*On 27/06/2015 at 18:36, ***xxxxxxxx** wrote:

non decaying light till a specific point is not physically correct even if it has a correct falloff, and you don't see it in other render engines because it is fake.

Where are you getting "non decaying" from? That's where I'm getting lost. The light in C4D decays with whatever falloff you configure in the GUI ("Inverse Square" for physically accurate). The light doesn't stay at a constant brightness until it hits the edge of the radius (that would be "Inverse Square Clamped"). It exceeds the intensity by several factors (at the source of the light), and that amount is determined by the radius so that it meets the target intensity at the edge of that radius.

You can test this by rendering out an omni light near a flat surface with a large radius (enough to white out the render). If the render output is set to a 32-bit format, you can see that the values simply exceed white (255/255/255 or 1/1/1).

I believe what you're describing is actually the "Inverse Square Clamped" option (which MAXON themselves states is not physically accurate). This option will clamp the light output at the max intensity until you hit the edge of the radius, but that is a completely different option from "Inverse Square (Physically Accurate)".

Anyways, thanks for your help. I really do appreciate it, but I'm still kinda lost. I understand the basis of the equations, but they're just not working for me even after I've rearranged them to try and calculate the actual brightness of the light object given a radius (and the intensity at that radius). This is what is puzzling me- it's like nothing I try lines up with what I'm reading on Wikipedia, Google, and here.

Just to try and clarify what I'm trying to figure out here, if you have a light object with the following settings, then they approximate the listed candela output (which I've just dialled in by hand)... I'm trying to calculate the exact correlation between these, but I can't seem to figure out what that formula is.

Light intensity: 100%

Light decay/radius: 1000mm

Light falloff: Inverse Square (Physically Accurate)

Approx. equivalent candelas: 1000

Light intensity: 100%

Light decay/radius: 2000mm

Light falloff: Inverse Square (Physically Accurate)

Approx. equivalent candelas: 1824.5

Light intensity: 100%

Light decay/radius: 3000mm

Light falloff: Inverse Square (Physically Accurate)

Approx. equivalent candelas: 2580

Light intensity: 100%

Light decay/radius: 4000mm

Light falloff: Inverse Square (Physically Accurate)

Approx. equivalent candelas: 3294

Light intensity: 100%

Light decay/radius: 5000mm

Light falloff: Inverse Square (Physically Accurate)

Approx. equivalent candelas: 3979

Light intensity: 100%

Light decay/radius: 6000mm

Light falloff: Inverse Square (Physically Accurate)

Approx. equivalent candelas: 4640

Light intensity: 100%

Light decay/radius: 7000mm

Light falloff: Inverse Square (Physically Accurate)

Approx. equivalent candelas: 5284

Light intensity: 100%

Light decay/radius: 8000mm

Light falloff: Inverse Square (Physically Accurate)

Approx. equivalent candelas: 5913

This doesn't seem to follow the formula of i = c / d^2, which is what puzzles me. I've tried rearranging that to solve for c (the output power at distance d with a known intensity at that distance), but the numbers don't line up.

-CMPX