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Is it possible to make a remote thermometer using a standard IR LED as a transmitter and an IR receiver diode as a receiver?

The range is about 5 meters. I know there are chips available for this purpose but I’d like to understand the principle.

Do the temperature sensor chips use only receiving diodes?

How is it done in thermo-vision hunting cameras so it is able to sense over 1km?

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  • \$\begingroup\$ what function would the LED serve? \$\endgroup\$
    – jsotola
    Commented Nov 20, 2023 at 7:59
  • \$\begingroup\$ @jsotola I thought about increasing the reflection from object surface so it would be easier to sense. :) \$\endgroup\$
    – Michal Podmanický
    Commented Nov 20, 2023 at 8:02
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    \$\begingroup\$ the object emits IR .... that is what's being measured \$\endgroup\$
    – jsotola
    Commented Nov 20, 2023 at 8:07

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No, IR photodiodes are only sensitive to near-infrared light. To measure temperature, you need to measure far-infrared energy, then use blackbody radiation equations. https://en.wikipedia.org/wiki/Infrared_thermometer

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  • \$\begingroup\$ Oh. I see. The receiver has to catch the wide wavelength range. \$\endgroup\$
    – Michal Podmanický
    Commented Nov 20, 2023 at 8:21
  • \$\begingroup\$ Depends on the temperature that you're attempting to sense. A microwave radio receiver can sense temperature. en.m.wikipedia.org/wiki/Cosmic_microwave_background \$\endgroup\$
    – D Duck
    Commented Nov 20, 2023 at 9:03
  • \$\begingroup\$ @DDuck Thank you. That’s interesting. \$\endgroup\$
    – Michal Podmanický
    Commented Nov 20, 2023 at 9:54
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    \$\begingroup\$ @MichalPodmanický Near IR LEDs can be used (though you'd prefer photodiodes) if the temperature you are looking at is high enough. Say 400 C or higher, just as a guess. I've done it with a narrow band around 860nm. But this brings up another point. Wide band isn't good. To use the BB radiation curve you want a very narrow slice. And you also need to know the average emissivity over the target view. That's shape-dependent as well as material (though it's basically 1 inside an integrating sphere in all cases.) \$\endgroup\$
    – periblepsis
    Commented Nov 20, 2023 at 10:10

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