To differentiate among geological materials and vegetation using therm
al infrared spectra, radiometric measurements require corrections for
focal temperature variations and reflected radiation. A laboratory tec
hnique is described for the determination of spectral emissivity signa
tures for natural geological samples using a Fourier Transform Infrare
d Spectrometer and the self-emission of the scene. The approach descri
bed does not require control of the sample temperature. A quantitative
assessment of uncertainties is presented. Comparison of the emissivit
y measurements from natural geological specimens with hemispherical re
flectance data gave an average difference of about 0.01 in emissivity,
except near strong features of the spectra. The technique is applicab
le to field measurements of high-emissivity objects.