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Colour Gamut - in layman’s terms
By David Harradine

How much do you know about colour gamut, clipping and RGB working space? David Harradine provides some insight.

The term colour gamut refers to the range of colours a device can reproduce, the larger or wider the gamut the more rich saturated colours available. As colour gamuts become smaller it is generally these rich saturated colours that are the first to suffer, a phenomena technically referred to as clipping. This clipping phenomenon is most apparent when converting from RGB to CMYK, with many of the rich saturated colours that were available in RGB no longer being available in CMYK.

sRGB Vs ProPhoto.gif

 sRGB vs ProPhoto

A98 Vs sRGB.gif

 A98 vs RGB

A98 Vs ProPhoto.gif

A98 vs Prophoto

Display devices like monitors also have gamuts or ranges of colour they can reproduce. So in order to accurately preview your images you would ideally like your display gamut to be at least as large, if not larger, than your printer’s gamut, otherwise clipping will be occurring in your preview.

Another area where gamut comes into play is in your choice of RGB working space. Three of the most common RGB working spaces are sRGB, Adobe RGB 1998 and Pro Photo RGB, with their greatest point of difference being their size or gamut. sRGB is the smallest of the three, Adobe 98 is medium to large and Pro Photo is extremely large. There are many other RGB working spaces out there and you can even create your own if you like, but it’s useful to understand these three first, as all others can be understood in relation to them.

Before we look at each of these spaces individually it is also worth noting that apart from an elite few wide gamut monitors, no monitor can display any colours beyond that of the sRGB colour space.

Common Colour Spaces
sRGB was designed to be the lowest common denominator colour space, so most colour managed applications will default to this RGB space unless told otherwise. sRGB is the ideal colour space for the web and low end printing devices as it’s gamut is the easiest to manage. However, it should be noted that most ink jet printers and commercial CMYK processes can print colours that lie outside of the sRGB gamut.

Adobe RGB 1998 was designed to be a wider gamut and therefore a more professional colour space than sRGB. Adobe 98 can pretty much contain all of the colours achievable in commercial CMYK and has become a default photographic RGB space in recent years. However, most ink jet printers these days can still print colours beyond even the Adobe 98 colour space.

Pro Photo RGB was originally designed (by Kodak) as an archiving space for wide gamut film scans. The logic behind Pro Photo, or ROMM RGB as it was originally called, was that all possible colours were contained even if they could not be viewed on screen or in print, so it was insurance for the future and all the wonderful super wide gamut devices that may one day arrive.

More recently with the boom in digital photography and the Raw workflow, Pro Photo RGB has become a popular working space for those printing to high end devices and looking to squeeze every drop of colour out of the process.

sRGB Vs Adobe 98
sRGB can serve many users very well and if you are perfectly happy with your sRGB output then there is really no need to change it. However, Adobe 98 is a larger space and many of those extra colours are available on most ink jet printers and in most commercial CMYK processes. But of course these colours must also be present in your image to begin with for any benefit to be realised.

Adobe 98 Vs Pro Photo
As stated already Pro Photo is enormous. Most monitors can barely display half of the colours that can, in theory, be present and the highest end widest gamut monitors on the market can still only display about 65 percent of it. However, many of these colours that the monitor can’t show you can be printed, especially on today’s high-end ink jet photo printers and papers. So Pro Photo is quite the double-edged sword that you would need to be sure you could benefit from before using.

One popular theory is that Pro Photo can preserve differences between colours, therefore detail, that smaller spaces will map to the same colour. However, this is extremely difficult to prove with real world examples.

Conclusion
The only real solution is to do some tests and chose the RGB working space that best suits your needs. Despite much of the technical theory, if you cannot see any clear benefits using a larger colour space then why bother? I would recommend, if printing, using at least Adobe 98 as your standard working space. I would then suggest testing Pro Photo and seeing if you believe there is a meaningful advantage.

Many respected commentators recommend always working in Pro Photo to ensure you always reproduce the most possible colours, but the down side of this, is that half the possible colours in your image cannot be viewed on screen.

Only testing and careful evaluation can reveal what is best for you, your equipment and your expectations.

David Harradine is a photographer, trainer and Photoshop beta tester, who regularly runs training seminars throughout Australia and New Zealand on Photoshop, Digital Photography and Colour Management.
For more on David’s current training events please visit 
www.whack.com.au

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