Color Management and Why We Need It - Color Management for Photographers - page 24

Fig. 1-8 It is somewhat
difficult to show a three-
dimensional color sphere
on a flat page, yet this
illustration courtesy of X-
Rite does a great job of
showing how hue,
saturation, and lightness
are plotted in such a
space. In this globe, white
to black is plotted
vertically running through
the center of the sphere.
Saturation runs from the
center of the sphere
outward with gray in the
center of the sphere and
full saturation running the
equator. Hue (color) runs
360 degrees around the
equator.
notable experts that suggest editing in this space within Photoshop, but
there are progressively less reasons to do so, and some would say it's just
wrong. LAB is a very useful color space for color management, though,
because it is device-independent. LAB is a universal color space transla-
tor that forms a foundation for color management systems. See Chapter
9, Tutorial #1: “Photoshop's Color Picker and Color Model.”
Sidebar
Delta-E: One benefit of a perceptually uniform color space like CIELAB is that it allows us to
put a value on the difference between two colors as a human would perceive them. Just how
close are these two similar colors? If I had three solid color patches that looked quite similar,
which two match the closest? Using LAB, we can calculate these differences and assign a value
to them. This calculated value is delta-E ( D E). A delta-E of 1 is considered to be a single, just-
noticeable difference. That is, if we had two color patches side by side (not touching) and the
delta-E was less than 1, the average human observer would say they matched. Many in the
print trade consider a delta-E of approximately 3 to 9 as an “acceptable match.” I put accept-
able match in quotes because there are clients who would demand a closer match (lower delta-
E value). A value of 1 is barely visible, whereas a value of 3 to 6 is visible but still acceptable
to most viewers. As the delta-E values get higher, it becomes easier to see the differences. Under-
standing delta-E is important, as we will use this unit of measure later when looking at such
issues as process control. To complicate matters, there are a number of differing methods of
calculating delta-E. Original or simple delta-E was not uniform in all areas. Some newer methods
such as delta-E CMC and delta-E 2002 produce a more perceptually uniform result.