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FAQ -Emissivity
What is Emssivity?

1. Emissivity is a term used for the ability of a surface to radiate energy. Most (90% of typical applications) organic materials and painted or oxidized surfaces have an emissivity of 0.95 (pre-set in the thermometer).
2. Inaccurate readings will result from measuring shiny or polished surfaces. To compensate, cover the surface to be measured with masking tape or flat black paint.
3. Allow time for the tape to reach the same temperature as the material underneath it. Measure the temperature of the tape or painted surface.

Any reference information regarding emssivity?
What should the emissivity setting be when measuring furnace temperature?

The emissivity setting of the IR-thermometer should be 1.00. The furnace can be seen as a blackbody cavity, i.e. the effective emissivity approaches 1.00 due to multiple reflections of thermal radiation inside the furnace. The corrections and errors on the measured surface temperature are small with surrounding surface at similar temperature.

How can I correct the emissivity setting of my IR-thermometer looking through a window?

Three effects must be considered: absorption, reflection or scattering of light/thermal radiation. The ideal window has only low reflection losses of light, i.e. the window material has a small refractive index. The best way to compensate for the effect of the window is to calibrate the IR-thermometer with the window in the same geometry and conditions as in the application, e.g. decrease the emissivity setting of the IR-thermometer until the same reading is obtained looking through the window as without the window. Be aware of things becoming fairly complex when the window is hot and absorbs in the spectral range of the IR thermometer.

Why is emissivity important?

Knowledge of surface emissivity is important both for accurate non-contact temperature measurement and for heat transfer calculations. Radiation thermometers detect the thermal radiation emitted by a surface. They are generally calibrated using blackbody reference sources that have an emissivity as close to 1 as makes no practical difference. When viewing 'real' more reflective surfaces, with a lower emissivity, less radiation will be received by the thermometer than from a blackbody at the same temperature and so the surface will appear colder than it is unless the thermometer reading is adjusted to take into account the material surface emissivity. Unfortunately, because the emissivity of a material surface depends on many chemical and physical properties it is often difficult to estimate. It must either be measured or modified in some way, for example by coating the surface with high emissivity black paint, to provide a known emissivity value. The NPL provides a service for measuring the emissivity of samples (for further information see the NPL website) which is used by customers when they need valid surface temperature measurements or heat transfer calculations.

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