Light Unit


What is the light unit in UE4?

Edit: I’m more specifically thinking about Directional and Sky lights to simulate real world values for outdoor lighting. Not the point lights.

Those lights don’t actually use a real world value for their intensity. I kinda wish they did, but I agree with others who have said that there are too many factors to give an exact value to sunlight. What you should do is turn off Auto Exposure, turn off its bias, and then find what seems to be a good brightness value for your scene. That will give you a good starting point based on your specific need. Epic uses a sun intensity of 3.14 for many of their demos, but I think Koola has used 6 for some of his demos, and I’ve seen 10 for many others. They all look good. It’s an artistic decision. Maybe someone else has a different solution.

Thank you StephaBon, that is exactly where the problem is. You cannot find a good starting point. Because when you are trying to do so you are actually looking at an environment. Say the environment was authored darker than it should have been then you need to be using overkill value for the sun/sky lights or the other way around. But if we knew in the first place the values we are using for our global lighting are correct and are representing the look we have in mind then we were comfortable to tweak all the materials until we achieve the desired look.

The only way we could achieve more close to realistic values for sun/sky lights I think would be having a library of ~30-40 physically accurate materials placed in the scene and only then we can do what you suggested since the materials are all 100% pbr accurate. (the materials in starter content are using pbr rendering system, but are not accurate with values. Only the metallic ones are accurate with values since those values are the most common in pbr charts and if someone author those very basic materials wrong it’d be too bad. But grass, moss etc. are not accurately done).

For point lights and spot lights the lighting units are in lumens. But for directional lights I don’t know, it would need to be a pretty high value but I’m guessing that it’s still physically based.

I’m new to Unreal but been working for years in proprietary engines and PBR rendering workflows. Would be great to get an answer from Epic regarding the sun values in Unreal and how the “intensity” parameter maps to nits / lumens / etc? I could apply the logic that I have with other engines to derive the sun values based on real world measurements. That’s a longer term plan, but if Epic doesn’t provide specifics, this is probably the next logical step.

I imagine a decent way of determining sun brightness for PBR materials would be to buy an x-rite color kit and a 50% grey sphere, go outside and photograph it at specific times throughout the day to get sunlight at different intensities and times of day, then use the information from your photos to match 50% grey from the x-rite color kit in different lighting conditions inside unreal. Would be nice if epic games did this and released the results. I imagine it’s good info to have anyway for in-house development.

The problem with that is you’d need a very good camera (which is expensive) with all correct settings otherwise the result is very different.
It’s added to the wishlist.