In blog # 015 on Feb 4, 2011 I discussed “radiosity”. Radiosity is all of the infrared radiation coming into your camera in the defined band width that your infrared camera operates in (likely 7.5µm to 14µm band width). That would include emitted, reflected and transmitted infrared energy waves. Remember, emitted is the only one that counts as giving an accurate reading of the surface temperature. Reflected bounces off the surface and comes straight into your detector array. Transmitted would be the infrared radiation that is coming through the material and into your camera’s detectors. An example would be the infrared radiation that transmits through thin plastic. Rarely will you experience “transmission” of energy waves because almost everything is opaque to IR waves in the 7.5µm to 14µm band width. But you will always have the emitted and reflected energy waves coming into your camera. Let me repeat that: you will always have the emitted and reflected energy waves coming into your camera. The infrared camera cannot distinguish between emitted and reflected radiation. That would be the job of the thermographer. As a result, we have to be aware of false positives and false negatives. What is a false positive or a false negative? “False” means it is not true! “Positive” in the context that we are talking about is where your infrared image is indicating the presence of something that is not there in reality. “Negative” in the context that we are talking about is where your infrared image is indicating the absence of something that is there in reality. Since reflected and transmitted energy waves are not representative of the true surface temperature, those energy waves are “false” as an indicator of a surface temperature. Two examples of a false “positive” would be where the camera is indicating that something is “warm” when in fact it is not. Or your infrared camera is indicating that something is “cool” when in fact it is not. Notice the image below. There appears to be a hot spot on the gutter. What you are seeing is a reflection of the energy from my forehead. That’s a “false positive”. (No jokes, please). The infrared camera is showing it as a relatively warm spot when in fact it is not any warmer than the adjacent area.
How do you know it is a reflection? Easy! As I move to the left, the reflection moves with me.
Now, look at the window. The left side is open and thus shows heat loss. The right side is glass and we see the upper portion as warmer and the lower portion as cooler. Why is the bottom portion cooler? Remember, although glass has a high emissivity (low reflectivity), it is a smooth, specular surface. (see blog # 018 on Feb 11) So, we are seeing a reflection of the sky, which is very cold. The upper portion is a reflection of the overhang (awning). The reading is false.
An example of a “false negative” would be where the infrared image indicates the absence of something that is really there. For example, let’s say your moisture meter definitely verifies the presence of moisture. You are expecting your infrared camera to show a drop in temperature due to evaporative cooling. However, your camera shows no temperature difference. A conclusion that there is no moisture is both false and negative. This is what happens when the wall surface is a non-permeable vinyl wall paper or tile where moisture doesn’t come through from the other side. Or you might have a situation where the relative humidity is so high that there is no evaporative cooling. In both cases, your image is false and negative. It is saying that there is not a problem when in fact there is.
Let’s look at another example. This south wall consists of filled and unfilled cinder blocks. That’s a fact. The first image was taken at 7am and clearly shows both filled and unfilled blocks. The second image was taken around 10am. Equilibrium was reached by 10am and you see no indication of a difference in the blocks. If you looked at this wall only at 10am you would conclude that the entire wall is filled. What you are seeing is equilibrium and that would be a false negative. You would be concluding that unfilled blocks are not a part of this wall when in fact they are.
Another example would be when you are looking for insulation issues. Let’s say there is missing insulation in the wall. If you have a significant temperature difference from outside surface to inside surface (18° or more) your infrared camera should pick up missing insulation rather dramatically. But if the outside surface temperature is 70° F and the inside surface temperature is 70° F you will not be able to pick up the missing insulation. Your image will indicate that there is no problem, when in fact there is. So, that’s an example of a “false negative”.
Hopefully you are more comfortable with this subject now and you will be aware of false positives and false negatives.
How’s your comfort level in thermography?
(The above comments represent my opinion).
Rod Hoff / Restoration Consultants Inc
Thermography Instructor / IR camera sales
3284 Ramos Circle, Sacramento CA 95827
toll free 888-617-3266 ext 301
Provider of Fluke TiS, TiR, TiR1, Ti, Ti25, TiR27, Ti27, TiR29, Ti29, TiR32, Ti32, TiR3, and TiR4 infrared cameras.
See DEMO www.moistureview.com/demo.html