IES data in Corona & C4D Lights

[Exactly]

Hi,

Not sure what is happening here but I seem to get different results when using the same IES file in a Corona light compared to a native C4D light.
The attached image shows 3 setups. Each setup uses a different IES file as I wanted to see if it was the IES file causing the issue and I am starting to think it is not.

The light on the Right is the Native C4D light and the one on the Left is the Corona light.
I have also had to adjust the intensity of the Corona light to match the IES lumens. It is like the Corona Light does not read the lumens value in the IES file.
The scene on the far right has had the lumens value of the C4D light reduced by about 80% to make it match the Corona light.
Light distribution patterns seem to be different too.

Hopefully someone can let me know why I am seeing these differences as I want to make sure the light I use in my scene is displaying the IES file as it should do.
This is all done using R20 and the latest Corona 4.

Thanks

[Exactly]

Is anyone able to offer any information on this?
It is starting to cause me a few headaches with some client work.

Thanks.

[houska]

Hello, Exactly

Sorry for not answering right away.

I checked the parsing of the .ies data format in the core source code and we are not using several of the parameters in there - for example the "number of lamps" or "lumens per lamp" parameters. The light geometry is not taken into account either, so each of the faces will have the same .ies distribution regardless of its normal. which might lead to unexpected results if you use anything else than an area light. Also, the light intensity changes with scaling of the geometry, which is not what you might expect. Some of the issues with the geometry you can mitigate using the "Keep sharp pattern (fake)" option, some of them are more problematic...

That all said, the .ies light distribution works well and judging from your images it's even higher-resolution than in C4D. My suggestion would be to use the Corona Light with Light Type set to Area. Then the only issue will be the different intensity, which you can match using another light that is not using an .ies profile.

Hope this helps!

[Exactly]

Thanks for the detailed reply.

I played with the "keep sharp pattern" and checked all my Corona lights were set to area (they were).

I guess saying what looks better between the Corona light and the C4D light is subjective. It also depends on the IES file as I prefer the C4D light on the left and right scenes but the Corona light in the centre.
Either way, accuracy is what I am searching for and sadly some lights just have poor light distribution and we can only talk to the light manufacturer for that :-)

Do you plan on updating the code to make use of the IES parameters you are not using at present? Like I said, accuracy is what I am after here.

Thanks.

[houska]

Unfortunately, there are no plans on updating the IES code as far as I know. The light distribution parsing works well, so there's no need to change that.

As for the other parameters, they really only have niche uses and currently we have other issues to care about than supporting the whole IES feature set.

[houska]

Interesting fact: I actually found a bug where the light's intensity won't update when you resize the light using the scene's gizmos, so I will fix that for the next daily.

Also, note that all the photometric units work with a constant conversion ratio between the radiometric and photometric quantities, not taking into account the light's color (or rather, presuming a 555nm green light). Note that even if we wanted to do this conversion more accurately, we could still only use a heuristic. Indeed, exact radio<->photo conversion is impossible without a spectral renderer.