What’s an IES light?
IES (Illuminating Engineering Society) is a file format (.ies) that you can plug into your Cinema 4D lights to give them physically accurate falloff, dispersion, color temperature, spatial emission, brightness, direction, and spread. As far as I understand it – IES files are an industry standard for lighting manufactures so that they can document the distribution of their lights intensity. They save these IES files to store the lights photometric data and publish them online, so basically they are just making a digital representation of their lights. So by creating an IES light, depending on which .ies file you choose, you are getting all of that lights attributes.
Cinema 4D started supporting IES lighting data in R12 (Visualize and up) so if you are on any version before this you are out of luck.
Where to get more
Cinema 4D R12+ (Visualize and up) includes around 850 IES files in the content browser that you can use, just run a search for “IES” in the search bar. But there are also many different free ones out there that you can download and use for your projects. The best collection of IES lights that I found was over at c4dcafe where the user whthawk provides a HUGE collection of 19,370 ies files that are all clearly organized in a nice .lib4d file that you can just toss into your content browser. Follow this link to download his collection. 19,000 may seem like a lot, and I guess it is, but since downloading those I’ve never needed to go further than my content browser to find anything that I’ve ever needed.
Also, if you just do a quick search for IES lights I’m sure you’ll be able to find thousands of other free ones out there, I’m still trying to get through the 19,370 I got in the whthawk collection.