How to read the temperature from a thermal image and trust the number
A thermal camera doesn't measure temperature. It measures infrared radiation and works out a temperature from it, using assumptions about the surface you pointed it at. Get the assumptions wrong and the number is wrong, however good the camera.
To read a temperature from a thermal image you need a radiometric file, which stores a raw value for every pixel, and software that converts those values with the camera's calibration and the object parameters: emissivity, reflected apparent temperature, distance, and air temperature and humidity. Hovering over a pixel in such software gives that pixel's temperature. The colours in the image are only a rendering; never read a temperature off the palette by eye.
Colours are not temperatures
The ironbow or rainbow picture is the camera mapping a temperature span onto a palette. Change the span from 20–40 °C to 20–80 °C and the same breaker turns from yellow to purple without getting any colder. Two images with different spans can't be compared by colour at all, which is why reports should lock one palette and span across the images being compared.
Emissivity: the setting that matters most
Emissivity is how well a surface radiates compared with a perfect emitter, from 0 to 1. Most building materials, paint, tape, wood and plastics sit around 0.90–0.95, and readings on them are reliable. Bare, shiny metals sit far lower, polished aluminium as low as about 0.05, and act like mirrors: the camera sees the reflection of whatever is around them, not their own temperature.
Practical rules:
- Measure on a high-emissivity spot: paint, insulation, a piece of electrical tape stuck on beforehand.
- Treat readings on bright metal as unreliable, whatever the camera says.
- When you change emissivity in software, the temperature should move. If it doesn't, the file isn't radiometric.
Reflected temperature, distance and atmosphere
Reflected apparent temperature is the temperature of the surroundings that the surface reflects. It matters more as emissivity drops; a quick way to measure it is to aim at crumpled aluminium foil with emissivity set to 1.0.
Distance, air temperature and humidity account for the infrared absorbed by the air between the camera and the target. Across a room it barely matters; from a drone at 40 m it does.
Radiometric files store all of these with the raw data, so you can correct a value set wrongly in the field afterwards. HeatDeck recomputes every reading when you change them, per image or across a batch.
Spot, box and line: which to use
- Spot for one point you can describe, such as a terminal or a fixing.
- Box with max, min and average for a component or area; the max finds the hot spot so you don't have to aim precisely.
- Line for a profile across a wall, pipe or panel, to see where a temperature changes.
Make sure the target fills several pixels. A spot smaller than the camera's resolution averages the target with its surroundings and reads low.
Read temperatures from your own images
Hover for the temperature, draw spots and boxes, correct emissivity afterwards. Free for 10 images a day.
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FAQ
Can I get temperatures from a normal thermal JPEG?
Only if it is radiometric. A plain JPEG holds colours only; the temperature data has to be stored in the file.
What emissivity should I use?
About 0.95 for most painted, organic and building surfaces. Avoid measuring bare, shiny metal; stick tape or paint on it if you need a reading.
Can HeatDeck be used to measure body temperature?
No. HeatDeck is for building, electrical, mechanical and solar inspection and is not a medical device.