SkyModels

Preetham et al.

http://www.cs.utah.edu/~shirley/papers/sunsky/sunsky.pdf

Javascript implementation for O3D : http://www.patapom.com/O3D/patapi/skydome.js (and http://www.patapom.com/O3D/patapi/skydome_SHCoefficients.js for SH coefficients)

Nishita et al. (1993)

http://nis-lab.is.s.u-tokyo.ac.jp/~nis/cdrom/sig93_nis.pdf