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By default, light sources are considered holdouts are do not illuminate other holdout objects directly. This prevents double contribution between 3d lights used to illuminate non-holdout geometry and the illumination already captured on the plate image.
To create a holdout light (i.e. a light source that does directly illuminates illuminate holdout objects), create a light using use "__illumholdout" as the light group suffix. This tells integrators integrator plug-ins to consider this light as a non-holdout light source when illuminating holdout geometrydetermining illumination.
Light "PxrSphereLight" "spherelight" "string lightGroup" ["light__illumholdout"]