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I use two objects of different sizes to give the same self-illumination parameter, why the large object looks brighter than the small object. And the brightness difference varies with the size difference of the object. How does this work, by volume or by surface area? How can I control the internsity value to make two objects of different sizes look the same bright?

The material conversion for unity materials default to "surface brightness"
Which means that as the object grows in size, it will emit more light.

Possible solutions to your goal:
- You can create an octane material in unity with an emission node that does not have surface brightness.

- Another thing you could do is calculate the intenstiy difference on your second material by multiplying the intensity by the X in the following formula:
X = ModelScaleOne^2 / ModelScaleTwo^2
Fully formula would be: IntensityOne = IntensityTwo * (ModelScaleOne^2 / ModelScaleTwo^2)

- If your model is a sphere you could also create two point lights, and use the "octane light override" node to change its emission settings. There you can turn off surface brightness

ChrisHekman wrote:The material conversion for unity materials default to "surface brightness"
Which means that as the object grows in size, it will emit more light.

Possible solutions to your goal:
- You can create an octane material in unity with an emission node that does not have surface brightness.

- Another thing you could do is calculate the intenstiy difference on your second material by multiplying the intensity by the X in the following formula:
X = ModelScaleOne^2 / ModelScaleTwo^2
Fully formula would be: IntensityOne = IntensityTwo * (ModelScaleOne^2 / ModelScaleTwo^2)

- If your model is a sphere you could also create two point lights, and use the "octane light override" node to change its emission settings. There you can turn off surface brightness

Many thanks to "ChrisHekman". Is "ModelScaleOne" surface area or volume?

wyh880123 wrote:

ChrisHekman wrote:The material conversion for unity materials default to "surface brightness"
Which means that as the object grows in size, it will emit more light.

Possible solutions to your goal:
- You can create an octane material in unity with an emission node that does not have surface brightness.

- Another thing you could do is calculate the intenstiy difference on your second material by multiplying the intensity by the X in the following formula:
X = ModelScaleOne^2 / ModelScaleTwo^2
Fully formula would be: IntensityOne = IntensityTwo * (ModelScaleOne^2 / ModelScaleTwo^2)

- If your model is a sphere you could also create two point lights, and use the "octane light override" node to change its emission settings. There you can turn off surface brightness

wyh880123 wrote:Is "ModelScaleOne" surface area or volume?

Just the transform scale of the object. As long your scaling is uniform in all directions, you can ignore surface area and just use your scaling value in that formula.