Are "Nikon sRGB" and "Nikon Adobe RGB" the same as the Real Thing?
If you're a Nikon shooter and have installed Capture NX or other Nikon software, you may well have been puzzled by having both an sRGB color profile and one named "Nikon sRGB." The same confusing situation exists with "Adobe RGB" versus "Nikon Adobe RGB." Are the Nikon versions the same and the real ones? If so, why did Nikon provide theirs, and either way, which ones should you be using?
The short answer is that the Nikon variants are, for all practical purposes, the same as their "official" counterparts and they can be used interchangeably. The full answer though is a bit more complicated.
The official versions are just that — official standards. And in terms of color management history, both are relatively old standards, having been developed in the early days of digital color. That "1998" at the end of Adobe RGB (1998) is ages ago in computer years, and sRGB is even older.
The full name for the official sRGB profile is "sRGB IEC61966-2.1." Taking each part of this name in turn, "IEC" stands for the International Electrotechnical Commission. While the ISO or International Organization for Standardization is responsible for coordinating standards efforts worldwide for most types of standards, the IEC has dominion over electrical and electronic standards. The standard known as IEC61966 was issued by the IEC technical committee for audio, video and multimedia systems and equipment. The standard document is titled "Multimedia systems and equipment — Colour measurement and management," and part 2.1 of that standard covers "Colour management — Default RGB colour space — sRGB." Being international, the wording of the standard uses "colour" rather than the Americanized "color," but the meaning is the same.
The origins of the name "Adobe RGB (1998)" (or simply "Adobe RGB") is a bit easier to figure out, having been developed and named for the same company that makes Photoshop. No government of international bodies here, just the biggest name in digital imaging software. By the way, it is not correct to shorten Adobe RGB further still to "aRGB" so resist the temptation to try to follow sRGB's lead.
The ICC or International Color Consortium has their own standards that play a role in all this as well. Most significantly, they produce the standard that defines just what an ICC profile is. As such, both sRGB and Adobe RGB have to conform to the ICC profile standard. The International Color Consortium though continues to work on this standard, releasing updated versions periodically to incorporate feedback on areas that may be slightly ambiguous or unclear, and also to add support for new advances in the understanding of color science.
Any given ICC profile has to be written to conform to a particular version of the profile standard. The current version of the ICC standard is version 4, but most software is written to the requirements of version 2. Both sRGB and Adobe RGB are version 2 compliant profiles. Oddly, there is no version 3 of the ICC standard.
Just like the real thing, the Nikon version of sRGB is attempting to conform to IEC61966-2.1, just as Nikon Adobe RGB is intended to conform to Adobe's specification for Adobe RGB. They have the same white and black points, the same gamma and "tristimulus" red, green and blue colorant values. In all major aspects they are the same thing as the real versions. The actual differences between what Nikon gives you and the official versions are relatively minor.
The most obvious difference is in their names. Adobe in particular gets rather upset if you name a profile Adobe RGB. Even if a profile you distribute conforms to the same standard as the real thing, Adobe won't let you name it Adobe RGB so it has become common practice to do what Nikon did and name your profile with an obviously similar name that still avoids the licensing issues that would come from naming it the same thing that Adobe names theirs. Of course Nikon could simply not distribute their version at all, but they have no way of knowing whether you already have the real thing, so including their version means they can be sure, even if the name is slightly different. In addition to sRGB and Adobe RGB variants, I also have Nikon Apple RGB, Nikon ColorMatch RGB, and several other Nikon-branded versions of well known standard profiles on my system.
In terms of Adobe RGB versus Nikon Adobe RGB, that's really the difference I know of. Yes, the header in Adobe's version says that Adobe is the preferred Color Management Module (CMM) for their profile and Nikon says that Nikon CMM is preferred for theirs, but that's meaningless for normal use. Other minor differences do exist, but are also not significant in practical terms.
Nikon sRGB does have at least one additional difference in how its red, green and blue response curves are defined. The official sRGB uses 1024 "steps," or data points to map input values to output values for each color while the Nikon version of sRGB uses 4096 steps. This makes Nikon's version somewhat more accurate, or at least a bit smoother, in terms of response curves, but the difference is generally negligible since your Color Management System will interpolate additional values between the steps for both. If you graph the gamut for sRGB versus Nikon sRGB, you won't see any differences. Having more steps though, the Nikon profile is bigger than the official one, weighing in at over eight kilobytes as opposed to around three for the official sRGB. This will make images in which you embed it correspondingly larger, but the difference is negligible for normal sized photographic images in jpeg format. Adobe RGB doesn't use steps at all to define its curves, sticking to the simpler mathematical curve defined by the basic 2.2 gamma curve instead. Nikon Adobe RGB does likewise. Both versions of Adobe RGB are tiny compared to either sRGB.
So, which should you use? The choice is yours, but keep in mind that only Nikon users will have the Nikon versions. If you open an image tagged with a Nikon profile on a system that doesn't have it, many programs will be smart enough to automatically substitute the real thing, but this isn't always true. If you send someone a Nikon-tagged image, they may be greeted by a profile warning and have to manually assign sRGB or Adobe RGB as appropriate.