This article may also help explain the anomili of the unexpected white paint vs. bare (shiny) aluminum temperature difference.
Shiny Metals and Non-contact Infrared Temperature Measurements [top]By Joe DeMonte
ASNT/PdM Thermal Infrared Level 3
Senior Thermography Course Instructor
Infrared Training Center
Here is an article that I put together to show the untrained user of “spot radiometers” why there can be significant errors in measurements. Using infrared devices for non-contact temperature measurement may seem like an easy task. Just squeeze the trigger on the “laser pyrometer” and point it at the target for accurate temperatures, right? Wrong!! Different materials have different efficiencies at radiating. I will discuss briefly the important measures that must be taken to accurately measure temperatures with these devices.
Take a look at an aluminum block heated up on a hot plate. I am viewing this hot plate with an infrared camera that detects the same radiation as a typical spot radiometer such as a Raytek or 3M “laser Pyrometer”. It produces an infrared picture of the heat and allows me to obtain temperatures from thousands of points across the thermal map.
Al block heated on a hot plate Temperature of the Tape on the Al block
In the next image, I have placed a simple piece of 3M Super 88 black electrical tape on the surface of the aluminum block.
It is not possible for the tape to actually be hotter than the block. The source of heat is at the bottom of the block and needs to conduct upwards through the aluminum and then outward through the tape. What we are seeing is the difference in the efficiency of both the aluminum and tape to emit IR radiation. The actual value of this efficiency may range from 0.0 to 1.0 and is termed emissivity. The tape has an average emissivity of 0.95 for the wavelengths of my infrared camera. Take a look at the next image for another temperature measurement of the block with the tape still attached.
As you can see, the temperature of the block as read by an infrared device is nearly 100°F lower than that read on the tape! We would have the same results with a spot radiometer. Check out these pictures of the same block with a spot radiometer:
Temperature on the tape! Temperature on the block!
Next, I will correct for emissivity using my infrared camera by setting it to 0.95 and take the same measurement of the tape.
Temperature with Emissivity Correction
Proper compensation for emissivity results in another correct temperature rise. We can now adjust the infrared camera to give us a better understanding of the aluminum temperature now that we know the actual temperature is 203°F, not 191°F or 103°F as previously thought.
There is much more to this than meets the eye, and the basic course that I teach on infrared science/certification lasts for a full four days. We just have to remember that accurate temperatures on clean, unpainted, and uncoated metals are not easy with infrared devices. Take your temperature measurements from highly corroded metals, or from other higher emissivity targets and you will have less error in your reading. If you want accuracy from a chromed piece or other clean metal, just place a piece of electrical tape on it and you will be much better off than without. But remember that this kind of tape will melt at around 200°F!
IF YOU DON'T HAVE AN ACCURATE TEMPERATURE ON ONE OBJECT, YOU CANNOT GET AN ACCURATE DIFFERENCE IN TEMPERATURE BETWEEN TWO OBJECTS EVEN WHEN THE EMISSIVITY IS EQUAL BETWEEN THE TWO!!
Emphasis by me. This may suggest that the temperature of the bare aluminum is actually higher than what is read by the IR thermometer and closer to the reading from the white paint. The difference in emissivity may also explain the low temp of the black rubberized coating. Either way, I would suspect the underside of the white aluminum will be lower than the underside of the bare aluminum.