The emissivity of real materials is often unknown, for the following reasons:
- there is limited published emissivity data
- the exact material composition is unknown
- the emissivity of a material can change in use due to heating in air (oxidation) or other effects (see Influencing factors)
Because of the above factors, there are often large uncertainties in estimating emissivity values. Emissivity and reflectivity data are commonly used to deduce temperatures resulting from use of radiation thermometers or thermographic instruments. If these are your applications, you have two main options:
- make emissivity measurements. Often it is possible to prepare small samples of the material you are working with. In such a case, contact one of the measurement institutions listed in Measurement services in this technical area or, for a fuller search, look in Measurement services in the left-hand menu of the website. Your material will be characterised according to your specific needs (for example, if you are using an IR thermometer, the normal spectral emissivity at wavelengths corresponding to the working band of your thermometer can me measured) and you will be in the best position for using your IR thermometer reliably.
- use educated guesswork. There are often data trends or bands of possible values that may help the user estimate a property value. An example is illustrated in the two figures below taken from the Thermophysical properties of matter data book (eds Touloukian & DeWitt - you will find the separate volumes in Docs & artefacts). The figure on the left shows the possible values of normal spectral emissivity of Inconel - you will see there is a very wide spread - and the figure on the right shows an analysed version of the data, providing a convenient overview to the user for clearer interpretation. For example, the grey band with values around 0.9 represent the analysed data for heavy oxidised Inconel. Other bands are for different material conditions: “as received”, “sandblasted”, “moderate oxidation”, etc. The use of such analysed data may significantly reduce the uncertainty of your measurements.
Normal spectral emissivity of Inconel alloy
"Analysed" normal spectral emissivity of Inconel alloy - helpful when trying to assign an emissivity value to a particular material
In many industrial applications reflected radiation is another problem that can affect temperature measurement accuracy. Thermal radiation reflected from the surroundings will increase the measured signal (assuming the radiation detector is sensitive to those wavelengths) and this leads to an overestimation of the true material surface temperature. Because a quantitative prediction of the reflected radiation is difficult to obtain, some techniques have been developed that help users to reduce these effects. Some good examples of possible solutions for the steel, aluminium, glass and plastics industries may be found in part IV of Theory and Practice of Radiation Thermometry by De Witt and Nutter (the book details can be found in Docs & artefacts). Problems related to thermographic applications are also discussed.
If you want to measure emissivity or some other IR & optical property you could be faced with specific problems related to the different Measurement methods. In such a case, we suggest you ask one of the emissivity experts that have been contributing to evitherm (Consultancy). Alternatively, select from a fuller list of consultants by choosing Services and under "Service type" choose "Consulting".