CxF is an ISO standard that has been published in June 2015 as ISO 17972-4. To be more precise it is a series of standards from which part 4 has been published first. CxF/X-4 covers spot colors, the other standard parts will cover other color, like process colors etc.

CxF stands for Color Exchange Format and allows for embedding spectral data (measurements) into a PDF file. The presence of such data can potentially improve results when colors have to be simulated on a device that does not have that colorant. That obviously makes the most sense for spot colors, e.g. when they have to be printed on digital printer like on an ink jet machine. The same is true when the spot color has to be proofed.

CxF/X-4 defines 3 conformance levels:

• CxF/X-4b - is the least demanding level and requires a measurement for a single solid (100%) spot color patch.
• CxF/X-4a - requires a minimum of 3 measurements (3 spot color patches), recommended are a total of 11 measurements.
• CxF/X-4 - requires that patches on black background have to be measured in addition to the measurements on white background which are the same as for CxF/X-4a . Again 3 measurements are required - on white and black substrate (a total of 6), but recommended are 11 (22). Printing on black background shows how that spot color appears when printed in conjunction with other colors at the same spot of the substrate.