Left: Sketch of an integral number count with a power-law fit (dashed red line) at an arbitrary flux or luminosity limit. Right: Arbitrary spectrum of a source at the flux or luminosity limit, where the red part corresponds to the part of the spectrum integrated in a given band X. In the most general case survey, it is always possible to fit a power law at the flux or luminosity limit. However, here the spectra are not a power law, so for a photometric survey the spectral index α cannot be defined as S_ν ∝ ν^−α over the band.