Stainless steel reflecting sky temp

This is extracted from a recent message board post:

"I'm curious as to how to adjust for stainless steel reflecting sky temp. The apparent reflected temp of the ground and surroundings is approx 50F. The bottom half of the drum is reflecting this (SP4). The top half is reflecting the sky (SP5). 

In my software, if I set the emissivity to 1, the sky temp in the picture shows -34F.
SP4 and SP5 *should* be the same temp, within a few degrees, based on the operation of the equipment. If I enter 50F as RAT for SP4, I have to enter -95F as RAT for SP5 to get the two temperatures to match.

Why is it -95F and not -34F for SP5? I want to make sure I'm understanding and looking at the image correctly. "

Click image to enlarge

 

Using Cavities to Solve Emissivity Problems, Here's the Drill

by Bernie Lyon

Here are two illustrations that I have created to demonstrate how the cavity effect works.

Referring to Figure 1, the “ideal” block has an emissivity of 0.2 and a reflectivity of 0.8  We are assuming that these values are constant and do not change with angle. This is not the case. If thermal radiation strikes a single surface, 20% of the radiation is absorbed and 80% is reflected. (Keep in mind 20% of the radiation – Not the temperature!) If it strikes another surface before exiting, 20% of what remains is absorbed and 80% of that is reflected, and so on.

Therefore, with multiple reflections caused  by cavities, the effective emissivity, or emittance, increases, while the effective reflectivity decreases. This is the reason why holes and cavities appear dark.  They reflect less visible light.

Quick and Easy Solution to Emissivity Problems

"We've received the camera and so far it's been good in that department. I am concerned however about emissivity. In my application, I'm filming the inside of a computer system which is a small, tightly packed space with different materials and emissivities. Because thermography is something new to me, I'm not sure how much of a role emissivity plays in throwing off my measurements. I could attach a thermocouple to each component to verify the readings on the thermal cam, however that's very time consuming and given the limited amount of time I have with this instrument, it doesn't make sense. I've read that there's emissivity sprays that will help with this issue but don't know if that solution is any better or cost effective. Do you have any cheap and quick solutions for my problem?"

Yes, emissivity can play a big role in your temperature measurement accuracy, especially if you are trying to measure shiny metals. Here is what I would do:

Measure your reflected apparent temperature

1. Set camera emissivity to 1.0
2. Use some slightly crinkled aluminum foil and place next to your target(s).
3. Using the area mode of your camera set to average temperature, read the apparent average temperature of the aluminum foil. (Be careful that you don't reflect your own heat reflection of the foil).
4. Enter this temperature as the reflected temperature in your camera.

Measure your target(s)

1. Use some Liquid Paper brand correction fluid.
2. Place a small amount on your targets.
3. Set your camera emissivity to 0.95
4. Measure the temperature of your target(s)

Cautions and Caveats

• When using these techniques make sure that the correction fluid will not damage your components.
• Use a small amount of correction fluid. Changing the emissivity of small shiny components from less than 0.1 to 0.95 will greatly affect the heat transfer from these components, and thus affect their temperature. So the smallest amount of emissivity improver that you can measure is the amount to use. This is easy on FLIR systems cameras, just make sure that the emissivity improver is large enough to fill the cursor circle on the spot tool. Anything smaller will not be measured correctly.
• There are more sophisticated ways of doing this automatically for all materials on a PCB through software. Please contact a FLIR Systems Scientific Segment Engineer regarding Altair software.
• If you don't understand emissivity and reflected apparent temperature terms, get some training! A good place to start is this web based course, Thermography Basics.