The high-dynamic-range image (HDRI) is used in CG to create more realistic scenes than the more simplistic lighting models used.
An HDRI is created when several photos at varying exposures are taken of the same subject, ranging from very dark (underexposure) to highlight only the brightest parts of the scene, to very bright (overexposure) to capture the absolute darkest parts of the scene. When these images (typically five or seven images) are compiled into an HDR image, you get a fantastic range of bright to dark for that one subject or environment.
mental ray can create an environment map in your scene to which you assign an image, usually an HDRI. That environment map uses the brightness of its image to cast light in your scene.
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