The Inverse Square Law and Cinema 4D
"the inverse-square law states that as a light source is moved away from a surface it illuminates, the illumination decreases in an amount inversely proportional to the square of the distance. Thus the illumination of a surface by a source of light 2 ft away is 1/4 of the illumination at 1 ft from the source. Conversely, for two light sources, one at 1 ft from a surface and the other at 2 ft, to give the same illumination to the surface, it would be necessary for the source at 2 ft to have an intensity 4 times that of the source at 1 ft."
The black line graph below shows the falloff of light in Cinema 4D when choosing Inverse Square falloff in the light dialog. The red line shows true Inverse Square falloff graphed. A real light, though it falls off quickly, penetrates deeply. The attenuation curve is exponential. Cinema's falloff curve kills the light completely long before it is natural or realistic to do so.
Both curves result in a falloff of 75% of the light from 100 meters to 200 meters, which is correct. According to the inverse square law, the light at 200 will be one forth as strong at 400 meters. But in Cinema, the light attenuates 75% again by the 300 meter mark. It is attenuating 75% every 100 meters, which is why is dies so quickly. In the real physical world, light attenuates 75% every time the distance doubles, which is why its dim light travels so far despite the fact that most of its strength falls off relatively quickly.
Cinema probably uses the simpler finite curve in order to render faster. But it is not a very realistic approximation of light falloff. To get a more accurate approximation of light in Cinema, use two lights, the first providing the majority of the light with its limited falloff range, and a second much weaker light with a very long range to represent the long range falloff. That is what I did for the render below. The light falls off realistically (using inverse square) but never dies.
Hopefully Cinema will offer a more true to life and realistic light falloff curve in the future. Though hardly a buzzword in the world of 3D yet, I believe that truer inverse square falloff lighting is an important "next step" toward achieving greater realism in 3D lighting. Of course nothing will ever substitute for skill and hard work.
Cinema XL6.3 Project file: You can download the sit file here. There was only one criteria for the falloff distance setting and the light's brightness setting: Make the grayscale image return correct values from 100% to 0% at the starting distance of 100 units (inches). You can use other starting distances, but as long as you set the falloff and brightness by this criterion, your results will return the same curve.
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This movie shows the falloff of light as the board moves from 100 to 400 meters