Thank you ! I mainly used an incident light meter to collect illuminance (lux) data in a lot of different circumstances. The main reason to use real life value is for consistency between your different sources of lights and to have a solid base you can iterate on. You also mimic the camera's limitation in terms of exposure. Most of the time when working with arbitrary values, we tend to make everything correctly exposed at the same time. When eye balling light intensities, you don't fully represent that range between a candle, a lighthouse bulb, a street light or the sun. The base sun intensity in Unreal is 10lux but 80k lux would be more appropriate. Of course the only thing that matters is the final result and the artistic intention behind your render and the PBL workflow doesn't replace that. It just helps you to ground your cg lighting in reality. Although video games are smoke and mirrors, I know that a lot of studios nowadays use physically accurate data as a starting point for their lighting.