If an IES profile is selected, the distribution of emitted light is specified by the information in the IES profile. This is an easy way to get realistic emission profiles for a given light fixture, without creating a detailed model of the fixture.
Please note that only IES files with vertical angles below 90 degrees are currently supported.
See also the IES tutorial in the Techniques section of this manual.
Some freely available IES profiles are available to download from http://www.indigorenderer.com/dist/ies-profiles.zip