Colour Basics
IN TOUCH EVERY DAY
www.mondigroup.com
Colour Basics
Introduction
Visible Color
Colour management involves using software and hardware
to achieve optimal colour reproduction between devices,
including digital cameras, scanners, monitors, proofers and
printers. Colour management uses standard International
Color Consortium (ICC) profiles, which contain information
about the colour characteristics of each device in the colour
reproduction system. Most production software programs,
such as Adobe Photoshop and Illustrator are ICC-aware and
can understand and apply this profile information within your
files.
Visible Colour Spectrum
Prior to colour management, skilled operators used drum
scanners to adjust and customize colour for specific printing
presses and papers. The process was highly specialized
and very expensive. Today, colour management enables
almost anyone to get high-quality colour from various input
devices such as digital cameras, as well as a variety of
output devices, including desktop, large-format colour
printers, digital production and conventional printing presses.
RGB Colour Model
The benefits of colour management include:
- Colour is optimized during input, so scans and digital
photographs closely match the original.
- Colours appear on-screen as they will look when printed.
- Colour is optimized upon output, enabling an original
photo or scene to be matched closely on any printer,
with any substrate or ink.
- Digital proofs simulate final printed products, which
means proofs from low-cost or one-off devices will
mimic a larger high-cost format printing press.
reproduction. Both systems use a small number of primary
colours that combine to produce a large number (gamut) of
colours.
- Professional users have an accurate starting point for
manual colour corrections.
The visible colour spectrum, or rainbow, encompasses
light wavelengths from approximately 380 to 730 nm. When
divided into three primary colours, the rainbow will break
down into red, green, and blue (RGB).
- Novice users get good plug-and-play colour without a
lot of expense or colour management knowledge.
Additive colour models. The primary additive colour model,
RGB, uses combinations of red, green and blue light to
produce a multitude of colours. When mixed in various
combinations, these three colours can create millions of
distinct colours. If they are mixed in equal quantities, the
colour is perceived as white.
Colour Theory
Additive and Subtractive Colour
To understand how colour management works, it is helpful
to understand the additive and subtractive systems of colour
IN TOUCH EVERY DAY
2
www.mondigroup.com
Colour Basics
The human eye is one device that uses the RGB colour
model. Here`s how it works. Imagine a red car sitting in under
a street light. All of the colours of light are shining on the car,
yet we only see red. This is because the surface of the car
is absorbing (or subtracting) the green and blue rays of the
white light, and reflecting the red. So, when you look at the
red car, your eye receives the reflected red light and sends
a message to the brain: this car is red.
Many digital devices also work in RGB: scanners and digital
cameras capture images in RGB, and computer monitors
display images in RGB. These devices work by combining
combinations of red, green and blue light to capture or
represent a multitude of colours.
The Colour Models
Additive
Red
Magenta
Yellow
White
Blue
Green
Cyan
Subtractive colour models. Vice versa, subtractive colour
systems work by subtracting, or absorbing, selected colours
of light that are reflected off or transmitted through them. The
CMY colour model manipulates reflective colours by using
the colours that sit opposite RGB on the colour wheel: cyan,
magenta, and yellow. It is important to understand how the
RGB and CMY colour models interact.
Subtractive
Magenta
Blue
Cyan
Black
Red
In the RGB colour model, red, green and blue mix to create
CMY.
— - Green and blue mix to make cyan (C).
Green
Yellow
— - Red and blue mix to make magenta (M).
Remembering the Mixes
— - Red and green mix to make yellow (Y).
In the CMY colour model, magenta, cyan and yellow mix to
create RGB.
Write RGB across from CMY in a column. The opposites fall across from each other.
To determine the mixes, eliminate the opposite. For example, to find out which two
RGB colors create magenta, eliminate the opposite of magenta, which is green. The
remaining colors, red and blue, mix to create magenta.
— - Magenta and yellow mix to make red (R).
— - Cyan and yellow make green (G).
— - Cyan and magenta make blue (B).
Colour in technology
Subtractive colour printing. Cyan, magenta and yellow are
used in all dye and ink on paper processes, including digital
inkjet and offset printing. Digital imaging devices that use
these inks work by subtracting, or absorbing, the RGB
colours.
Here is an example of how subtractive printing works. To
IN TOUCH EVERY DAY
3
R
C
G
M
B
Y
print a red circle, we need a white light source, a white sheet
of paper and a pair of eyes to view it. The paper appears to
be white because all of the RGB lights from the light source
www.mondigroup.com
Colour Basics
are bouncing off the paper and into the eye. Printing magenta
ink on the paper will cause the green light to be absorbed,
leaving behind the red and blue light. The eye perceives this
colour as magenta.
Spot Colours
Adding a layer of yellow ink will subtract the blue light as
well, leaving behind just the red light. From this, the human
eye will perceive the printed circle as red.
Due to impurities, printing all three CMY inks will create a
grayish-brown colour instead of black. To add density and
correct the colour of image shadows in subtractive colour
printing, a fourth colour, black (K, which stands for Key) is
added. Including black ink is a more cost-effective way to
create images, text and line art. These printers are often
called 4-colour, or CMYK printers.
Colour separations. CMYK colour separations were first
made on graphic art cameras and photographic enlargers
using RGB filters. In the 1980s a technique called gray
component replacement (GCR) was developed, in which
combinations of CMY colours (which produced a muddy
colour) were removed from the colour separations and
replaced with black. A related process, under colour removal
(UCR), replaces CMY colours in neutrals and shadows only,
with black.
Specification books like this PANTONE Plus Formula Guide provide ink formulas
to help achieve spot colours on press and visually compare sample and target.
PANTONE colours can be simulated with CMYK tint
percentages that have been developed by PANTONE, but
CMYK simulations are unable to completely match most of
the original PANTONE colours. This is know as the PANTONE
Bridge books.
Multi-colour printing. Although process-colour printing uses
three primary colour inks and black (CMYK), it is possible
to add more colours. A fifth, sixth, or even seventh colour
can extend the colour gamut to a much wider range of
colours. Additional colours may include orange, blue, green
or red. PANTONE Hexachrome is a six-colour system, which
includes orange and green or XG whith additional Green,
Violett and Orange (7 colours), for printing with an extended
colour gamut.
Total ink coverage. In CMYK printing it is often important to
limit the total amount of ink that prints on the page. Excessive
ink can lead to long drying times, smudging, set off of ink
onto the sheet above, and excessive ink usage.
When making UCR or GCR separations, the total amount
of ink in a particular image area can be quantified as total
ink coverage. On a scale of 0–to 400%, each colour makes
up one hundred percent. So, one hundred percent of each
colour, C+M+Y+K, would equal 400% total ink coverage. The
standard total ink coverage range for CMYK offset printing is
typically between 200% and 320%.
A number of large-format inkjet printers use a six-colour
system consisting of CMYK, plus light cyan and light magenta.
The lighter process colours enable finer tonal gradations
in highlights to be printed, with a less conspicuous dot
structure, but they do not enhance the gamut.
Spot colours. Ink colours that are selected from a specification
book, such as PANTONE, are called spot colours. Each
colour is mixed in a can and placed in a printing unit on
the press. PANTONE colours are mixed from 14 primary ink
colours according to a recipe book. Spot colours can be
used on multi-unit presses where CMYK plus a spot colour
may be printed, and can also be used in packaging where
product colours are used.
IN TOUCH EVERY DAY
Colour Management
Colorimetry
Colourimetry is a way of measuring and quantifying the
colour of an object based on a standard light source and
4
www.mondigroup.com
Colour Basics
a standard model of human vision. Three basic types of
colour measurement instruments are used in the graphic
arts. In increasing sophistication, they are densitometers,
colourimeters, and spectrophotometers.
Densitometers. Densitometers measure colour through red,
green, and blue filters that are similar to those used for colour
separation. Cyan is measured through a red filter, magenta
through green, and yellow through blue. To measure black,
a special visual”colour filter balances the light source and
provides even illumination across the spectrum. Since the
filters make their complimentary colour inks look dark,
densitometers are sometimes said to see”in black-andwhite.
A colour reflection densitometer shines light onto a surface,
such as a printed press sheet with a colour target, and
measures the amount of light returned to its sensor; no
colour is measured.
Colorimeters. Colorimeters also measure colour through
red, green, and blue filters. However, unlike densitometers,
they calculate values in three-dimensional colour spaces
that more closely represent human vision. It has been said
that they see in colour. Most monitor measuring devices are
colourimeters.
Spectrophotometers. These devices measure light reflectance
across the visible spectrum of light, from approximately
380–to 730 nm wavelengths. This very precise data can
then be converted into densitometric or colourmetric data.
The spectrophotometer is the most useful measurement
Colour Measurement Devices
Spectral Data. Spectral data can be plotted as a spectral
curve, providing a visual representation of a colour’s
fingerprint. Light’s wavelengths and reflectance intensity
provide two absolute points of reference for plotting a curve:
the 350 nanometers of different wavelengths comprise
the horizontal axis, and the level of reflectance intensity
comprises the vertical axis.
To compute spectral data, spectrophotometers examine a
number of points along the wavelength axis, then determine
the amount of reflectance intensity at each wavelength. This
is the most complete and infallible description of a colour,
which can be achieved.
So far, we`ve studied light; objects; how objects affect light
to generate different colours; and how a spectrophotometer
can be used to directly measure how different objects affect
light. To completely define colour as we know it, we must
conclude by studying the viewer—the human eye and other
devices that sense or render colour.
CIE Standard Observer. The CIE Standard Observer is a
mathematical model of human vision that is based on a small
sample of individuals with normal vision who were tested by
the Commission Internationale de l`Eclairage (CIE) in 1932.
The individuals sat in front of a white board with a semi-circle
of spectral light projected on half of it. The other half of the
board contained a mixture of red, green, and blue additive
primary light colours that the individual could adjust with
dimmers. Each individual was asked to dial in the amount of
RGB primaries that he or she felt provided the best match to
the spectral colour. The results provided a map of the entire
spectrum of RGB primary colours, and therefore a model of
human vision.
Most colourimeters and spectrophotometers enable the user
to select from two Standard Observers:
i1 Display Pro
Exact
- The 2° Standard Observer is the original Standard
Observer developed by the CIE in 1932, where test
subjects’ vision was focused on a 2° circle of light. The
subjects’ vision was focused on the fovea, the area
of the retina most sensitive to colour. The 2° observer
is like viewing your thumbnail at arm’s length and is
most commonly used for graphic arts purposes, where
i1 Pro 2
IN TOUCH EVERY DAY
udevice because it can be used for monitor, projector and
printer profiling as well as spot colour measurement.
5
www.mondigroup.com
Colour Basics
viewers look at objects at close distance and scrutinize
the colour.
Colours of White Light
- The 10° Standard Observer, developed by the CIE in
1964, used a 10° circle of light, and allowed additional
light to fall outside the fovea; covering more of the retina.
The 10° observer is like viewing your palm at arm’s
length and is more commonly used for matching colours
that people will see over a larger area and at a greater
distance, such as automobile and house paint.
CIE Standard Illuminants
CIE Standard Illuminant. The CIE defined several standard
illuminants called A, B, C, and D. These illuminants are
characterized by spectral power distribution curves, which
are like spectral curves for light sources.
Spectral Power Distribution Curves
The whiteness of light is characterized on the Kelvin scale,
which represents the absolute temperature of a theoretical
black body that changes colour as it is heated. This concept
is visually represented in a flame, which changes colour as it
gets hotter. Relatively low temperature candlelight is yellow,
while a much hotter welder’s torch is blue. Similarly, warm
looking incandescent lights have a colour temperature of
2000–to 3000K, while bluish computer monitors are closer
to 7500K.
Illuminant D50
Illuminant A
Illuminant F2
Illuminant F11
The graphic arts standard viewing condition according ISO
12647-2:2013 (Graphic technology -- Process control for
the production of half-tone colour separations, proof and
production prints -- Part 2: Offset lithographic processes) is
D50, or 5000K. This approximates noon daylight.
Colorimetric Spaces. Colour is measured in three-dimensions
using colourimetry. The position of a colour in colourimetric
space is calculated by using a CIE standard illuminant
(usually D50 or D65), a CIE Standard Observer (2° or 10°),
and the spectral colours reflected (or transmitted) by the
object to be measured.
CIE Spectral power distribution curves represent the color of light sources. CIE
Standard Illuminants D50 (daylight) and A (incandescent light) are compared with
F2 (cool white fluorescent) and F11 (TL84 daylight fluorescent) bulb.
are x (red-green), y (blue-yellow), and Y (luminance), the
diagram is also known as CIE xyY colourspace. This space
is often used to graph and compare the colour gamuts of
different reproduction processes, such as a colour monitor
or CMYK printer. CIE xyY colour space is not uniform, so it is
not helpful for comparing one colour to another.
Basic colorimetric measurement is expressed as three
values - X, Y, and Z - each representing the relative amounts
of red, green, and blue primary colours reflected by the
object and detected by the standard observer.
XYZ values are graphed in a three-dimensional plot known as
the CIE Chromaticity Diagram, a horseshoe-shaped diagram
representing all of the colours perceived by the human eye.
Since the colour values in the CIE Chromaticity Diagram
IN TOUCH EVERY DAY
CIE L*a*b* (also written CIELAB) is a mathematical
transformation of the CIE xyY colour space that provides a
circular, more even space. The values represent red-green
(a*), blue-yellow (b*), and luminance (L*). CIELAB is much
6
www.mondigroup.com
Colour Basics
better for comparing colours.
Colour Tolerances. Verification between colour specifications
and actual colour results is achieved by using tolerances
that are based on numeric colour measurement data.
Colour tolerancing involves comparing the measurements of
several colour samples (the colour output) to the data of a
known colour standard (the specification or input). Then, the
closeness of the samples to the standard is determined. If a
sample`s measured data is not close enough to the desired
standard values, it is unacceptable and adjustments to the
process or equipment may be required.
CIE L*a*b* Colour Space
While control limits and colour tolerances are separate
considerations, the production workflow and print job should
be set up with both parameters in mind. In general, a project
should never have customer specifications that cannot be
achieved within the printer`s control limits.
The amount of closeness”between two colours can be
calculated using a variety of colour tolerancing methods.
These methods calculate the distance”between two sets of
measurement coordinates within a three-dimensional colour
space such as L*a*b*.
Sphear Tolerancing Method. Two colours can be compared
in CIELAB colour space by measuring the distance between
them, classicaly called delta E (∆E76). Distance is calculated
from the three coordinates of each sample using the algebraic
distance formula. If two colours have a delta E of 1 or 2, the
difference between them is barely perceptible. Higher delta
E values have more noticeable colour differences.
Tolerancing in L*a*b*
The size of the tolerance sphere is determined by customer’s
specifications for acceptable colour difference. A typical
customer tolerance in the graphic arts industry usually lies
between 2 and 6 ∆E76 (ISO 12647-x series of standards
for Offset, Flexo, Gravure, etc.) set colour distance only
for CMYK colours). This means, for example, that samples
outside the tolerance box lie more than ∆E76 = 6 units away
from the standard. Tolerances of less than ∆E76 = 2 units are
typically unachievable given normal process variation, while
a high tolerance could result in visible mismatches between
specifications and results (highly dependent on the image).
Differences between colours in an image that are within ∆E76
= 4 units of each other often are not visible to most viewers.
Delta E76 is the total color difference. Colours that fall within the tolerance circle pass, and
those that fall outside the circle fail.
Elliptical Tolerancing Methods. Our eyes accept colour
IN TOUCH EVERY DAY
7
Colour is a three-dimensional model
matches inside elliptical regions, as opposed to the
spherical”regions used in the CIELAB tolerancing method.
For this reason, the CIELAB method can often provide
misleading results. For example, an acceptable”colour that
falls within a CIELAB tolerance might actually lie outside the
elliptical region of acceptability.
www.mondigroup.com
Colour Basics
To try to more closely approximate perceived colour
distances, three alternative formulas to CIELAB have been
introduced: delta E CMC, delta E 1994, and delta E 2000
(elliptical tolerance). Each revision in the formula attempted
to find a number that more accurately measured colour
difference as perceived by the eye. Many of these more
advanced CIELAB systems are available on today’s
measurement instruments.
The CMC, CIE94, and CIE2000 are not new colour spaces
- they are simply tolerancing systems that are based on the
L*a*b* colour space. The calculations mathematically define
an ellipsoid around a standard colour in the colour space.
This ellipsoid consists of a semi-axis that corresponds to the
attributes of hue, chroma, and lightness. It represents the
area of acceptance in relation to the standard, the same way
the CIELAB “sphere” defines acceptable difference limits.
The size of the ellipsoid varies depending on its position
in the colour space—for example, in the orange region,
ellipsoids are narrower, while in the green region, ellipsoids
are wider. Also, ellipsoids in high-chroma regions are larger
than those in low-chroma regions.
Newer delta E formulas tend to produce lower values for
saturated colours and higher values for neutral colours,
which accurately represents the eye`s sensitivity.
Delta E76
Delta ECMC
Delta E
1994
Delta E
2000
ColourBasics. Rev1 ©2016 X-Rite, Incorporated. All rights reserved.
delta E CMC, delta E 94 and delta E 2000 are revisions to the delta E formula
that attempt to more accurately measure color difference as it is perceived by
the eye.
Mondi Paper Sales GmbH
Marxergasse 4A
1030 Vienna,Austria
Tel: +43 1 790 13 0
Fax: +43 1 790 13 960
Email: [email protected]
www.mondigroup.com/printing