An IES file is the measurement of distribution of light (intensity) stored in ASCII format. IES profiles are useful for creating life-like lighting, as they can represent the physical distribution of light from any light source.
Here is an example of lights with different IES profiles (From Maya Zest free IES files):
Any of these profiles can be attached to a mesh light in Reality and this will make the light behave according to the IES profile. Yes, that means you can turn a little mesh light plane into a round spot light with a well-defined light cone!
IES profiles makes is possible to control the light cones and light spill to a degree that is not possible using standard mesh lights and it will take your renders to the next level.
In the tutorial I show how to make a rather simplistic (primitive-only) simulation of a flashlight in a otherwise low-light scene with a bunch of low-emissive light sources. All to demonstrate the amount of control you can achieve with IES profiles:
As a bonus I also show how to create the in-air visible light beam shown above using a render effective localized fog/dust volume.
Finally, it contains links to free IES resources and some great tools to view and generate IES profiles.
You can grab the tutorial here:
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Hope you find it useful and let me know, if you have any questions or find any errors.
Updated with better quality PDF.
Updated Dropbox links