I am involved in a DLI (daylight integral) study in for which I have spectral transmission data available. I’m assuming generic properties for other opaque materials as this is still early stage. This is my modelling approach:

Calculate spectral average (400 - 700 nm) of transmission (without any weighting applied, I cover this later)

I then run an irradiance study (to output W/m2 instead of lux) and convert to micro mol / m2 / s assuming a CIE D65 illuminant with the following equation:

My question though, is whether I should also weight the spectral power by the (normalised) spectral transmission… I think yes! I emphasise normalised transmission such that the total power remains the same, just that it’s weighted more heavily on the wavelengths with more transmission and vice versa.

However, my spectral transmission changes depending on the angle of incidence, so in any case this would be an approximation (i.e. using normal transmission).

Hi @jroberts , when you refer to spectral transmission data, are you implying that you have a glazing (glass) material in your scene ? If so, I think that you don’t have to weight the spectral power by the spectral transmission as this will be handled by Radiance. The spectral weights (RGB channel values) for the sky can be incorporated into the gensky/gendaymtx/gendaylit commands and the spectral weights for the transmission can be incorporated into the glass material.

Actually, I have water as a dielectric. I have modelled the transmission based on water transmission data in the photosynthetic range (400-700 nm).

My question is more about the conversion from W/m2 to mol/m2/s, since as I understand it, it depends on the spectral distribution (which is a function of the source distribution - CIE D65, and the transmission distribution, as not all wavelengths are equally transmitted).

Hi @jroberts , if your sensor was just behind a glass material or even behind a dielectric material then as per my understanding you won’t have to do anything extra as the three channel radiation received at the sensor will be SPD weighted by the transmission distribution by Radiance while ray tracing. In the paper that you mentioned before, we were only dealing with urban contexts so we didnt have anything transmitting.

Anyway, assuming that you are measuring this radiation inside the dielectric (water), I am not sure if the spectral weighting will still be done by Radiance. Perhaps @Greg_Ward can clarify?