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====== PerezSunSky ======

A physically coupled sun + anisotropic Perez sky model represented as 73 directional light sources: 72 sky elements (6 rings &times; 12 azimuths) with Perez-weighted power densities, plus 1 direct sun source.

====== Inputs: ======

  * for solar position via Goudriaan's algorithm:
    * latitude
    * dayOfYear
    * hourOfDay
    * minuteOfHour
  * diffuse horizontal irradiance [W m⁻²].
  * direct normal irradiance [W m⁻²].

====== Sky model ======
The Perez all-weather sky radiance distribution (Perez et al. 1993) is:

$$  f(θ, γ) = (1 + a·exp(b / cos θ)) · (1 + c·exp(d·γ) + e·cos²γ)$$


where $θ$ is the sky-element zenith angle and $γ$ is its angular distance from  the sun.  The five coefficients $(a-e)$ are selected from a lookup table indexed by the sky clearness $ε$. 

The 72 directional lights are normalised so that 

$$ \sum_i(powerDensity_i·sin(elevation_i)) =  dhi$$

====== References ======

  * Perez, R., Seals, R., Michalsky, J. (1993). All-weather model for sky  luminance distribution. Solar Energy 50(3), 235–245.
  * Perez, R. et al. (1990). Modeling daylight availability and irradiance components from direct and global irradiance. Solar Energy 44(5),  271–289.