Methods and devices are provided for reducing motion artifacts when monitoring
volume changes in blood vessels. Light having a first wavelength and light having
a second wavelength are transmitted through a human appendage, toward the epidermis
of a patient, or through tissue within the body of a patient. A portion of the
light having the first wavelength and a portion of the light having the second
wavelength is received. A first signal is produced based on the received portion
of light having the first wavelength. A second signal is produced based on the
received portion of light having the second wavelength. One of the first and second
signals is subtracted from the other to produce a plethysmography signal that is
representative of volume changes in blood vessels of patient tissue.