An Etymological Dictionary of Astronomy and Astrophysics

The normalized interstellar extinction at a given wavelength. It is defined by f(λ) = A(λ)/A(Hβ) - 1, where A(λ) is the extinction at the given wavelength and A(Hβ) the extinction at Hβ, with f(Hβ) = 0. It is used to → de-redden observed fluxes:

I(λ)/I(Hβ) = F(λ)/F(Hβ).10^c(Hβ).f(λ),

where I represents the flux in the absence of extinction and F the observed flux affected by extinction, c(Hβ) being the → reddening coefficient.

See also: → reddening; → function.