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How far can I measure with a thermal imaging camera?

17 December 2018

To make an assessment about the best thermal imaging camera for an application several factors need to be considered, including resolution, instantaneous field of view (IFOV), lenses, the size of the object and more, says Andrew Baker.

Firstly, understanding spot-size ratio is important. This 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 with a thermal camera.

For the most accurate temperature measurement, it is necessary to get as many pixels from the camera’s detector as possible onto the target. This will give more detail in the thermal image. As the camera moves farther and farther away from the object being measured the ability to measure temperature accurately is reduced. The higher the resolution the camera has, the more opportunity there is to get more pixels on a target from farther away with accurate results. Digital zoom doesn’t improve accuracy, so higher resolution or narrow field of view is key. 

Consider, for example, how to get an accurate temperature measurement of a 20mm target from 15m distance away with your thermal camera. It is important to know both the field of view and the resolution. For this example, let’s say the camera’s resolution is 320 x 240, and the lens has a 24-degree horizontal field of view (FOV).

The first step is to  calculate IFOV in milliradians (mrad) with this formula: 

IFOV = (FOV/number of pixels*) x [(3.14/180)(1000)]
(*Use the number of pixels that matches the direction (horizontal/vertical) of the FOV)

Because the lens has a 24-degree horizontal FOV, divide 24 by the camera's horizontal pixel resolution –  in this case, 320. Then multiply that number by 17.44, which is the result of (3.14/180)(1000) in the equation above. 

(24/320) x 17.44 = 1.308 mrad

Knowing that the IFOV is 1.308 mrad, it is then necessary to find the IFOV in millimetres with this formula: 

IFOV (mm): (1.308/1000) x 15000** mm = 19.62mm
(**The distance from the target)

So what does this number mean? The spot size ratio is 19.62:15000. This number is the measurable size of one single pixel (1 x 1). To put it in more simple terms, this calculation shows that the 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 can be misleading because it is not necessarily the most accurate. This can be because it only gives the temperature of a very small area within a single pixel. As previously mentioned, it is important to get as many pixels as possible on the target to ensure accuracy. One or two pixels may be enough to qualitatively determine that a temperature difference exists, but it may not be enough to 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, so do make sure that the hot area where the spot value requested is at least 3 x 3 pixels. To do this, multiply the theoretical spot size ratio in millimetres by three, which gives a spot size ratio of 3 x 3 pixels instead of 1 x 1. This number is going to be more accurate. 

Multiply IFOV in millimetres (19.62) by 3 = 58.86mm. This means it is possible to measure a 58.86mm spot from a distance of 15m. 

Now let’s say you want to measure a 20mm spot. How far you can accurately measure that specific spot size? Cross-multiplication is needed: 

IFOV in mm: Distance in mm (15m = 15,000mm)
5.886:15000
20mm : x
15000*2 = 58.86*x
300000/58.86 = x
x = 5096.8mm 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 helps to demonstrate whether a thermal camera is capable of accurately measuring temperature at the distances required. When measuring 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 it is possible to 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. To make these calculations easier, FLIR has created a FOV calculator for each of its cameras on http://flir.custhelp.com. 

Andrew Baker is sales director North Europe at FLIR Systems 


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