You can simulate light emitted from an infinitely distant (environment) sphere to create image-based lighting more easily and efficiently.
Image Based Lighting is a technique to create photo-realistic images. With image-based lighting, you use an environment texture (an image file) to illuminate the scene. Typically, the image is a photograph of a real environment, either a panoramic image or a photograph produced by taking pictures of a chrome ball (to capture the surrounding environment).
(The alternative method, where you surround your scene with a real, finite geometrical sphere, is supported. If a finite distance to the environment is required (for example, for a closed room), you can do this. However, because the shape is actual geometry, mental ray processes it as such. If tessellated, it becomes part of acceleration structures (BSP tree) where it (due to its encompassing nature) can slow rendering down significantly and interactive Maya workflow can be hindered. With the workflow below, no geometry is created (or exported on .mi export); instead, IBL is represented by a set of shaders. You create the desired IBL node, then set its attributes to achieve the look you want.)
See Render infinitely distant (sky-like) illumination and reflection.
