Page 116 - Graphic Design and Print Production Fundamentals

104 Chapter 4. Colour Management in the Graphic Technologies
information about the file. This is followed by a key to the colour data that follows. The actual colour
information comprises a row and column ID to identify the specific colour patch and then the numerical
data that goes with the patch.

The key information is contained in the second section where the data format is laid out. This indicates
what colour spaces are captured in the data for each colour patch. The first four values are CMYK; the
next three are XYZ (a device independent colour space like Lab); the next three are Lab; then RGB; and
finally there is a wavelength measurement for each of the 10 nanometer slices of the visible spectrum
(380-730 nanometers; violet to red).

The CMYK values for each patch are fixed in the software, coming from either the IT8 or ECI
specification. These CMYK values have to remain constant because they are our point of reference for
all devices. When we read the patch with the spectro, it uses the spectral information (each of the 10
nanometer slices) to calculate the XYZ and Lab values that describe the colour appearance of the colour
swatch. By matching the measured Lab value to the predetermined CMYK value supplied in the original
pdf, we have the raw material to build a translation table describing the device’s colour behaviour. Once
we have this look up table (LUT), we can predict the Lab equivalent for any CMYK value produced
on the device. Correspondingly, when we are given the Lab values from another device that we want to
match, we can produce the appropriate CMYK values to provide that colour appearance.

Device Dependent Versus Device Independent Colour Spaces

We’ve defined CMYK, RGB, and Lab colour spaces, and we’ve seen how the first step in colour
profiling is to measure output from a device to define a relationship between the CMYK device
dependent and the Lab device independent numbers. Establishing this relationship between device
dependent (RGB or CMYK) values and device independent (Lab) values is a fundamental component of
the colour management process.

We call the CMYK and RGB colour spaces device dependent because the results we get from specific
RGB or CMYK values depend entirely on the device producing them. When we tell a device to produce
“50% cyan,” we are saying, “Give me half of as much cyan as you can produce.” Since the capacity and
colour appearance of cyan from any two devices will not be the same, it should not be surprising that a
specification of “half that much” will also produce colour events that do not match. Similarly, the RGB
values on a monitor or projector simply specify some proportion of the full red, green, and blue signals
that the device can produce. Since there is no common starting point between two monitors in terms of
what a full red, green, or blue signal is, then providing the same RGB values to those two monitors will
in no way provide an opportunity to generate the same colour appearance.

For RGB devices, RGB values simply identify the volume of signal for each channel. For printers,
proofers, and presses the CMYK percentages dictate what proportion of pigments are deposited. The
numbers associated with specific RGB and CMYK colours only have colour meaning when attached to
a particular device. There is no inherent consistency between any two devices based on providing the
same RGB or CMYK values.

So if the device dependent colour spaces don’t give us any consistency or control of colour between
devices, where can we turn? Enter the device independent colour spaces of Lab and XYZ. We