How to get surface illuminance values?

Good day,
I am simulating light propagation from a spot light over a landscape in UE5. The spot light is the sole source of light. I would like to compare the surface illuminance (in lux) computed by UE5 versus values that I’ve got from a photometry simulator (Tracepro) and real world measurements.
I found that I can “probe” the surface using the HDR histogram overlay in the viewport and get a discrete lux value. Ideally, I would like to see the illuminance values over my entire landscape (as a colormap). Or maybe I could write a piece of code to probe the landscape over a grid and export the gridded lux values. Then I would create the illuminance map outside UE5. Not sure which class and methods should be used to build the code to get the lux values.
I would be happy to hear suggestions, if any. Thanks!

Hi, did you solved the problem? I’m facing the same and haven’t found help about it.

Hi, Not yet. It does not seem trivial. If I found a solution, I will post it here.

Try looking into loushang gpu lightmass code.
In theory, the values are calculated and used for the shading to occur.

Also the eye occlusion system messes with things / there is a built in post process you have to remove to “see” the real scene.

In the case of checking numbers, if that post process is in place before you check your values are bound to be off…

Thanks for the hints! I will look into that.

Hi dgingras - I am also researching how to do this - have you managed to develop a solution at all? Would be very interested to know. I am a lighting designer using EU now to simulate my designs. If I had the ability to show a lux map - I could potentially not have to use alternative ‘lighting design software’. Thanks