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Photoplethysmographics using phase-division multiplexing

  • US 5,349,952 A
  • Filed: 03/05/1991
  • Issued: 09/27/1994
  • Est. Priority Date: 03/05/1991
  • Status: Expired due to Term
First Claim
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1. An instrument for evaluating the concentration of a constituent in an object by measuring the transmission of light of two wavelengths therethrough, comprising:

  • (a) first and second light emitter means for emitting light at respective, first and second different wavelengths;

    (b) modulator/driver means for driving the light emitter means simultaneously with respect first and second carriers which vary as a function of time, the carriers being continuous time-varying signals of the same carrier frequency and having a phase difference other than 0°

    degrees and other than an integer multiple of 180°

    degrees, wherein said first and second light emitter means have at least one time period when they are both substantially emitting;

    (c) detector means disposed to receive light from the first and second light emitter means after it has passed through the object, for generating a resulting detector signal carrying information relating to transmission of the object at both wavelengths;

    (d) demodulator means for generating, in response to the detector signal, in a first channel, a first demodulated signal which is a sum of a component proportional to the object'"'"'s transmission at the first wavelength and one or more carrier modulated components, and, in a second channel, a second demodulated signal which is a sum of a component proportional to the object'"'"'s transmission at the second wavelength and one or more carrier modulated components; and

    (e) a demodulated signal filter, coupled to said first and second channels, wherein said demodulated signal filter filters out carrier modulated components of the signals from the first and second channels.

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