Handheld instruments
How far can you measure with a thermal imaging camera?
In this article, Andrew Baker, FLIR Systems, discusses the factors you need to consider if you have bought a thermal imaging camera to use over distance
aybe you have bought a camera but are unsure of how it can be used over distance. Or you could be about to buy a camera but need guidance on which model will accurately measure the temperature of your application without breaking the budget. To make that assessment, thermographers need to consider several factors such as resolution, instantaneous field of view, lenses, the size of the object and more. You can compare it to an eye test. When you look at the eye chart from the optician’s chair, you may be able to see that there are letters on the smallest line – but at what distance can you still read the letters (i.e. “measure” them)? If you have 20/20 vision, you can define the smallest letters at greater distances. In this case, 20/20 vision would be equivalent to a high-resolution thermal imaging camera. If your vision is not perfect, you can improve it with glasses (i.e. adding a magnifying glass to the camera) or getting closer to the eye chart (i.e. reduce the distance to your target). Understanding spot-size ratio is especially
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important. Spot-size ratio is a number that tells you how far you can be from a target of a given size and still get an accurate temperature measurement. For the most accurate temperature
Instrumentation Monthly October 2018
measurement, you want to get as many pixels from your camera’s detector as possible onto your target. This will give you more detail in your thermal image. As you move farther and farther away from the object you want to measure, you lose the ability to measure temperature accurately. The higher the resolution your camera has, the more likely you are to get more pixels on a target from farther away with accurate results. Digital zoom does not improve accuracy, so higher resolution or narrow field of view is key here. Let us say you are looking to get an accurate
temperature measurement of a 20mm target from 15m away with your thermal camera. How
do you figure out whether your camera can do this? You will need to check the spec of your camera and know both the field of view and the resolution. For this example, let us say your camera’s resolution is 320 x 240, and your lens has a 24-degree horizontal field of view (FOV). You first need to calculate instantaneous field of
view (IFOV) in milliradians (mrad) with this formula: IFOV = (FOV/number of pixels*) x [(3.14/180)(1,000)]
*Use the number of pixels that matches the direction (horizontal/vertical) of your FOV Since your lens has a 24-degree horizontal FOV, you will divide 24 by the camera's horizontal pixel
IFOV is an angular projection of just one of the detector’s pixels in the IR image. The area each pixel can see depends on your target distance for a given lens
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