At one point or another you may have found yourself troubleshooting light fixtures that were not casting light in a rendering. More often than not these may have been recessed can light fixtures.
Before we get into the 2 approaches for troubleshooting light fixtures, I want to pass along a technical solution which includes items to verify for rendering issues:
Rendering or material appears black
The most common cause for a recessed can rendering issue is a floor object directly above the ceiling object. Because the light fixtures are hosted on the ceiling, they will cut an opening in the ceiling. They will not cut an opening in the separate floor object.
If the floor object is close to the recessed can, and overlapping the geometry, the light source definition will be buried inside the floor. Because of this no light will be cast when the scene is rendered.
There are 2 approaches below for this scenario:
1. Temporarily hide the floor object if it is not visible in the scene
The easiest approach would be to temporarily hide the floor object. If the ceiling object covers it in the view, simply turning it off would remove the obstruction.
2. Create voids in the floor around the recessed can light fixture
By creating an in-place component void extrusion, and cutting the void out of the floor object, we can create an opening around the recessed can fixture. This will ensure there is no obstruction for the light source and will render the scene as expected.