Color Space, Gamut Conversion, Inks, and Proofs in Photoshop

Images are captured in RGB color space with a scanner or digital camera. When you output to devices such as monitors or projectors or for use on websites, your images will remain in an RGB color space and gamut (range of reproducible colors). But when you print your images, they must be converted to a print-oriented color space and gamut. Historically, this color space and gamut has been a four-color process (CMYK—cyan, magenta, yellow, and black), and remains so for the majority of commercial printing. Expanded gamut commercial printing with inks in addition to the standard four is available but not widely used. In recent years, expanded gamut printing with seven or eight inks (including cyan, magenta, yellow, and black, plus light cyan, light magenta, and various versions of black ink) has become the norm for high-quality, photo-quality ink-jet printing. While it is most common for clients to perform the RGB to CMYK conversion prior to submitting final documents and images, there are some circumstances, such as when printing with expanded gamut inks, or if the client and printer agree that the printing company should perform the conversion, when images will be submitted in a RGB color space.

This is a good time to say a word about proofs. There are basically two types: composition and contract proofs. Composition proofs simply show the content or composition of the image. Contract proofs promise a “match” of the color that you see. For our discussion here, we are focusing on contract proofs. While only proofs actually created in the final output device, such as a digital press, can truly offer a match, proofing technologies, and even soft proofing, have capabilities have dramatically improved in recent years. However, color management and proofing capabilities vary widely from one printing company to another, so the type and “closeness” of the match of the proof should be discussed. While usually it is the printing company providing their client the contract proof for approval, some fully color savvy and managed clients, such as photographers, may be the ones providing the print company with the proof to match. Who is creating what kind of proof and what kind of match is expected should be discussed and clear prior to job submission. Clarity on these responsibilities and expectations will produce better results and less finger pointing later.

One final proof point: Some image such as duotones cannot be proofed accurately, because the final printing ink is not available in proofing devices. Gamut conversion for these expanded gamut many ink devices is too complicated and device specific to be accomplished prior to output, and is nearly always performed on the fly either at a RIP or through a printer driver. Unlike for RGB to CMYK conversions, Photoshop does not provide any color profiles for accomplishing RGB to 6-8 color gamut conversions. There are no standard inksets, paper or output conditions that can be used to create useful profiles, and each expanded gamut circumstance requires a custom conversion. So you will want to keep in mind whether you want to keep your image in RGB color space or convert it to CMYK or another color gamut for commercial printing.

Although you can leave your image in RGB color space and allow the printing company to perform the conversion to CMYK for you, you may achieve better (and less expensive) results if you perform the conversion yourself, and particularly if your are creating and providing a contract proof for matching purposes. Where and when these color space conversions and proofing will occur and who should perform them is still a topic that should be discussed between client and printing company. Aglimpse at the future: It is this author’s opinion that, for reasons related to image use flexibility and editability, submission of RGB color space images will become increasingly common and will ultimately become the standard practice. So it is a good idea to keep this dialog open between client and printing company, and always be clear who has responsibility for color gamut conversion and who will be providing what kind of contract proofs.