[Radiance-general] illuminance on external facade

[Ery Djunaedy]

Folks,

This may not be a relevant question for Radiance list, but I ask it anyway.

If I have the solar incident (irradiance, direct and diffuse) on the 
external facade, is there a simple way to calculate/estimate the 
illuminance on the facade? The magic number 179?

Thanks,

Ery