Showing the creation of the “feature mask,” ﹩{{{\boldsymbol{ \psi }} }_{m}}(\lambda )﹩, for H₂O (top) and CO (bottom). In blue are the first principal components of the molecules while in green are the spectral features selected for the mask. The horizontal red line indicates the threshold parameter η, Equation (11). Spectral features above this threshold are taken to be “significant features of the absorber” and included in the feature mask. The vertical red lines show the major cuts in the feature mask. For the case of H₂O, being an absorber/emitter across a broad wavelength range, most wavelengths are included in the feature mask ﹩{{{\boldsymbol{ \psi }} }_{{{{\rm H}}_{2}}{\rm O}}}(\lambda )﹩. CO only absorbs/emits in discrete wavelength ranges and only those will be included in the feature mask of CO, ﹩{{{\boldsymbol{ \psi }} }_{{\rm CO}}}(\lambda )﹩.