Method for accurate measurement of infrared emissivity for opaque low-reflectance materials

Accurate determination of infrared (IR) emissivity is important for non-contact temperature measurement and for energy balance evaluation in systems that exchange radiation. A method for accurate measurement is proposed based on active modulation of the background radiation. The hemispherical directional reflectance is measured as a proxy for directional emissivity using an IR camera and an integrating sphere, while the background radiation is modulated using an IR emitter and a mechanical shutter. Measurement of the apparent temperature observed by the camera under two different illumination conditions allows the extraction of reflectance and emissivity. The accuracy of the measurement and its sensitivity to surface properties are analyzed, showing uncertainty values as low as 0.004 in some cases. Example measurements of natural and artificial surfaces are presented.