ICC profiles and colour spaces

Colour space: which colours are in play

A pixel value such as R 200, G 40, B 40 does not by itself define a particular colour. It is the colour space that says how saturated the most saturated red, green and blue are, and what the white point is. The range of colours a space or device can contain is called its gamut.

Space	Origin	Where you meet it
sRGB	standard IEC 61966-2-1	the web, most screens, the default space of cameras and labs
Adobe RGB (1998)	Adobe	cameras (optional), print workflows; wider in the green–cyan tones
Display P3	Apple, based on cinema DCI-P3	phones and computers with wide-gamut screens

In the diagram above, Display P3 has more saturated red and green than sRGB, while Adobe RGB reaches further into greens and cyans. All three have a D65 white point.

A printer’s gamut is a different shape

A printer and paper do not have a triangular gamut. Its shape depends on the inks and on the paper. Good pigment printers on glossy paper exceed sRGB in some cyan and yellow tones, while they cannot print some very saturated blues and violets from the screen. Matt papers have a smaller gamut than glossy ones, especially in dark and saturated colours.

That leads to an important point: colours outside the print gamut have to be squeezed inside somehow. How that happens is decided by the rendering intent.

The ICC profile

An ICC profile is a file that describes how a particular device reproduces colour. The profile format is defined by the International Color Consortium. There are profiles for:

• colour spaces (sRGB, Adobe RGB, Display P3), which are embedded in files,
• screens, created by calibration,
• printers, always for a combination of a specific printer, inks, paper and settings.

Fine Art paper manufacturers make profiles for common printers available on their websites. A lab that prints on several papers should have a profile for each. If a file has no embedded profile, software and labs usually interpret it as sRGB.

Rendering intents

When colours are converted from one space into a smaller one, the conversion follows the chosen rendering intent:

Intent	What it does	When
Perceptual	compresses the whole range of colours to preserve the relationships between them; also changes colours that could have been printed	photographs with many saturated colours outside the gamut
Relative colorimetric	keeps colours inside the gamut exactly (relative to the paper’s white point) and clips those outside to the nearest printable colour	most photographs; with black point compensation
Saturation	puts saturation before accuracy of hue	charts and business graphics, not photographs
Absolute colorimetric	simulates the paper colour as well	proofs, simulating another paper

Black point compensation with the relative colorimetric intent maps the darkest black in the file to the darkest black of the paper. Without it, detail in deep shadows can block up. Which intent is better for a particular photograph is shown by soft proofing — see Calibration and soft proofing.

Which space to use

• Sending a file to a lab: ask which space the lab accepts. If you don’t know, send sRGB with the profile embedded. It is the safest choice.
• Printing yourself with paper profiles: you can edit in a wider space (Adobe RGB) and convert to the printer’s space only when printing.
• Phone pictures: these are often in Display P3. Good software converts them correctly when printing; when sending to a lab, convert them to sRGB if the lab does not support P3.
• Never assign a profile instead of converting. Assigning changes the meaning of the numbers and the colours shift. Converting preserves the appearance.