Over the years I have been involved in the infrared industry I have seen some mistakes made and problems missed using spot temperature measurements on IR cameras, luckily these were rectified using the IR software when the reports were written… (would not have been the case had we been using “report by exception” techniques!).
I have always trained my thermographers to use area measurement at all times, in particular the “area max temperature function” this way we ensure that we do everything to make sure we don’t miss anything that may be detrimental during the inspection, and this is more important when inspecting through IR Windows!
There are instances when the use of spot temperature measurement techniques are very effective, especially when trying to compare one item to another in an IR image to see what the temperature difference is, or when trying to correlate one component to another, etc… However as a rule I have always found it best practice to use the area maximum temperature function whilst conducting electrical surveys using IR cameras.
Many people are not aware that the detector in an infrared camera actually only reads electromagnetic radiation it receives in a specific range of wavelengths. In order to display this in a useful reading the camera makes several calculations in order to convert the actual data to a temperature. The emissivity and transmissivity (sometimes depending on the camera manufacturer) have to be manually entered into the camera’s menu. if this value is entered incorrectly the actual temperature will be exponentially different (see Stefan-Boltzmann’s Law) than the displayed temperature. The old saying of “well as long as it is consistently wrong the change will be noted” is not entirely correct either, as the difference between phases will also be exponentially wrong. The error is going to be worse as the temperature rises – if the differential between the measured temperatures is significant then the displayed temperatures could be significantly different!
So that phase imbalance that looks like it is only a couple of degrees different could actually be upwards of 30 degrees! The visual setup of the camera could be the only other way of determining the severity of a potential defect. As anyone who has spent some time looking through a camera will tell you the visual component can be significantly altered (both to make things look better than they are as well as to show “elevated” differentials.) Depending on the level, span and range setup on the camera it would be very easy to miss a severe problem.
With this in mind it is easy to see why there are so many infrared problems that aren’t caught. In order to ensure that our customers “See What You’ve Been Missing”, all IRISS polymer based windows utilize the same grade and thickness of polymer, so that if you have correctly setup your camera for one IRISS window you can know without any doubt that it is “calibrated” for all IRISS windows. Our Fixed And Stable Transmission (FAST) is exactly what the name implies – unchanging! The ambient temperature outside, length of time in the field, relative humidity or barometric pressure have no effect on transmission rate and therefore no effect on your readings!
Every infrared camera defines its Field of View (FOV) across a horizontal/vertical axis.
You have two ways to determine the Field of View (FOV) on your camera:
- You can calculate the FOV using the formula: 2 x the tangent of ½ the angle x distance
- You can measure (and “map out”) the practical FOV with a quick field test to check your math!
The practical FOV test is quick, relatively easy, and in no way requires a scientific calculator!
The practical FOV test is a simple method to determine what can be seen at set distances with your camera, the lens, and IR Windows.
- Find a long workbench, counter-top or a 6 foot folding table. Layout a piece of plain paper along the entire length of the table.
- Set the camera on the table and mark a “zero” line across the table width. This zero line should be far enough on the table so that the camera cannot accidentally fall off the table. The zero line is where the camera lens touches the line.
- Draw a straight line the length of the paper along the center.
- Label this line with 6 inch increments marked out from the zero line.
- Label the lines from 0 to 36 inches. You can go further if you have a panel depth deeper than 36 inches, but usually 36 inches is sufficient.
- Place the camera lens at the zero line with the straight line going down the center of the paper in the middle of the camera lens.
Now, it is time for a coffee break. Not really, you need two heat sources. Some people use coffee cups – good excuse for a break. You can use any known heat source. Some people use hot plates if they are in a lab, a griddle, or you can purchase inexpensive candle warmers.
- Place the two heat sources at a distance from the camera that is typical of the targets you will monitor. For example, if your targets are 18 inches from the panel, then place the two heat sources at the 18 inch mark.
- Move one heat source from the center until it appears just inside the edge of the image in the camera display.
- Move the other heat source in the opposite direction until it appears just inside the camera display on the other side.
The distance between your two heat sources is the maximum FOV using your camera and lens. At the defined distance.
- You can draw a line from each side of the camera lens at the zero line to the heat source on the same side. This gives you the FOV for any distance from the zero line to the heat sources.
- If you are using an IR Window, subtract the camera lens diameter from the FOV. Next add the diameter of the IR window. This gives you the Maximum Horizontal Window FOV. For example, you have a FOV of 8 inches, at 18 inches on the center line. The camera lens is 1.75 inches. The camera FOV is then 6.25 inches. If you are using a 4 inch IR Window, then add 4 inches for a total Maximum Horizontal Window FOV of 10.25 inches.
You should repeat the above process with the camera lying on its side to determine the Vertical FOV.
When finished, roll up your map and save it for future reference.
You can create a table for the distances along the center line and the Horizontal and Vertical FOV for different window sizes in your plant.