Why is the solar BSDF used to calculate the solar irradiance?

The solar BSDF characterizes the optical property of a CSF in the near-infrared wave band. It is used to calculate the solar irradiance or heat gain calculation with Radiance. However, if an object absorbs visible light, it also produces heat. Why was only near-infrared light considered when calculating solar irradiance or heat gain?

Short answer: because Radiance is a lighting calculation, not a thermal model. If you need comprehensive thermal modeling, you will need different tools not included in Radiance.


Hi, Greg

Thanks for your reply!

Firstly, I want to confirm whether the solar BSDF is measured in NIR only or NIR+Visible wave band.
Secondly, If the solar BSDF is measured in the NIR wave band, is it feasible to calculate the sum of irradiance in NIR and Visible wave band by two Radiance models, respectively?

Unfortunately, I don’t know the answer to this question. I believe the BSDF files contain a reference standard that serves as the basis for the measurements, and this can vary from one file to another. I’m wondering if @JCJonsson could offer more insight.


The solar BSDFs generated by WINDOW and similar XML files present in the CGDB are calculated from spectral data in the 300-2500 nm band and weighted according to NFRC 300. So it is not the case of only containing NIR data.
We have dabbled with using Vis and NIR matrices but there should be no evidence of that, so sorry if you uncovered that specific strata of history.
I can imagine exceptions that would make the above statement false; it is not impossible to make WINDOW use a user defined wavelength band for the solar calculations. But it is unlikely.

Hope that helps,