If you have the average luminance of the sky, then you should be able to multiply by pi to get the corresponding average Lux value (Note that in my above post I actually deduce that Unreal’s skylight units are already in cd per square meter and not Lux as stated by ). You can test this in unreal by placing a perfectly white, 100% rough, 0% metallic, 0% specular, ball in the scene and checking its luminance values with the pixel inspector. A directional light with a lux value of 3.14 (pi) will produce a luminance of 1 at the highlight. Creating a perfectly white sky sphere (make sure it’s luminance is 1 when hovering over sky with the pixel inspector if using bp_sky_sphere, as it has weird output) and using it as the input for a skylight, will produce a ball with luminance values of 1 spread over the entire surface of the ball.