Measurement of colour

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Measurement of colour

  1. 1. Measurement of Colour
  2. 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. 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. 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 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
  5. 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. 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. 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. 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. 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. 10. Three dimensions of colour • Hue • Chroma or saturation • Lightness
  11. 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 -100 -80 -60 -40 -20 0 20 40 60 80 100 -100 -80 -60 -40 -20 0 20 40 60 80 100 b* a*
  12. 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. 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. 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 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 Printdensity
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