How to choose an infrared pyrometer correctly?
A non-contact infrared pyrometer measures an object's thermal radiation and converts it into a temperature. This guide explains how emissivity, the distance-to-spot ratio and the spectral range work together so you can select the right instrument.
View infrared pyrometersWhy is emissivity so important?
Emissivity ε describes how efficiently a surface emits thermal radiation - a value between 0 and 1. A pyrometer measures the radiated energy but needs to know the emissivity to calculate the correct temperature. If the set value is wrong, the reading deviates significantly.
Matte, dark surfaces such as painted metal, rubber or plastic have a high emissivity around 0.90 to 0.95. Bare, shiny metals like aluminium or stainless steel often sit below 0.20 and reflect stray radiation - here measurements are unreliable without correction.
- Human skin, water, wood: ε around 0.95.
- Paint, plastic, ceramic, rubber: ε around 0.90 to 0.95.
- Oxidised metal: ε around 0.60 to 0.85.
- Bare aluminium, polished metal: ε below 0.20 - critical.
What does the distance-to-spot ratio (D:S) mean?
The distance-to-spot ratio (D:S) tells you how large the measured circle on the object is at a given distance. At 50:1 the instrument measures a 10 mm spot from 500 mm away. The larger the first number, the smaller the target it can read from a distance.
Crucially, the target must completely fill the measuring spot. If the spot is larger than the object, the cooler or hotter background enters the reading and distorts the result. For small components or long distances you therefore need a high D:S ratio.
Which spectral range fits your material?
The spectral range is the band of wavelengths in which the sensor detects infrared radiation. Standard instruments work in the long-wave band from 8 to 14 µm and cover most everyday applications from -50 to about 1000 °C. Special cases call for other wavelengths.
- 8‑14 µm: universal standard for non-metallic surfaces and moderate temperatures.
- 1.6 / 2.2 / 3.9 µm (short wave): for hot metals, glass and flames at high temperatures.
- 5.0 µm: specifically for glass surfaces.
- 3.43 µm: tuned to thin plastic films such as polyethylene.
For shiny metals a short-wave instrument offers a decisive advantage: in this band the emissivity is higher and more stable, so the reflection error drops. For glass or plastic film the right wavelength prevents the pyrometer from measuring straight through the material.
Frequently asked questions
Can I measure shiny aluminium with a standard instrument?
Only to a limited extent, because the low emissivity below 0.20 causes large errors. Apply matte tape or use a short-wave pyrometer designed for metals.
How far can I stand from the object?
That depends on the D:S ratio. At 50:1 and a 20 mm target, about 1 m is the maximum before the spot grows larger than the object.
Do I always have to set the emissivity?
For accurate values, yes. Many instruments are fixed at 0.95, which suits matte surfaces. For metals or glass you should correct the value or use a reference surface.
What is the difference between a pyrometer and a thermal camera?
A pyrometer measures a single point as one temperature value, while a thermal camera delivers a full image of many points. For single readings the pyrometer is faster and cheaper.
Looking for the right infrared pyrometer?
We stock non-contact pyrometers with different D:S ratios, adjustable emissivity and spectral ranges for metal, glass and plastic.
Calibrated
Instruments with traceable accuracy available.
Correctly specified
We match D:S and spectral range to your task.
Wide range
From -50 to over 1000 °C depending on model.
Expert advice
Our measurement specialists support your choice.