Electronic – Optical heart rate sensor vs bioimpedance sensor


I didn't know a better place to ask this, so here it goes. For activity trackers, Fitbit has the Charge HR with an optical heart rate monitor, while Jawbone has the UP3 with a bioimpedance sensor for heart rate.

Can you please explain the differences between the two. Also, diagrams would make it more understandable.

Info about optical sensor:
Also, are any diagrams from here relevant?

On Jawbone's blog, there is some info:

Bioimpedance measures the resistance of body tissue to tiny electric
current to enable the capture of a wide range of physiological signals
including your heart rate. If you've ever measured your body
composition such as fat content, this is very similar. Our choice to
use bioimpedance technology sets us apart in three key ways:

Battery life. Because bioimpedance requires significantly less power
compared to optical sensors for same level of accuracy, we can deliver
a smaller form factor and longer battery life Physiological signals. A
single platform utilizing bioimpedance sensors captures a wide range
of signals: heart rate, respiration rate and galvanic skin response
(commonly known as skin conductance) Updatable technology. Given the
versatility of the sensor platform, we are able to (and we will)
unlock exciting new features with a simple, free, over-the-air
firmware updates in the coming months

Best Answer

The optical sensors (photoplethysmography) rely on the change of volume of blood in a digit or earlobe (based upon the pulse) changing the light absorption characteristics as detected by an LED/photodiode pair. They're pretty easy to use, and hard to get wrong.

Bioimpedance plethysmography is much the same, but relies on a very small electrical signal provided at one point on the body and received at another.

It's pretty well described in A Bio-Impedance Measurement System for Portable Monitoring of Heart Rate and Pulse Wave Velocity Using Small Body Area Min-Chang Cho, Jee-Yeon Kim, and SeongHwan Cho, in Proc. IEEE EMBC, 1997, pp. 2072–2073. (though this will be hard to get outside of a university)

My understanding is that the bioimpedance measurement is trickier, as electrode placement becomes pretty important.