How Far Can You Measure with a Thermal Camera?

A common question is “What’s the farthest target my thermal camera can measure?” The answer to this question is complicated, but essentially the distance you can measure depends on your camera’s resolution and field of view (FOV). A camera with a higher resolution will get you more pixels on the target you’re trying to measure, improving accuracy even from a distance. A narrow field of view—something like 6°—will magnify distant objects, again putting more pixels on distant targets.

When measuring temperature with a thermal camera, you want at least 3 × 3 pixels on your target to ensure you’re getting an accurate measurement. A general rule of thumb is to make sure the area you’re trying to measure overfills the spot size measurement tool on your camera. However, just because you can see something on your viewfinder does NOT mean you’re able to accurately measure it.

How do you know whether you can measure a target accurately? When is it important to calculate the Distance to Size Ratio, and when can you probably skip it?

The following types of applications generally will require long-distance measurements:

Inspecting roofs or other high objects from ground level

Monitoring power lines

Monitoring electric substations and transmission lines

Drone monitoring and mapping

Drone solar farm inspection

Distance inspections when it’s dangerous or impractical to get closer to the target.

These applications will likely require you to calculate the distance:size ratio of your camera.

The following types of applications generally don’t require long-distance measurements:

Inspecting the inside of a house or building for air leaks, missing insulation, etc.

Detecting moisture issues

Inspecting a breaker box or other home wiring

Monitoring transformers, motors, etc. in situations where it’s safe and practical to approach equipment

Vehicle maintenance and repair

Measuring large targets when you can easily ensure the target fills more than 3 × 3 pixels

Be aware that using the zoom function on your thermal camera only enlarges the pixels: it does NOT enhance your ability to measure small targets. If the target is particularly small, you may be able to get closer to make sure you have at least 3 × 3 pixels to measure. Be sure to follow all safety precautions and do not approach environments or equipment unless it is safe to do so.

If you have decided that you do need to perform distance measurements, what kind of thermal camera do you need? And which thermal cameras won’t suit your application?

“Entry level” cameras is a term used to refer to lower-resolution cameras. These cameras are great for certain applications when you’re able to get close to your target and especially when the temperature difference you’re looking for is obvious. Lower-resolution options should not be used for distance measurements, especially if it’s important for your application to have accurate temperature measurements.

Higher-resolution cameras will be better suited to making measurements at a distance. Cameras especially suited for distance work include the FLIR T840 and FLIR T860, which include telephoto lens options for temperature measurements of small targets from a distance.