“±2°C or ±2%”: what a radiometric module’s accuracy figure means on a real target
Almost every radiometric module datasheet has a line like “accuracy ±2°C or ±2% (typical)”. It is a fair figure, but it describes the module looking at a calibration source in a lab. On a real target, other things usually matter more. Here is what to check, with MINI 640-2 Radiometric as the example.
Read the line itself first
“±2°C or ±2%” gives a fixed figure and a proportional one. At 50°C, 2% is 1°C; at 300°C, 2% is 6°C. Check which figure applies at your target temperature. Note the word “typical” too, and the ranges: this core measures from −20°C to 150°C in one range and from 100°C to 650°C in the other. Use the range that covers your target with the most margin.
Emissivity is usually the biggest error
A thermal camera measures radiated energy, and surfaces at the same temperature radiate differently. Painted, plastic and oxidised surfaces are usually around 0.9 or higher; bare polished metal can be below 0.1 and mostly reflects its surroundings. Measure shiny metal with the emissivity set for paint and the reading can be badly wrong. Apply the right emissivity for the material in your calculation, or put a patch of high-emissivity tape or paint on the spot you measure.
Reflections
Low-emissivity surfaces act as mirrors in the infrared. A hot lamp, the sun or your own body can appear as a hot spot that is not there. Change the viewing angle a little: a real hot spot stays where it is, a reflection moves.
Spot size
Each pixel reports an average over everything it sees. If the target is smaller than about 3 × 3 pixels, background mixes in and pulls the reading toward it. Check with distance and IFOV: for this core with the 9.1 mm lens (IFOV 1.3 mrad), at 5.5 m, the far end of its sharp range, a target of about 2 cm or larger is enough (calculated).
Distance, air and windows
Over a few metres indoors, air has little effect. Over tens of metres outdoors, humidity and distance absorb part of the signal, and a protective window in front of the lens always does. Correct for these in your software, or check against a reference under your real conditions.
Ambient temperature and warm-up
Accuracy figures assume a stable module. On this core, measurement is specified for ambient temperatures from −10°C to +70°C, while imaging works down to −40°C. After power-up, or after a quick change in ambient temperature, let the readings settle before you trust them. If the product runs outdoors through winter, keep the module inside that measuring range.
Non-uniformity correction
Uncooled cores need periodic correction. With a shutter, the image freezes briefly each time; with shutterless correction it keeps running. This core supports both, with shutterless as standard. For continuous monitoring, ask how often correction runs in the mode you will use and how readings behave around it.
How to check it on your bench
Point the module at something whose temperature you know: a calibrated blackbody if you have one, or a matte black surface measured with a good contact probe. Check at the distances and temperatures of your application, not just at one point.
Tell us what you need to measure.
Send the target, its temperature range, the distance and the conditions. We reply within 24 hours on working days.