Spectral indices and spectral curvature for each spectral class. The top panel shows the cumulative histograms of the spectral index α_{[ 1.4−4.85 ]}, while the middle panel shows α_{[ 4.85−10.45 ]}. Spectra are classified as steep, flat, or inverted. The bottom panel shows the spectral curvature (C), defined as the difference between α_{[ 4.85−10.45 ]} (higher frequency) and α_{[ 1.4−4.85 ]} (lower frequency).