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By Andrew Baker, Sales Director North Europe, FLIR Systems

Maybe you’ve 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 isn’t 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 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 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 doesn’t improve accuracy, so higher resolution or narrow field of view is key here.

Let’s say you’re looking to get an accurate temperature measurement of a 20 millimetre target from 15 metres away with your thermal camera. How do you figure out whether your camera can do this? You’ll need to check the spec of your camera and know both the field of view and the resolution. For this example, let’s say your camera’s resolution is 320 x 240, and your lens has a 24-degree horizontal field of view.

You first need 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 your FOV

Since your lens has a 24-degree horizontal FOV, you’ll divide 24 by the camera’s horizontal pixel resolution — in this case, 320. Then you’ll 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, you then must find your IFOV in millimetres with this formula:

IFOV (mm): (1.308/1000) x 15000* mm = 19.62 mm

*The distance from your 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 tells you that your camera can measure a 19.62 mm spot from 15 metres 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 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. We recommend 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 15 metres away.

Now let’s say you want to measure a 20 millimetre spot. How far you can accurately measure that specific spot size? You need to use a little cross-multiplication:

IFOV in mm: Distance in mm (15 m = 15000 mm)


20 mm : x

15000*2 = 58.86*x

300000/58.86 = x

x = 5096.8 mm or approximately 5.1 m

You can measure a 20 mm spot from approximately 5 m 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 don’t consider the spot size ratio.

To make the calculations quicker, FLIR has a FOV calculator for each of its cameras on 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.

FLIR Systems specialises in technologies that enhance perception and awareness.  The company brings innovative sensing solutions into daily life through its thermal imaging and visible light imaging technology and systems for measurement, diagnosis, location and advanced threat detection.  Its products improve the way people interact with the world around them, enhance productivity, increase energy efficiency and make the workplace safer.

FLIR Systems has six operating segments – surveillance, instruments, OEM and emerging markets, maritime, security and finally, detection. Of these six, ‘instruments’ is of greatest interest to trade and industry and the second largest segment in the company’s portfolio. This division provides devices that image, measure and assess thermal energy, gases and other environmental elements for industrial, commercial and scientific applications.

These products are manufactured across five production sites, three in the USA and two in Europe; Sweden and Estonia.

A model to suit every application and budget
The options that FLIR Systems provides for measuring temperature and studying thermal performance have never been greater.  Not only does the company offer a huge range of models to suit all thermal application needs but the technology is also affordable and very easy to use.  Thermal cameras now come in various shapes, sizes and degrees of sophistication and FLIR continues to invest heavily in the development of new and complementary technologies to differentiate itself from competitors.

An important milestone in the development of thermal imaging has been the introduction of the FLIR Lepton® core, a micro longwave detector, the size of a mobile SIM.  This has allowed thermal imaging to be repackaged to meet the needs of an even wider audience and, in combination with another new technology called Infrared Guided Measurement – IGM™ – has led to the development of a range of test and measurement meters with imaging capability.

Another important growth area for FLIR thermal imaging is in continuous monitoring to assure quality and safety.  Through its introduction of discrete fixed mounted thermal cameras which are fully compliant industry standard plug-and-play protocols, FLIR Systems has provided industry with infrared machine vision which is instantly ready for quick and easy network installation.

Protecting assets and people from fire is an area for which thermal imaging is least known but, thanks to FLIR Systems’ development, it is now one of the most cost-effective methods available.  Its application flexibility and rapid return on investment present an attractive proposition for any site or safety manager.

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