What color space do LUTs use in Lightroom and Camera Raw?

My answers are not definitive because my understanding might not be totally correct, but anyway, this is my impression of it…

What input colour space is implemented when using LUTs in Camera Raw? 

By @David Carr

In Lightroom and I assume Camera Raw too, the “working space” is ProPhoto RGB, so I assume a LUT is mapped to that.

What is "space" in the options? Is this going into the LUT or coming out of it? Same for the "gamut" options.

By @David Carr

Do you mean the Camera Raw options shown in the picture below? Those are called “Workflow” options but what that means in practical terms is output options. For example, regardless of the color specs of the currently edited file, the Workflow options determine what Camera Raw will set the specs to if your next step is clicking the Open button that sends the image to Photoshop. In other words, when in Photoshop you inspect the image received from Camera Raw, the color space, bit depth, etc. will be what the Workflow options are set to in Camera Raw. 

If you click the underlined workflow options text at the bottom, it opens Camera Raw Preferences to the Workflow panel, and changing A, B, C, or D in either place changes it in both places.

A secondary consequence of the Workflow Options is that they provide a form of soft-proofing. For example, if you have the Workflow Options set to ProPhoto RGB and view a raw image with highly saturated colors, then enable the highlight/shadow clipping warnings, and then you switch the Workflow Options Gamut down to smaller gamuts such as  sRGB or (Camera Raw only) one of the CMYK profiles, you’ll see the warnings cover more of the image and also you’ll see the colors change. 

If you use the Save Options dialog box in Camera Raw to export a copy, then the settings in the Save Options dialog box control the export, not the Workflow Options. In other words, Save Options in Camera Raw is basically the same as Export in Lightroom Classic. 

So far, nothing above is LUT-specific, because Workflow/Save Options are not LUT-specific. 

Camera-Raw-18-Workflow-Options.jpg

What kind of LUT actually functions correctly in Camera Raw? I'm assuming that a film emulation LUT that expects Cineon Log as an input won't. I'm trying to understand if LUTs intended for video can bes used on stills, and how.

By @David Carr

As far as I know, the key is to separate the concept of LUTs from the concept of Adobe raw profiles. First, about LUTs: Something like a Cineon Log LUT is designed to properly interpret images that the video camera rendered to a non-raw format with a specific gamma curve optimized for capturing maximum dynamic range, and the purpose of the LUT is to normalize that for editing. But when working with stills that are camera raw images, those are in raw format so a cinema LUT designed for non-raw media won’t really work with it. I guess it could work if you have a still with the exact specs the input LUT is designed for…

In addition, in both video and still editing, other LUTs are also used for creative “looks”. You should be able to apply those to any image because they’re about color finishing (grading) later in the pipeline, not input color interpretation/correction before editing. 

A LUT in a common LUT file format cannot be used in Lightroom/Camera Raw as is. This is why, to use a LUT in Lightroom/Camera Raw, you need to include it in a custom Adobe raw profile: Not in a LUT file format, but taking advantage of a raw profile being able to include a LUT. So you have to create an Adobe raw profile to get a LUT into Lightroom/Camera Raw.

About the pipeline: The thing that makes Adobe raw profiles different from presets is that a profile is early in the pipeline, setting the baseline or starting point for the choices you make in the other Edit panels. One way you see this is that if you change which profile is applied in Lightroom/Camera Raw, the image appearance changes but the values of the slider options don’t change. The profile determines initial colors, saturation, contrast, etc. In this way, applying a profile can, for example, change how far you can increase or decrease saturation because the profile can change the initial saturation. (In contrast, a preset can change the values of the options sliders. So a preset can be applied on top of a profile.)

Because a LUT can only be applied as part of a profile, it’s applied at the point in the pipeline where the profile is applied. 

There are some color features that are applied later in the pipeline. For example, if you choose the Adobe Monochrome profile to convert to B&W, changes you make in the Color panel will not add color. But if you go down to the Curve panel and start editing individual RGB channel curves, or you edit the Color Grading panel, then you can introduce color. Structuring it this way lets someone use the Basic, Color, and B&W Mixer panels adjust how original image colors affect to a B&W conversion, and then with that tuned monochrome image, use Color Grading and (less commonly) RGB curves to create a traditional split-toning effect. 

You can create a PROFILE from a LUT in Photoshop and use (after importing) the Profile in Camera Raw and Lightroom Apps.

https://insider.kelbyone.com/tip-tuesday-use-a-lut-in-camera-raw/

https://www.youtube.com/watch?v=lDoqQyt9YrA

https://www.youtube.com/watch?v=atX7yKhcBCY