IES stands for Illuminating Engineering Society, it is the organization responsible for creating and maintaining industrial standards for design and manufacturing of artificial light sources.
In 3D rendering, an IES file or “photo-metric file” is a text file containing a physical description of a light source’s beam spread , pattern and intensity, allowing for faithful depiction of the light source in 3D renders.
Most modern 3D rendering software support IES lights, that is allow loading IES files into the software and lighting the 3D scene using the light source described in the IES file.
Lighting manufacturers make measurements of their light fixture model’s physical light output and create IES files available for download on their websites.
This allows architects, lighting designers, and interior designers to download the files and realistically visualize the light sources effect on their projects.
CG artists use IES lights to add realistic spotlight beam patterns to their renderings and animations, such that can’t be created using regular simple 3D light sources.
Examples of IES lights rendered with V-Ray for 3ds max:
IES Spotlights for Blender & Cycles