Getting semi transparency in the alpha channel on semitranparent objects?

[demodunk]

So, I have a plastic helmet that is transparent plastic with an IOR of 1,2.
How do I get the alpha channel to not be a black and white cutout but taking the refraction and reflections in consideration?
The manual says to use "opacity" for the the opacity to show up in the alpha channel but the only opacity option is the "cut out opacity" in the geometry tab and that's not what I'm after.

[lee_griggs · Accepted solution]

Transmit AOVs

[demodunk]

Wow, thank's Lee! Exactly what I was looking for!

[demodunk]

Seems that this won't work with an Arnold sky though. Even if I set the camera to 0 in the main tab.
Arnold seems to recognize the Arnold Sky as a background no matter what...

[lee_griggs]

Skydome light?

[demodunk]

No I use an HDR texture via HDR-link.

[demodunk]

The only workaround I have right now is to render the beaty and then disable Arnold sky and render the alpha.

[Stephen.Blair]

You have to set Transmission to 0 for the skydome light (you're using an HDR? Then you must be using a skydome light, no?)

[demodunk]

No, I use Arnold Sky in texture mode. If I was to set transmission to 0 on it, the transparent object won' see that light.

[demodunk]

Hey! It works when using a Sky dome instead, Awesome!
Thank's a million for the help!