Though you can render Maya’s area light source with mental ray for Maya, it can be used only as a direct light source. If you want to render an area light source with mental ray for Maya to achieve indirect illumination (for example, to create the most realistic soft shadows), you must use a mental ray for Maya area light.
You create a mental ray for Maya area light by turning a Maya area light into a mental ray area light. See Create a mental ray area light.
The mental ray for Maya area light works by casting rays from sample points on the light to create smooth shadows.
