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What does "Correction for lumen units" mean?

When Use Inverse Squared Falloff is turned on, Intensity of point/spot light seems to be internally multiplied by 16.0.

 // Correction for lumen units
 DeferredLightUniformsValue.LightColor *= 16.0f;

What is the meaning of this 16.0f ?

This value results in white diffuse material lit by 1963 lm point light with distance of 100 cm returns color 1.0.

 1963 * 16 / 100^2 / PI  = 1

In other words, white diffuse material lit by 156 lx returns color 1.0.

 1963 / (4 * PI) = 156

Is this a magic number without any physical meaning? If so, why this value is chosen?

Product Version: UE 4.9
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asked Oct 29 '15 at 11:12 AM in Rendering

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tenten
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2 answers: sort voted first

f at the end means number type is "float", not "double" which is default fraction number type when you use ".x" (x being faction number) at the end

Other then that i guess it's to correct units used in rendering libraries to be closer in resemblance of real world unit of lumen

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answered Oct 29 '15 at 05:05 PM

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Shadowriver
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avatar image tenten Nov 01 '15 at 01:57 AM

Hi Shadowriver,

Yes, I also think 16 is to be closer in resemblance but I don't have any idea how this value is derived. Is this based on knowledge of camera or something?

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I know this is a somewhat old thread, but I would like to inform you about the rationale behind this x16 factor, as I just got a reply from Epic on this regard.

Basically, it is a compensation factor to make things look bright enough at default exposure settings.
In the case of point light falloff contribution, the expected 1/(4pi) distribution factor has been omitted.

For the interested reader, this is the question: https://udn.unrealengine.com/questions/280050/clearing-confusion-with-photometric-measurements.html

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answered Mar 11 '16 at 01:21 AM

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slomp
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