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WASI manual version 3
23
2.9 Sky radiance
The same parameterization as for Ed(λ) is also implemented for Ls(λ):
Ls(λ) = [ α* · tA(λ) + β* · (λ/λR)-4.09 + γ* · (λ/λM)ν + δ* · tC(λ) ] · E0(λ).
(2.26)
The functions E0(λ), tA(λ), (λ/λR)-4.09, (λ/λM)ν, and tC(λ) are those of eq. (2.23). Parameters of
Ls(λ) are the weights α*, β*, γ*, δ*, which represent the relative intensities of the four abovementioned light sources for a radiance sensor, and the exponent ν.
This model of Ls(λ) has been included for modeling specular reflection at the water surface.
Its usefulness has been demonstrated (Gege 1998b). Capillary waves at the water surface, and
moreover gravity waves, spread greatly the sky area that is reflected into the sensor, and
change the angle of reflection. Consequently, measurements of Ls(λ) are frequently not reliable. For these cases, and if no Ls(λ) measurement is available, eq. (2.26) can be applied. If
the user selects the wavelength-independent model of surface reflections, Ls(λ) = Ed(λ)/π is
set.