BACKGROUND TO SHADE SORTING
Sorting using the concept of colour space
Shade sorting may be carried out visually or instrumentally.
Visual shade sorting employs human eye judgement
for separating dyed goods into groups. It is an
unreliable method since different observers may produce
inconsistent results. Even with a single observer, decisions
will change over time. The advantages of this process are
its low cost and simplicity in implementation. As the
demand for quality is increased, the need for a more
reliable instrumental shade sorting system has become
urgent. However, the reliability of any system depends on
a great extent on correct implementation, involving, for
example, consistency in sample presentation and strict
control in the operation of the instrument [1–4].
The most widespread method is the 555 system, a
three-number rating ‘cube’ system based on chromaticity
charts developed by Simon in 1961 [5]. The 555 system is
referred to as a fixed grid form of shade sorting because it
divides the colour space into cubes, which are oriented in
a fixed position and parallel to three-dimensional axes.
The L*, a* and b* values of CIELAB 1976 represent a threeaxis
opponent colour space in which to allocate the colour
population [6]. The central cube is designated as 555 and
the centre of this cube is the standard colour. There is no
dead space and also no overlapping portion between
cubes [7–10].
In general practice, the size and shape of the 555 sorting
block can be varied in three dimensions as DL*, Da* and
Db* tolerances, in order to yield an optimum condition for
shade sorting. Smith published a reference guideline to set
these tolerances [11]. However, it is unclear whether this
calculation has ever been done or not.