Mapping Human Settlements Using the Middle Infrared (3-5 m): Advantages, Prospects, and Limitations
Green leaves exhibit very low reflectance — 2 to 5% — comparable to that of liquid water, and plant canopies are likely to exhibit even lower reflectance due to multiple scattering (Salisbury and D’Aria 1994; Figure 16.1). MIR is better able to penetrate anthropogenic haze and smoke (particle radii
0.1 to 0.4 nm) than red wavelengths, although it is worse for dust (particle radii
1000 to 2000 nm) (Kaufman and Remer 1994). Soils are MIR bright, reflecting up to one third of the incident radiation (Salisbury and D’Aria 1994; ASTER Spectral Library 1998; Figure 16.2). Rocks are generally less reflective than soils but exhibit more definite spectral signatures (Salisbury and D’Aria 1994; ASTER Spectral Library 1998). Cellulose, lignin, hemicelluloses, and other structural plant molecules exhibit enhanced reflectivity between 3.6 and 4.3
m (Elvidge 1988; Salisbury and D’Aria 1994), but less than soils (Salisbury and D’Aria 1994; Snyder et al. 1997; Figure 16.1). View angle dependence of emissivity is greater in the MIR than thermal IR (Labed and Stoll 1991), but there is little change in the spectral features (Snyder et al. 1997). View angle dependence becomes an issue when the scene is composed of objects of sharply contrasting reflectance and emission characteristics (Smith et al. 1997). Changes in spectral emissivity across 3 to 5
m are greater than the sensitivity to temperature changes (Wan and Dozier 1996).