Specifications

The Digital Fine Print Course
Refining a Fine Print
Copyright Les Walkling 2012
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Working in L*a*b*
One of the advantages of working in the CIE LAB colour space is that it separates the
image into a Lightness channel (L*) and two colour channels (a* and b*). This encourages
the editing of tone independently of colour.
L*a*b* is also a device independent and very large colour space. But this can also be a
disadvantage if only subtle editing changes are required because of the relatively large
tonal jumps between adjacent levels in the a* and b* channels. It is also not possible to
judge colour balance and colour clipping by visually inspecting the a* and b* histograms.
‘RGB Luminosity’ can be enhanced by first duplicating the RGB image, mode changing to
LAB, discarding the two colour channels a* and b*, and then converting the L* channel
into greyscale, and then back into RGB, before moving it onto the original RGB image as
a separate layer blended on Luminosity. The RGB Luminosity is then replaced with that of
the Lightness channel, which results in expanded tonal separation through out the image.
L*a*b* L* L*a* (magenta to green) L*b* (yellow to blue)
+
=
+
RGB L* RGB + L*