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Comment by JorgeGT | Hacker News Reader

Yep, a lot of people nowadays is using IR cameras without any calibration or understanding of the physics behind the measurement. The truth is, the camera requires an emissivity coefficient to associate the received radiation with the emitter body temperature. There are tables for different types of materials, but the best is to calibrate against a surface thermocouple or similar.

A corollary of this is, when you see an IR picture of a product with very different surfaces like rubber, shiny metal, matte paint, plastic, glass, etc. you know that almost for sure the measurement is unreliable because at most they calibrated the camera for the emissivity coefficient of one of the surfaces. And they vary quite a lot...

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Indeed, but note it usually works well enough, because most surfaces have high IR emissivity (in particular with organics like plastics, papers, woods, fabrics, etc.; soils and liquids as well).

The one exception you have to be careful is with unpainted metal (high reflectivity). If polished it can be so low that even calibration doesn't help -- because you'd need extremely precise calibration that isn't practical and because reflections and noise/other heat sources will pollute you readings. In that case a simple sticker (or thermocouple reading) would be more adequate.

This is a more accurate representation of RPi 4 temps[0].

[0] https://www.raspberrypi.org/magpi/raspberry-pi-specs-benchma...

>people nowadays is using IR cameras without any calibration or understanding of the physics behind the measurement

The four factors are Emissivity (ε), Absorptivity (α), reflectivity (ρ) and transmissivity (t). Emissivity is only one facet. Ideal is high Emissivity and low reflectivity.

https://en.wikipedia.org/wiki/Radiation_properties

Certainly, in most cameras the factors are in fact accounted for by setting the emissivity factor, a reflected apparent temperature, the distance to the object and the relative humidity. Thermography is certainly not an straightforward "point and click" process...

As a once-infrared-astronomer, I can certainly confirm your statement: Thermography is certainly not a straightforward "point-and-click" process.

Six years of grad school...

Sounds like a cool profession, what are you up to nowadays?

I thought that username seemed familiar… [0]

[0]: https://en.wikipedia.org/wiki/Clifford_Stoll

Yup, if you look into his comment history he confirmed he's the same person

Oh wow what a pleasant surprise! Such an interesting person.

Check out the Numberphile videos he's been in on YouTube: https://www.youtube.com/playlist?list=PLt5AfwLFPxWJeBhzCJ_JX...

Damn, are you the Cliff Stoll? I must say I thoroughly enjoyed your book back in the day. Upon reflection, it may have even affected my career (for the best).

Cheers!

Emissivity is mostly what matters in this case.

Whether the material is reflective or transmissive just means you'll be "seeing" IR sources reflected off the target or from behind the target, respectively. You do have to keep that in mind if there's an object not in ambient temperature in those locations (e.g. a clear sky is of very low temperature, a person is around 35C).

Absorptivity is generally equal to emissivity (except for things that have wildly different emissivity in their blackbody IR spectrum vs the ambient IR spectrum).

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