I have an idea how to make one using infrared and ultrasound sensors and getting a transfer function as if it was a black box. Once you figure out the software, making the device should be very cheap.
Being a dirt- and oxide-sensitive surface effect, emissivity is difficult to characterize:
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This shows the 20:1 range of variation for bare aluminum:
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Right now it's 32F outside in the shade. A brushed, anodized aluminum plate that I washed and left to dry in the back of my truck measures
59F in the early morning sun, with a fine type K thermocouple held in contact with a Q-tip.
The simple way to quantify and integrate the various heat loss paths is to measure a controlled heat -input- and equilibrium temperature or heating/cooling rate, which answers the original question by a different method.
I figure the heat loss from my house by recording the indoor cooling rate with the heat off versus the outdoor temperature. The answer is around 2% of the difference per hour, a piecewise approximation of an exponential process. When the sun is out the indoor temperature is stable at about 15-20F above outdoors.
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