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I've been reading a bit on realistic lighting in Blender and was introduced to realistic physical light values, as well as color management through Filmic Blender and the Exposure slider.

This excellent tutorial suggests that the value for direct sun lighting outdoors is a whopping 441 units. This completely blows up the lighting in the scene, making it look bathed in nuclear fire.

A swipe of the Exposure slider in color management fixes that. However, if I'm not mistaken, this also messes with other light-related things which are organized around more "relaxed" numbers, including...

The Emission shader.

I have a character with glowing eyes. In my "regular", unrealistic, "if it looks good it works" lighting setup, I used a Strength of about 4 to control the emission on her eye textures (combined with the Diffuse via an Add shader), which made them look sufficiently glowy. However, with sun lighting cranked all the way UP and exposure all the way DOWN, it feels like they barely glow anymore.

Blender docs say the following about Emission shader strength:

Strength of the emitted light. For point and area lamps, the unit is Watts. For materials, a value of 1.0 will ensure that the object in the image has the exact same color as the Color input, i.e. make it ‘shadeless’.

This confuses me. Does this mean that Emission material strength doesn't change regardless of the lighting? How can you even go beyond "shadeless"? Does this mean Emission materials don't use Watts as a unit, and so I can't apply the realistic lighting technique?

TL;DR

  • Do materials need to be changed according to the scene (lighting, exposure), or is the simulation realistic and materials only need to be configured for their realistic physical properties?
  • Is it normal that realistic light values completely destroy the scene when using default Exposure, and it needs to be dialed way down?
  • Is there a way to calculate realistic, physically correct emission for materials? (Ex. the emission of an LED lamp on a monitor)
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Strength of the emitted light. For point and area lamps, the unit is Watts. For materials, a value of 1.0 will ensure that the object in the image has the exact same color as the Color input, i.e. make it ‘shadeless’.

This is junk documentation, written by someone that clearly doesn't understand scene referred values.

A material albedo value of 1.0 means that 100% of the light is reflected back. This does not result in the exact same value as the colour input, which depends on the transfer function and other potential sweeteners in the transform from the scene referred to the display referred domain.

While I can't answer the wattage question any better than the other answers on this site already well written, I can speak to exposure. Exposure is always a creative choice. Glowing eyes will likely always be less glowing than a screaming ball of fire in the sky that is gargantuan. That is a creative issue that could be negotiated with creative compositing or creative exposure within your scene.

Does this mean that Emission material strength doesn't change regardless of the lighting?

Emissions are scene referred. That is, the effective range of a value is some potentially infinitely small value to some potentially infinitely large value. If you set an emission to 146281.8, it is that value. How that value ends up in your image is a complex byproduct of the aforementioned transform from the scene referred to the display referred domain. That is, think of the transform as a sort of film stock. Various film stocks may have different colour responses, different dynamic ranges, and a few other aesthetic elements that will take a physical scene quantity of light and convert them into an aesthetic output.

Do materials need to be changed according to the scene (lighting, exposure), or is the simulation realistic and materials only need to be configured for their realistic physical properties?

Starting with physically plausible values is a great entry point. Ultimately however, materials should serve the creative needs.

Is it normal that realistic light values completely destroy the scene when using default Exposure, and it needs to be dialed way down?

Is it normal for a camera to take an overexposed photo when set to manual? Same idea applies here; exposure is a creative choice, as is balancing lighting in a scene, either starting with realistically plausible values or creatively lighting a piece with artificial sources.

Is there a way to calculate realistic, physically correct emission for materials? (Ex. the emission of an LED lamp on a monitor)

Yes. This is a duplicate question with more than a few good answers on this site.

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I have received an answer by /u/Edgarska on Reddit which, I find, answers everything.

That 441 strength seems correct, the reason it was never used before was because of the lack of the filmic view, but now that it's there we can use real sun values. When you take pictures in sunny weather you also have to adjust exposure accordingly.

As for emission shaders, they use the same unit as the sun lamp, W/m2 .

To answer the questions, if you're using the principled shader then it should be ready to use in any lighting situation as long as your textures are correct (no albedo values outside the realistic values), but if you're using the other shaders then you might have to adjust them per scene.

As for the strong light destroying the scene with default exposure, that is the point of having control over the exposure, you wouldn't use the same camera settings for a picture in the sunlight as you would in an interior shot.

I would also recommend installing the "real camera" addon, it enables you to use real camera settings so the picture comes out just as it would if you were using a real camera in real lighting conditions.

Yes, there is a way to get realistic values for real light sources, although you have to hope the manufacturer makes those numbers available, or you have a way of measuring it. Most laptop screens are equivalent to about a 5-10 on emission strength, so you can use that as a starting point.

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    $\begingroup$ That's really interesting. Thanks for coming back to post that. $\endgroup$ – Scott Milner May 12 '18 at 17:12

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