Let’s define a few important acronyms that I am sure everyone has seen on infrared camera specification sheets, but may not understand very well: FOV, IFOV, and IFOVmeasurement.
These are not just any acronyms—these describe very important performance specifications of your thermal imager. It is important to understand not only what they mean, but what impact they will have on the day-to-day use of your infrared camera.
OK… so here are the basics:
FOV, or Field of View, is the largest area that your imager can see at a set distance. It is typically described in horizontal degrees by vertical degrees—for example, 23º X 17º. (These “degrees” are units of angular measurement, not to be confused with the degrees of temperature measurement.) Essentially, it is like a rectangle extending out from the center of your camera’s lens extending outward. The farther away you look, the larger the rectangle becomes. Itching for an analogy? Think of Field of View as the windshield that you are looking out as you drive your automobile down the road. You can see everything from top of the windshield to the bottom, and from the left to the right.
IFOV, or Instantaneous Field of View (otherwise known as Spatial Resolution), is the smallest detail within the FOV that can be detected or seen at a set distance. What does this mean? It means that at certain distance, you may not be able to see certain small details if your Spatial Resolution is not good enough. To continue the driving analogy, think of IFOV as the ability to see a roadside sign in the distance (through your windshield). You can see that it is a sign, but you may not be able to read it when it first becomes recognizable. IFOV is typically measured in units called milliradians (mRad). Milliradians are small fractions of an angular degree.
IFOVmeasurement, or Instantaneous Field of View Measurement (otherwise known as Measurement Resolution), is the smallest detail that you can get an accurate temperature measurement upon at a set distance. Going back to our analogy again… when you see a sign in the distance, and you cannot read it, what do you do? Pretty obvious, huh? You either move closer, or you use some type of optical device, such as binoculars, to effectively “bring you closer”. The same is true in infrared thermography. Assuming that you do not have the power to make the object itself bigger, in order to “read” the temperature measurement more accurately, you usually need to be closer to the object… either physically or optically. IFOVmeasurement, or Measurement Resolution, is also typically specified in milliradians, and it is often two to three times more than the specified Spatial Resolution.
So what impact does all of this have on your daily thermal imager operation and inspection work? It means that you need to understand the capabilities of your camera, and work within its physical and optical limitations to see potential problems and to obtain more accurate temperature measurements (when it is important to your application). If the motor that you are supposed to be inspecting is not within your Field of View, then you are going to need to move so you can see it. If you are too far away to see the distinct differences between different sets of feeder wires in a lighting control circuit, your thermal images will not do you much good—you’re probably operating outside of the Spatial Resolution capabilities of the camera. And if you can barely make out the individual feeder wires, you will most likely not be able to get a reasonable temperature measurement because you’re outside of the Measurement Resolution of the imager, too! Move closer, or put on a lens that effectively does the same thing.
Infrared imagers, like photographic cameras, can often have different kinds of optional optics and lenses that will allow you to change your Field of View, Spatial Resolution, and Measurement Resolution. Telephoto lenses magnify the scene and bring you “optically closer”, but generally make the Field of view narrower. Wide-angle lenses, on the other hand, give you a much wider Field of View, but you may not be able to see the same level of detail. Telephoto lenses are great for seeing things at a distance, or seeing smaller objects. Wide-angle lenses are excellent for seeing “the big picture” without having to back up. Knowing what kind of lenses are best for specific applications will often save you time, and produce better infrared images for your analysis and reporting.
If you would like to learn more about the FOV, IFOV, and IFOVmeasurement specifications of your particular Fluke thermal imager model, go to the free Field of View Calculator on the Fluke website, choose your imager and lens combination, set the units and distance to your target, and voilà!—the program will give you a visual drawing of what we have discussed here today. You have the choice of viewing it online, or you can download it!
If you guys value definitions like the ones on today’s post, we would love to continue to offer definitions and practical examples of other important thermography terms. At a later time, we may actually discuss more detailed information related to today’s terms as well…so comment, let us know what you think, and stay tuned!