What is a Color Profile?

A "color model" is defined elsewhere as a means of associating names or numbers with colors. A "color profile" (as the term is used by the ICC and other color management authorities) is an association between two different color models.

That's pretty abstract, so lets try to make it more concrete. Offset printers have a very strong habit of expressing color in CMYK. By printing a set of CMYK patches, and measuring them with a colorimeter, we can construct a table that records which CIE LAB numbers are associated with which CMYK values.

Here is a table in which the CMYK (in percentages) sent to a printer are next to the CIE LAB measured by a colorimeter.

 
Description CMYK CIE LAB
A red 0, 100, 91, 0 50, 71, 54
A darkish blue 100, 69, 0, 0 32, 34, -71
A yellow 6, 0, 91, 0 95, -17, 99
A green 100, 0, 79, 9 56, -81, 32

Here is a table in which the RGB voltages (0-255) sent to a monitor are next to the CIE LAB measured by a colorimeter.

 
Description RGB CIE LAB
A red 203, 0, 23 50, 71, 54
A darkish blue 0, 46, 160 32, 34, -71
A yellow 255, 243, 0 95, -17, 99
A green 0, 139, 74 56, -81, 32

If the above tables were stored in a computer, and had hundreds of entries rather than just four each, they would be "device color profiles". That's all a "device profile" is, nothing more or less.

Notice that in the above two tables, each color has exactly the same CIE LAB numbers. The two tables could therefore be combined, to form what is called a "link profile", establishing a relationship between color on a CMYK printer and on an RGB monitor. Here again, a real link profile in a computer would have hundreds or thousands of entries, rather than just four.

Description CMYK CIE LAB RGB
A red 0, 100, 91, 0 50, 71, 54 203, 0, 23
A darkish blue 100, 69, 0, 0 32, 34, -71 0, 46, 160
A yellow 6, 0, 91, 0 95, -17, 99 255, 243, 0
A green 100, 0, 79, 9 56, -81, 32 0, 139, 74

If that seems too simple, sorry to disillusion you. That's about all that a link profile contains. Color scientists may endlessly debate which points should be recorded, how many points should be recorded, and the detailed mathematics of how these tables should be stored and interpolated, but they're just tables of numbers as shown above.

The more points are stored in the table, the more precision can be expressed in the color matching function. The ICC Profile format, which is just a file format for storing these tables, is flexible. Many monitor profiles are distributed are only 10 K bytes in size, while the link profiles used by programs like Wasatch SoftRIP are commonly over 700 K bytes.

The numbers in these tables can be skewed about very flexibly to deal with color reproduction involving "out of gamut" colors, the final and most interesting issue in color management.