenwtional search report and to be republished
`upon receipt of that report
`
`For two-leuer codes and 01her abbreviations, refer to the "Guid(cid:173)
`ance Notes on Codes anti Abbreviations" appearing at the beght(cid:173)
`ning of each regular issue oftiU! PCT Gazelle.
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`~ (54) Title: PHOTOPLETHYSMOGRAPHY
`
`100
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`~
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`107
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`l/)
`t(cid:173)
`~
`~
`~ (57) A bstract: A photoplethysmograph device includes a ligh t source for illuminating a target object. A modu lator drives the light
`~ source such that the outpu t intensity varies as a function of a modulation signal at a modu lation frequency. A detector receives light
`
`i:::: from the target object and generates an electrical output as a function of the intensi ty of received light. A demodulator with a local
`Q oscillator receives the detecto r outpu t and produces a demodulated output representative of the modulation signal. 'l'he demodulator
`Q
`is insensitive to any phase difference between the modulation signal and the oscillator of the demodu lator. From the demodulated
`~ outpu t, a signal indicative of blood volume as a function of time and I or b lood composition is generated. A number of demodulators
`0 may be provided to derive signals from multiple light sources of different wavelengths, or from an array of detectors. The plethys(cid:173)
`> mograph may operate in a transmission mode or a reflectance mode. When in a reflectance mode, the device may use the green part
`
`~ of the optical spectrum and may use polarising fi lters.
`
`Apple Inc.
`APL1037
`U.S. Patent No. 8,942,776
`
`i
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`PCT/GB2007 /001355
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`PHOTOPLETHYSMOGRAPBY
`
`The present invention relates photoplethysmography and in particular to a method and
`
`apparatus for measuring pulse rate, breathing rate and blood constituents in the human
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`5
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`or animal body.
`
`The word plethysmography is a combination of the Greek words Plethysmos,
`
`meaning increase, and graph, meaning write. A plethysmograph is an instrument,
`
`method or apparatus used to measure the variations in blood volume in the body.
`
`10
`
`P~otoplethysmography (hereinafter also referred to as 'PPG') refers to the use of light
`
`to measure these changes in volume, and therefore a photoplethysmograph is an
`
`instrument, method or apparatus that uses light to perform these measurements.
`
`Although the human or animal body is generally assumed to be opaque to light, most
`
`15
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`soft tissue will transmit and reflect both visible and near-infrared radiation.
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`TherefOJ;e, if light is projected onto an area of skin and the emergent light is detected
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`after its interaction with the skin, blood and other tissue, time varying changes of light
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`intensity having a relation with blood volume, known as the plethysmogram, can be
`
`observed. This time varying light intensity signal will depend on a number of factors
`
`20
`
`including the optical properties of the tissues and blood at the measurement site, and
`
`the wavelength of the light source. The signal results because blood absorbs light and
`
`the amount of light absorbed, and hence the intensity .of remaining light detected,
`varies in relation with the volume of blood illuminated. Variation in the
`plethysmogram is caused by the variation in blood volume flowing in the tissue.
`
`25
`
`This technique was introduced in 1937 by Hertzman. He was the first to use the term
`
`photoplethysmography and suggested that the resultant plethysmogram represented
`
`volumetric changes of blood in the skin's vessels.
`
`30
`
`The plethysmogram is usually described with respect to its AC and DC components.
`
`The absorption of light by non-pulsatile blood, bone and tissue is assumed to be
`
`constant and gives rise to the DC component. The DC component represents ~he
`
`volume of non-pulsatile blood belo~ the sensor, plus light reflected and scattered off
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`1
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`wo 2007/122375
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`PCT/GB2007 /001355
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`the skin, bone and other tissues. The AC component is caused by the time varying
`
`absorption of light caused by temporal changes in blood volume below the sensor.
`
`Changes in the blood volume can be caused by cardiovascular regulation, blood
`
`5
`
`pressure regulation, thermoregulation and respiration. Thus the plethysmogram can
`
`be analysed to determine infonnation on such parameters as pulse rate, breathing rate,
`
`blood pressure, perfusion, cardiac stroke volume and respiratory tidal volume. These
`
`can be observed as periodic and non-periodic changes in the amplitude of AC and DC
`
`components in the plethysmogram. This has been described in more detail in Kamal
`
`10
`
`et al: 'Skin Photoplethysmography - a review', Computer Methods and Programs in
`
`Biomedicine, 28 (1989) 257-269). The plethysmogram can also be analysed to
`
`determine blood constituents. One such technique is pulse oximetry, which
`
`determines the relative amount of oxygen in the blood. Other blood constituents can
`
`also be measured by using photoplethysmography.
`
`15
`
`There are two modes of photoplethysmography, the transmission mode and the
`
`reflection mode. In transmission mode the light source is on one side of the tissue and
`
`the photodetector is placed on the other side, opposite the light source. The use of
`
`transmission mode is limited to areas where the tissue is thin enough to allow light to
`
`20
`
`propagate, for example the fingers, toes and earlobes of a human subject.
`
`In reflection mode the light source and photodetector are place side-by-side. Light
`
`entering the tissue is reflected and a proportion of this is detected at the photodetector.
`
`This source-detector configuration is more versatile and allows measurements to be
`
`25
`
`performed on almost any area of tissue. However, the use of reflectance mode is
`
`much harder to design than transmission because the signal level is significantly lower
`
`at the most effet:tive wavelengths. Thus, considerable attention must be given to
`
`maximising signal-to-noise ratio. As a result, the most common PPG sensors use
`
`transmission mode and hence are restricted to positions where light can pass through
`
`30
`
`tissue.
`
`As a photodetector is used to measure light from the source, the photoplethysmograph
`
`can also respond to interfering signals from other sources of light, for · example
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`2
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`PCT/GB2007 /001355
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`fluorescent lighting and computer monitors. The sensor must also respond to changes
`
`in the light propagating through tissue, i.e. the plethysmogram. These physiological
`changes contain frequency components between DC and 25 Hz. However, it is
`desirable for the sensor not to respond to ambient light noise. Accordingly, the
`
`5
`
`photoplethysmograph should reject ambient
`
`light noise while detecting the
`
`plethysmogram in the bandwidth of interest.
`
`A second source of interference is other electrical apparatus. Other electrical devices
`
`can generate radio frequency signals that a photoplethysmograph can detect. It is
`
`10
`
`desirable to minimise the sensitivity of the system to interfering sources of this nature.
`
`A
`
`third source of interference
`
`is
`
`the electrical noise generated by the
`
`photoplethysmograph itself. Such noise can be generated by electronic components,
`
`and can include thermal noise, flicker noise, shot noise, as well as noise spikes, for
`
`15
`
`example, harmonics generated by missing codes in an analogue-to-digital converter.
`
`It is also desirable to minimise the sensitivity of the system to interference from these
`
`sources.
`
`A known technique for reducing the noise generated by these three sources of
`
`20
`
`interference is to drive the sensor's light source with a carrier modulated at a
`
`frequency that is not present, or dominant, in the ambient light, electrical radio
`
`frequency signals, · or photoplethysmograph system noise. This can be done by
`
`modulating the sensor's light source with a square wave, by pulsing it on and off. The
`
`detected signals are then band pass filtered to attenuate interference outside the
`
`25
`
`frequency
`
`range of interest.
`
`Subsequent demodulation will
`
`recover
`
`the
`
`plethysmogram. In general, any periodic signal such as a sine wave may be used to
`
`modulate the light source.
`
`Though modulated light photoplethysmography exists in the prior art, there are still
`
`30
`
`critical limitations in how it has been applied, especially in terms of suitable signal
`
`conditioning circuits for attenuating or removing noise, and demodulation. For
`
`example, EP0335357, EP0314324, W00144780 and W09846125 disclose modulated
`
`light photoplethysmography. However, they use a demodulation method . and
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`apparatus that requires the modulating and demodulating carrier phase to be
`
`synchronised. Error in the synchronisation timing will add noise to the demodulated
`
`signal (timing jitter or phase noise). The prior art also fails to make full use of band
`
`pass filter characteristics to remove ambient interfering light, by still relying on a
`
`5
`
`separate channel to measure ambient light, and later subtracting it from the signal,
`
`which adds further complexity and is arguably less efficient at attenuating
`
`interference. These limitations reduce immunity to broadband and narrowband noise
`
`from sources such as fluorescent lighting, computer monitors, sunlight, incandescent
`
`light, electrical RF interference, thermal noise, flicker noise, and shot noise.
`
`10
`
`A further limitation in the prior art is the choice of wavelength for reflectance mode
`
`sensors. Both reflection mode and transmission mode sensors use light sources in the
`
`red and/or infrared part of the spectrum, wavelengths between 600nm and lOOOom.
`being typical. However, red I infrared reflectance sensors do not function well
`
`15
`
`because light at red and infrared wavelengths is poorly absorbed by blood. This
`
`results in low modulation of the reflected signal and therefore a small AC component.
`Therefore red I infrared reflectance probes give poor results when compared to
`
`transmittance probes. It has been shown in Weija Cui et al: "In Vivo Reflectance of
`
`Blood and Tissue as a Function of Light Wavelength", IEEE Transactions on
`
`20 Biomedical Engineering, Volume 37, No 6, June 1996), that a larger plethysmogram
`AC component amplitude can be recorded if a reflectance mode sensor uses light of
`
`wavelengths between 500nm and 600nm (green light).
`
`A continuous non-modulated green light photoplethysmograph was described in WO
`
`25
`
`9822018Al. However, the objective of this invention was reflectance pulse oximetry,
`
`and the patent does not explain the steps necessary to produce a reliable
`
`photoplethysmograph suitable for measuring the plethysmogram AC and DC
`
`component. Such a green light sensor would be necessary to reliably detect the AC
`
`component, for example heart rate, but moreover the breathing signal, which is
`
`30
`
`extremely small and was not detected by this system.
`
`In Benten et al: "Integrated synchronous receiver channel for optical instrumentation
`
`applications" Proceedings of SPIE - The International Society for Optical
`
`4
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`WO 2007/122375
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`PCT/GB2007/001355
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`Engineering, Volume 3100, 75-88, 1997), a modulated
`
`light reflectance
`
`photoplethysmograph is described that uses a switching multiplier to systematically
`change· the gain of the signal path between + 1 and -1. This is the equivalent of
`
`mixing the modulated signal with a square wave. to recover the plethysmogram.
`
`5 However, similar to the other prior art described previously, this method needs the
`
`modulating carrier and demodulating local oscillator signals to be in-phase.
`
`It is an object of the present invention to provide an improved plethysmograph.
`
`10 According to one aspect, the present invention provides a photoplethysmograph
`
`device comprising:
`
`a light source for illuminating a target object;
`
`a modulator for driving the light source such that the output intensity varies as
`
`a function of a modulation signal at a modulation frequency;
`
`15
`
`a detector for receiving light from the target object and generating an electrical
`
`output as a function of the intensity of received light;
`
`a demodulator for receiving the detector output, having a local oscillator and
`
`producing a demodulated output representative of the modulation signal and any
`
`sidebands thereof, in which the demodulator is insensitive to any phase difference
`
`20
`
`between the modulation signal and the oscillator of the demodulator; and
`
`means for generating, from the demodulated output, a signal indicative of
`
`blood volume as a function of time and I or blood composition.
`
`According to another aspect, the present invention provides a method of generating a
`
`25
`
`plethysmogram, comprising the steps of:
`
`illuminating a target object with a light source;
`
`driving the light source with a modulator sucb tbat the output intensity varies
`
`as a function of a modulation signal at a modulation frequency;
`
`receiving light from the target object with a detector and generating an
`
`30
`
`electrical output as a function of the intensity of received light;
`
`receiving the detector output in a demodulator having a local oscillator and
`
`producing a demodulated output representative of the modulation signal and any
`
`5
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`PCT/GB2007 /001355
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`sidebands thereof, in which the demodulator is insensitive to any phase difference
`
`between the modulation signal and the oscillator of the demodulator; and
`
`generating, from the demodulated output, a signal indicative ofblood volume
`as a function of time and I or blood composition.
`
`5
`
`According to another aspect, the present invention provides a photoplethysmograph
`
`device comprising:
`
`one or more light sources each for illuminating a portion of a target object;
`
`one or more modulators for driving the light sources such that the output
`
`10
`
`intensity of each light source varies as a function of a modulation signal at a
`
`modulation frequency;
`
`one or more detectors for receiving light from the target object and generating
`
`one or more electrical outputs as a function of the intensity of received light;
`
`a plurality of demodulators each for receiving one or more of the electrical
`
`15
`
`outputs and producing a demodulated output representative of the modulation signal
`
`of one of the modulated light sources and any sidebands thereof, to thereby produce a
`
`plurality of demodulated outputs corresponding to the plurality of light sources and/or
`
`plurality of detectors; and
`
`means for generating, from the demodulated outputs, plethysmogram signals
`indicative of blood volume as a function of time and I or blood composition for each
`
`20
`
`of the demodulator outputs.
`
`According to another aspect, the present invention provides a method of generating a
`
`plethysmogram, comprising the steps of:
`
`25
`
`illuminating a portion of a target object with one or more light sources;
`
`driving the light sources with one or more modulators such that the output
`
`intensity of each light source varies as a function of a modulation signal at a
`
`modulation frequency;
`
`receiving light from the target object with one or more detectors and
`
`30
`
`generating one or more electrical outputs as a function of the intensity of received
`
`light;
`
`receiving one or more of the ~lectrical outputs with a plurality of
`
`demodulators, each producing a demodulated output representative of the modulation
`
`6
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`

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`wo 2007/122375
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`PCT/GB2007 /001355
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`signal of one of the modulated light sources and any sidebands thereof, to thereby
`
`produce a plurality of demodulated outputs corresponding to the plurality of light
`
`sources; and
`
`generating, from the demodulated outputs, plethysmogram signals indicative
`of blood volume as a function of time and I or blood composition for each of the
`
`5
`
`demodulator outputs of the pixel array.
`
`According to another aspect, the present invention provides a photoplethysmograph
`
`device for non-contact use, comprising:
`
`10
`
`a light source for illuminating a target object via a first polarising filter;
`
`a modulator for driving the light source such that the output intensity varies as
`
`a function of a modulation signal at a modulation frequency;
`
`a detector for receiving light from the target object via a second polarising
`
`filter having a different polarisation state than the first polarising filter, the detector
`
`15
`
`adapted to generate an electrical output as a function of the intensity of received light;
`
`a demodulator for receiving the detector output and producing a demodulated
`
`output representative of the modulation signal and any sidebands thereof; and
`
`means for generating, from the demodulated output, a signal indicative of
`blood volume as a function of time and I or blood composition.
`
`20
`
`According to another aspect, the present invention provides a method of generating a
`
`photoplethysmogram, comprising the steps of:
`
`illuminating a target object with a light source via a first polarising filter;
`
`driving the light source with a modulator such that the output intensity varies
`
`25
`
`as a function of a modulation signal at a modulation frequency;
`
`receiving light from the target object with a detector via a second polarising
`
`filter having a different polarisation state than the first polarising filter, the detector
`
`generating an electrical output as a function of the intensity of received light;
`
`receiving the detector output with a demodulator and producing a demodulated
`
`30
`
`output representative of the modulation signal and any sidebands thereof; and
`
`generating, from the demodulated output, a signal indicative of blood volume
`as a function of time and I or blood composition.
`
`7
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`

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`According to another aspect, the present invention provides a photoplethysmograph
`
`device for non-contact use, comprising:
`
`a light source for illuminating a target object with optical radiation of
`
`wavelength less than 600 nm;
`
`5
`
`a modulator for driving the light source such that the output intensity varies as
`
`a function of a modulation signal at a modulation frequency;
`
`a detector for receiving light from the target object and adapted to generate an
`
`electrical output as a function of the intensity of received light, the light source and
`
`detector being disposed laterally adjacent to one another on a substrate such that the
`
`10
`
`active surfaces thereof can be directed towards substantially the same point on a
`
`surface of the target body;
`
`a demodulator for receiving the detector output and producing a demodulated
`
`output representative of the modulation signal and any sidebands thereof; and
`
`means for generating, from the demodulated output, a signal indicative of
`blood volume as a function of time and I or blood composition.
`
`15
`
`According to another aspect, the present invention provides a method of generating a
`
`photoplethysmogram, comprising the steps of:
`
`illuminating a target object with optical radiation of wavelength less than 600
`
`20
`
`nm from a light source;
`
`driving the light source with a modulator such that the output intensity varies
`
`as a function of a modulation signal at a modulation frequency;
`
`receiving light from the target object with a detector to generate an electrical
`
`output as a function of the intensity of received light, the light source and detector
`
`25
`
`being disposed laterally adjacent to one another on a substrate such that the active
`
`surfaces thereof can be directed towards substantially the same point on a surface of
`
`the target body;
`
`receiving the detector output with a demodulator and producing a demodulated
`
`output representative of the modulation signal and any sidebands thereof; and
`
`30
`
`generating, from the demodulated output, a signal indicative ofblood volume
`as a function of time and I or blood composition.
`
`8
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`

`

`wo 2007/122375
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`PCT/GB2007 /001355
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`The invention provides a modulated light photoplethysmograph device. In selected
`
`embodiments, it combines the features of modulated light, band pass filtering, and IQ
`
`demodulation to give a plethysmogram of perfuse tissue. When used in reflectance
`
`mode, light in the blue and/or green portion of the optical spectrum is used which
`
`5
`
`gives a larger pulsatile signal and improved signal to noise ratio.
`
`Selected embodiments of the invention provide improved reliability through the
`
`reduction of noise when the photoplethysmograph device is used in transmission
`In addition, the choice of light in the blue I green portion of the optical
`mode.
`
`10
`
`spectrum (i.e. wavelengths ofbetween 400 run and 600nm) gives improved reliability
`
`through the reduction of noise and the increase in AC component signal amplitude,
`
`when the photoplethysmograph device is used in reflection mode.
`
`Selected embodiments can be applied to different photoplethysmography techniques
`
`15
`
`including
`
`single wavelength
`
`photoplethysmography, multiple wavelength
`
`photoplethysmography, pixel array photoplethysmography, and non-contact
`
`photoplethysmography.
`
`Embodiments of the present invention will now be described by way of example and
`
`20 with reference to the accompanying drawings in which:
`
`Figure 1
`
`is a
`
`functional block diagram of a single wavelength
`
`photoplethysmograph device;
`
`Figure 2 is a functional block diagram of a demodulator suitable for use in the
`
`photoplethysmograph device of figure 1;
`
`25
`
`Figure 3 is a
`
`functional block diagram of a multiple wavelength
`
`photoplethysmograph device;
`Figure 4 is a schematic plan view of a pixel array photoplethysmograph
`
`device;
`
`Figure Sa is a schematic side view of a non-contact photoplethysmograph
`
`30
`
`device with polarising filters;
`
`Figure 5b is a plan view of a polarising filter for use with the reflectance mode
`
`photoplethysmograph device of figure 7;
`
`9
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`PCT/GB2007 /001355
`
`Figure 6
`
`is a
`
`functional block diagram of a single wavelength
`
`photoplethysmograph device;
`Figure 7 is a schematic plan view, side view and end view of a reflectance
`
`mode photoplethysmograph device;
`
`5
`
`Figure 8 is a circuit diagram of a transimpedance amplifier suitable for use in
`
`the photoplethysmograph devices described herein;
`
`Figure 9 is a circuit diagram of a band pass filter circuit suitable for use in the
`
`photoplethysmograph devices described herein;
`Figure 10 is a circuit diagram of a light source chiver circuit suitiibl~ for use in
`
`10
`
`the photoplethysmograph devices described herein;
`Figure 11 is a process flow diagram illustrating a demodulation algorithm
`
`suitable for use in the photoplethysmograph devices described herein;
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`Figure 12 is a functional block diagram of a light source brightness control
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`loop suitable for use in the photoplethysmograph devices described herein;
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`15
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`Figure 13a is a photoplethysmogram showing a combined AC and DC output
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`of a photoplethysmograph device;
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`Figure 13b is a photoplethysmogram showing the magnified AC component
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`from figure 13a;
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`Figure 14a is a photoplethysmograro showing combined pulsatile and
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`20
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`breathing signal;
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`Figure 14b is a photoplethysmogram showing the breathing signal of figure
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`14a only;
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`Figure 15a is a photoplethysmograro showing a breathing signal only;
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`Figure 15b is a corresponding breathing signal as measured by an oral
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`25
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`thennistor;
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`Figure 16 is a photoplethysmogram recorded using a green light source of
`wavelength 510 nm;
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`Figure 17 is a photoplethysmogram recorded using a red light source of
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`wavelength 644 nm;
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`30
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`Figure 18 is a functional block diagram of an alternative demodulator suitable
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`for use in the photoplethysmograph device of figure 1;
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`Figure 19 is a functional block diagram of an alternative demodulator. suitable
`
`for use in the photoplethysmograph device of figure 1;
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`Figure 20 is a functional block diagram of an alternative demodulator suitable
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`for use in the photoplethysmograph device of figure 1.
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`Single wavelength photoplethysmograph device
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`5 With reference to figure 1, a photoplethysmograph device 100 comprises a driver
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`circuit 101 which is coupled to energise a light source 102 with modulated drive
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`signal such that the output intensity of the light source varies as a function of a
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`modulation signal having a specific modulation frequ~ncy (fm) and modulation
`
`amplitude (M1(t)). The waveform driving the light source is therefore a modulating
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`10
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`carrier characterised by its frequency and amplitude.
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`The light source 102 is configured to illuminate a target object 103 such as an area of
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`tissue of the human or animal body. The light source 102 preferably comprises one or
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`more light emitting devices each of a given wavelength or range of wavelengths.
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`15
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`A photodetector 104 is configured to receive light from the target object 103 after its
`
`interaction therewith. Depending on the relative positioning of the light source 102,
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`the target object 103 and the photo detector 104, this . received light may be one or
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`more of light that bas been transmitted through the target object, light that bas been
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`20
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`reflected from the surface of the target object, and light that has been scattered by and
`I or reflected from structures or fluids within the target object. The photodetector will
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`generate an electrical current that is a function of, e.g. proportional to, the amount of
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`light incident to its active area.
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`25
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`A detector 105 may be provided to convert the electrical current from the
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`photodetector 104 to a voltage that is proportional to the current. The detector 105
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`may incorporate an amplifier (not shown). The gain of that amplifier can be rolled off
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`at a frequency greater than the modulation frequency. The detector 105 and amplifier
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`can, with careful design, minimise the noise at the input to a band pass filter 106
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`30
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`coupled thereto. In a general sense, the photodetector 104 and detector 105 functions
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`may be P!Ovided by any detector capable of receiving light from the target object and
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`generating an electrical output that is a. function of the intensity of the received light.
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`PCT/GB2007/00 1355
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`The band pass filter 106 may be provided for attenuating signals outside a bandwidth
`
`of interest. The filter bandwidth is preferably centred on the modulation frequency fm
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`and is sufficiently wide to pass the modulating carrier and sidebands caused by
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`plethysmogram amplitude modulation, but narrow enough to attenuate frequency
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`5
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`components of interference and noise. To reduce noise, the bandwidth of the band
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`pass filter 106 should be as narrow as possible. It need only be wide enough to pass
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`the upper and lower sidebands of the plethysmogram, typically but not limited to 50
`Hz. The band pass filter 106 may incorporate an amplifier (not shown) to provide
`
`additional gain. The band pass filter 106 and amplifier are preferably designed tu
`10 minimise noise at the input of the following stage, namely a demodulator 107. It will
`
`be appreciated that the provision of a band pass filter 106 is not always necessary but,
`if employed, an increase in signal-to-noise ratio (SNR) may result.
`
`A preferred arrangement of demodulator 107 is shown in more detail in figure 2. The
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`15
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`demodulator 107 is adapted to demodulate the output of the band pass filter 106 and
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`hence recover a plethysmogram from the detected light received from the target
`
`object. The preferred demodulator 107 uses a method that is insensitive to the phase
`difference between the modulation carrier and a demodulation carrier. In other words,
`the demodulator is insensitive to any phase difference between the modulation signal
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`20
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`and an oscillator in the demodulator, as will be explained later. Thus, it is
`
`unnecessary to maintain a predetermined phase relationship between the modulation
`
`and demodulation process.
`
`The demodulator 107 may comprise a multiplexer 210 for splitting the modulated
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`25
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`signal Ml (t) into two channels. A first channel processes a first modulated input
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`signal M1 (t)a and a second channel processes a second modulated input signal
`
`Ml(t)b. The first modulated input signal Ml(t)a is provided as input to a first
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`multiplier 201 together with an output of a first demodulator local oscillator (LO)
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`signal 204, D1(t). The frequency of the local oscillator signal 204 is preferably
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`30
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`substantially equal to the frequency of the modulation signal and therefore equal to
`
`the modulating carrier frequency of input signal Ml (t). The result of the
`multiplication of Ml (t)a with the first LO signal 204 is an I ('in phase') signal. In the
`
`second channel, the second modulated input signal is multiplied, using a multiplier
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`206, with a second demodulator local oscillator (LO) signal that also has a frequency
`
`preferably substantially equal to the frequency of the modulation signal. However,
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`the second demodulator LO signal is phase shifted by phase shifter 205 with respect
`
`to the first demodulator LO signal. The phase difference between the first
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`5
`
`demodulator LO and second demodulator LO is preferably 90 degrees. The result of
`the multiplication of M1(t)b with the second demodulator LO signal is the Q
`('quadrature phase') signal. It will be understood that the local oscillator, although
`
`shown as a producing a sine wave output, could produce other waveforms of the
`
`required frequency.
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`10
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`The separate I and Q signals are preferably separately low pass filtered in filter
`
`elements 202 and 207 respectively to remove unwanted harmonics and products of the
`
`multiplication process. Optionally, the resulting signals may be decimated in
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`decimators 203 and 208 respectiv