Part 3: What is the size and distance to your target of interest?
To get the best thermal imagery and most points of measurement on your thermal object of interest, you should select a lens that fills the field of view with the target of interest. At the same time, you typically want to optimize your spatial resolution to make sure the smallest object detail you need to see matches your instantaneous field of view.
Spatial resolution is the same as instantaneous field of view (IFOV). Both are the smallest physical detail you can detect on your target and are based on the smallest area a single camera (detector) pixel covers. The closer you are to an object, the smaller the area a pixel will detect. As you move farther away, the single pixel covers a larger area of interest (see Figure 1).
Figure 1: Field of view and instantaneous field of view
Field of View (FOV)
As you’ll notice, the field of view also changes as you view objects from farther away. Similar to spatial resolution, this means you will have fewer pixels on a target when imaging from a distance than you will from closeup. Ideally, you want the object to fill the field of view, but sometimes this is not possible due to the heat of the target and the danger it could pose for the camera or the operator.
Once you have defined the desired field of view and spatial resolution you can select the best lens or set of lenses for your application. The math required to determine these values by hand can be daunting, so FLIR has developed a free online field of view calculator to assist you with this process (www.flir.com/custhelp/calculator). To use the online tool, simply enter the object size, distance to the object, and prospective lens. The calculator will compute the field of view, spatial resolution, and number of pixels on the object of interest—making the lens selection process very easy.