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What you need to know about spot size ratio before purchasing a thermal imaging camera

Thermal inspection being performed with a portable FLIR camera
Thermal inspection with portable camera
FLIR A50/70
Fixed monitoring system installation

What thermal camera is right for your application?

Which one will accurately measure your target AND fit within budget?

How far from the target can the camera be and still measure temperature accurately?

When considering thermal imaging cameras for monitoring or inspection, it is critical to get these questions answered. To make the most of an investment in thermal imaging technology, you want to be sure it is the right tool for your application. In this post, we share what you need to know about spot size ratio.

According to a technical note published by FLIR Systems, you can compare spot size ratio to an eye exam at the optometrist’s office. When looking at the eye chart from the exam room chair, you may be able to see that there are letters on the smallest line – but at what distance can you still read those letters? Those with 20/20 vision can determine the smallest letters at greater distances. In this analogy, 20/20 vision is compared to a high-resolution thermal imaging camera. To read the smallest letters, those who do not have 20/20 vision need corrective lenses or must move closer to the eye chart. The same is true for thermal imagers. Cameras are available with different lenses and resolution to allow for the most accurate temperature measurement based on the setting.

So, what is spot size ratio?

It is a number that tells you how far the camera can be from a target of a given size and still obtain accurate temperature measurement. Accuracy depends on getting as many pixels from the camera’s detector as possible onto your target. The higher the camera resolution, the more likely to get a greater number of pixels on a target from farther away with accurate results.

Does digital zoom improve accuracy?

No. Higher resolution (or narrow field of view) is key.

How can you verify a camera will achieve accurate measurement in your setting?

You need to know some details about your application and the camera specs.

  • Size of the target area, or spot, you want to measure
  • How far away the camera will be (whether in a fixed position or held by a person)
  • Camera field of view (FOV)
  • Camera resolution

Once you have this information, you can do some calculations to determine the spot size ratio. Or, it’s much easier to use this FLIR online FOV calculator. Select the camera you are interested in. Then, click “FOV Calc.” for its spot size ratio.


Now you know the importance of spot size ratio. You have the understanding to ensure the thermal imaging camera you are considering is capable of accurately measuring temperature at the required distance. Whether for fixed system installation or a thermography survey using a portable camera, Viper experts will help ensure measurements are obtained from accurate measurement range.


Interested in learning more about calculating spot size ratio yourself? Click here for the FLIR technical note. Viper also highly recommends professional certification and training from the thermography experts at ITC.

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