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I want to measure body temperature and heartbeat rate only with a IR receiver and transmitter. How could I do that? Is it an accurate method? Or I have to use IR temperature sensor for this.

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"IR" refers to a range totally different wavelengths, which need totally different materials to detect and even to be transparent to.

Pulse rate:

  • Near-to-visible IR (NIR) 0.9um,
  • transparent to skin, can see inside
  • sensing done by reflection: LED emits NIR, travels into body reflects off blood vessels, and back out to Photodiode

Temperature sensing:

  • 5-10um Thermal IR (TIR),
  • measures heat emitted from skin surface
  • emission energy proportional to surface temperature
  • does NOT use an LED emitter. skin IS the emitter

You need different devices.

Why?

Photodiodes (NIR and Vis) work by converting photons to electrons. This doesn't work at TIR as the silicon becomes transparent beyond 1.1um and doesn't capture the energy. (From another perspective the TIR photon energy is too low)

TIR Temperature sensors tend to work by the incoming TIR energy heating up the sensor, and measuring the relative temperature change. This makes them pretty slow reacting.

It is possible that your skin temperature pulses with blood flow, but seems likely that the sensor will be too slow to track this. Even more significantly, your surface temperature is probably well low-pass filtered by the blubber layer and changes slowly - the temperature ripple has very low amplitude after filtering.

Someone else may know more about the transparency depth of TIR into skin.

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  • \$\begingroup\$ What if I use two receiver and one transmitter? \$\endgroup\$ – Panda Mar 13 '18 at 19:32
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    \$\begingroup\$ the transmitter (LED) is not used to measure temperature. for temperature, the skin IS the transmitter \$\endgroup\$ – Henry Crun Mar 13 '18 at 19:41
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IR temp sensing is done by integrating a Bolometer sensor with current in a differential Bridge.

IR Heart beat detection is done by transmission of IR with an adaptive threshold of 2/3 peak or so.

... technically described as "Photoplethysmography with an IR phototransistor via transmissive absorption"

So temperature is integrated emissions only and blood flow BPM is transmissions of pulsed pressure changes that affect absorption of IR with AGC.

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