The manpage of the rpict for -i option writes that:
Boolean switch to compute irradiance rather than radiance values. This only affects the finalresult, substituting a Lambertian surface and multiplying the radiance by pi. Glass and other
transparent surfaces are ignored during this stage. Light sources still appear with their original
radiance values, though the −dv option (above) may be used to override this.
However, I did not quite understand it regarding the sentence “This only affects the final result, substituting a Lambertian surface and multiplying the radiance by pi”. For example, for direct sunlight calculation in sun-coefficient method, the reflectance of indoor surface is modeled as zero, if the irradiance were obtained via multiplying the radiance by pi, then the irradiance should be pi*0. Obviously, the calculated irradiance using rpict is not zero otherwise the material map doesn’t work. I guess whether the indoor surface was replace by a lambert surface of a certian reflectance and then multiplied the radiance by pi.