Oce VarioPrint i Series, Руководство по эксплуатации

Страница: 261 / 402

Learn about colour differences
What are colour differences
When you have printed a specific source colour or a spot colour, it can be required to check if the
reproduced colour is the close enough to the source colour. Or, you maybe want to know how
accurate the printer reproduces colours on specific media in general. Therefore, you need to
know how you express and calculate a colour difference.
This topic describes several calculation methods and explains how they can be used in colour
evaluations.
The comparison of colours always occurs in the same device-independent colour space, such as
the CIELAB colour space. You literally can draw a line between two colours in a CIELAB colour
space to show how close or different they are. Therefore, a colour difference is also named the
distance between colours. The CIELAB colour space represents the colours as we see them. A
value that describes the difference automatically indicates if the difference will likely be visible or
not.
The difference between two colours or two groups of colours can be expressed by a colour
metric. This topic first describes the CIELAB colour spaces and then describes the colour metrics
that are derived from them.
CIELAB colour values
The CIELAB colour space has been developed by the Commission Internationale d'Eclairage (CIE)
and represents a three-dimensional sphere in which every colour is unique by its location. The
Lab colour value describes the location of the colour in the CIELAB colour space.
The VarioPrint i-series profiles, colour validation tests, spot colour libraries, and media family
calibration use the CIELAB colour space to define colour values and colour differences in a
device-independent way.
To read the CIELAB colour values of printed colours, you use a spectrophotometer. PRISMAsync
Print Server software can process the measured Lab values into further calculations.
There are two variants of the CIELAB colour space: CIE L*a*b* and CIE L*C*h*.
• The
CIE L*a*b* colour space is used to describe colours and to make colour differences visible.
The CIE L*a*b* colour space is based on the fact that the human eye transforms visual
information into three signals: light - dark, red - green, and yellow - blue.
-L
+L
-b
+b
-a
+a
Lab (60, 80, 25)
[282] CIE L*a*b*
The CIE L*a*b* colour space components are the following:
Learn about colour differences
Chapter 10 - Print what you expect 261