MacPhun's Luminar 2018 is designed a standalone app and is itself a viable Lightroom alternative, but it can also be used as a Lightroom plugin if you want to keep using Lightroom but also want the option of using Luminar' impressive collections of presets and filters.
While I'm calling it a plugin here, the way that Luminar and Lightroom can work together is actually as an external editor. It establishes a connection so that you can quickly send images from Lightroom to Luminar and then back to Lightroom in a round-trip editing process. The link smooths out that process. But it doesn't import Luminar's presets and filters into Lightroom itself.
How to Install Luminar 2018 as a Lightroom External Editor
The installation is handled not through Lightroom, as you might expect, but through the Luminar app. I'll assume you've already installed Luminar as a standalone app. If you haven't go ahead and do that. You can download it here.
And it's simplest to start the process with Lightroom closed, so go ahead and quit Lightroom if you have it open. Then start Luminar in its standalone mode.
In the Luminar app, go to the top menu, then Luminar 2018 > Install Plugins....
The next popup is where you choose which plugins to install. There are versions for Photoshop, Lightroom, Photoshop Elements, and Apple Aperture. It will be doing a quick scan in the background to try to find which of those host apps you have installed on your computer and then will make the corresponding Install buttons active. It might take a moment to do this. In this example, both Photoshop and Lightroom are installed as potential host apps for the Luminar plugin.
Once you hit whichever of the available install buttons you want, it will quickly handle in and change the status in the right column.
You can now go ahead and fire up Lightroom and it should automatically detect the new Luminar plugin features.
Sending Images from Lightroom to Luminar 2018
Luminar's Lightroom plugin adds two sets of items to Lightroom. One set consists of new export presets that are accessible through the regular export feature (File > Export).
If you're using Luminar's sister HDR app, Aurora, as a Lightroom plugin, that's how you'd do it. A nice feature is that you can specify templates and presets as part of the export, which also means that you can incorporate those into an export preset to speed things up for next time.
But while you can, technically, do it that way through the export function, there's a good reason not to. And that is that that method doesn't work with sending multiple images at once.
So a better option is to use Lightroom's Edit In feature. Select the image or images you want to send to Luminar, and then right click. You'll get a long drop-down menu. About a third of the way down you should see the Edit In item. When you move the mouse over that, you'll get a flyout menu with a list of the available external editors.
Choose Luminar 2018. You'll then get a popup where you can specify some options like filetype, colorspace, and resolution.
Once you hit the Edit button, it will send the newly generated file to Luminar for editing. If you've selected multiple images, it will send one at a time, loading the next one after you hit the Apply button in Luminar.
About Luminar 2018
Luminar 2018 has become one of the leading Lightroom alternatives. For now, it's focused on image processing. Image management is under development and is slated for inclusion during 2018. For image processing, it includes a wide selection of filters and presets built in, includes the ability to use layers, and it's compatible with Windows and Mac.
To get $10 off Luminar 2018, use the coupon code HAVECAMERA during checkout. It works for both new licenses as well as upgrades from previous versions.