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Using Physically Correct Brightness in Cycles


Johnson Martin writes:

In order to better understand the lighting system in the Cycles render engine, I've decided to dive into understanding how Cycles uses physically based units to accurately measure the intensity of emissive materials. On the surface, the values used by Cycles seem somewhat arbitrary. But digging deeper I've found that once again, Cycles has a facet of accuracy quite underestimated by the common user.

So let's dig in.

About Author

Johnson Martin

I'm Johnson, 3d artist and writer. I'm currently a student, exploring the world of creative arts and a part time 3d Artist at Martin Media. I run Topology Guides, a blog that gives tips and tricks for 3d modelers. l also write for BlenderNation and occasionally, the Sketchfab blog.


  1. Dragon Venom Animation on

    Thank you for the article. It's always nice to get a better understanding of the basis for the wide range of value settings in blender.

    • Didn't notice your comment. Sorry!

      I'm not sure if that is 100% true. Sure, our eyes don't always view colors as they are, but this does not negate that color should be reproduced correctly.

      Also, the moon has no direct kelvin value since it is not an emitter. Realistically, it would be the Kelvin value of sunlight, around 4000-5500k (the moon is also dielectric , so the reflection of light is more or less pure). Often times it seems as though the Moon is white or even blue in photos, however, this is generally due to improper white balance.

  2. Edwhard hitten on

    Personaly I tried kelvin values and didn't worked for me at all.
    Instead I use the chart created by Jeremy Birn in the article on my blog. I took every color and converted to an RGB value, then made a group node with all the values and when it's applied to light sources in my scenes (without textures and only white/gray materials) the light color feels right immediatly.

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