This document discusses how materials modify light and how color is measured. It explains that light can be scattered, reflected, refracted, transmitted, absorbed, or diffracted when interacting with materials. Color measurement is based on spectroscopy, which measures light absorption across the electromagnetic spectrum. The CIE L*a*b* color space is commonly used to define color in a device-independent way using lightness, chroma, and hue. Print density is also measured to characterize color in printing and is calculated from spectral data using virtual filters.

1. Measurement of
Colour

2. HOW MATERIALS MODIFY LIGHT
• Scattering
• Scattering is general term which consists of
• Reflection
• Light is reflected at fibre and pigment
surfaces in the surface and inside paper
structure
• Refraction
• Light penetrates fibres and pigments and
changes direction
• Refraction occurs whenever there is a
change of refractive index at a surface
boundary.
• Diffraction
• Light meets particles or pores which are as
large as or smaller than the wavelength of
the light
• Particles that are smaller than 1 um
Light Reflection
Absortion
Transmission
Refraction
Reflection
Diffraction

3. HOW MATERIALS MODIFY LIGHT
• Transmission
• Light passes through the material essentially
unchanged = transmitted
• Material is said to be transparent
• Absorption
• All wavelengths of the visible spectrum are
absorbed by the material through which the
light passes
• it appears black and is said to be opaque.
• If part of light is absorbed, material is said to be
colored and to some degree transparent.
• If no selective absorption occurs – the same
amount of scattering at each wavelength-
material appears white
Light Reflection
Absortion
Transmission
Refraction
Reflection
Diffraction

4. HOW MATERIALS MODIFY LIGHT
Dichroism
• Shift of hue with concentration of a
colorant (ink film thickness).
• Example: Magenta
• Spectral peaks in two different parts of
spectrum
• Changing the concentration or ink film
thickness, can change the colour
substantially.
• Ink film has bluish-red = magenta
appearance
• Red reflectance about twice as great
as that of blue.
Magenta
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a
b

5. PRINCIPLES OF SPECTROSCOPY AND
DENSITOMETRY
• Science of the measurement of color - spectroscopy.
• Spectroscopy
– Used to measure all electromagnetic spectra, not just visible.
– Unsaturated molecules absorb UV light- UV spectrum is
measured
– Technique of measuring absorption (transmission) by molecules.
• Densitometry
– used in multicolour printing
– broad-band filters are used.

6. Measuring devices
• Tabletop devices
– Used in paper industry (Elrepho, ColourTouch)
– From the spectral data several optical measures
are calculated
– colour, brightness, opacity, whiteness
– Measurement geometry: d/0°
– Light source: Pulsed Xenon lamp, D65 filtered
– UV levels: D65 and C, automatically controlled

7. Measuring devices
• Handheld devices
– Used in printing industry (X-rite, Techkon)
– From the spectral data several optical measures
are calculated like colour, print density
– Measurement geometry: 45°/ 0°
– Light source: Gas filled tungsten, type A
illumination
– UV levels: Specified by light source

8. Colour measurements are based on
spectral response
• Different variables can be
calculated from spectrum as
– Colour expressed in
L*a*b*colour space
– Print density which uses
filters to block unwanted
wavelengths White paper

9. Colour spaces
• CIE L*a*b* colour space is the most commonly used device
independent colour space in the graphic art industry
– Colours are defined by the perception of human visual system
– In CIE L*a*b* colour space colours can be defined with chroma,
lightness and hue

10. Three dimensions of colour
• Hue
• Chroma or saturation
• Lightness

11. L*a*b* colour values
• Primary colours such as Cyan,
Magenta, Yellow and Black
are controlled by
increasing/decreasing ink
amount
• Secondary colours such as
Red, Green and Blue are
controlled by trapping
behaviour and print densities
of primary colours
– Printing inks must be partly
transparent in order to create
secondary colours
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12. Print density
• Print density measures only darkness not colour
directly  calculated where absoption of light is
strongest
• Earlier actual physical filters were used – nowadays
print density is calculated from spectrum using
virtual filters
• Densitometric measurements
– print density (D)
– dot gain (%)
• calculated according to Murray-Davis
equation
• solid print density and halftone print
density are measured
)101(
)101(
% solid
halftone
D
D
Dot 





13. Print density cont...
• Densitometric measurements
– trapping (%)
• how well next ink layer transfer on previous printed ink
layer
• The ratio of ink transfer on printed surface to ink transfer
on unprinted surface.
Microscopic pictures of cyan, magenta and blue printed areas
ColourPrinted2nd
ColourPrinted1stColourSecondary
%
D
DD
Trap


Preucil equation

14. Print density
• Print density is the
has logarithmic
relation to
reflectance
D= log10 1/T
• Measuring device
can be zeroed to
paper
• Relative
measurement
• Or absolute white
• Absolute
measurement
0
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The amount of ink on paper, g/m²
Printdensity