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Measurement of
Colour
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
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
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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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.
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
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
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
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
Three dimensions of colour
• Hue
• Chroma or saturation
• Lightness
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 -100
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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
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
Colour Printed 2nd
Colour Printed1st ColourSecondary %
D
DDTrap
Preucil equation
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
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The amount of ink on paper, g/m²
Pri
nt
den
sit
y