Introduction

The region of the spectrum in the vicinity of 10 μm wavelength is called the thermal infrared. It is important because many materials have strong vibrational absorption bands there (Chapter 3). In most remote-sensing measurements these bands can be detected only through their effects on the radiation that is thermally emitted by the planetary surface being studied. Many substances have overtone or combination bands at shorter wavelengths, and although the latter bands are observed in reflected light, their depths and shapes may be affected by the thermal radiation that is emitted by the material. Hence, even though the primary subject of this book is reflectance, it is important that the effects of thermal emission be discussed. It will be seen that most of the preceding discussions of reflectance also apply to emissivity at the same wavelength because of the complementary relation between the two quantities.

Figure 13.1 shows the spectrum of sunlight reflected from a surface with a diffusive reflectance of 10%, compared with the spectrum of thermal emission from a black body in radiative equilibrium with the sunlight, at various distances from the sun. Clearly, thermal emission can be ignored at short wavelengths, and reflected sunlight at long, but at intermediate wavelengths in the mid-infrared the radiance received by a detector viewing the surface includes both sources.