HEMISPHERICAL EMISSIVITY BY INFRARED REFLECTANCE

摘要

Emissivity and absorptivity are key parameters inmodels for thermal analysis. Absorptivity depends onthe spectrum of the incoming radiation in the ultra violet(UV) and visible (VIS) range and is thus largelyindependent of temperature. Emissivity characterises theoptical properties of surfaces towards infrared radiation,and is therefore temperature dependent. Very commonlythe emissivity is determined at room temperature, I.e.Between about 3 – 30 μm. With a few exceptions, thetemperature of any exposed surface of spacecrafts willsignificantly differ from that temperature. The spectralproperties of respective materials will therefore have aneffect on the emissivity under real operationalenvironment. For example metals may undergo aminimum in reflectance around 1 μm, having an impacton emissivity at high temperatures. Polymers on theother hand may effect the emissivity at both extremes,high and low, due to specific absorption bands.Typical methods for the determination of temperaturedependence of emissivity are the dynamic thermalmethod or infrared reflectance. Due to the ease ofsample preparation and speed, the infrared methodsrepresent a very versatile approach. The hemisphericalemissivity can be determined collecting total reflectanceinfrared spectra using a Fourier transformed infrared(FTIR) spectrometer coupled with an integrating sphere.Due to the spectral limits of beam splitters anddetectors, several wavelength ranges need to beacquired independently. By that it is possible to coverthe reflectance from the UV/VIS to the far infrared(FIR). Once the spectrum is measured, emissivity can becalculated for any temperature needed as long as mostpart of the blackbody radiation falls into the spectralrange measured. This approach allows multipleapplications, and examples are presented for:? Emissivity on nozzle at high temperatures withworking conditions between 1000 and 1300 K. Asignificant increase in emissivity can be observed athigher temperatures.? Emissivity of black coatings for cryogenicapplications, where for temperatures down to 30 K awavelength range of at least 300 μm need to becovered. The difficulty of using an integrating sphere in the FIR is overcome by a variable angleaccessory, where incident and reflected angles canbe set independently. This is supporting theassessment of specularity/diffusity of the samplespecimen.