I wonder about those IR thermometers so prevalent during the past year. Previously, I recall seeing them at deli salad bars, used to monitor the sour tureens.
I understand the physics, the relationship between spectrum and temperature. What is the accuracy, considering the problems of focus and reflection, and ambient noise?
What's the internal sensor?
The cheap ones are pretty terrible. Not very accurate, and the laser pointer basically lies about the field of view and what it's really pointed at.
Emissivity and ambient reflections can fool even a good IR thermometer, but a pot of soup will have an emissivity near 1 so that's not bad.
A thermally reflective surface reflects the temperature of the ceiling or something, and the IR thermometer doesn't know how to handle that.
That's a cool demo. Didn't know about the emissivity vs reflection details other than the ones that just show a temperature can be very wrong at times. Are liquids in general highly emissive?
Water and ice are almost 100%... almost black at thermal IR wavelengths. Most organics are too.
Aim your IR toy into a bucket of ice and water and see what it says.
Oh, they've been around for a decade or two.
Probably a small black thermistor bead. You can do ratio measure from a resistance, so that's a stingy battery-saving solution; it doesn't take regulated power.
They may not be totally accurate due to surface albedo, but for keeping recipes consistent, they're MUCH more convenient than stick-a-probe systems, and this one
Some of the pistol types are... slow, erratic, and kill batteries soon.
With transparent materials, depth or thickness matter. If it's optically thick enough, it is a black body.
Joe Gwinn
Cheap thin black lawn bags are essentially transparent to thermal IR. To see inside a box, replace one side with the plastic to block convection.
Water has the issue of surface reflectivity depending on the angle as do other substances like plastic. I was attempting to measure the temperature of my freezer and it varied a lot until I realized the plastic packaging would reflect the room temperature and even shiny printed cardboard would do the same thing. I had to point the gadget into a corner behind other packages to get a true reading. The freezer itself is not so reflective.
I sometimes point an IR thermometer at the sky on clear nights. I've seen some pretty low temperatures that are most likely from space with some interference from moisture in the air. One night it was so clear and dry the device didn't give a number. It was something that reminded me of the IEEE floating point format of NaN.
The question was why an IR thermometer could be used to work with a pile of ice or a large pot of soup.
Joe Gwinn
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