IES light profiles, developed by the Illuminating Engineering Society of North America, enhance renderings with light distribution that is more accurate. An IES file is basically the measurement of distribution of light (intensity) stored in ASCII format. Think of it as a digital profile of a real world light. If lighting is important to you, use a renderer that includes IES Presets. You will also find that many lighting manufacturers provide IES profiles for the fixtures they make available via their websites.