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

0

0,5

1

380

400

420

440

460

480

500

520

540

560

580

600

620

640

660

680

700

720 nm

-120

-100

-80

-60

-40

-20

0

20

40

60

80

100

120

-120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120

a

b

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

-80

-60

-40

-20

0

20

40

60

80

100

-100 -80 -60 -40 -20 0 20 40 60 80 100

b*

a*

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

0

0,25

0,5

0,75

1

1,25

1,5

1,75

2

2,25

0 0,5 1 1,5 2 2,5 3

The amount of ink on paper, g/m²

Pri

nt

den

sit

y