An infrared thermometer measures the thermal radiation emitted by a surface and converts it to a temperature reading without any physical contact. Making it the tool of choice for measuring hot, moving, hazardous or hard-to-reach surfaces. From checking electrical distribution boards for overheating components to verifying food display cabinet temperatures for SFA compliance, infrared thermometers are one of the most versatile instruments in any facility maintenance or quality control toolkit.
How Infrared Thermometers Work
All objects above absolute zero emit thermal radiation as infrared energy, with intensity and spectral distribution related to temperature by Planck's law of blackbody radiation. An IR thermometer's optical system focuses infrared energy from the target onto a detector (typically a thermopile or pyroelectric detector), converted to an electrical signal and then a displayed temperature. The optical system defines the field of view via the distance-to-spot (D:S) ratio — a 12:1 ratio measures a 100 mm spot at 1.2 metres. The measurement spot must be smaller than the target surface or the reading gets contaminated by background temperature; small targets at distance need a higher D:S ratio (50:1 or more).
Understanding Emissivity
Emissivity, the single most important concept in infrared thermometry and the most common source of error, is the ratio of infrared radiation emitted by a real surface to that emitted by a perfect blackbody at the same temperature, on a scale from 0 to 1. A perfect blackbody (1.0) emits maximum radiation; real surfaces read lower. Organic materials, painted surfaces, skin and most non-metallic surfaces have high emissivity (0.85–0.98), while polished or bare metals have very low emissivity (0.05–0.3), causing dramatically wrong readings if the instrument's emissivity setting doesn't match.
Common Material Emissivities
| Surface | Typical Emissivity | Notes |
|---|---|---|
| Human skin | 0.98 | Very consistent across races |
| Black paint (matt) | 0.96–0.98 | Good target for reference checks |
| Concrete, brick | 0.90–0.95 | Suitable for IR measurement |
| Plastic (most types) | 0.85–0.95 | Good emissivity in most cases |
| Oxidised steel | 0.60–0.80 | Adjust emissivity setting |
| Polished aluminium | 0.02–0.10 | Unreliable. Use contact measurement |
| Polished stainless steel | 0.10–0.20 | Unreliable. Use contact measurement or cover with tape |
For polished metal surfaces, either apply matt black tape or paint to the measurement spot and allow to reach thermal equilibrium, then measure the tape's surface. This approach gives accurate readings even on highly reflective materials. This technique is commonly used in electrical panel inspections and motor bearing temperature checks.
Applications of Infrared Thermometers
Electrical maintenance and safety: thermal surveys of switchboards, distribution boards, motor control centres and cable terminations identify hot joints, overloaded conductors and failing components before outages or fires, a recognised predictive maintenance technique under MOM workplace safety expectations. IR thermometers give a fast first-pass survey; for full documentation, a thermal imaging camera (see our thermal imaging guide) provides a 2D temperature map suitable for engineering reports and insurance/compliance records. HVAC and building services: checking supply/return air temperatures at grilles, pipe surface temperatures, motor and bearing temperatures, and AHU electrical components — including identifying cold spots on ductwork that may cause condensation in Singapore's humidity. Food safety: fast, non-contact checks of display cases, cold counters, hot-holding equipment and cooking surfaces, but since IR reads surface not core temperature, and SFA requirements for cooking/holding refer to internal temperature, any suspect reading needs follow-up with a contact probe thermometer. Industrial process monitoring: moving webs, rollers, extruder dies, mould surfaces and conveyors where contact measurement is impractical, without needing slip rings or wireless transmitters.
Limitations of Infrared Thermometry
IR thermometers have important limitations that users must understand:
- Surface measurement only: IR thermometers cannot see through transparent materials (glass, plastic films in the visible spectrum). They measure the surface of the material, not the interior.
- Reflective surfaces: Polished metals, glass and other highly reflective surfaces can give grossly inaccurate readings due to reflected thermal radiation from nearby objects.
- Atmospheric interference: Steam, smoke, dust and high humidity can absorb or scatter infrared energy, reducing accuracy at longer distances.
- Small targets at distance: The measurement spot must be smaller than the target. At long distances, even a high D:S ratio instrument has a large spot that may include background areas.
- Unknown emissivity: If the emissivity of the target material is unknown, measurement error is introduced. Always set the instrument's emissivity to match the target material.
Choosing an Infrared Thermometer
Key selection criteria for IR thermometers include:
- Temperature range: Ensure the instrument covers the full range you need. For electrical work: -20 °C to +500 °C is typical. For furnace or kiln inspection: up to +1000 °C or higher may be needed.
- D:S ratio: Higher is better for small targets at distance or for precision work. 12:1 is adequate for most HVAC and electrical work; 50:1 or higher for industrial targets at distance.
- Adjustable emissivity: Essential for professional use. Basic fixed-emissivity instruments are suitable only for measurement of surfaces with known emissivity close to 0.95.
- Laser pointer: Dual or circle lasers help define the measurement spot. Ensure the laser pattern matches the actual measurement spot at your working distance.
- Data logging: For trending or documentation, logging capability (to PC or memory) is valuable.
- Ruggedness: For field use in Singapore's industrial environment, an IP-rated, drop-rated instrument from a reputable brand is worth the investment.
Fluke infrared thermometers, available through Unitest Instruments, cover the full range from general-purpose models to professional instruments with adjustable emissivity, data logging and high D:S ratios. Browse our product range to find the right model. Our team can advise on the best instrument for your specific application.
Calibration of Infrared Thermometers
Like all measurement instruments, IR thermometers require periodic calibration to verify accuracy. Calibration involves comparing the instrument's reading against a calibrated reference blackbody radiator at known temperatures across the instrument's operating range. Our SAC-SINGLAS accredited temperature calibration laboratory (LA-2023-0845-C) provides calibration for infrared thermometers. Standard turnaround is 3–5 working days. Contact us to arrange calibration.