WHAT IS COLOUR?
Colour is defined as the perception of a radiance modified by the surroundings; therefore it is the
combination of a source of luminance, an object and the observer as shown in the following
schematic:
The sensation of colour essentially exists in the human brain, not in the actual material (colour is
the light that can be processed by the human eye). Light enters the eye through the cornea, focuses
on the light-sensitive retina and subsequently the image is received by the brain through the
optical nerve. Light - sensitive cells of the retina are distinguished in rods and cones. Rods are
sensitive to luminosity variance and allow us to see in the dark, however they are unable to
determine colour; cones on the other hand are able to determine colour. Cones are in turn
distinguished into different cones sensitive to red, blue and green colour (Thomas Young's Theory
of Colour Vision). Their relative quantities in the eye are approximately 7,000,000 cones and
120,000,000,000 rods.
Colour perception is a ultimately subjective concept. The sensation of an illuminated object
depends from factors such as age, gender, inherited features, angle of observation, even the
particular mood at the moment.
According to the theory of opposite colours by Young-Helmholtz, Ewald Hering -1930, colour
radiance receptors in the human brain are organized in pairs of opposite - antagonistic colours (red
- green , blue- yellow, white-black). Thus, each receptor may only respond for one of the colours
in each pair, not for both at the same time. The above mentioned theory has been experimentally
confirmed in 1966 at the University of Berkeley by Prof. Russel de Valois.
It is nowadays possible to construct the spectral response curve for the eye as well as any other
instrumental light detector all over the visible spectrum.
Each object reflects a percentage of the electromagnetic radiance that incides thereupon.
Electromagnetic radiance spectrum is wide. The human eye can only sense a small part thereof
and this is called light. The three features determining the colour of any object are the light source,
the object and finally the observer. Three elements are required for the perception of light: relative
spectral power distribution curve: wavelength (nm) vs. relative power); spectral reflectance curve:
(wavelength (nm) vs. reflectance %); and detector's spectral response curve: (wavelength (nm) vs.
relative response % ). Combination of the above may yield the signal or stimulus that is converted
to colour by the brain.
Spectral power spectral
spectral
stimulus of color
Distribution
reflectance
response
In the period between 1928-1931 spectral response curves were determined for a reference
observer regarding primary colours (red, green and blue) representing the mean response of a
multitude of persons having normal sight. All colours can be produced by mixing primary colours
in varying quantities. The required quantities of red, green and blue colour (r, g, b respectively) to
generate another colour are called tristimulus values. New standards have been defined in 1931,
specifying that each colour hue can be determined by three numbers: Χ, Υ, Ζ (standard observer
CIE 1931, 2°). A supplementary standard has been adopted in 1964, according to which the
observation angle became 10 °, and spectral response curves of the detector, x, y, z, are in
correspondence to the optical response of the human eye.
Spectral energy spectral
standard CIE
Distribution
reflectance object observer
Source (CIED65)
stimulus
values
In 1666, Isaac Newton divided the visible spectrum in colour zones using a glass prism. Visible
spectrum comprises of electromagnetic radiation whose wavelength varies between 380 and 780
nm (1 nm = 10-9 m) and is broken down as follows:
Up to - 380 nm
UV (ultraviolet)
380nm-450nm
violet
450nm-490nm
blue
490nm-560nm
green
560nm-590nm
yellow
590nm-630nm
orange
630nm-780nm
red
780 nm and above
IR (infrared)
A significant effect in colour technology is metamerism. Two objects are called metameric and
the relevant effect is called metamerism when they exhibit different reflection curves but the same
Χ, Υ, Ζ colour values for a specific luminance. This coincidence will nevertheless cease to exist
when source of light or observer changes.
Non - metameric objects are called the ones having the same reflection curves (hence identical
trichromatic values). These coincide in chromatic terms (irrespective of luminosity or observer).
Organic pigments and dyes determine the colour of an object. These chemical substances serve
to modify appearance through selective absorption and / or reflection of light energy incident on
the object. This effect is dependent upon specific regions called the chromophores (C=C, C=O,
C=N, Ν=Ν). Their presence in the molecular structure leads to absorption of light radiance.
Different combinations of these regions result in varying energy levels absorbed as well as the
generation of
different colours. other chemical regions, e.g. OH, Cl, Br, NH2, CH3 (auxochromes) cannot
regulate absorption levels, yet they may aggravate these variations. The colour they yield is even
influenced by the method of chemical treatment of these compounds. The colour of inorganic
pigments is influenced by key factors such as chemical composition, degree of oxidation and
crystalline structure.
To avoid metamerism in colour production, specific and stable pigments or their combinations
must be used.
ADDITIVE AND SUBTRACTIVE COLOUR MIXING
According to additive theory, main colours in light are red, green and blue. Visible spectrum is
divided into three equal regions (700-600 nm for red colour, 600-500 nm for green and 500-400
nm for blue). It is impossible to produce filters marginally clipping solar light. Dashed lines
indicate the actual spectrum of each colour while continuous line represents the ideal spectrum. A
secondary colour can be derived by adding two basic colours (red and green give yellow, green
and blue give cyan, blue and red give magenta). Addition of all three basic colours will
theoretically produce white colour (in practice however, a grey shade is produced).
According to the subtraction theory, colour is produced by the selective absorption of visible
spectrum from the pigment or the ink dye. I.e. if the range of blue is absorbed then yellow is
produced, if the range of green is absorbed then magenta is produced and lastly if the range of red
is absorbed then cyan is produced. In theory, it is possible to produce any colour by using these
three colours, whereas mixing of all three will yield black colour. In practice, four-colour
processes also utilize black colour; prints are often produced using six colours or basic inks are
even modified (masking of the primaries).
DESCRIPTION OF OPTICAL SENSATION OF COLOUR
Several methods exist to describe each colour by using the three following elements.
A. Hue - (h) - is the chromatic character of each colour. It determines whether the colour is red,
yellow, green etc. It is identified by the prevalent wavelength perceived by the observer
B. Intensity, strength, purity, vividness, saturation, chroma - (C) - is a colour's degree of purity /
intensity / vividness
C. Lightness, value, brightness, tone - (L) determines the amount of luminous energy reflected
by a coloured sample. It is the property of a colour that expresses its relation with light;
furthermore it is the lightness of a hue, in other words its degree of brightness or darkness.
Hence, colour is illustrated by three coordinates (hue and saturation are at the same level whereas
lightness is a vector at a right angle to the former).
With a few exceptions, objects do not emit coloured light; they just seem coloured when lit. Thus,
an objects can be described as white in colour terms when it reflects almost all its incident light
under common daylight. On the other hand, an object absorbing all incident light seems to be
black and in case only part of the incident light is absorbed, then it looks coloured. Thus, the
colour of an object looks yellow when it only absorbs blue light whereas it looks blue green (or
cyan) when it absorbs red light.
LIGHT SOURCES
Production of light is performed by incandescence (a natural process) or luminescence (an
artificial process). In the first process, light is produced by a heat source (a metallic plate heated to
the incandescence temperature will produce light). Artificial light (also called cold light) is the
product of sources other than heat, at room temperature or at even lower temperatures. Quantum
physics declares that artificial lighting is the product of electron motion from the lowest energy
level (ground state) to the excitation level (high energy state). The reverse process (i.e. electron
transition from a higher to a lower energy state) will produce energy in the form of photons. In
case both phases occur in very short intervals (a few microseconds apart) the effect of
fluorescence is observed, whereas occurrence of these phases in larger periods (a few hours) the
effect of phosphorescence is observed.
When comparing colour comparisons, light source and spectral energy allocation must be stable
and specifically determined. Hence, miscellaneous light sources with specific characteristics have
been proposed by CIE (International Commission of Illuminant). Light source constitutes a light
with spectral energy distribution (that can be experimentally determined), while the term
Illuminant refers to a spectral energy distribution curve in relation to wavelength (without any
physical significance). All standard light sources have their respective illuminance while standard
illuminances do not necessarily correspond to any specific light source. Thus, lighting as a term
has replaced light source.
Sunlight is the best natural way to observe an object but not the ideal light source to judge a color
(due to variability in solar radiance). Artificial light sources are regulated and precisely specified;
quantification of light sources is made on a Kelvin scale (°K).
The following are the most common illuminance types:
A. Illuminance Α: simulates a standard incandescent lamp (tungsten, 2854 ° Κ)
B. Illuminance Β: resembling sunlight at midday (4800 ° Κ)
C. Illuminance C: resembling average daylight (6500 ° Κ)
D. Illuminance D65: resembling average daylight in the northern hemisphere (6700 ° Κ)
E. Illuminance F11: resembling indoor lighting using a TL 84 fluorescent lamp under narrow
energy distribution at 4000 ° Κ
F. Illuminance F2: resembling indoor lighting by CWF fluorescent lamp (4230 ° Κ)
PIGMENTS - DYES
Chemists use a series of pigments to reach the desired hue. Two categories of pigments exist:
organic and inorganic pigments. Organic polymer soluble dyes are also commercially available.
Inorganic pigments are based on oxides, salts and oxidized metal complexes. Molecular structure
of most inorganic pigments is simple; special treatment is required so that they are appropriate for
use in ink systems.
Organic pigments are carbon - based; commonly they do not contain metallic substances.
Appropriate pigments are produced depending on the requirements of each application. In
practice, combinations of the individual pigment types and categories are used to achieve any hue
in printing processes. The following table lists comparison data on these pigment types
DYES
INORGANIC PIGMENTS ORGANIC PIGMENTS
cheap materials
average to high cost
costly materials
high stability (physical chemical)
varying stability
low stability
high opacity
low to medium opacity
low opacity
dull hues
vivid hues
vivid hues
easy processing
difficult processing
difficult processing
easy dispersion
difficult dispersion
no dispersion phase is necessary
(disperse dyes)
INTERACTION BETWEEN LIGHTING AND THE MATERIAL
Behaviour of light incident on a material (paper, plastic, metal, ceramic, fibre etc) is influenced by
a series of chemical and physical characteristics. Transparent materials allow deflection of a part
of electromagnetic energy (diffused from linear motion) while the remaining lesser part is specular
reflected. Opaque materials reflect and absorb incident light. Semi - transparent material allow
incident light to pass through the mass of the object while the remainder is absorbed and reflected.
incidence beam
reflected light (diffuse)
scatter
reflected light (specular)
Dye particles
incidence beam
reflected light
diffuse
specular
scatter
absoption
transmitted light
Appearance of materials essentially refers to an examination of their colour and geometric features
(sheen, shape, surface finish). Energy received from an object is divided into re-emitted energy
and specular energy - gloss. Colour spectral curve of a material represents its colour; it is a
diagram upon which the reflected - transmitted part of incident energy is projected, in proportion
to its wavelength in the region between 380 - 700 nm. The following figure shows such a curve:
Y-axis shows percentile reflection while X-axis shows wavelength in nm.
COLOUR MANAGEMENT SYSTEMS
Measurement of an ink colour over an opaque substrate is performed using a set of instruments
comprising of the following modules: light source of samples, spectrum filtering system and a
photo receptor.
Α. Colorimeters Light comes from a statement incandescent lamp; using three or four filters and
aided by a light sensor, values for main colour constituents are obtained (three stimulus values:
blue, green and red)
sample
Light source
tristimulus filters
meter
photo receptor
Power supply voltage is regulated to ensure stable current conditions during measurement.
Reflected light is measured by a photodiode; the value obtained is read on a galvanometer.
Ceramic sample items are used as calibration specimen.
B. Densitometers An advanced colorimeter form used to analyse four-colour process printing into
four basic colours (black, cyan, magenta, yellow). The following filters are used:
red to determine cyan
green to determine magenta
blue to determine yellow
special filter to determine black
The above are high quality filters. Colour intensity (density) is a logarithmic (not linear) function
of the reflected energy.
R1: energy
reflected from sample
Rw: energy reflected from white paper
light source
illuminant beam
receiver
ink film
substitute (paper)
receiver
substrate - filter
light source
ring mirror
Measuring spot
Sample is lit by the light source. Light beam penetrates the ink layer; its strength diminishes.
Substrate absorbs and reflects a portion of the remaining light. Reflected radiance again passes
through the ink layer; its intensity diminishes and is finally intercepted by the photo receptor.
C. Absorbency and reflectance spectrophotometers
Absorbency spectrophotometers are used for colour measurement on liquids and suspensions;
reflectance spectrophotometers are used in the paint, textile and plastics sectors. They comprise of
the following parts:
- light source (incandescent lamps of tungsten or halogen, xenon lamp)
- light beam direction and focus control device
- light detector and reflected radiance metering device over any wavelength (photodiodes, photomultipliers etc.)
- analog - to - digital converter for electric signals
- results recorder
Spectrophotometers are classified into the following types based on lighting and object
observation methods:
45° / 0°
0° 745°
diffusion / 0 -10°
0-100/ diffusion
detector source
source detector
sample
source
detector
sphere
sample
detector
source
0 -10° / diffusion
diffusion / 0 -10°
Method of angular measurement 45° / 0° or 0° / 45° is used to control level substrates, not threedimensional bodies or metallic colours; in such cases the diffusion sphere method is used. Object
chromatic examination by the diffusion sphere includes the sheen factor parameter. Furthermore,
several instruments applying the diffusion method may measure colour on transparent substrates
or remove the specular energy component- gloss from such a measurement.
Particular attention is required for measurement preparation. Samples due for inspection shall be
representative of the material in question; they shall be level, free from bends or other marks and
their surface shall not be contaminated by grease etc. or even fingerprints.
COLOR VARIATION MEASUREMENT
Several models have been tried for colour measurement, viewing and comparison. Optical
methods have been initially employed based on pantone scale, atlas Munsell. Attempts started in
1915, when prof. Α. Η. Munsell published the first colour atlas. Other papers have been published
since then, e.g. the DIN system, Colour Harmony Manual (Oswald), NCS atlas etc. All the above
surveys share the following common principles:
1. Y-axis indicates luminance
2. distribution of hues around the axis is indicated by circle patterns
Tables thus created exhibit a limited useful life (they rarely exceed five years). Colour differences
between test samples are significant while a closed lighting generator must be always provided to
avoid metamerism errors.
When electronic methods based on spectrophotometers are employed, colour is shown according
to coordinates:
- L, a, b (rectangular coordinates)
- L, C, h (cylindrical coordinates)
L: lightness (L= Ο black colour, L = 100 white colour)
a: red-green component - abscissa of the red - green axis
b: yellow - blue component - abscissa of the yellow - blue axis
C: saturation (C = 0, grey shade)
h: hue
The following formulae are mainly used to calculate the total variation of two colours, ∆Ε:
In 1970, CMC (Colour Measurement Committee of the Society of Dyers and Colourist) proposed
the following colour variation calculation formula:
A new formula was proposed in 1994 to calculate the colour variation between two objects:
KL, KC, KH parameters are assumed equal to one for printing. They only vary in measurements for
the textile industry (KL=2, KC= KH = 1).
• SL= 1
• Sc = 1 + 0.045 C*
• SH = 1 +0.0015 C*
A value of ∆Ε < 1 corresponds to an acceptable colour variation, barely perceptible by
an average observer.
A value of ∆Ε < 2 is a dominant condition that can be achieved during the production
process.
CAUTION
Instruments supervise hues and shall be only used as ancillary devices that do not substitute the
judgement of an experienced observer. Each instrument must be subjected to regular maintenance
in consultation with the manufacturer. The following table provides a comparison between the two
hue generation methods:
OPTICAL METHOD
COMPUTER AIDED METHOD
Empirical method
Preparation of reference samples, reflectance curves
reading and storage
Visual sample assessment
Sample measurement
Design of compositions based on
experience and the past
applications library
Data entry, calculation of potential composition formulae
and optimal selection
Correction estimate
Correction calculation