An apparatus includes a first electromagnetic sensor configured to generate a first sensed signal based at least in part on detection of a first signal having a first wavelength. The apparatus includes a second electromagnetic sensor configured to generate a second sensed signal based at least in part on detection of a second signal having a second wavelength different from the first wavelength. The apparatus includes a processing circuit configured to generate a plethysmogram based at least in part on the first sensed signal and the second sensed signal. The apparatus may include a first emitter configured to emit an optical signal having the first wavelength. The apparatus may include a second emitter configured to emit a reference signal. The first wavelength may be a human-blood-sensitive and human-skin-penetrable wavelength and the second wavelength may be at least one of a human-blood-insensitive wavelength and a human-skin-impenetrable wavelength.