[klickverbot]

Physicist here too. What exactly do you disagree with? The parent comment is sound – thermal imaging cameras typically under-read on shiny metal surfaces. Their emissivity/absorptivity at relevant wavelengths is low, and reflectivity is high. Thus, their own Planck spectrum is (approximately) scaled down by their emissivity, and consequently the radiation in the measured MIR band is mostly what is reflected, which tends to come from the room-temperature environment.

A polished piece of metal makes a shitty black body. This is also why shiny metal (foil) is used to curb unwanted radiated heat transfer everywhere from thermos flasks and cryostats to space probes. (The lower emissivity further improves the efficiency of multi-layer insulation.)

[tagrun]

Let me try again. Assistant Professor of Physics here (not a grad student).

Yes, reflectance of room temperature aluminum at those wavelengths is pretty good (not true for all metals BTW). Yes, this usually makes it hard to distinguish thermal radiation and reflected radiation with metals. What are you trying to say though? That whatever comes off from a metal must always be a reflection coming from somewhere else?

> Thus, their own Planck spectrum is (approximately) scaled down by their emissivity, and consequently the radiation in the measured MIR band is mostly what is reflected, which tends to come from the room-temperature environment.

I don't know what you mean by "Planck spectrum is (approximately) scaled down" (as "Planck spectrum" only refers to thermal radiation and is generated in a separate process from reflected photons [one is governed by the conduction band whereas the other is governed by everything up to Fermi level] and you can't hope to suppress thermal radiation by simply shining random environmental light on a metal --there is no such thing as "scaling down" of thermal radiation unless you engineer such property), but there is just no way that 10 micron photons at that intensity could be coming from a room-temperature environment.

So your blanket statements about metals aside, the hot area in that picture is due to a very specific signal which can't be due to something that's reflected from the environment. No significant fraction of those 10 micron photons coming off from that localized the area around the CPU could have originated from the environment --assuming that those pictures aren't taken in a hot oven and someone focused the thermal radiation on to the heatsink to get that amount of intensity.

And as I mentioned, that's pretty trivial to test. If those 10 micron photons were coming from the environment as you or the parent comment suggest, the thermal camera would report ~60C even when you look at Pi 4 when it is cooled (again, this is something can use as "dark frame" and subtract off from all readings if you're trying to be more accurate). This is clearly not the case, though, as you can see in the video on the blog post.

[michaelt]

  What are you trying to say though?

In the first image in the linked article, the thermal camera picture has a scale at the bottom. On the scale shown white and red are hottest (66°C) and blue and black are coldest (23°C). The CPU is black (23°C), and the PCB directly adjacent to it is white (66°C).

kees99 and klickverbot are saying it's unlikely the CPU is actually 23°C, especially given the author's statement the CPU was around 60°C, and that it's well known taking thermal camera images of things with different emissivities will produce inaccurate results.

kees99 is also saying, given that the thermal image doesn't accurately measure the temperature of the CPU, the article's statement that the metal casing helps isn't really warranted.

[tagrun]

CPU is at 23C? Are we looking at the same image?

The CPU is the heat generator there, and is in contact with metallic regions around 60C (the red ring, if you compare to the real picture and follow the metallic bevels), where heat conductivity abruptly drops, which is what I've been talking about from the beginning. Since the heat is generated by the CPU and flows to the metal casing and to the PCB, the CPU can't be lower than 60C.

I agree that the reading for the inner region of the metal casing (which is not the CPU) must be off, and it's probably because the emission intensity there isn't strong enough and the camera software is mixing the emission and reflection when inferring the temperature (which gives physically incorrect results because the spectrum won't obey Planck's law, but the error depends on how different the temperatures are, and gets much stronger as they drift apart) rather than doing something like a "dark frame" subtraction (which is doable in principle).

Accuracy concerns aside, though, everything we see there (when you consider the physical context) supports the fact that the metal casing helps spreading the heat (which is obvious, it's a material which high heat conductivity, and there wouldn't be any need to put it there otherwise).

Even the 60C reading must be off by some for the same reason (given the regions appearing at around 70C), of course, but I assume OP doesn't care about that level of accuracy.

[michaelt]

I agree it's true metal conducts heat better than plastic, and that a metal package is a conventional choice for that reason.

I disagree that the thermal image provides evidence of those truths

Does the image prove the CPU has a low temperature? No, the image reports the temperature inaccurately. Does the image prove the package has no hotspots? No, it wouldn't show hotspots if they were there. Does the fact the PCB gets hot tell us much? Not really, you'd expect heat to conduct from the package and balls to the PCB no matter what the package was made from.

[tagrun]

If that cap didn't spreading the heat as well, what you'd be seeing on the thermal camera would be something that glows around 60C-70C (because clearly, the camera's software can more or less resolve the room temperature reflection from 60C thermal radiation), and the color would be more or less uniform in the region above the CPU. There wouldn't be such a strong observed temperature gradient.

Which is the evidence you're looking for.

[arpa]

Genuinely curious person here. What is the key reason to use photon size instead of wavelenght? To stress the quantifiableness of the radiation being capured by the thermal camera? To be honest it is the first time i've seen photon size semicasually mentioned in a conversation. Then again i only had physics in high school.

[acqq]

> What is the key reason to use photon size

Photon size wasn't used. Micron is an unofficial name for 1e-6 m, the lengths of 0.7e-6 m to 1e-3 m correspond to the wavelength of infrared radiation:

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

So "those 10 micron photons" there mean "the photons of the radiation with the wavelength of 1e-6 m."

[arpa]

Ah! Makes so much more sense, thanks!

[tagrun]

They're the same thing: de Broglie wavelength would be the closest thing if you want to assign an effective "size" to a particle.

[arpa]

That actually makes a lot of sense after a bit of Wikipediaing. Thank you for broadening my knowledge.