Handheld instruments
An illustration of field of view at 2.6 mRad vs. 1.36 mRad. Courtesy of the Infrared Training Centre
resolution — in this case, 320. Then you will multiply that number by 17.44, which is the result of (3.14/180)(1,000) in the previous equation. (24/320) x 17.44 = 1.308 mrad
Knowing that the IFOV is 1.308 mrad, you then
must find your IFOV in millimetres with this formula: IFOV (mm): (1.308/1,000) x 15,000* mm = 19.62 mm
*The distance from your target So what does this number mean? The spot
size ratio is 19.62:15,000. This number is the measurable size of one single pixel (1 x 1). To put it in more simple terms, this calculation tells you that your camera can measure a 19.62mm spot from 15m away. This single-pixel measurement is called “theoretical spot size ratio.” Some manufacturers list theoretical spot size ratio in their product specifications. While this may be considered the true spot size ratio, it is misleading because it is not necessarily the most accurate. This can be because it only gives you the temperature of a very small area within a single pixel. As previously mentioned, you want to get as many pixels as possible on your target for the greatest accuracy. One or two pixels may be enough to qualitatively determine that a temperature difference exists, but it may not be enough to
As you move further and further away from the object you want to measure, you lose the ability to measure temperature accurately
provide an accurate representation of the average temperature of an area. A single pixel measurement may be inaccurate
for various reasons: • Thermal cameras can develop bad pixels • Objects reflect – a scratch or solar reflection
would cause a false positive and a false high reading • The object that is hot – say a bolt head –
might be close to the same width as a pixel but those are square whereas a bolt head is hexagonal • No optics are absolutely perfect – there are
always some distortions in optical systems which impact measurements Due to a phenomenon called optical
dispersion, radiation from a very small area will not give one detector element enough energy for correct value. FLIR recommends making sure that the hot area where the spot value requested is at least 3 x 3 pixels. Just multiply your theoretical spot size ratio in millimetres by three, which gives you a spot size ratio of 3 x 3 pixels instead of 1 x 1. This number is
going to be more accurate. So if you multiply IFOV in mm (19.62) by 3, you get: 58.86 mm
This means you can measure a 58.86 millimetre spot from 15m away. Now let us say you want to measure a 20 millimetre spot. How far can you accurately measure that specific spot size? You need to use a little cross-multiplication: IFOV in mm: Distance in mm (15m = 15,000mm) 5.886:15,000 20mm : x
15,000*2 = 58.86*x 300,000/58.86 = x
x = 5,096.8mm or approximately 5.1m You can measure a 20mm spot from
approximately 5m away from the target with your 320 x 240 resolution camera Other manufacturers may not use this
number when they discuss IFOV or SSR; but in truth, this number will give you a more accurate temperature reading on an anomaly. Ultimately, spot size ratio matters because it will help you understand whether your thermal camera is capable of accurately measuring temperature at the distance that you need it to. If you need to measure small targets from long distances, knowing the spot size ratio of the camera and whether you are standing within accurate measurement range is crucial. If you are planning a thermography survey, think about whether you can get close enough to a target to get an accurate reading. Accurate should be interpreted as “good enough for proper interpretation.” This does not necessarily even mean within the accuracy specification of your camera. You can make the mistake of being off by several – even hundreds – of degrees if you do not consider the spot size ratio. To make the calculations quicker, FLIR has a
FOV calculator for each of its cameras on
http://flir.custhelp.com. Just click on the FLIR camera series you are using, which will take you to a list of all the cameras in that series. Click on “FOV Calc.” next to the correct camera, and it will show you your camera’s spot size ratio.
In the ideal situation the projected target should cover at least one pixel. To ensure accurate readings it is advisable to cover a wider area to account for the optical dispersion of the projection
26 FLIR Systems
www.flir.com October 2018 Instrumentation Monthly
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