Colour differences are the differences between the requested colours and reproduced colours. These colour differences can be caused by factors such as printer environment, printer condition, printer gamut, media, and output profile. This topic explains how PRISMAsync compares and represents colour differences.
Requested colours, printed colours and measured colours can all be defined with three numerical values. These three numerical values form together a three dimensional colour space. This makes it possible to calculate colour differences by calculating the distance between two colours in a colour space. For example between a requested colour and the reproduced colour.
The Commission Internationale d'Eclairage (CIE) offers the following CIELAB variants to define a colour space:
CIE L*a*b* (CIELAB), in which:
L* represents the lightness (from 0 to 100)
a* represents the green to red axis (from -128 to +127)
b* represents the yellow to blue axis (from -128 to +127)
CIE L*a*b*CIE L*C*h* (CIELCh), in which:
L* represents the lightness (from 0 to 100)
C* represents the chroma or colour saturation, represented by the distance from the lightness axis (from 0 to > 100 at the edge)
h* represents the hue or colour, represented by the angle with respect to the a-axis (from -180° to +180°)
C* and h* are calculated from the a* and b* coordinates of the CIE L*a*b* model. The same colour is thus defined in two different notations: CIE L*a*b* and CIE L*C*h*.
CIE L*C*h*
The CIELAB colour space is device independent and based on how humans perceive colour differences. These are the main reasons why PRISMAsync uses the CIELAB colour space to indicate colour differences using the colour metrics described below.
The most general CIE metric to express the difference between two colours in CIELAB is Delta E. PRISMAsync Print Server uses two variants of Delta E:
Delta E 1976, △E76, based on the calculated distance between two colours.
Delta E 2000, △E00, based on the calculated distance between two colours and the human perception of colour differences.
Difference between two colours in CIELAB The following Delta E00 range illustrates how the values of Delta E may be interpreted for a specific colour evaluation.
Delta E00 smaller than 1 is not visible by humans.
Delta E00 between 1 and 2 is visible through close observation.
Delta E00 between 2 and 10 is visible at a glance.
To indicate more colour difference information than the total colour difference Delta E, the hue error △H (Delta H) is used.
You can imagine Delta H as which colour difference is left over when the lightness and the chroma differences are ignored.
△L (Delta L) is the lightness difference between the two colours. |ΔL| is the absolute value of △L (Delta L).
The weighted |ΔL| puts more emphasis on the colour differences for light greys, where the K value is lower than 50%. The goal of weighting is to reduce the importance of the lightness factor for very dark greys. Humans usually do not identify errors in the very dark grey colour area. This weighting definition is in accordance with the IDEAlliance G7 specification.
Close to the grey axis, in other words for near neutral colours, the usage of Delta H is not appropriate enough. Instead, the usage of the △Ch (Delta Ch) metric is proposed.
The metric that is called chromaticness can reveal grey balance errors.
The weighted ΔCh puts more emphasis on the colour differences between composite greys, where C is lower than 50%. The goal of weighting is to reduce the importance of the grey balance for very dark greys. These dark greys errors are difficult to handle and are usually printed with black toner. This weighting definition is in accordance with the IDEAlliance G7 specification.