On Mac OS 10.11, as options for the output profile I only get the profiles provided by RT 5.6 and some screen profiles (including my own MacBook Pro profile). So copied the ProPhoto ICM file to the ~/Library/ColorSync/Profiles folder to install it.
But while ColorSync shows it now under “User”, after restarting RT it is still not listed among “output profiles”.
What can I do?
In ICC parlance, “output” profiles are printing profiles. ProPhotoRGB profiles are typically marked as “Display” profiles. They could also me marked as a “ColorSpace” profile, but that tends to restrict their usefulness.
http://rawpedia.rawtherapee.com/Color_Management#Output_Profile
Here is described the list of bundled output profiles, which include a “RT_Large*” set that the docs describe as “similar to ProPhoto”. I just inspected RT_Large.icc from the RT source tree, compared it with Elle Stone’s LargeRGB-elle-V4-g10.icc, which is constructed from the ProPhoto specification, and the primaries are identical in the two.
I’m guessing you want to save a large-bitdepth TIFF for import in another program… ?? If so, RT_Large will work just fine for your intent.
I know this list, and so I recently used the one with “large” and the latest version: “RTv4_Large”
I’m guessing you want to save a large-bitdepth TIFF for import in another program… ?? If so, RT_Large will work just fine for your intent.
Correct, I will use 16 bit uncompressed TIFFs as my digital negative (result of a focus stack), and for this I prefer industry standard profiles, even if they are copyrighted.
Now I see that I can “install” it on Mac OS for RT by just copy it to /Library/ColorSync/Profiles/Displays
It is not a good choice to use this path, because Mac OS saves display profiles there, and never output profiles for images. It would be preferable to use (~)/Library/Application Support/ColorSync/profiles directly.
Questions remain:
I’m not a RT dev, so take this with a grain of salt. The semantics of the word “output” imply a rendition that will be used for display somewhere. ProPhotoRGB is definitely not a profile for that purpose. Export for downstream software is not the predominant use of Output…
Some folk might want to use another profile; ProPhotoRGB is challengingly large. Me, I’d use Rec2020, no good reason… 
Oh gee, just noticed you’ve joined us anew… Welcome!
Do you mean there may be a data loss because values will become “more dense” within the 16 bits?
And I guess transformation with “perceptual” intent is are what’s going on most of the time. I had never problems with either display or exports to sRGB JPEG.
BTW: I just noticed that there are some real color geeks here! And this great! 
Probably you are right. The transparent one is ProPhoto, the other one is a temporary ICM file Nikon ViewNX-i creates during export.
“lost”, in the output to a monitor or printer that can’t handle the gamut. Export for your purposes is not the primary use. I get the efficiency of saving the internal image already in the working profile, and the RT devs may be able to provide insight into how that might happen.
And that’s where I need to leave it; the folks who develop RT will be better-equipped to explain what it does and why. I just pitched in because I happened to have tools handy to pick at some of the profiles.
It does support it, it just doesn’t use it by default.
Why would it?
Wrong assumption. The image at the end of the pipeline is apparently in L*a*b*.
https://github.com/Beep6581/RawTherapee/blob/dev/tools/color_management.svg
