The Extract Features Tool is used to extract surface and atmospheric features from CHRIS images. Before you can perform the Feature Extraction on the CHRIS product, you have to apply the TOA Reflectance Computation first.
The measured spectral signature depends on the illumination, the atmosphere, and the surface. The figure below shows CHRIS Mode 1 band locations compared with the spectral curve of healthy vegetation, bare soil, and the atmospheric transmittance. The spectral bands free from atmospheric absorptions contain information about the surface reflectance, while others are mainly affected by the atmosphere.
At this step, rather than working with the spectral bands only, physically-inspired features are extracted in order to increase the separability of clouds and surface covers. These features are extracted independently from the bands that are free from strong gaseous absorptions, λi ⊂ BS, and from the bands affected by the atmosphere, λi ⊂ BA. A detailed analysis of the extracted features can be found in the ATBD of the cloud screening.