Illuminating Engineering Society (IES) profiles are a lighting industry standard method of diagramming the brightness and falloff of light as it exits a particular real world light fixture. They have been used in the CGI industry for many years in photometric lighting. Unreal Engine 4 can use IES profiles to get realistic lighting results based on profiles directly from light manufacturers.
IES profiles render very fast and do not have a significant effect on performance, making them a more ideal option for shaping your light than using a Light Function, which is discussed in Example 3.2. Profiles of various fixtures can be downloaded from light manufacturer websites. See the IES Profile Documentation for more information.