(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY l PCT)
`
`(19) World Intellectual Property Organization -
`International Bureau
`'
`
`(43) International Publication Date
`1 November 2007 (01.11.2007)
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`(10) International Publication Number
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`W0 2007/ 12237 5 A2
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`(51) International Patent Classification:
`AME Sill-£55 (2W601)
`A618 5’08 (2006.01)
`A613 5.3024 (2006.0l)
`
`(74) Agent: CHARIG, Raymond; liric Potter (Ilarkson 1.].1’.
`Park View House. 58 The Ropcwalk. Nottingham NS]
`SDD (GB).
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`(21)
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`IllTvemaTvim'laj APPHWlion Nun‘beri V _
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`11 Apfil2007t|1.04.2007)
`[ingfish
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`(22) International Filing Date:
`(35) Filing Language:
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`English
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`t81) Designated States (titties: otherwise indicated. for every
`kind of national protection available}: AE. AG. AL. AM.
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`{84) Diningnatcd' states (unless otherwise indicated. for every
`kind nfmgimirit protection available): ARl'PO (BW. Gll,
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`GA. GN, GQ, GW. ML. MR. NE. SN. TD. TG).
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`(26) Publication Language:
`I
`.
`_
`‘3") Pr'"“‘-" "3”"
`“3
`11 AW" 300“ U "0430“)
`“5073705
`(71) Applicant [for all designated States except US}: THE
`UNIVERSITY OF NOTTINGHAI“ [GBIGBR Univcp
`my Park‘ leingham NG? 2RD (GBI
`[72) Inventors; and
`[75)
`lnventorst'Applicants (for US only): CROWE, John
`[(33033]; School of Electrical and Electronic Engineer
`ing. The University of Nottingham. University Park,
`Nottingham NG7 2RD (GB). GRUBB. Mark lGBKiBI;
`School of Electrical and lilectronic Engineering. The
`University of Nottingham. University Park, Nottingham Published:
`NG? 2RD (GB). HAYESflfliL, Barrie [GBJ'GB]; School
`without international scrim}! report and to be republished
`of Electrical and Electronic Engineering. The University
`upon receipt of that report
`of Nottingham, University Park, Nottingham NUT 2RD
`(GB). MILES. Nicolas lGBt’GB]: School ot'lilectrical and
`Electronic Engineering, The University of Nottingham,
`Universin Park. Nottingham NG't' 2RD (GB).
`
`For terJetter codes and other abbreviations. refer to the "Guide
`mice Notes on Codes and Abbreviations " appearing tit the begin-
`ning ofeach regular issue ofthe PCT Gazette.
`
`(54) Title: PHO’t‘om.n't'HstOGRAPHv
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`(57) Abstract: A photoplethysmograph device includes a light source for illuminating a target object. A modulator drives the light
`source such that the output intensityr varies as a function of a modulation signal at a modulation frequency. A detector receives light
`from the target object and generates an electrical output as a function of the intensity of received light. A demodulator with a local
`: oscillator receives the detector outth and produces a demodulated output representative of the modulation signal. The demodulator
`c is insensitive to any phase difference between the modulation signal and the oscillator of the demodulator.
`lirom the demodulated
`N output. a signal indicative of blood volume as a function of time and for blood composition is generated. A number ol'dcmodulators
`may be provided to derive signals from multiple light sources of different wavelengths, or from an array ofdelectors. The plethys-
`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 spectnim and may use pelarising filters.
`
`0001
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`Apple Inc.
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`US. Patent No. 8,923,941
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`Apple Inc.
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`PHOTOPLETHYSMOGRAPHY
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`The present invention relates photoplethysrnography and in particular to a method and
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`apparatus for measuring pulse rate, breathing rate and blood constituents in the human
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`or animal body.
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`The word plethysrnography is a combination of the Greek words Piethysmos,
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`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.
`Photoplethysmography (hereinafter also referred to as 'PPG') refers to the use of light
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`to measure these changes in volume, and therefore a photoplethysmograph is an
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`instrument, method or apparatus that uses light to perform these measurements.
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`Although the human or animal body is generally assumed to he opaque to light, most
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`soft
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`tissue will
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`transmit and reflect both visible and near-infrared radiation.
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`Therefore, if light is projected onto an area of skin and the emergent light is detected
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`after its interaction with the slcin, blood and other tissue, time varying changes of light
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`intensity havhig a relation with blood volume, known as the plethysmogram, can be
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`observed. This time varying light intensity signal will depend on a number of factors
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`including the optical properties of the tissues and biood at the measurement site, and
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`the wavelength of the light source. The signal results because blood absorbs light and
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`the amount of light absorbed, and hence the intensity of remaining light detected,
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`varies in relation with the volume of blood illuminated.
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`Variation in the
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`plethysmogram is caused by the variation in blood volume flowing in the tissue.
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`This technique was introduced in 1937 by Hertzman. He was the first to use the term
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`photoplethysmcgraphy and suggested that the resultant piethysmograrn represented
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`volumetric changes of blood in the skin's vessels.
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`The plethysmogram is usually described with respect to its AC and DC components.
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`The absorption of light by uon—pulsatile blood, bone and tissue is assumed to be
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`constant and giVes rise to the DC component. The DC component represents the
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`volume of non-pulsatile blood below the sensor, plus light reflected and scattered off
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`the skin, bone and other tissues. The AC component is caused by the time varying
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`absorption of light caused by temporal changes in blood volume below the sensor.
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`Changes in the blood volume can be caused by cardiovascular regulation, blood
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`pressure regulation, fliermoregulation and respiration. Thus the ple’diysmogram can
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`be analysed to determine mforrnation on such parameters as pulse rate, breathing rate,
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`blood pressure, perfusion, cardiac stroke volume and respiratory tidal volume. These
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`can be observed as periodic and non-periodic changes in the amplitude of AC and DC
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`components in the plethysmogram. This has been described in more detail in Kamal
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`et at:
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`'Skz'n Phoroplerhysmography — a review’, Computer Methods and Programs in
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`Biomedicine, 28 (1989) 257-269).
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`The plethysmogram can also he analysed to
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`determine blood constituents.
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`One such technique is pulse oximetry, which
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`determines the relative amount of oxygen in the blood. Other blood constituents can
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`also be measured by using photoplethysmography.
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`There are two modes of photoplethysmography, the transmission mode and the
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`reflection mode. In transmission mode the light source is on one side of the tissue and
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`the photodetector is placed on the other side, opposite the light source. The use of
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`transmission mode is limited to areas where the tissue is thin enough to allow light to
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`propagate, for example the fingers, toes and earlobes of a human subject.
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`In reflection mode the light source and photodetector are place side-by—side. Light
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`entering the tissue is reflected and a proportion of this is detected at the photodetector.
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`This source—detector configuration is more versatile and allows measurements to be
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`performed on almost any area of tissue. However, the use of reflectance mode is
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`much harder to design than transmission because the signal level is significantly lower
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`at the most effective wavelengths. Thus, considerable attention must be given to
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`maximising signal-to-noise ratio. As a result, the most common PPG sensors use
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`transmission mode and hence are restricted to positions where light can pass through
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`tissue.
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`As a photodetector is used to measure light from the source, the photoplethysmograph
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`can also respond to interfering signals from other sources of light, for' example
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`fluorescent lighting and computer monitors. The sensor must also respond to changes
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`in the light propagating through tissue, i.e. the plethysmo gram. These physiological
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`changes contain frequency components between DC and 25 Hz. However, it is
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`desirable for the sensor not to respond to ambient light noise. Accordingly, the
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`phot0plethysmograph should reject ambient
`plethysmogram in the bandwidth ofinterest.
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`light noise while detecting the
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`A second source of interference is other electrical apparatus. Other electrical devices
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`can generate radio frequency signals that a photoplethysmograph can detect.
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`It is
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`desirable to minimise the Sensitivity of the system to interfering sources of this nature.
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`A third source of
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`interference
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`is
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`the
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`electrical noise generated by the
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`photoplethysmo graph itself. Such noise can be generated by electronic components,
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`and can include thermal noise, flicker noise, shot noise, as Well as noise spikes, for
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`example, harmonics generated by missing codes in an analogue-to-digital converter.
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`It is also desirable to minimise the sensitivity of the system to interference from these
`SOUICB$
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`A known technique for reducing the noise generated by these three sources of
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`interference is to drive the sensor's light source with a carrier modulated at a
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`frequency that is not present, or dominant,
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`in the ambient
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`light, electrical radio
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`frequency signals, 'or photoplethysmograph system noise. This can be done by
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`modulating the sensor’s light source with a square wave, by pulsing it on and off. The
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`detected signals are then band pass filtered to attenuate interference outside the
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`fiequency range of
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`interest.
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`Subsequent demodulation will
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`recover
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`the
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`plethysmogram.
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`In general, any periodic signal such as a sine wave may be used to
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`modulate the light source.
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`Though modulated light photoplethysmography exists in the prior art, there are still
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`critical limitations in how it has been applied, especially in terms of suitable signal
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`conditioning circuits for attenuating or removing noise, and demodulation. For
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`example, EP0335357, EPG314324, W00144780 and W09846125 disclose modulated
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`light phot0plethysmography. However,
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`they use a demodulation method. and
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`apparatus that requires
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`the modulating and demodulating carrier phase to be
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`synchronised. Error in the synchronisation timing will add noise to the demodulated
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`signal (timing jitter or phase noise). The prior art also fails to make full use of band
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`pass filter characteristics to remove ambient interfering light, by still relying on a
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`separate channel to measure ambient light, and later subtracting it fi'om the signal,
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`which adds
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`filrther complexity and is arguably less efficient at attenuating
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`interference. These limitations reduce immunity to broadband and narrowband noise
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`from sources such as fluorescent lighting, computer monitors, sunlight, incandescent
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`light, electrical RF interference, thermal noise, flicker noise, and shot noise.
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`A further limitation in the prior art is the choice of wavelength for reflectance mode
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`sensors. Both reflection mode and transmission mode sensors use light sources in the
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`red andfor infrared part of the spectrum, wavelengths between 600nm and lOOOnrn
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`being typical. However, red l infrared reflectance sensors do not function well
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`because light at red and infrared wavelengths is poorly absorbed by blood. This
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`results in low modulation of the reflected signal and therefore a small AC component.
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`Therefore red / infrared reflectance probes give poor results when compared to
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`transmittance probes. It has been shown in Weija Cui er al: “In Vivo Reflectance of
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`Blood and Tissue as a Function of Light Wavelength”,
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`IEEE Transactions on
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`Biomedical Engineering, Volume 37, No 6, June 1996), that a larger plethysmogram
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`AC component amplitude can be recorded if a reflectance mode sensor uses light of
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`wavelengths between 500nm and 600nm (green light).
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`A continuous non-modulated green light photoplethysmograph was described in W0
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`9822018Al. HUWever, the objective of this invention was reflectance pulse oximetry,
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`and the patent does not explain the steps necessary to produce a reliable
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`photoplethysmograph suitable for measuring the plethysmogram AC and DC
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`component. Such a green light sensor would be necessary to reliably detect the AC
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`component, for example heart rate, but moreover the breathing signal, which is
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`extremely small and was not detected by this system.
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`In Benton er al: “Integrated synchronous receiver channelfor optical instrumentation
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`applications” Proceedings of SPIE — The International Society for Optics]
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`Engineering,
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`Volume
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`3100,
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`75—88,
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`1997),
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`3. modulated
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`light
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`reflectance
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`photcplethysmograph is described that uses a switching multiplier to systematically
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`change‘the gain of the signal path between +1 and —1. This is the equivalent of
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`mixing the modulated signal with a square wave to recover the plethysmogram.
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`However, similar to the other prior art described previously, this method needs the
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`modulating carrier and demodulating local oscillator signals to be iii-phase.
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`It is an object of the present invention to provide an improved plethysmograph.
`
`According to one aspect,
`
`the present
`
`invention provides a photoplethysmograph
`
`device comprising:
`
`a light source for illuminating a target object;
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`a modulator for driving the light source such that the output intensity varies as
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`a function of a modulation signal at a modulation frequency;
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`a detector for receiving light from the target object and generating an electrical
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`output as a function of the intensity of received light;
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`a demodulator for receiving the detector output, having a local oscillator and
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`producing a demodulated output representative of the modulation signal and any
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`sidebands thereof, in which the demodulator is insensitive to any phase difference
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`between the modulation signal and the oscillator of the demodulator; and
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`means for generating, from the demodulated output, a signal indicative of
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`blood volume as a function of time and f or blood composition.
`
`According to another aspect, the present invention provides a method of generating a
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`plethysmograrn, comprising the steps of:
`
`illuminating a target object with a light source;
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`driving the light source with a modulator such that the output intensity varies
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`as a fimction of a modulation signal at a modulation frequency;
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`receiving light from the target object with a detector and generating an
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`electrical output as a function of the intensity of received light;
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`receiving the detector output in a demodulator having a local oscillator and
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`producing a demodulated output representative of the modulation signal and any
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`sidebands thereof, in which the demodulator is insensitive to any phase difference
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`between the modulation signal and the oscillator of the demodulator; and
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`generating, from the demodulated output, a signal indicative of blood volume
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`as a function of time and / or blood composition.
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`According to another aspect, the present invention provides a photoplethysmograph
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`device comprising:
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`one or more light sources each for illuminating a portion of a target object;
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`one or more modulators for driving the light sources such that the output
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`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
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`one or more electrical outputs as a function of the intensity of received light;
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`a plurality of demodulators each for receiving one or more of the electrical
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`outputs and producing a demodulated output representative of the modulation signal
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`of one of the modulated light sources and any sidebands thereof, to thereby produce a
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`plurality of demodulated outputs corresponding to the plurality of light sources and!or
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`plurality of detectors; and
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`means for generating, from the demodulated outputs, plethysmogram signals
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`indicative of blood volume as a function of time and f or blood composition for each
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`of the demodulator outputs.
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`According to another aspect, the present invention provides a method of generating a
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`plethysmogram, comprising the steps of:
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`illuminating a portion of a target object with one or more light sources;
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`driving the light sources with one or more modulators such that the output
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`intensity of each light source varies as a function of a modulation signal at a
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`modulation frequency;
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`receiving light from the target object with one or more detectors and
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`generating one or more electrical outputs as a fimction of the intensity of received
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`light;
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`receiving one or more of the electrical outputs with a plurality of
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`demodulators, each producing a demodulated output representative of the modulation
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`signal of one of the modulated light sources and any sidebands thereof, to thereby
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`produce a plurality of demodulated outputs corresponding to the plurality of light
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`sources; and
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`generating, from the demodulated outputs, plethysmogram signals indicative
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`of blood volume as a function of time and X or blood composition for each of the
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`demodulator outputs of the pixel array.
`
`According to another aspect, the present invention provides a photoplethysmograph
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`device for non—contact use, comprising:
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`alight source for illuminating a target object via a first polarising filter;
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`a modulator for driving the light source such that the output intensity varies as
`
`a function of a modulation signal at amodulation frequency;
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`a detector for receiving light from the target object via a second polarising
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`filter having a different polarisation state than the first polarising filter, the detector
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`adapted to generate an electrical output as a function of the intensity of received light;
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`a demodulator for receiving the detector output and producing a demodulated
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`output representative of the modulation signal and any sidebands thereof; and
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`means for generating, from the demodulated output, a signal indicative of
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`blood volume as a function of time and f or blood composition.
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`According to another aspect, the present invention provides a method of generating a
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`photoplethysmogram, comprising the steps of:
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`illuminating a target object with a light source via a first polarising filter;
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`driving the light source with a modulator such that the output intensity varies
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`as a function of a modulation signal at a modulation frequency;
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`receiving light from the target object with a detector via a second polarising
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`filter having a different polarisation state than the first polarising filter, the detector
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`generating an electrical output as a function of the intensity of received light;
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`receiving the detector output with a demodulator and producing a demodulated
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`output representative of the modulation signal and any sidebands thereof; and
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`generating, from the demodulated output, a signal indicative of blood volume
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`as a function of time and f or blood composition.
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`According to another aspect, the present invention provides a photoplethysmograph
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`devicc for non-contact use, comprising:
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`a light source for illuminating a target object with optical
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`radiation of
`
`wavelength less than 600 nm;
`
`a modulator for driving the light source such that the output intensity varies as
`
`a function of a modulation signal at a modulation frequency;
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`a detector for receiving light from the target object and adapted to generate an
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`electrical output as a function of the intensity of received light, the light source and
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`detector being disposed laterally adjacent to one another on a substrate such that the
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`active surfaces thereof can be directed towards substantially the same point on a
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`surface of the target body;
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`a demodulator for receiving the detector output and producing a demodulated
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`output represtative of the modulation signal and any sidebands thereof; and
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`means for generating, from the demodulated output, a signal indicative of
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`blood volume as a function of time and/ or blood composition.
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`According to another aspect, the present invention provides a method of generating a
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`photoplethysmogram, comprising the steps of:
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`illuminating a target object with optical radiation of wavelength less than 600
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`20
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`nm fi'om a light source;
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`driving the light source with a modulator such that the output intensity varies
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`as a function of a modulation signal at a modulation fi‘equency;
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`receiving light from the target object with a detector to generate an electrical
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`output as a function of the intensity of received light, the light source and detector
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`being disposed laterally adjacent to one another on a substrate such that the active
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`surfaces thereof can be directed towards substantially the same point on a surface of
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`the target body;
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`receiving the detector output with a demodulator and producing a demodulated
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`output representative of the modulation signal and any sidehands thereof; and
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`generating, from the demodulated output, a signal indicative of blood volume
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`as a fitnetion of time and f or blood composition.
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`The invention provides a modulated light photoplethysmograph device.
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`In selected
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`embodiments, it combines the features of modulated light, band pass filtering, and IQ
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`demodulation to give a plethysmogram of perfuse tissue. When used in reflectance
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`mode, light in the blue and/or green portion of the optical spectrum is used which
`
`gives a larger pulsatiie 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
`
`mode.
`
`In addition, the choice of light in the blue K green portion of the optical
`
`spectrum (i.e. wavelengths of between 400 nm 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
`
`including
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`single wavelength
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`photoplethysmography, multiple wavelength
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`photoplethysmography,
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`pixel
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`array
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`photoplethysmography,
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`and
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`non-contact
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`photoplethysmography.
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`Embodiments of the present invention will now be desoribed by way of example and
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`with reference to the accompanying drawings in which:
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`Figure
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`1
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`is
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`a
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`fiinctional block diagram of
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`a
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`single wavelength
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`photoplethysmograph device;
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`Figure 2 is a functional block diagram of a demodulator suitable for use in the
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`photoplethysmograph device of figure 1;
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`Figure 3
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`is
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`a
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`functional block diagram of a multiple wavelength
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`photoplethysmograph device;
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`Figure 4 is a schematic plan view of a pixel array photoplethysmograph
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`device;
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`10
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`15
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`20
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`25
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`Figure 5a is a schematic side view of a non—contact photoplethysmograph
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`30
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`device with polarising filters;
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`Figure 5b is a plan view of a polarisng filter for use with the reflectance mode
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`photoplethysmo graph device of figure 7;
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`Figure
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`6
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`is
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`a
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`functional
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`block diagram of a
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`single wavelength
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`photoplethysmograph device;
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`Figure 7 is a schematic plan View, side view and end View of a reflectance
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`mode photopiethysmograph device;
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`Figure 8 is a circuit diagram of a transimpedance amplifier suitable for use in
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`the photoplcthysmograph devices described herein;
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`Figure 9 is a circuit diagram of a band pass filter circuit suitable for use in the
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`photoplethysmograph devices described herein;
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`Figure 10 is a circuit diagram of a light source driver circuit suitable for use in
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`10
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`15
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`the photoplethysmograph devices described herein;
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`Figure 11 is a process flow diagram illustrating a demodulation algorithm
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`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 photoplethysrnograph devices described herein;
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`Figure 13a is a photoplethysrnogram 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 photoplethysmogram showing combined pulsatiie 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 photoPIethysmogram 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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`thermistor;
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`Figure 16 is a photoplethysmogram recorded using a green light source of
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`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 fimctiona] block diagram of an alternative demodulator suitable
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`for use in the photoplethysrnograph device of figure 1;
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`Figure 19 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 20 is a functional block diagram of an alternative demodulator suitable
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`for use in the photoplethysmo graph device of figure 1.
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`Single wavelength photoplethysmogmph device
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`With reference to figure I, a photoplethysmograph device 100 comprises a driver
`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 frequency (fm) and modulation
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`amplitude (Ml(t)). The waveform driving the light source is therefore a modulating
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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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`A photodetector 104 is configured to receive light from the target object 103 after its
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`interaction therewith. Depending on the relative positioning of the light source 102,
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`the target object 103 and the photodetector 104, this_received light may be one or
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`more of light that has been transmitted through the target object, light that has been
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`reflected from the surface of the target object, and light that has been scattered by and
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`f 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, eg. proportional to, the amount of
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`light incident to its active area.
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`10
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`15
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`20
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`25
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`A detector 105 may be provided to convert
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`the electrical current
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`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.
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`In a general sense, the photodetector 104 and detector 105 functions
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`may be provided 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 ofthe received light.
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`The band pass filter 106 may be provided for attenuating signals outside a bandwidth
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`of interest. The filter bandwidth is preferably centred on the modulation frequency fin
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`and is sufficiently wide to pass the modulating carrier and sidebauds caused by
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`plethysmogram amplitude modulation, but narrow enough to attenuate frequency
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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.
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`It need only be wide enough to pass
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`the upper and lower sidebands of the plethysmograrn, typically but not limited to 50
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`Hz. The band pass filter 106 may incorporate an amplifier (not shown) to provide
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`additional gain. The band pass filter 106 and amplifier are preferably designed to
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`10
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`minimise noise at the input of the following stage, namely a demodulator 107. It will
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`be appreciated that the provision of a band pass filter 106 is not always necessary but,
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`if employed, an increase in signal—to-noise ratio (SNR) may result.
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`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
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`object. The preferred demodulator 107 uses a method that is insensitive to the phase
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`difference between the modulation carrier and a demodulation carrier. In other words,
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`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.
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`Thus,
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`it
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`is
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`unnecessary to maintain a predetermined phase relationship between the modulation
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`and demodulation process.
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`The demodulator 107 may comprise a multiplexer 210 for splitting the modulated
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`25
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`30
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`signal M16) into two channels. A first channel processes a first modulated input
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`signal Ml(t)a and a second channel processes a second modulated input signal
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`M1(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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`substantially equal to the frequency of the modulation signal and therefore equal to
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`the modulating carrier
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`frequency of input signal M16).
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`The result of the
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`multiplication of Ml(t)a with the first LO signal 204 is mil (‘in phase’) signal. In the
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`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
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`preferably substantially equal to the fi'equency of the modulation signal. However,
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`the second demodulator LO signal is phase shifted by phase shifter 205 with respect
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`to the first demodulator LO signs].
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`The phase difference between the first
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`demodulator L0 and second demodulator L0 is preferably 90 degrees. The result of
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`the multiplication of M1(t)b with the second demodulator LO signal
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`is the Q
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`(‘quadrature phase') signal.
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`It will he understood that the local oscillator, although
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`shown as a producing a sine wave output, could produce other waveforms of the
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`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
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`elements 202 and 207 respectively to remove unwanted harmonics and products of the
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`multiplication process. Optionally,
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`the resulting signals may be decimated in
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`decimators 203 and 208 respectively to reduce the sample rate. The results of this are
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`15
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`the I’ and Q’ signals.
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`The I’ and Q’ sig