How to get surface illuminance values?

[flyskyosg]

Good day,

I am simulating light propagation from a spot light in filament. My scene may have multiple light sources like this. I would like to get the surface illuminance (in lux) computed by filament.

How should I convert the power parameters of the light for use in ray tracing calculations of the scene?

How to get surface illuminance values?

I would be happy to hear suggestions, if any. Thanks!