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To create a light source using Cycles, I create a plane (or other mesh shape) and set the strength until it looks good. When I scale that light source, say make it three times bigger, the total amount of light increases. Highlights blow out and the scene sometimes goes almost all white. So, I reduce the emission shader's strength.

It seems that the strength is interpreted as luminous watts per square meter. Is this how it works? Wouldn't it be better to be able to alter a luminous object, to control softness and depth of shadows, without changing the overall brightness contributed by that source to the scene. Is this possible to do this easily, and am I understanding what is happening correctly?

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  • $\begingroup$ Actually it is working this way becuse the nature of ray-tracing. When a ray hits an emission, the it "takes the strength back to the ray source". Rays from a material will hit the light source more probable when the visible surface is bigger. So more rays from that material will "bring light back". It would cost much calculation, and would not be natural to do this precisely on the level of the renderer. $\endgroup$ – Róbert László Páli Jul 9 '13 at 22:35
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Set the emission using a driver

Example driver: "0.1 ** (x * y * z)" where x, y, z are the respective scale axis's

See this answer for the basics of setting up a driver.

Result

A scaling cube with diminishing emission

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  • $\begingroup$ What is the rationale for the expression you have for the strength in the driver? Róbert's formula seems more logical to me. (Strength / scale ^ 2) $\endgroup$ – hjaarnio Jul 8 '13 at 23:31
  • $\begingroup$ @hjaarnio, Purely artistic — I set up the animation and then tweaked the expression for desired result. $\endgroup$ – Aldrik Jul 9 '13 at 8:58
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I am not sure, I cannot check it from this machine, but I think, it may be able to do it using nodes. When all the three coords are scaled with the same amout, then theoretically the same total light amount comes out when you set the emission to:

desired strength / scale square

Where desired strength is what you like when scale is one. So in case you can acces the scale from the nodes, you are saved. I will check this when I get home.

I did not Find a node for scale. But you can use the method Aldrik suggested, with the equation I have written, and your result will be the desired: Scaleing the light source will only change the shadows blur but not the lights total strength.

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