Maxwell Render

It's difficult to tell because it may depend on that particular IES file and how they measured the distribution. But in most cases a 2-3cm sphere is good. It will give you a precise light pattern. The bigger you make the sphere the blurrier the light pattern will get.

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Next Limit Team
Merchant of happiness

Thanks Mihai.

I´m right , that the IES-Lights distribute the light in direction along the negative Y axis ?

Best,
Tom

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Maxwell 2.7.20/Realflow 2012/Cinema 4D Studio R14/MacPro 12 core / Nvidia 4000 / 48 GB
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Maybe noob question, but, how to make emitter geometry with "no light", i.e. sphere visible to camera and reflections only but which doesn't light scene - reverse of what IES geometry emitter is (hidden to cam and reflections but giving light to scene)?

No, its not a noob question, a solution is to put a pure black surface just in front of the light, and make that object hidden to cam, and hidden to reflections if you want that also...

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Materials, Arch-viz, C4D Dynamics

Thanx alot, now it's possible to model light with bulb and put IES in front of it to light diffuse, so you get diffuse from IES and reflective/scattering effect from "bulb". It should work nicely, I have to check it. Still when IES is top and bottom type you will get shadow on wall from black cover, but it's rare example. Only thing left is to match emitter and IES.

Generally if you make something hidden to gi, you also want to make it hidden to refl/refraction, because they overlap, i.e. reflections become diffuse gradually theres not a distinct cut off. (reflections fade to 0 around roughness 66).

This uses 3 emitters. The first one for is an ies spotlight applied to a flat disc hidden to cam (has soft alpha'd edge).
Second is what Bubbaloo described, (an mxi emitter with using a 32bit pic of a headlamp), this is a bit bigger, positioned just behind the first and also hidden to cam, it fills in the reflection.
Then behind that is what the camera sees, which can be made however you need it to get a nice look. This is 'wrapped' in pure black water-tight geometry which is hidden to cam to isolate that from the rest of the scene.. so you can really go crazy with it, because it wont add noise to the rest of the scene.

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Materials, Arch-viz, C4D Dynamics

It look really nice and it seems to get *that* effect using ies is sometimes more complicated then modelling silver/aluminium reflector i.e. from lamp drawings and bulb with glass hidden to gi. Advantage of 3-emitter method is that it will denoise lot faster cos there are no caustics from front flashlight element isn't it?
I wonder why do you use flat disc instead of sphere for IES, when IES profile (spherical) is mapped to flat it can give weird effect.

Think of the flat disc, as a part of a larger ies sphere, because I'm using a spotlight type ies file, I only need that frontal part.. and when you put an ies on a large sphere you get a softer ies effect which renders faster.. but I'm just using the bit of the sphere i need, (which is roughly a disc) which is much more convenient, while still rendering fast.

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Materials, Arch-viz, C4D Dynamics

Indeed, part of sphere is nice idea, how have you managed to make part of larger profile?