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
This is an advanced application. Unless you really know what you're doing, do not attempt measuring temperatures on materials that have very low emissivities. The possibilities for significant errors are large.
Nevertheless, I will illustrate the steps I have taken to properly measure the reflected temperatures, calculate the emissivity of the steel, and of course, measure the temperature of the steel.
Click image to enlarge
- OK, first things first. I changed the color palettes so I could more easily see exactly what is being reflected by the steel. I used the iron bow palette for this.
- Secondly, instead of using spots I prefer to use areas. This enables me to average out individual objects that might be reflected. I set the areas to read average temperatures.
- Now, since the shell of the cylinder is obviously quite warm, I prefer not to measure reflected temperatures from the shell but rather from objects that are more closely allied to the reflected temperatures. I've chosen the clear sky above for the sky reflected temperature and I have chosen what appears to be a support structure at the bottom to more closely reflect ground temperature. I set the emissivity to 1.0 on both of these areas in order to obtain an accurate reading. As you can see, the sky reflected temperature is about -39° F and the ground reflected temperature is approximately 43° F.
- Now that I know the reflected apparent temperatures, I can enter these into the area's for the steel.
- Now all we have to do is determine the emissivity of the steel. The assumption is that area's three and four are identical in temperature. That means I have to iteratively enter various values of the emissivity until the temperatures are identical. Since I know the steel is very reflective, I started with an emissivity of 0.5. Since the temperature readouts were not identical I then used a value of 0.25, etc. At a value of 0.17 I found that both temperatures read the same at approximately 170° F.
- So there you have it. Remember, don't try this at home; let thermography professionals perform calculations like this. One really needs to understand the physics involved to extract data like this from a thermogram. Different situations call for different tactics. I would recommend knowledge at a minimum of a level II certification to handle these types of situations.
