Transcript Colour Order Systems - University of Manchester

Colour Order Systems
Dr Huw Owens
© Huw Owens - University of Manchester :
Introduction
• How can we adequately describe colour?
• How can a colour be defined unambiguously?
• Approximately how many colours are discernable to
the human visual system?
• Can we impose some sort of systematic order to the
naming of colours?
• How can the idea of a colour be communicated?
• Can we specify colours precisely?
• Which colour am I?
• Daybreak, Desert Glass, Sophisticated Lady,
Surrender, Whimsical, Hepatica, Mignon, Nuncio,
Nymphea, Pomp and Power
© Huw Owens - University of Manchester :
Colour descriptions in textiles
• It depends what is adequate for you application.
• Approximately 1.6 million discernable colours to the
human visual system.
• Verbalise colour descriptions for discussion and policy
making.
• To promote and sell colour ideas.
• To reproduce colour accurately across a wide range of
products.
© Huw Owens - University of Manchester :
Verbal descriptions of colour - Precision
• Everyday use
• 20-100 colour description terms cover most
possible needs.
• Professional/artistic use
• At this level there is a need to define the colour
specified.
• Physical samples of the colour are often used to
define the colour precisely.
• Scientific Colour Management
• All colours are given a precise NUMERIC colour
definition. Samples can be measured to provide a
precise specification.
© Huw Owens - University of Manchester :
Describing Colour
• The Desert Island Experiment (Judd, 1975)
• Suppose a person with normal colour vision and no
experience of dealing with colours is idling away
their time on a desert island, surrounded by a large
number of pebbles of similar texture but having a
wide variety of colours. Suppose they wanted to
organise these pebbles in some orderly way,
according to their colour. How can we describe
colour in terms of what they might do?
© Huw Owens - University of Manchester :
Desert Island Experiment
• One possible way would be for out
experimenter to think about colour in terms
of the common names red, blue green etc
and separate those out without hue – that is
those that are white, grey or black. Thus
they separate the chromatic pebbles from
the achromatic ones.
• The observer may find that the achromatic
pebbles could arranged in a logical order in a
series going from white to light grey to dark
grey to black. This arrangement in terms of
lightness, provides a place for every
achromatic pebble. (value, whiteness or
blackness)
© Huw Owens - University of Manchester :
Desert Island Experiment
• The chromatic pebbles differ from one another in
Red
several ways not just by differences in lightness.
• Our experimenter could separate them by hue,
into different piles they call red, yellow, green,
Yellow
Blue
and blue. Each pile may be subdivided as finely
as they want, for example, yellow-green, green
Green
and blue-green piles.
• Each group of pebbles of a given hue could be
separated by lightness just as the achromatic Yellow- Green BlueGreen
Green
pebbles were. The red pebbles could be
Pinks
separated into a series staring with the lightest
pinks and becoming gradually darker, ending
Light red
with the dark cherry reds. Each red pebble
Medium red
would be equivalent in lightness to one of the
Dark red
grey pebbles in the achromatic series.
Very dark red
© Huw Owens - University of Manchester :
Desert Island Experiment
• But the pebbles also differ in another way other than
lightness and hue. For example, a brick red could be
compared to a tomato-red colour. They are the same
hue (neither is yellower or bluer red than the other).
They also have the same lightness. (being equivalent
in lightness to the same medium-grey stone taken
from the achromatic pebbles)
• This third kind of difference relates to how much the
stones differ from grey – in crude terms how much
colour they contain. The stones with a single hue and
a single lightness that vary in their hue are said to
have varying chroma.
© Huw Owens - University of Manchester :
Definitions
• Hue: Attribute of visual perception according to which
an area appears to be similar to one of the colours,
red, yellow, green and blue, or to a combination of
adjacent pairs of these colours considered in a closed
ring (CIE 17.4).
• Lightness: Attribute by which a perceived colour is
judged to be equivalent to one of a series of greys
ranging from black to white (ASTM E 284).
• Chroma: Attribute of colour used to indicate the
degree of departure of the colour from a grey of the
same lightness (ASTM E 284).
© Huw Owens - University of Manchester :
Industry Terminology
Lightness
• Textile dyers use the terms “brighter”, “duller”,
“weaker” and “stronger” to represent specific changes
in lightness and chroma. Changes in dye
concentration relate to stronger or weaker colours,
and one may need to change the choice of dyestuff to
increase a colour’s “brightness”.
Chroma
© Huw Owens - University of Manchester :
Colour Space
• Three dimensional colour space.
• In order to turn this into a useful colour space, two
descriptions are needed.
• Firstly, we need a coordinate system to specify
which colour lies at which point in the colour space.
• Secondly, we need a notation system to describe
colours with reference to other colours within the
colourspace.
© Huw Owens - University of Manchester :
Colour Notation Systems
• Collections of samples are often used to provide
examples of colour products. (e.g. Patches of paint,
swatches of cloth, pads of papers, printings of inks,
etc.)
• Munsell (1905)
• Pantone
• Natural Colour System (NCS)
• OSA Uniform Color Scales System
• The Colorcurve system
• These systems provide a set of symbols that denote a
colour precisely.
© Huw Owens - University of Manchester :
Natural Colour System (NCS)
• Colours are ordered by reference to the four primary
hues of red, green, yellow and blue.
• Colours are further modified by adding in proportions
of black (swarthy) and white.
© Huw Owens - University of Manchester :
The Natural Colour System
• The NCS system is based on single hue triangles with
white, black and a pure colour at the corners.
© Huw Owens - University of Manchester :
The Munsell Colour Notation System
• In 1905 Albert Munsell invented a complete colour
description system.
• This system consists of:• A set of master physical samples whose colours are
the basic reference colours. These are carefully
spaced out as to cover colourspace evenly and as
completely as possible.
• A colour notation by which each colour can be
described and located.
• Commercially available colour atlases, which
contain carefully made copies of the original master
reference colours.
© Huw Owens - University of Manchester :
The Musell Colour System - Descriptors
• The Munsell system follows closely the hue, chroma
lightness approach to colour description. Munsell uses
slightly different terminology.
© Huw Owens - University of Manchester :
The Munsell Colour System - Dimensions
• Hue – The quality of colour. Described using terms such as
Blue, Yellow, Green and Red etc.
• Value (Lightness) – The quality described by the terms
light\dark etc., relating any colour to a grey of the same
lightness.
• Chroma – The quality that describes how the colour differs
from a grey of the same lightness.
• 10 principal hues and 10 steps of value. The 10 hues are
arranged in a circle – Red (R), yellow-red (YR), yellow (Y),
green-yellow (GY), green (G), blue-green (BG), blue (B),
purple-blue (PB), purple (P), red-purple (RP). Each
principal hue is divided into 10 subhues.
• Value varies between black(0) and white(10). Achromatic
colours are notated with the prefix “N”.
© Huw Owens - University of Manchester :
The Munsell Colour System – Value & Chroma
• Value or lightness is
defined by reference to
an equal visual step grey
scale where black=0 and
white=10.
• Chroma is on a variable
length scale of equal
perceptual steps ranging
from zero to 20.
© Huw Owens - University of Manchester :
Differences between the NCS and Munsell Systems
• The NCS differs from the Munsell system in two
important respects:• Colours are defined by reference to measured
numeric colour specifications, not master samples.
• Many more colours (16,000) are defined than can
be put in a typical colour atlas.
© Huw Owens - University of Manchester :
Acceptance
• The NCS colour atlas is published (2nd Edition 1996)
as a Swedish National Standard, and has achieved
World-wide status as a colour reference standard.
The colour spacing is based on 60,000 visual
observations, giving uniform colour scales.
© Huw Owens - University of Manchester :
Advantages of colour order systems
• They are easy to understand because they usually
have actual samples that can be seen.
• They are easy to use. In most circumstances side by
side comparisons are made without the need for
instrumentation.
• The number and spacing of the samples can be
adapted for different applications, and different
arrangements of the samples can be used for different
purposes.
• Most colour-order systems are calibrated in terms of
CIE tristimulus values and these can therefore be
obtained for colours if required.
© Huw Owens - University of Manchester :
Disadvantages of colour-order systems
• There are many different colour-order systems in use
and there is no simple means of transferring the
results from one system into another.
• There are gaps between the samples which means
interpolation often needs to be used.
• The visual comparison between samples is only valid
if it is done using the same illuminant (with the same
spectral power distribution), geometrical arrangement
(as was used for the original judgements) and
calibration adopted for each system.
• Different observers make slightly different matches on
the same colour (observer metamerism).
© Huw Owens - University of Manchester :
Disadvantages to colour-order systems
• There may be differences between samples in the
collection actually being used and those for which the
calibration of the system applies. (manufacturing
tolerances, fading etc.)
• Some colours may lie outside of the gamut of the
samples available in the system. This can happen in
the case of colours of very high chroma.
(pigment/dye stability – fluorescent colours).
• Most colour-order systems cannot be used for selfluminous colours, such as light sources
© Huw Owens - University of Manchester :
Which colour am I?
• Daybreak, Desert Glass, Sophisticated Lady,
Surrender, Whimsical, Hepatica, Mignon, Nuncio,
Nymphea, Pomp and Power.
• Light purple.
© Huw Owens - University of Manchester :