A photoplethysmograph device includes a light source for illuminating a target object. A modulator drives the light source such that the output intensity 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 output and produces a demodulated output, insensitive to any phase difference between the modulation signal and the oscillator, indicative of blood volume as a function of time and/or blood composition. A number of demodulators may be provided to derive signals from multiple light sources of different wavelengths, or from an array of detectors. The plethysmograph 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 polarizing filters.