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`PHOTODETECTOR‘SIZE CONSIDERATIONS IN THE DESIGN OF A
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`NONINVASIVE REFLECTANCE PULSE OXIMETER FOR TELEMEDICINE
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`APPLICATIONS
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`C. Pujary, M. Savage, Y. Mendelson
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`Department of Biomedical Engineering, and Bioengineering Institute
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`Worcester Polytechnic Institute, Worcester, MA 01609.
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`compromising
`Abstraci—Low power management without
`provided an easy way to simulate various PD areas by
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`signal quality is a key requirement in the optimal design of a
`connecting in parallel multiple PBS to the common
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`wearable pulse oximeter.
`This paper
`investigates
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`summing
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`a
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`advantage gained by using a photodetector with a larger active
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`Additional circuitry, consisting of amplifiers and band pass
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`area. Preliminary in viva experiments have demonstrated that
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`filters, were used to produce two different signals (a
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`by increasing the area of the photodetector it is possible to
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`pulsatile AC component and a non-pulsatile DC component)
`reduce the overall power requirement of a wireless sensor.
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`from each PPG. Analog data streams were digitized at
`intended for telemedicine applications.
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`501—12 for 30s periods by a National Instrument DAQ card
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`Keywords—pulse oxinieter, telemedicine, wearable sensor
`installed in a PC matting LABYIEW 6.0 software.
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`1. INTRODUCTION
`In- Vitro Experiments
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`Dark Tests: To test the background noise level generated
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`by each PD, we performed a series of dark measurements by
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`switching off the LEDs in the sensor and blocking ambient
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`light from reaching the six PDs.
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`Multiple Photodetectors Tests: Each PD was randomly
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`connected through the hub: ‘to determine
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`uniformity of the illuminating field incident on the PDs. To
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`produce a constant level background illumination, a signal
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`composed of a DC bias voltage modulated by a small lI-Iz
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`AC sine wave was generated by a programmable function
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`generator. The signal from the function generator was
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`applied to a separate LED that was used to simulate a
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`typical PPG signal. The external LED was attached to a
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`translucent flat medium serving as an optical difi'user. The
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`surface of the diffuser was positioned at a distance of 30cm
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`away from the planar surface of the sensor.
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`In- Viva Experiments
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`A series of in vivo experiments were performed to
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`determine the signal improvement gained by using different
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`PD areas. The prototype sensor was attached to the base of
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`a volunteer’5 finger and the peak currents supplied to the red
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`and infrared LEDs were adjusted to 3m and 1.9mA,
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`respectively As the driving currents were adjusted, ,the
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`output of each amplifier was monitored to assure that (i) a
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`distinguiShable and stable PPG was observed when a single
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`PD was employed, and (ii) maximal PPG signals were
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`produced without causing amplifier saturation when all 6
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`PDs we're connected in parallel through the hub.
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`important
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`significantly lower compared to the typical driving currents
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`employed in commercial pulse oximeters.
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`Commercially available sensors used in reflectance
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`mode pulse oximeters employ a single photodetector (PD)
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`element typically with an active area of about 12-15mm2.
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`The light
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`reflectance pulse oximeter depends on the incident light
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`intensity, absorption by skin, reflection by bones, tissue
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`backseattering and the amount of blood in the vascular bed
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`Compared to transmittance mode pulse oximetry,
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`reflected photoplethysmograms
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`lower amplitudes.
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`Low power management without compromising signal
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`quality is a key requirement in optimizing the design of a
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`wearable telesensor. One approach to lowering the power
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`consumption of a wireless pulse oximcter, which is
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`dominated by the current required to drive the LEDs, is to
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`reduce the LED duty cycle [2]. Alternatively, lowering the
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`current supplied to the red and infiared LEDs can also
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`reduce power consumption. However, with reduced current
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`drive, there is a direct impact on the quality of the detected
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`'PPGss Mendelson et a1. [3] showed that a concentric array
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`of PDs could be used to increase the
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`backseattered light detected-by a reflectance type pulse
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`oximeter sensor. In this paper We investigate the advantages
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`of increasing the PD area and minimizing LED driving
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`currents to optimize the overall power requirement of a
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`reflectance mode pulse oximeter
`intended for 'future
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`applications in telemedicine.
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`II. METHODOLOGY
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`Experimental setup
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`To study the effect of different PD areas, we constructed
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`and tested a prototype reflectance sensor employing 6
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`identical (3mm x 4mm) PDs. The" equally spaced PDs were
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`' concentrically arranged in a 18mm diameter planar
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`configuration around a pair of red and infrared LEDs. Each
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`PD was individually connected to‘a central hub. The hub
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`III. RESULTS
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`The Rent Mean Square (RMS) values corresponding to
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`the amplitude of the AC components measured frOm each
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`PD are plotted1n Fig.1. During darkness the average noise
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`0-7803-7767-21031'31 7.00 ©2003 IEEE
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`Page 1
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`VALENCELL EXHIBIT 2012
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`IPR2017—00318
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`Page 1
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`VALENCELL EXHIBIT 2012
`IPR2017-00318
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`9
`DCortpletedarhtess
`} IConatant light ilbani'tltion i
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`UCahullIed :5
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`PHOTODETECTOROUTPUT(V)
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`6-PDs
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`i i E !
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`I l
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`PHOTODETECTOROUTPUT(V)
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`5-PDs
`4-PDs
`2-PD!
`PIX
`PDQ
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`1-PD
`P03
`PBS
`F135
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`INDIVIDUAL PHOTODETECTORS
`MULTIPLE PHOTODETECTORS
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`Fig. 1. Individual photodetectar perfonnance under complete darkness and
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`a constant light illumination.
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`In
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`generated by the
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`comparison, the average PPG amplitude measured in vitro
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`by the 6 PBS under a spatially uniform illumination field
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`produced by the external LED source was 0.647V.
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`Fig. 2 shows the signals detected in vitro with multiple
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`combinations of PBS using the
`simulated uniform
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`illumination produced by the external LED. The right-side
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`bars represent the measured RMS values corresponding to
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`different PD areas. For comparison, the left-side bars were
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`computed
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`0.647n, where
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`corresponds to the number of PBS connected in parallel. The
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`trend observed provide sufiicient evidence of a linear
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`improvement in signal intensity as a filnction of an increase
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`in the active area of the PDs.
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`Fig. 3 shows the magnitude of the pulsatile red and
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`infrared PPGs measured
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`different PD
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`IV. DISCUSSION
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`Minimizing the current required to drive the LEDs is a
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`critical design consideration in optimizing the power
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`consumption of a wearable pulse oximeter. However,
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`reduced LED driving currents has a direct impact on the
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`incident light intensity produced by the sensor and could
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`lead to deterioration in the quality of
`the PPGs.
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`Consequently,
`it could result
`in unreliable and therefore
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`inaccurate calculation of oxygen saturation.
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`From the data presented in Fig. 3, it can be observed that
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`the overall increase in the reflected PPG signals achieved in
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`viva when all 6 PBS were connected in parallel is smaller
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`and does not follow the same linear relationship observed in
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`vitra as shown in Fig. 2. This deviation was most likely
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`caused by the non-uniform backseattered light distribution
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`emanating from the finger.
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`V. CONCLUSION
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`The data presented in this study demonstrate that the
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`driving currents of the LEDs in a reflectance pulse oximeter
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`canbe lowered significantly without compromising the
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`Fig. 2. Signal improvement observed in-virro with multiple photodetectars.
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`'i
`iElRedLED
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`4-5]
`41
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`2.51
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`I
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`PHOTODETECTOROUTPUT(V)
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`3-l’Ds
`é-FDa
`l—PD
`2-?De
`4-FDs
`S‘PDs
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`MULTIPLE PHOTODETECTORS
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`Fig. 3. Signal improvement observed in vivo with multiple photodetectors.
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`quality of the PPGs simply by increasing the overall size of
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`the PD in the sensor. Hence, with reduced LED driving
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`currents, maximizing the backseattered light collected by
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`the sensor and optimized digital switching techniques, a
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`very low power consuming sensor can be developed thereby
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`extending the overall battery life of a pulse oximeter
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`intended for telemedicine applications.
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`ACKNOWLEDGEMENT
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`This research was supported in part by Department of
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`Defense Cooperative Agreement DAMDl7-03-2-0006.
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`REFERENCES
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`[1] Y. Mendelson, B. Ochs, “Noninvasive Pulse Oximetry
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`Utilizing Skin Reflectance Photoplethysmography,” IEEE
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`Transactions on Biomedical Engineering, vol. 35, no. 10,
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`pp. 798-805, Oct. 1988.
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`[2] S. Rhee, B.H. Yang and H. Asada,” The Ring Sensor: A
`
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`new Ambulatory Wearable Sensor for twenty Four Hour
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`Patient Monitoring,” Proc. of the 20th Annual International
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`Conference ofthe IEE Engineering in Medicine and Biology
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`Society. Hong Kong. Oct. 1998.
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`[3] Y. Mendelson, 10. Kent, BL. Yocum and MJ. Birle,”
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`Design and Evaluation of new reflectance pulse oximeter
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`sensor,” Medical Instrumentation, Vol.22, NO. 4, pp. 167-
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`173, Aug. 1988.
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