Wavelengths of RGB rendered colors and conversion to radiometric units

Development Rendering
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Hello everyone,

I see that in unreal photometric units are used in the rendering pipeline. I wonder:

  • What are the wavelengths and the bandwidth around each color of the RGB channels? Is this defined somewhere? I couldn’t find a clear definition in the documentation.

  • If someone would like to use radiometric units instead, what would be the way to convert (from lumens to watts)? I tried CIE’s photopic curve but the green wavelength is weighted by a factor that is way too smaller than the R and B, which makes the output look weird.

Sorry for the too technical question and have a good day! :slight_smile:

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Someone might be an expert on the specifics, but what I can say is that the end color is influenced mostly by two things. The color space and tone mapping.

The color space is typically sRGB, and this is likely responsible for your experience with green. sRGB modifies the linear color values into a non linear range that better represent our eyes sensitivity to that spectrum.

Once that has been done, the colors go through tone mapping and post processing. Usually this is to make the image aesthetically pleasing or to match filmic tone mapping, and is rarely kept at natural values.

Long story short, both of those things will screw up any objectivity of the values unless you take them into consideration. Since it’s just math, you can easily convert to and from sRGB, linear or other color spaces, for example.

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Thanks a lot @BananableOffense for the insights !

Indeed, I should have clarified it… I’m already looking at linear RGB and tonemapping is turned off. I’ve also set the exposure accordingly (see Auto Exposure in Unreal Engine | Unreal Engine 5.5 Documentation | Epic Developer Community) so I’m sure I’m getting luminance (in cd/m2) as an output, which I was able to also validate with a simple analytic set up :slight_smile:

My questions are more regarding the physical meaning of the channels (like, can they be interpreted as the sum of the light energy over given wavelengths and, if so, what are the band of wavelengths).