How to open a FLIR image on a Mac and actually read the temperatures

A FLIR radiometric JPEG is two files in one. The part every image viewer understands is an ordinary JPEG: the coloured rendering the camera produced, palette and all. Hidden in the file's metadata segments is a second block in FLIR's own format, holding the raw 16-bit sensor value for every pixel, the camera's calibration constants, and the object parameters that were set when the picture was taken.

Preview reads the first part and ignores the second. That is why a FLIR image looks right on a Mac but behaves like a screenshot: the colours are fixed, the scale can't be changed, and there is no reading under the mouse. The temperatures are still in the file. You just need something that decodes them.

The thermal block typically contains:

• The raw thermal image, one value per pixel, usually 160×120 to 640×480 depending on the camera.
• Planck calibration constants (often labelled R1, R2, B, F and O) that convert raw sensor counts into temperature for that specific camera.
• Object parameters: emissivity, object distance, reflected apparent temperature, atmospheric temperature and relative humidity.
• The visual photo on cameras with a second lens, plus palette and measurement overlays.

Because the raw values and calibration travel with the file, the temperatures can be recomputed later with different emissivity or distance. That is the whole point of a radiometric image, and the part a plain viewer throws away.

Run Windows. A virtual machine with FLIR Thermal Studio works, at the cost of a Windows licence, a Thermal Studio subscription ($216–432 a year) and an Apple Silicon Mac running an ARM build of Windows.

Do it by hand. exiftool can dump the raw thermal image and the Planck constants from a FLIR JPEG, and a short Python script can turn them into a temperature array. It is accurate if you apply emissivity and reflected temperature correctly, and it is a fine route for one-off research. It is slow for an inspection with 200 images and a client waiting.

Use a native app. HeatDeck opens FLIR radiometric JPEGs directly on macOS: drag in a folder, hover for temperatures, draw spot, box and line measurements, change emissivity and distance, lock a palette across the batch and export a PDF report. It also opens InfiRay / Topdon .irg files in the same batch.

Not every FLIR-branded JPEG carries temperature data. Some entry-level cameras and phone apps save a non-radiometric JPEG, and editing a radiometric JPEG in a photo editor and re-saving it strips the thermal block. If an app shows no temperatures for a file, check it with exiftool yourfile.jpg | grep -i planck: no Planck constants means no radiometric data, and no software can recover it. Always keep the original files from the camera.