A scene that demonstrates how to use the imager_light_mixer can be found here.

The light mixer operates on light groups not individual lights, so first you need to create groups via the Light Manager.

Light groups displayed in light_mixer. Lights without a light group display as default.

Add Layer

Adds an existing light group as a layer in the light mixer.

Light Manager

Opens the Light Manager window where you can create, edit and delete light groups.

Mute

Mute an individual layer by Ctrl + click on the mute button (exclusive mute).

Solo

Solo an individual layer by Ctrl + click on the solo button (exclusive solo).

Light Group

More information about light groups can be found here.

Intensity 

 

Exposure 

Tint

Output to separate AOV

When enabled, creates a new additional AOV in which the mixed result will be written into, instead of overwriting the beauty.

Note that this feature is available only in multi-layer Arnold drivers (e.g. EXR) and when rendering to the Picture Viewer, but in single-layer Arnold drivers (e.g. jpeg, png, etc).

AOV Name

The name of the AOV in which the mixed result will be written.