A handheld infrared thermometer gives an instant surface temperature without touching anything. That makes it ideal for motors, switchgear connections, bearings, pipes and furnaces. It also makes it easy to get a confidently displayed wrong answer. Two things cause most errors: emissivity and spot size. Blackbody sources and contact measurements are how you check the readings.
How an IR Thermometer Works
Every surface emits infrared radiation that increases steeply with temperature. The thermometer focuses part of that radiation onto a detector and converts the signal to temperature, assuming the surface emits a known fraction of what a perfect emitter (a "blackbody") would emit at the same temperature. That fraction is the emissivity, ε, between 0 and 1.
Emissivity
Most non-metals — paint, plastics, rubber, ceramics, oxidised or dirty metal — have high emissivity, around 0.9 or above, and many fixed-emissivity thermometers are preset to 0.95. Bare, polished metals have low emissivity and also reflect radiation from their surroundings, so an IR thermometer pointed at polished aluminium or a bright copper busbar can read far below its true temperature, or show the reflected temperature of something else.
Ways to deal with it:
- Use an instrument with adjustable emissivity and set it for the material.
- Apply high-emissivity tape or matt paint to a small area and measure that.
- Measure an oxidised or painted part of the same component.
- Check one point with a contact probe and adjust emissivity until the IR reading matches.
Spot Size
The distance-to-spot ratio (D:S) tells you how large an area the thermometer averages. At 12:1, the spot is 1 cm across at 12 cm and 10 cm across at 1.2 m. If the target is smaller than the spot, the reading averages target and background together. Get close enough that the target fills the spot with margin to spare.
MEWOI's IR thermometer range runs from compact fixed-emissivity models such as the 810 and 330S to the MEWOI303B (−20 to 550°C, emissivity adjustable from 0.1 to 1, D:S 12:1) and high-temperature models with adjustable emissivity that reach 1350°C, 1650°C and 2200°C. Check both the temperature range and whether emissivity is adjustable before choosing a model.
Blackbody Sources
A blackbody source is a cavity or coated plate held at a precisely controlled temperature, with known, high emissivity. Pointing an IR thermometer or thermal camera at it shows how far the instrument's reading departs from the known temperature. This is how radiation thermometers are calibrated by comparison and checked in the field.
MEWOI's blackbody sources focus on the body-temperature range used for checking fever-screening cameras and medical IR thermometers. The supplier lists the HTY and EZ669 at emissivity ≥0.99 over 35–45°C and 35–50°C, the HTX2 at 0.97 ±0.02 over 35–45°C, and the ITM80B and C50 covering 5–50°C for thermal cameras and IR thermometers. The ITM80B has a 70 × 70 mm source and stability of ±0.05°C per hour. For industrial temperatures above that range, a higher-temperature blackbody is needed.
When to Use Contact Measurement Instead
Where results need traceable accuracy, or the surface is shiny, enclosed or must be logged over hours, thermocouples and RTDs are usually better. Safety standards' temperature-rise tests on luminaires, plugs, sockets, appliances and tools are made this way: thermocouples are fixed at defined points and logged until temperatures stabilise.
LISUN's TMP-8 and TMP-16 multiplex recorders log 8 or 16 K-type thermocouples from −40 to 300°C, with 0.5% accuracy, and plot temperature-rise curves for certification records. For spot checks of liquids and process materials, MEWOI's CW2800 probe thermometer covers −50 to 330°C with a stainless steel probe up to 2 m long.
Frequently Asked Questions
Can an IR thermometer measure through glass? Generally no. Ordinary glass is opaque at the wavelengths most IR thermometers use, so you measure the glass surface instead.
Is a thermal camera calibrated the same way? Yes. The camera views a blackbody at known temperatures. Large-area plate blackbodies suit cameras, whose field of view is wider than a spot thermometer's.