PSA: Emissivity, Reflectivity, & Improper Use of Thermographic Imagers

By Published December 20, 2016 at 8:30 am
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Thermal cameras have proliferated to a point that people are buying them for use as tech toys, made possible thanks to new prices nearer $200 than the multi-thousand thermal imaging cameras that have long been the norm. Using a thermal camera that connects to a mobile phone eliminates a lot of the cost for such a device, relying on the mobile device’s hardware for post-processing and image cleanup that make the cameras semi-useful. They’re not the most accurate and should never be trusted over a dedicated, proper thermal imaging device, but they’re accurate enough for spot-checking and rapid concepting of testing procedures.

Unfortunately, we’ve seen them used lately as hard data for thermal performance of PC hardware. For all kinds of reasons, this needs to be done with caution. We urged in our EVGA VRM coverage that thermal imaging was not perfect for the task, and later stuck thermal probes directly to the card for more accurate measurements. Even ignoring the factors of emission, transmission, and reflection (today’s topics), using thermal imaging to take temperature measurements of core component temperatures is methodologically flawed. Measuring the case temperature of a laptop or chassis tells us nothing more than that – the temperature of the surface materials, assuming an ideal black body with an emissivity close to 1.0. We’ll talk about that contingency momentarily.

But even so: Pointing a thermal imager at a perfectly black surface and measuring its temperature is telling us the temperature of the surface. Sure, that’s useful for a few things; in laptops, that could be determining if case temperature exceeds the skin temp specification of a particular manufacturer. This is good for validating whether a device might be safe to touch, or for proving that a device is too hot for actual on-lap use. We could also use this information as troubleshooting to help us determine where hotspots are under the hood, potentially useful in very specific cases.

That doesn’t, however, tell us the efficacy of the cooling solution within the computer. For that, we need software to measure the CPU core temperatures, the GPU diode, and potentially other components (PCH and HDD/SSD are less popular, but occasionally important). Further analysis would require direct thermocouple probes mounted to the SMDs of interest, like VRM components or VRAM. Neither of these two examples are equipped with internal sensors that software, and even the host GPU, is capable of reading.

It is minimally misguided, maximally lazy, to utilize thermographic imaging as a means to determine component temperatures. In fact, using a better cooler with this methodology would suggest a worse component temperature (large surface area cooling notwithstanding). Taking a thermographic image of a stout copper heatsink would suggest temperatures that look worse than, for instance, a larger surface area heatsink (even if the latter performs worse in actual sensor-level measurements).

Direct sensor monitoring simply must be performed for accurate component-level temperatures. If you really wanted to do something different, you could bore a small hole into a coldplate and stick a thermocouple in there, then use that for measurement of the IHS case temperature of a CPU. That’s fine. It’s not what we do, but it’s fine as long as it’s consistent. Infinitely better than thermal imaging a heatsink that obscures the chip, certainly.

But let’s move on from that point, as it seems a bit obvious.

What is perhaps less obvious is the challenge of emissivity, reflection, and transmission when dealing with measuring interfaces that are not necessarily understood by the technician. We recently discussed this with Bobby Kinstle of Corsair, previously introduced to GN’s readers in a thermal engineering talk.

Our below video has a practical demonstration using a tempered glass-clad case, a tortured system (100%/100% GPU + CPU load), and a thermal imager:

IR cameras use infrared wavelengths to produce colorful thermographs that provide some insight as to temperature. This is done by looking at the wavelength of the thermal radiation off of the device under test. Longer wavelengths indicate a lower temperature (protracted peaks), while shorter wavelengths indicate a higher intensity and closer peaks (higher temperature). The relationship between radiation intensity and temperature is called the Stefan-Boltzmann law.

Q = eσT^4
(radiation intensity [Q] = material emissivity [e] * Stefan-Boltzmann constant [sigma] * absolute temperature)

Thermal cameras see what’s emitted from the object, what’s reflected, and what’s transmitted to varying degrees.

emissivity-demonstration-1

In the video demonstration above, we show that attempting to image the system through the case side panel actually reveals the reflections in the glass, not the system internals. Andrew (video producer) can be seen standing off-screen, even though we’re pointing the imager at the case; my hand is visible as it moves around in space reflected on the enclosure.

This means that we’re not seeing system internal temperatures.

We can also see that there’s effectively zero resolution to the image – it’s just sort of a big, amorphous blob of red and blue colors. Looking at the temperature scale, we see that the maximum temperature isn’t even that hot – something like ~40C – despite CPU temperatures nearing 80-90C.

emissivity-thermocouple

This is because of emissivity and reflection. Electroplated copper has an emissivity of something like 0.03; it’s highly reflective, and without compensating through calibration of the camera, you’re just not going to get a measurement anywhere close to reality. Tempered glass experiences similar issues, as would polycarbonate or acrylic side panels, except in the realm of "reflection."

Pointing a thermal camera at a shiny, nickel-plated heatsink would also prove fruitless. Without understanding the emissivity or reflectivity of the surface (two separate things) and compensating for it within the camera (which is often not possible on the El Cheapo phone cameras), the measurement has no basis in reality.

Our video above further proves this point by using a trusted K-type thermocouple (calibrated in an ice and boiling water bath) to take a measurement of a shiny nickel-plated copper heatpipe. This measurement reveals temperatures 30~40C below the Seek Thermal camera’s stated temperatures (after removing the glass), and is a result of the emissivity of that nickel-plated surface.

In an experiment conducted by Fluke, one which we strongly recommend reading, the company stuffed a stainless steel block into an oven for three hours and then used thermal imagers. The block is highly reflective, given its stainless nature, and shows temperatures below the oven’s temperature. The block also reflects the environment (like the face and glasses of an off-camera onlooker), similar to our tempered glass test.

In its experiment, Fluke painted one half of the block matte black and left the other half stainless steel. This provides an ideal black body for the physics demonstration, and shows a temperature swing of around 100F between the two halves (despite being the same temperature, as the steel plate is one body that was heated through convection). Using a compatible camera, it would be possible to calibrate the emissivity index by taking a delta of these two halves, then programming it into the camera. Not all IR cameras can do this.

Understanding the material and emissive/reflective properties of the subject is critical to using a thermal imager properly. Even knowing these things doesn’t mean that the imaging is useful, either; again, as the first half of this article explained, imaging a system doesn’t necessarily provide us with critical information. It takes someone to analyze and interpret that data, then act on it further for more careful or component-specific analysis.

If this type of content is enjoyable, consider reading our previous PSA on power supply cabling. Our Patreon page is located at this link, for direct supporters, and YouTube is over here.

Editorial: Steve “Lelldorianx” Burke
Video: Andrew “ColossalCake” Coleman

Last modified on December 20, 2016 at 8:30 am
Steve Burke

Steve started GamersNexus back when it was just a cool name, and now it's grown into an expansive website with an overwhelming amount of features. He recalls his first difficult decision with GN's direction: "I didn't know whether or not I wanted 'Gamers' to have a possessive apostrophe -- I mean, grammatically it should, but I didn't like it in the name. It was ugly. I also had people who were typing apostrophes into the address bar - sigh. It made sense to just leave it as 'Gamers.'"

First world problems, Steve. First world problems.

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