Fast and accurate measurement of spectral emissivity with a portable field infrared spectrometer: ancillary equipment and methods

Validation of land surface emissivity (LSE) is very important for assessing the accuracy of remote sensing products and understanding the potential and limitations of retrieval methods. Currently, a commonly used method for obtaining in situ spectral emissivity at ground level is to use a portable Fourier Transform Infrared (FTIR) spectrometer, such as the Model 102F Portable Field Spectrometer. However, since complicated procedures are necessary to ensure the accuracy of measured spectral emissivity, the traditional measuring methods for the 102F FTIR, are usually time-consuming. Additionally, a time-consuming process also means the thermal environment can change, which can further decrease the accuracy of the measurement. A prior knowledge of the sample emissivity in certain wavelength intervals is also necessary for the calculations in the current software package for the 102F FTIR, which makes the performance of the estimated LSEs highly dependent on the accuracy of this prior knowledge. To overcome the limitations in measuring LSE with the 102F FTIR, this paper presents a new solution using well-designed ancillary equipment and an optimized temperature and emissivity separation algorithm. The ancillary equipment consists of 4 major parts and is mainly designed for rapidly switching the sample and the diffuse gold-plate using a remote control. As no extra manual adjustments are necessary in our method, it will obviously decrease the risk of thermal infrared environmental change during measurements. In addition, by using our ancillary equipment, the reflected environmental radiance over the height surface is now measurable instead of using the approximations at the ground. By using a Wavelet-Transformed Temperature and Emissivity Separation (WTTES) algorithm, the surface temperature and the spectral emissivity are now simultaneously obtained without any prior knowledge, thus making the retrievals more reasonable. According to the experiments, our method is more efficient and accurate for measuring the spectral emissivity through the 102F FTIR spectrometer.