Real world Lumens and Unity light Intensity

Hey Peeps,

I have seen a couple people ask similar questions a couple years ago but still no answers. Is there any direct correlation between the real world units of lumens and unity lights intensity property?

So I know theres no direct connection between lumens and light intensity, you have to take in to account light emission (candela) from a certain angular span (steradian) but maybe someone more intelligent than me has already done this?

 

Probable is just an brutal approximation. Exactly how PBR/PBS works, have nothing to do with real world lighting model, what's much more complex.

 

Bump. This is a good question, I feel like when this was first written no one really took this into consideration. I think the intensity should be based off of SI units like Candelas, or Lumens. In order to make realistic looking effects and lighting, we need to talk in terms that physicists use to describe the real world equivalent.

https://feedback.unity3d.com/suggestions/light-parameters-should-be-based-off-si-units

 

Absolutely.

 

Hello, is there any update on this topic? The project I am working on would really benefit from a real-world light scale or conversion.

 

Only the

HDRP

uses Physical Light Units (PLU):
https://docs.unity3d.com/Packages/c...inition@10.0/manual/Physical-Light-Units.html

The Built-in RP is considered legacy since 2018 (src) - you can still use it for the foreseeable future, but no new features
will be added to it.

 

I have a follow-up question: For URP, isn't there some sort of equation I can just do to the intensity value to end up with a corresponding "accurate-enough" (or at least the same as HDRP) Lumen/Lux value?
I mainly want to know how many lumens = x intensity on a standard spot/point light in URP.
Thanks!

 

Has anyone attempted something like this? Thanks!

 

This would be great to know if you want to go from HDRP to URP.

 

There is this asset that I have, but it's not as accurate as I would like. Does have some other helpful things like color temperature presets though!
https://assetstore.unity.com/packages/tools/lumenlights-88391

 

I don't know why but in the Spotlight Tunnel demo for HDRP and URP, they used 800 lux for HDRP project and 84 intensity for URP Project (Directional Light). So 1 lux = ~0.1 intensity in URP?

 

Wow, that's an interesting note, thanks for adding that!
If that's the case, I wonder what the range conversion should be. For lux, range is handled automatically in HDRP

 

I may sound dumb but what is the range?

 

Point and spot lights have a range parameter that determines how far the light will reach (in meters).
Directional lights don't have it. Because I'm handling directional lights with a day/night/sky system, I'm only looking for physical light units for points and spotlights in URP. Ideally set in Lumens, not lux for my purposes.

I know that 1 Lux = 1 lumen per square meter... But I'm too dumb myself to figure out what the range property should be set to.

If I want to set a light to 100 lumens, I would set the intensity to ten (*0.1 like you said), and the range also to ten? But maybe I'm oversimplifying it!

 

Well, the point lights in the Spotlight Tunnel URP project use a range of 4.29 and intensity of 4 while in the HDRP one they use the SAME RANGE but use 100 lumen intensity. Sooo, 1 lumen = 0.25 intensity? I don't know I might be completely wrong.

 

Interesting, thanks for the info. I don't have those demos on hand to test myself, so I appreciate it.

 

No problem

 

Please factor in tonemapping and exposure if making comparisons. These two will dramatically change how your lighting looks.

 

Ah, absolutely right, I forgot! Clearly this is much more complicated than originally thought...
Maybe an HDRP with no post effects enabled would be a good baseline, but that may be impossible based on how it works behind the scenes.

 

Oh, well the HDRP demo was using Auto Exposure but I've set it to fixed 3 EV to compare it with URP properly. And in URP I've set -3 EV Post Exposure in Color Adjustments PP Effect. Both EVs seem to work in the same way I guess. Both are EV100 according to Unity. And both projects were using ACES tonemapper. But in the baked lightmap view with lightmap exposure set to -3, HDRPs lightmaps looked more bright than URPs lightmap.

 

Note : Post Exposure in Color Adjustments PP Effect of URP is not in EV100 but in EV according to Unity docs. In the editor tho it says "Adjusts the overall exposure of the Scene in EV100" when you hover your mouse over it. I don't know which one is true.

 

Still nothing conclusive on this. Extremely frustrating, especially when trying to recreate HDRP visual effects in URP with VFX Graph.

The two SRPs seem to handle light intensity and emission in fundamentally different ways.

Edit: Ugh turns out I missed a small HDR checkbox on the camera that I could've sworn I had checked already.

 

I'm doing something like this, so It may be help.

https://github.com/8izips/LightPLU

 

This looks super promising!

 

Nice one! I've worked on something like that before. But yours seems to be a bit different. In my conversion, 2 intensity and 1 Post-exposure in URP was equal to 100k lux and -13 Exposure in HDRP.