Spectral properties of land surfaces in the thermal infrared: 1. Laboratory measurements of absolute spectral emissivity signatures

Abstract

A laboratory procedure to obtain absolute spectral emissivity signatures of soils in the thermal infrared is developed and discussed. Particular attention is given to correction for environmental radiation and to the estimation of the final accuracy attainable. The study covers the [3.0–5.5] μm and the [8.0–13.5] μm bands with a spectral resolution of 2.5% of λ. The method does not require any a priori assumption on the physical parameters, nor does it require any direct temperature measurement. Absolute emissivity signature ε λ can be measured within ±0.6%. Bidirectional reflectivity signatures, ρ b λ (0, 0), are obtained as well, against a reflectivity standard: a flame‐sprayed aluminum diffusor. It is spectrally flat and has been calibrated. SiC powder, SiO 2 sand, a number of soil samples, montmorillonite, and kaolinite have been measured. Good correlation is found between ε λ and ρ b λ (0, 0) as to the spectral information involved. An attempt to calculate the form factor F λ (0) from the emissivity and reflectivity signatures is made. F λ (0) is related to the bidirectional reflectivity. The accuracy, however, is poor. No spectral information can be inferred from the data. Agricultural soils have very similar signatures in the [8.0–13.5] μm band, dominated by the SiO 2 content. Above 10 μm, ε λ is rather flat and of the order of 0.97±0.01. Emissivity of clays is very flat and high above 8 μm. In the [3.0–5.5] μm band, emissivity ratios can be large. Characteristic spectral features can be resolved (CaCO 3 ). Thus simultaneous use of both thermal infrared atmospheric windows would certainly improve the interpretation of remotely sensed data.