Wide-format inkjet printer manufacturers are constantly trying to increase their devices’ printable colour gamut.
Assign or convert
With design software, signmakers can change the working space by either assigning or converting to a new ICC profile. In essence, this allows the size of the document’s colour gamut to be changed. There is a difference between the two options, however, which is important to understand.
Assigning a new profile takes the current CMYK or RGB values and changes the colour gamut to which they are relative. If a colour is defined as 100 per cent cyan relative to a small gamut, for example, and then a larger colour space is assigned, the defined colour will still be 100 per cent cyan, but relative to the larger space, which will cause a colour shift.
Converting the colour space, on the other hand, scales the gamut but keeps the colour values the same. The 100 per cent cyan relative to a small gamut, for example, would become a 50 per cent cyan when converted to a colour space that is twice as large. The colour remains the same visually, but relative to a larger space.
Assigning an ICC profile, then, should generally be avoided, as it can cause an unwanted colour shift. In some cases, however, there is no other choice. If a file comes into a colour-managed workflow without an embedded ICC profile, there is no reference for its colour values, so a profile must be assigned. This should only be a last option, however, and if any embedded ICC profile is available, it should always be honoured.
The only time an ICC profile does not need to be embedded is when the file has already been converted to a standard colour working space—a process referred to as ‘normalizing’—and the RIP is set to use that space.
Assigning a different working space can cause a colour shift in the file before any ink is even printed on a substrate.
RIP colour conversion
Colour is managed the same way in most wide-format RIP software platforms. When a file is brought into the workflow, the RIP should be configured to check first if the file has an embedded ICC profile (which, as mentioned, should then be honoured). If no ICC profile is found, the RIP should assign a default profile.
Once the RIP has a reference colour space defined by the profile, the RGB or CMYK values in the file can be converted through the PCS into L*a*b* values. These values are then converted again to CMYK, relative to the colour space of the output device.
Pantone colours set up as ‘spot colours’ in the file should be defined by L*a*b* values. These values will bypass the input ICC profile, which is typically smaller than the output gamut, and will be mapped from L*a*b* directly to CMYK, relative to the output device. This process ensures the closes match possible to the original Pantone colour.
Standardization misconceptions
One fairly common misconception about the wide-format printing workflow is that by assigning a common ICC profile—and thus a common colour working space—to all incoming work, the printed output will become more consistent. This is incorrect.
In fact, the opposite is true. Ignoring an embedded ICC profile and assigning a different colour working space may create a different reference from that of the file’s creation, which in turn can cause a colour shift in the file before any ink is ever printed on a substrate.
If the desired goal is to standardize colour spaces before files reach the RIP, this standardization has to be undertaken through conversion, rather than assignment. Today, there are systems available that act as a ‘colour server’ to automate the process, based on defined rules, to convert any incoming files to a standard colour space, while at the same time maintaining the original colour intent of each file.
Josh Hope is a colour management professional for HP, working in the large-format Scitex division. This article is based on a seminar he presented at the Specialty Graphic Imaging Association’s (SGIA’s) 2012 Expo. For more information, visit www.hp.ca and www.sgia.org.
