Hi Raphael,
Although lux (lumen/m2) is a unit of measuring for luminous flux, it is very difficult to convert solar radiation (Wh/m2) to lux. When we use lux, we are measuring the amount of visible light, or solar radiation within 400-700 nm (roughly) and there are different conversion factors for every wavelength. Also, the amount of luminous flux within this range in the indoor environment is dependent on your glazing’s SHGC and other material properties. Imagine how difficult this gets once you account for reflected light/radiation off of interior materials.
I’ll try to find some reference to help explain this more thoroughly. I hope this helps in the meantime.