A way to approximate complex lighting in ultra realistic renders.

All SH lighting techniques involve replacing parts of standard lighting equations with spherical functions that have been projected into frequency space using the spherical harmonics as a basis.

http://www.cs.columbia.edu/~cs4162/slides/spherical-harmonic-lighting.pdf

Spherical harmonics as used at Weta Digital

https://www.fxguide.com/fxfeatured/the-science-of-spherical-harmonics-at-weta-digital/

https://bellarender.com/

Bella works in spectral space, allowing effects such as BSDF wavelength dependency, diffraction, or atmosphere to be modeled far more accurately than in color space.

https://superrendersfarm.com/blog/uncategorized/bella-a-new-spectral-physically-based-renderer/