A photometric passband is the wavelength-dependent response used for a brightness measurement. An energy-weighted measurement is $I=\int\eta_\lambda F_\lambda\,d\lambda$, with <spectral flux density> $F_\lambda$. Photon-counting conventions can instead include a factor $\lambda/(hc)$; the calibration must use the same convention as the measurement.


Photometric passband

{title2=$F_\lambda,\ F_\nu$}

Spectral flux density is radiant power received per unit area and per unit wavelength or frequency. The two conventions satisfy $F_\lambda\,d\lambda=-F_\nu\,d\nu$ under $\nu=c/\lambda$, hence $F_\lambda=(c/\lambda^2)F_\nu$ when the arguments refer to the same photons. Integrating over a <photometric passband> gives its measured flux.


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 Incoming links: Spectral flux density