Radiometric properties of interest
(click to enlarge).

The (directional) emissivity (of a thermal radiator) (Ε) is defined as:

The ratio of the radiance of the radiator to that of a Planckian radiator at the same temperature.

Why emissivity is important

Spectral emissivity over the thermal infrared (3 µm to 60 µm) is a key property determining energy transfer. The reliable prediction of energy gains and losses to and from such structures as buildings, greenhouses, radomes, space vehicles, and industrial process plant has become an important aspect of energy conservation and control. For military applications the emissivity may be as important as the temperature in creating the "signature" of a target for passive infrared systems, while for infrared LIDAR for example, the reflectance at the chosen laser wavelength is the key property. Measurement of diffuse reflectance or of emissivity for these applications requires complete hemispherical irradiation or collection of radiation.

How "near ambient temperature" emissivity is measured at NPL

Over the thermal infrared spectral region, the spectral emissivity is related to, and most accurately determined from, the diffuse and regular (specular) components of reflectance and transmittance of the sample. The spectral emissivity, Ε(λ), at wavelength λ is given from Kirchhoff's law by:

Ε(λ)=1-[ρ_d(λ)+ρ_r(λ)+τ_d(λ)+τ_r(λ)]

where ρ_d(λ), ρ_r(λ), τ_d(λ) and τ_r(λ) are the diffuse transmittance, regular transmittance, diffuse reflectance and regular reflectance respectively.

A knowledge of the spectral emissivity over the thermal infrared spectrum enables total emissivity, Ε_T, to be computed for any temperature, T, over a range of temperatures of interest:

where c₂ is Planck's second radiation constant and the actual limits of integration needed depend on the temperature involved.

In order to calculate an emissivity we first need to determine the transmitted and reflected components. For many samples one or more of the components vanish, e.g. for opaque mirror-like samples the diffuse and regular components of transmittance and also the diffuse reflectance vanish so we need only determine the regular reflectance ρ_r(λ). The spectral and total emissivity can then be calculated using the above equations.

NPL has facilities to measure both diffuse and regular components of reflectance and transmittance and can therefore handle a wide range of sample types. However some samples have particular difficulties associated with them so please telephone to discuss your requirements.

References

1. CLARKE, F.J.J. Measurement of the radiometric properties of materials for building and aerospace applications. Proc. Soc. Photo-Opt. Instrum. Eng., 1980, 234, 40-47.
2. CLARKE, F J J, and LARKIN, J A. Measurement of total reflectance, transmittance and emissivity over the thermal IR spectrum. Infrared Physics, 1985, 25, 359-367.
3. CLARKE, F J J, and LARKIN, J A. Emissivity determined from hemispherical reflectance and transmittance throughout the thermal infrared spectrum. High Temp. - High Press., 1985, 17, 89-96.
4. CLARKE, F J J and LARKIN, J A. Improved techniques for the NPL hemispherical reflectometer. Proc. Soc. Photo-Opt. Instrum. Eng., 1988, 917, 7-14.