Photoplethysmograph (PPG) was developed in the 1960's and 1970's by
psychophysiology researchers. The instrument is used for assessment of peripheral blood
flow to the extremities. When the client is stressed their blood vessels constrict (less
blood flow in fingers). Photoplethysmograph is based upon the premise that all living
tissue and blood have different light-absorbing properties. PPG works by placing an
individual finger between two parts of a transducer consisting of a light source and a
photocell (which converts light to electrical energy). A beam of infrared light is projected
toward the photocell. The blood in the finger scatters light in the infrared range, and the
amount of light reaching the cell is inversely related to the amount of blood in the finger.
Hence, when blood vessels in the finger dilate, the increased blood flow allows less light
to reach the photocell, when blood vessels constrict, blood flow is decreased and
increased light reaches the photocell.
Pulse volume measurements are related beat-to-beat variations in the force of blood flow.
Phasic changes made on a beat-to-beat basis, are called pulse volume measurements and
are related to beat variations in the force of blood flow. These beat-to-beat changes in
peripheral blood flow reflect ANS regulatory activity and can be used for the purpose of
evaluation of sympathetic activity or stress. (“Measuring Stress” by S. Cohen, R. Kessler
and L. U. Gordon, 1995)
Using device right
Biocom HRM-02 is an ear-clip pulse wave sensor connected to PC via USB port.
Typically the ear-clip should be placed on earlobe.
In some cases when earlobe placement does not give good pulse wave signal the ear-clip can be placed on a finger.