🚀 go-pugleaf

I'm placing this question in the sci.physics group, since it
is a "how does it work question', of the type physicists (particularly
bio-physicists) might find interesting.

I recently purchased an exercise treadmill that comes with a heart beat
monitor. The monitor's probe attaches to one's EAR. The probe is a
clothes-pin like device that may contain light rather than pressure
sensors. I am making this guess that light sensitivity is involved because
the heart beat monitor refuses to function in bright sun. Also, the
sensors in the probe are recessed, do not come in contact with the skin,
and resemble two tiny light bulbs. 

If my assessment is incorrect, then what mechanism IS being used by these
inexpensive probes for pulse (heartbeat) detection? 

--Thanks,

  Bill Angel