Colour Scales & Values
Colour Scales
Colour Values
Spectral Data
Based on 84 calibrated glass colour
standards of different densities of magenta (red), yellow, and
blue, graduating from desaturated to fully saturated.
Sample colours are matched by a suitable combination of
the three primary colours together with neutral filters,
resulting in a set of Lovibond® RYBN units that define the colour.
Since several million combinations are viable, it is
possible to match the colour of almost any sample; it is
particularly popular for measuring the colour of oils and fats,
chemicals, pharmaceuticals and syrups.
The coordinates x, y, and z are derived
from X Y Z values by the following calculation such that x + y
+ z = 1
x = X/(X + Y + Z) y = Y/(X + Y + Z) z =
Z/(X + Y + Z)
The values of x and y can then be used to
pinpoint a colour in the x y coordinate system. The x and
y chromaticity coordinates are generally reported along with
the value of luminance factor Y.
The a* axis runs from green to red, the b*
axis runs from yellow to blue and L* (lightness) runs from
black to white. As the L*a*b* Diagram is a three
dimensional diagram, the colour difference between two points
can be obtained in all directions.
The coordinates L*, C* (chroma) and h (hue
angle) are derived from the following calculation:
L* = L*
C* = ÷(a*2 + b*2)
h = arctan (b*/a*) where h is the angle
measured from the positive a* axis in the anti-clockwise
direction.
The u’ v’ chromaticity
co-ordinates were derived to aid the prediction of the
magnitude of the perceived colour difference between two
objects that are found to mismatch in colour. These
modify the x and y chromaticity co-ordinates so that the colour
difference anywhere in the diagram will have the same
appearance of difference.
This colour difference is expressed as
Delta E where a value of 1.0 approximates to a just perceptible
colour difference.