An infrared (IR) thermometer measures surface temperature without touching the object, by detecting the infrared radiation every object emits in proportion to its temperature. This makes it the fastest way to check temperature on moving parts, energised electrical equipment, or surfaces that are hazardous, hot, or simply inconvenient to reach with a contact probe. This guide explains how non-contact temperature measurement works, its real limitations, and how to use it correctly.
How Non-Contact Temperature Measurement Works
Every object above absolute zero radiates infrared energy, and the intensity of that radiation increases predictably with temperature. An IR thermometer's optics focus the infrared radiation from a target spot onto a detector (a thermopile or similar sensor), which converts the radiation intensity into an electrical signal and, after correcting for the object's emissivity, displays it as a temperature. No contact with the surface is needed — the measurement happens at the speed of light.
Emissivity — The Number That Makes or Breaks Accuracy
Not every surface radiates infrared energy with the same efficiency at a given temperature. Emissivity is a 0-to-1 factor describing how effectively a surface radiates compared to a theoretical "perfect radiator" (emissivity = 1.0). Most IR thermometers assume a default emissivity — commonly 0.95, which suits most painted, oxidised, or organic surfaces — and this default is exactly where accuracy problems come from.
| Surface | Typical emissivity |
|---|---|
| Painted or oxidised metal, wood, concrete, human skin | 0.90 – 0.95 |
| Unpainted concrete, brick, rubber | 0.85 – 0.95 |
| Oxidised/rough steel, cast iron | 0.6 – 0.85 |
| Polished/shiny metal, stainless steel, aluminium foil | 0.1 – 0.3 |
Shiny metal is where IR thermometers lie the most
Distance-to-Spot Ratio
The measurement spot grows larger the farther the thermometer is from the target, and the distance-to-spot (D:S) ratio tells you exactly how much. A 12:1 ratio means that at 12 units of distance, the measurement spot is 1 unit in diameter — so at 1.2 m, the meter is averaging the temperature over a 10 cm circle. If that circle covers more than just the target (a small terminal, a narrow pipe), the reading blends in the temperature of the background as well, and will not represent the target accurately. Get closer, or use an instrument with a higher D:S ratio for long-distance spot checks.
What Infrared Thermometers Cannot Do
- See through surfaces: An IR thermometer reads only the surface temperature of what it's pointed at — it cannot read the internal temperature of a sealed enclosure, a liquid inside a tank, or a wire inside insulation.
- See through glass or clear plastic: Most glass and many clear plastics are largely opaque to the infrared wavelengths these instruments use — pointing one at an oven window or glass barrier will read the glass surface, not what's behind it.
- Measure steam or transparent gases: There is no solid surface to radiate from a specific point, so readings through steam or open flame are unreliable.
Common Applications
- Electrical panel and connection hotspot checks: Scanning busbars, terminals, and breakers for abnormal heating without touching live equipment — often the first screening step before a full thermal imaging survey.
- HVAC and refrigeration diagnostics: Checking supply/return air temperatures, coil surface temperatures, and duct hot/cold spots.
- Motor and bearing condition monitoring: Spot-checking motor casing and bearing housing temperature as an early warning of developing faults.
- Food safety and cold chain: Fast surface temperature checks on food and cold storage without contamination risk from a contact probe.
CIE supplies Vartech digital thermometers including non-contact infrared models for industrial and electrical maintenance use. Contact us to discuss the temperature range and D:S ratio suited to your application.