embed the profiles
All digital files should have embedded profiles (such files are called "tagged"), unless otherwise noted. Photoshop's color management policy should be set to "always preserve embedded profiles," and the "ask when opening" boxes should be checked to alert you to profile mismatches and missing profiles. When profile mismatches occur, you should elect to preserve the embedded profiles.
assigning vs. converting
Assigning profiles can change image appearance without changing the original image data. Assign when the original file has no embedded profile or the embedded profile is of dubious origin. Choose an appropriate profile with the help of the preview facility in Photoshop. While choosing the profile based on an image's appearance is necessarily a subjective decision, it enables a file to enter the workflow with a (hopefully well-chosen) definition of its intended color space.
Assigning a profile changes the appearance of an image but not its pixel values. Converting to a different color space does the opposite: It changes the pixel values while attempting to retain appearance. The purpose of converting is to transform the color space of the original file to the color space of the destination device as accurately as possible.
| NOTE: Every imaging device describes color in its own way, based on its particular characteristics. We might say each device has its own "language" for color. Some languages have a large vocabulary, others a smaller one, and some contain words simply not available in others. To make translations possible among all the languages, we need a universal "reference language" with a huge vocabulary. This is the role of CIELab, which is a theoretical color space containing every color the human eye can perceive. Continuing the analogy, ICC profiles are dictionaries that provide the CIELab equivalent for every word in a particular dialect — that is, for every color a specific device can represent. Embedding a profile in an image file is like sending the dictionary along with a written document: It gives the recipient information needed to understand all the words. It does not guarantee the recipient's language will have the words needed to translate the document perfectly, but if exact words don't exist, the recipient should be able to approximate the meaning with reasonable success. Converting a file from one profile to another, in our analogy, is writing out a translation. And, as with human languages, something is always lost in translation. It is therefore advisable to minimize the number of conversions that are done. Failing to embed a profile amounts to sending a document without revealing what language it was written in, forcing the recipient to guess. Computer programs don't guess, of course; they use a default setting and carry on blindly. A human operator, however, may try a number of different possibilities to see which one gives the most pleasing result. (This may or may not be the result that the sender intended.) This is done by assigning a profile to the image — instructing the program to act as though that profile had been embedded. Having settled on a profile, the human can then embed it to avoid further trouble. We are indebted to the Institute of Quality Assurance's Digital special interest group for this explanation. |
color space recommendations
- Open-ended uses: When the final use of an image is not known, as with images created for stock photography or when the client will use the images in several media, it's best to supply a file in the Adobe RGB (1998) color space, with the profile embedded. The European printing and publishing industry is standardizing on ECI-RGB, an RGB profile with a slightly different gamut than Adobe RGB (1998). However, like Adobe RGB (1998), ECI-RGB contains the entire gamut of the standard CMYK profiles.
- World Wide Web presentations: Convert images to sRGB and embed the sRGB profile before delivery.
- Images prepared for screen and electronic presentations: PowerPoint and similar programs vary in their abilities to read color profile tags, so it is best to convert images to sRGB and embed the sRGB profile before delivery.
- Display prints from professional digital color labs: A custom profile, obtained from the lab and used for your soft proofing, is the only way to have some assurance over final output. After adjusting your proof view, submit either an sRGB or (more rarely) Adobe RGB (1998) file with the profile embedded. If a lab does not have a custom profile, it's usually best to use the sRGB color space and embed that profile, though the inability to soft proof means less control over the output process. A better solution would be to find a lab that fully implements color management and provides custom profiles.
- Display prints from consumer digital-print vendors: There is a free database of ICC printer profiles for digital labs worldwide at the Dry Creek Photo site. The printers covered include models from Fuji Frontier, Noritsu, Agfa D-Lab, LightJet, Durst and Chromira, among others. Because most of these printers do not recognize embedded profiles, it is necessary to convert your files to their profiles, then save them with the profiles embedded. Converting to these profiles will give you the best color fidelity and allow you to soft-proof your digital files before committing them to print.
Labs that don't use profiles usually require conversion of submitted files to sRGB. Using the sRGB color space instead of a custom profile may yield less accurate color that doesn't take advantage of the full gamut such printers can produce. To avoid confusion on your end, it's still best to include the embedded profile, even if the lab will ignore it. - Offset printing: It's always best to begin by asking the printer or the client's production expert what file format, resolution and color space they prefer. RGB files contain many colors that cannot be reproduced by conventional CMYK printing. This can result in a printed piece different from the screen version or from the inkjet print of the file. There are three ways to avoid this confusion:
- If the photographer has the necessary experience and knowledge to convert from RGB, digital images can be delivered as CMYK data. This can be the best solution, since the photographer knows how the image should look.
- Accompany an RGB file with a cross-rendered guide print (see our page on guide prints and proofs) that includes only colors reproducible in CMYK.
- Deliver files in both CMYK and RGB. This allows the photographer to make artistic decisions about color rendering, yet also gives the printer more tools to recover from mistakes the photographer may have made in converting RGB to CMYK. Ideally, convert CMYK image files from RGB using the printer's CMYK profile (see below), and embed it in the file. If you can't get the printer's exact CMYK profile, you can deliver an RGB master file with an embedded profile and a ReadMe file that explains, "For accurate color, choose to preserve embedded RGB profile when opening this (these) file(s)." But keep in mind there is less color variation between different CMYK profiles than different RGB profiles. Hence, if the printer assigns the wrong CMYK profile it will likely look better than assigning the wrong RGB profile. The printer also has the option to convert from a general CMYK profile, such as US Web Coated (SWOP) v2 to a more specific CMYK profile, using either Photoshop>Edit>Convert to Profile, or more ideally, a device link profile that can convert from one CMYK profile to another without having to go through the three-channel Lab color space used by Photoshop's conversion engine.
- Inkjet and dye-sub printers: Choose a wide-gamut color space, such as Adobe RGB (1998) or ProPhoto RGB, for the source space. These printers have internal RGB-to-CMYK conversion algorithms, so they should be profiled in RGB and you should not convert images to CMYK before printing with them. Although manufacturer-supplied profiles have greatly improved, using custom RGB profiles for the printer-paper combinations in the print space will provide the best quality and the best match to a profiled monitor.
NOTE: You can bring desktop and wide-format printers into a color-managed environment with the help of profiles. If working with the manufacturer's printer driver, turn off all color management and print a copy of the color target file. Next, measure the printed target with a spectrophotometer to generate a profile for accurate output on a particular paper or other medium. Repeat this process for each paper stock you use. Most RIP (raster image processor) software offers profiles for a wide variety of papers. Many RIPs will also allow use of custom profiles.
We strongly recommend all offset printers adopt the ICC standards. Currently, there is tremendous variation among offset printers, and nothing can be taken for granted. For many years, offset printers used a "closed loop" color-management approach. They scanned film on drum scanners with software that output directly to CMYK. The CMYK was targeted to a proofing device, the customer approved the proof and the press was adjusted until its output matched the proof. Color settings were built into the workflow, not applied to files. This meant printers could not offer custom CMYK profiles for digital files created outside their shops.
Today, with 60 to 80 percent of images intended for print arriving as digital files, offset printers are moving away from the closed-loop workflow. A single printed piece often includes digital image files from many different sources. It is increasingly likely an offset printer will have a CMYK profile that describes its proofing device's color space. As a general rule, an offset printer should be able to match the output of its proofing device.
Many times, digital image files must go to several different printers, or a project is photographed and delivered before a printer has been chosen. In such cases, it may be best to deliver RGB master files. These should always have embedded RGB profiles to ensure accurate color when they are converted to each printer's specific CMYK profile.
Send RGB image files (especially Adobe RGB [1998]) to a printer only if the shop has experience converting RGB files to CMYK — and then only if the printer will provide a "random" or a contract proof. If a printer has a clear understanding of ICC profiles and requests RGB, it's best to submit files in Adobe RGB (1998), or possibly, the narrower-gamut ColorMatch RGB. European printers may prefer ECI-RGB. If a printer cannot ensure preservation of the embedded profile before converting to CMYK, it is better to provide files converted to a general-purpose profile, such as SWOP Coated V2 CMYK, ColorMatch RGB or sRGB, with the appropriate profile embedded in the image file.