Some targets are moving, electrically live, too hot or simply out of reach, so touching them with a probe is not an option. A non contact sensor reads the heat radiation they emit instead, provided you understand emissivity, spot size and the path in between.
Every object above absolute zero emits infrared energy, and the amount rises steeply with temperature. A radiation sensor collects that energy through a lens and converts it into a temperature reading within milliseconds.
What Is an Infrared Thermometer?
An infrared thermometer is a non contact instrument that measures surface temperature by collecting the thermal radiation emitted by a target. It belongs to the radiation family in types of temperature sensors, alongside thermocouples, RTDs and thermistors.
Inside every infrared thermometer, a lens focuses energy onto a thermopile detector. Electronics then correct for emissivity and ambient temperature.
The older disappearing filament instrument described in what is an optical pyrometer compared brightness by eye. Modern devices do the same job electronically and much faster.
Stefan Boltzmann Law and Emissivity
ε = emissivity, from 0 to 1
σ = 0.0000000567 W/m²K⁴, the Stefan Boltzmann constant
T = absolute temperature in kelvin
Doubling T from 300 K to 600 K raises W by 2⁴ = 16 times
Emissivity is the ratio of energy a real surface emits to what a perfect black body emits at the same temperature. Low emissivity surfaces also reflect more energy from their surroundings, which fools an infrared thermometer.
Optris explains that metals show low emissivity that depends strongly on surface condition, while non metals have high and fairly constant emissivity. Fluke notes that shiny metals need a low setting, while wood, paint and plastic fall under high.
Distance to Spot Ratio Explained
The distance to spot ratio, written D:S, tells you how large the measured circle is at a given distance. Fluke gives the example that a 12:1 unit measures a 1 inch spot from 12 inches away.
Example:
D = 1200 mm, ratio = 12
S = 1200 ÷ 12 = 100 mm
Target width = 200 mm
Target to spot = 200 ÷ 100 = 2.00, good fill
A common field rule is to keep the target at least twice the spot size. A fixed infrared thermometer with precision optics usually offers a much higher ratio than a handheld one.
Spectral Bands and Where Each Works
Optris states that wavelengths from 0.7 to 14 µm matter for infrared measurement. Atmospheric windows at 1.1 to 1.7, 2 to 2.5, 3 to 5 and 8 to 14 µm pass through air with little absorption.
General purpose band.
Smaller emissivity errors on metals.
Sees through furnace flames.
Reads thin plastic films.
Compares two wavelengths.
Steel mills pick a short wave infrared thermometer to cut metal errors. For electrically noisy spots, fiber optic temperature sensors are another option.
Handheld Units vs Fixed Process Sensors
A handheld infrared thermometer is ideal for spot checks, maintenance rounds and troubleshooting. Scanning of switchgear and busbars is often done with a thermal camera, as covered in infrared thermography of electrical panels.
A fixed infrared thermometer, or IR pyrometer, mounts on the process with an air purge collar and water cooling jacket. It sends a 4 to 20 mA or digital signal, much like a temperature transmitter, into the DCS or PLC.
6 Proven Tips for Accurate Readings
Fluke recommends letting the instrument sit in its operating environment for about 30 minutes before critical measurements. A cold unit carried into a hot plant will read wrong until its detector reaches equilibrium.
Common Errors in Non Contact Measurement
These sit on top of the general issues in temperature measurement errors. Ordinary glass or acrylic windows are opaque at 8 to 14 µm, so the sensor only sees the window itself.
When emissivity is unknown, compare against a contact reading using a surface pad thermocouple and adjust the setting until both match.
Spot Size Calculator
Close focus models have a minimum spot at their focal distance, so always check the optical diagram on the infrared thermometer datasheet.
- No contact, no contamination or wear.
- Fast response in milliseconds.
- Measures moving, live or distant targets.
- Very high temperature capability.
- Measures surface temperature only.
- Emissivity must be known.
- Affected by steam, dust and windows.
- Spot size limits small targets.
The infrared thermometer responds far quicker than the probes discussed in temperature sensor response time. For internal fluid temperature, a contact sensor from thermocouple vs RTD remains the right choice.
Check each infrared thermometer periodically against a black body source, one of the types of temperature calibrators used in labs and workshops.
Optris Non Contact Basics PDF
How an IR Thermometer Works Video
Infrared Thermometer FAQ
It collects infrared energy from the target through a lens and focuses it onto a thermopile detector. The detector output voltage rises with the amount of radiation it receives.
Electronics then apply the emissivity setting and compensate for the internal ambient temperature. The result is shown as surface temperature within a fraction of a second, even on moving targets.
Emissivity is how efficiently a surface emits thermal radiation compared with a perfect black body at the same temperature. It ranges from 0 to 1 and depends on material, surface finish and wavelength.
Painted, rubber and organic surfaces are near 0.95, which makes them easy targets. Polished metals can be below 0.1 and reflect most of the energy around them instead.
At 12 units of distance, the measured circle is 1 unit wide, so the spot grows as you step back. A 12:1 unit held 1200 mm away therefore averages a spot of about 100 mm.
The target must completely fill that spot for a true reading, ideally with some margin. Moving closer or choosing a higher ratio model fixes most small target problems.
Shiny metals emit very little radiation of their own and reflect a lot from their surroundings. The sensor then sees mostly reflected ambient energy, not the true temperature of the metal.
Apply high emissivity paint or tape where it is safe and allow it to reach temperature. Otherwise choose a short wavelength sensor that is designed specifically for metals.
Ordinary glass is opaque to the 8 to 14 µm band, so the instrument simply reads the glass surface. Viewing through a window needs special materials such as germanium, zinc selenide or calcium fluoride.
Short wave sensors can see through some quartz and glass windows used on furnaces. Always correct for window transmission losses when configuring the setup.
Yes, steam, smoke and dust absorb and scatter infrared energy along the optical path. The reading usually drops below the true surface value, sometimes by a large margin.
Use an air purge collar, shorten the sighting path or pick a two colour ratio pyrometer. Ratio types tolerate partial blocking of the view much better than single band units.
Handheld units are designed for spot checks and maintenance rounds rather than continuous duty. Their readings also depend on how the operator aims them each time.
Fixed process sensors give repeatable, continuous signals and come with purge and cooling accessories. They are the correct choice whenever the value feeds a control loop, alarm or trip.
Related Articles
- Types of Temperature Sensors
- What Is an Optical Pyrometer
- Infrared Thermography of Electrical Panels
- Temperature Measurement Errors
- Types of Temperature Calibrators
External References
- Basic Principles of Non Contact Temperature Measurement, Optris
- How to Get Great Results with an IR Thermometer, Fluke
- Infrared Thermometer, Wikipedia
What We Learn Today
- An infrared thermometer reads surface temperature from emitted radiation, following the Stefan Boltzmann law where energy rises with the fourth power of absolute temperature.
- Correct emissivity and a target at least twice the spot size set by the D:S ratio are the two biggest keys to an accurate reading.
- Pick the spectral band to suit the material, and keep steam, dust, reflections and ordinary glass windows out of the optical path.