IES stands for Illuminating Engineering Society. IES is the standard format, created for the electronic transfer of photometric data via the web. It has been widely used by many lighting manufacturers, and is one of the most widely used photometric data distribution industry standards. An IES file is basically for measuring the intensity of the ASCII format.
One can think of it as a digital profile of a light of the real world. In 3D software such as 3ds max, Maya, C4D, Softimage, it can be used to create lights with accurate shapes and physical form.
IES files of light are created by many lighting manufacturers, and can be downloaded for free from their sites. One of these is Lithonia Illuminazione (www.lithonia.com), which has an extensive library of IES files of different categories.
You can use an IES viewer to view these files in a schematic representation for easy understanding.
IESviewer is a most popular photometric viewer in the world.
It allows you to quickly view, find and convert photometric data.