I'm implementing the revised ASHRAE (Tau) clear sky model and am trying to find a source for the beam and diffuse optical depths described in the engineering reference: https://www.energyplus.net/sites/default/files/docs/site_v8.3.0/EngineeringReference/05-Climate/index.html
The depths appear to be based on the Beer-Lambert law but I can't find any documentation on how the diffuse and beam are calculated in the ASHRAE material. The goal is to produce the solar radiation estimates for use in a EPW file.
Question: Can someone provide documentation for calculating beam/diffuse optical depths? Or is it just better to use the Zhang-Huang model?
