Parameters used to characterize the filters

From the transmission curve, the central wavelength is computed assuming a source with an energy distribution of the form λ. F_λ(λ) = Cte in which case the energy distribution follows a similar form: ν. F_ν(ν) = Cte

In other terms, the central wavelength is the harmonic mean λ₀= ∫T(λ) dλ / ∫T(λ)/λdλ if T(λ) is the filter transmission.

The width is computed as an average between the WHM (difference between the most distant points where the transmission crosses the value 0.5) and the effective width λ_eff assuming a source with energy distribution having the property λ. F_λ(λ) = Cte. The formula used is

W = WHM/2 + σ. sqrt(3) if σ is the 2nd-order moment (σ²= ∫(λ–λ₀)²/λ. T(λ) dλ / ∫T(λ)/λdλ)

The value of W corresponds to the actual width if the filter has a rectangular shape (transmission equal to 0 or 1), and to 2.9σ for a Gaussian distribution.