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Use light linking to include/exclude pieces of geometry in the AOV. In this example, the PxrAovLight is "off" globally and "on" for the eye in the scene.  This linking setup creates a mask of the eye (red sphere).

Defining the Output AOV

Once you set an AOV Name in the PxrAOVLight attributes, use the Render Settings AOVs tab to define a new display to which PxrAovLight will write.

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