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.
Did physical lighting become obsolete already? Epic?
Thread dates back to January and there’s still no support.
Maybe we have to move this to the “Feedback for Epic” section and bump like crazy? 
Maybe even push this thread via Twitter? I think if we make enough people see and react to it, Epic HAS to respond at some point before it turns into negativity. I don’t like doing it like this, but man, if they continue to ignore this, let’s fight the good fight!
Twitter post is out! Share and retweet as if your lights depend on it!
#AllLightsMatter 
Retweeted 
Hey all,
Let me catch up on this thread and I’ll get back to you with one direction or another! Just so you know, I have not been avoiding it, just a super busy time with things going on, I apologize for that!
Hey guys,
I have pinged some of our teams on this. We’re in the thick of getting stuff ready for 4.20 so give us some time to put together a response. I know you’ve been waiting patiently and I promise I’ll stay on top of it, but I ask for a bit more time.
Thanks!
Thank you! Just a suggestion, it was mentioned above, but I’ll say it again. Please, make working presets. It’s a best way to show how everything works together. Scene with super simple exterior and interior. Light scenarios-bright day, cloudy, night. Artificial light in the interior. Dynamic and baked versions. I doubt if anyone will be unhappy with it.
so I may have found why the exposure isn’t working properly. I was looking through the ( https://.com/c/d1MUZwNk/218-physical-light-units ) today after the 4.20 pre-release came out and found this under the 4.19 physical light post. It looks like the EV100 option under metering mode is missing.
That’s just one issue with all of this. You can ignore the exposure in the Post-Process Volume and use the viewport’s EV100 slider as a workaround.
I’ve had decent results with interior lighting and EV100, but exterior lighting is still a little awkward. No clean way of measuring cd/m² for the Skylight/HDRI and intensities for the sun don’t feel correct even with the proper EV. While the Pixel Inspector is super useful, a viewmode for highlighting ranges, similar to false color, would be more beneficial I think.
Have a look at this for some context
Cheers!
So…completely ignore manual and auto-exposure in PPV and just use EV -1.2?
Any news on this topic?
Is somebody activelly using 4.19 in a scene with both interior and exterior lights?
vid on the topic was an eye opener in a lot of senses but even he couldn’t get around the interior lights (point and spot lights) problem.
Even then, I had to wrap my head around a lot of very complex topics, and I know I’m a noob on the subject of physical lights, but i’m not the n00best of n00bs, and I found some concepts… complicated to say the very least (like, aproximating the total brightness of the HDRI image based on pixel values that range from dunno, 3k to 20k)
Any word from Epic on some kind of examples, standards, documentation, something?
Maybe with 4.20 blaze it release around the corner we can expect some kind of feedback on this topic?
I’m currently working on a interior+exterior archviz project in UE 4.19… I have to eyeball everything and completely ignore the meaning of the values and units that I’m using in my artificial and natural lights.
Light eyeballing has been a huge bottleneck in my archviz workflow. I can’t stress enough the importance of proper physical light system in the engine. I would venture to say that this matter is equally (or even more) important for archviz in UE4 than the Unreal Studio stuff.
Yeah, I’m also working in Archviz projects but I think this problem not only affects the Archviz community.
Is like Daedalus says in his video, UE4 offers a light system (vital for any visualization purpose, cause… you want to see things in a screen after all, and if you want to see things on a screen you will have to simulate lighting at some point or another) and one part of that system is broken or so badly documented that is unintelligible.
Then, everything falls apart, your super careful crafted and delighted photogrammetry assets, your super complex multilayered materials… what do they represent? what are they reflecting or reacting to?
I’m gonna be a PBR “purist” here, the system needs a previsible camera, a previsible Lightning and a previsible Material simulation to be called, well, a system, at all.
And what bothers me is that Epic was taking the right steps towards that unified simulated system (EVEN if was a simulation at the end of the day, EVEN if wasn’t 100% accurate, EVEN if it was an approximation)
But on the verge of settling the issue, they just… gave up? and left us with a (apparently) broken or unfinished one.
And I don’t want to get over the Fortnite conspiracy theories wagon but… fortnite doesn’t uses realistic lighting and it prints money so maybe is that, they just gave up.
I found very strange that they let pass a whole numbered update without doing any change or documenting this absolutely CORE (for the engine) issue.
Even “obscure” topics like Distance Fields are better documented than this.
Man, I kinda-maybe-almost understood what a Signed Distance Function is by reading unreal’s help files and a couple of pages on the topic! And I really really SUCK at math and everything related!
Putting a lamp in your level should be FAR easier and predictable than that. Putting a Sun, a Camera with an aperture that simulates real cameras, an aperture bracket for the auto exposure, a skylight, setting an HDRI should be… working out of the box stuff! at least, “go to the help files, read and learn you n00b” stuff!.. not a dig-in-the-forums-and-talk-with-a-guy-who-worked-for-the-star-wars-franchise stuff! that’s my point.
Is light man, is like… the base for everything you do inside the engine… is not 100% Physically accurate? no problem, but give us the numbers! I want to know what numbers I’m punching in. A random Multiply between, dunno, 0 and 1000? OK… but then why… call it Candelas and Lumens and Lux stuff??? at least I know that a random 950 is 950 times stronger than 1 in a arbitrary scale.
Anyways… I get a little angry with this issue as you can see… I don’t understand the topic so deeply as people who writes here and not knowing where else to look for makes me very upset… I love the engine, and I really wish I could use it, I’m stuck with 4.18. And 4.19-20 look amazing (The GPU acelerated bootleg thing for baking light specially)
gonna try the 4.20 preview today and test some stuff. Maybe they fixed something? Somebody said the exposure thing is even worse now, they removed options or somehting… dang man!
Hey man,
any updates on this just yet? And apologies for pushing again…WE care!
Cheers,
Dadedalus
{Constructive feedback
I’m not sure if rendering engineers will happen to read this thread sometime in the future but in case they do, as you might be aware of, physially based lighting isn’t about Sun only, but it’s also about ambient light. With directional light being physically based, best we can do is to make surfaces that are under direct light look more close to reality. But there are parts of the game world where environments are covered in shadows. You can’t make skylight physically based because all it does is capturing the sky and projecting it everywhere, that includes anywhere it shouldn’t be projecting the sky light as well. i.e interiors, forest grounds, caves etc. which makes it impossible to achieve true physically based lighting, as you’re aware. You don’t want to work on a dynamic G.I solution and we’ve accepted that as an unchangeable fact. But Image Based Lighting is the perfect solution between flat skylight and dynamic G.I. It allows every different environment and even every interior to have their own unique ambient lighting. The type of ambient lighting that actually corresponds to the environment around it. Please consider letting the artists make better looking environments in UE4 by bringing IBL to the table. It’s a rather small feature compared to majority of features you’re releasing with every engine update.
End constructive feedback}