Temperature Measurement · Glossary

What is Emissivity?

Symbol: ε0 to 1Temperature Measurement

Emissivity is the ratio of radiation emitted by a surface to that of a perfect black body at the same temperature.

Emissivity Explained

A black body has ε = 1. Most non metals and painted surfaces are around 0.9 to 0.95. Polished metals have low emissivity and also reflect surroundings, so pyrometers read them badly.

Set the emissivity on the pyrometer to match the surface, or apply high emissivity tape or paint to create a known spot.

Emissivity: Typical Values

Black body	1.00
Water, most organics, paint	about 0.95
Oxidised steel	about 0.8
Polished aluminium	about 0.05

Calculators for Emissivity

CalculatorPyrometer Emissivity CalculatorFree online calculator with formula and worked example.CalculatorRadiation Heat Loss CalculatorFree online calculator with formula and worked example.

Frequently Asked Questions

What is Emissivity?

Emissivity is the ratio of radiation emitted by a surface to that of a perfect black body at the same temperature.

How do I find emissivity?

Measure the surface with a contact sensor and adjust ε until the pyrometer agrees.

Does emissivity change with temperature?

Yes for many materials, and with oxidation and surface finish.

Learn more on the blog

Thermocouple and RTD installation precautions

A practical guide on instrumentationblog.in with more detail on emissivity and how it is applied on real plants.

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