Colour and Colour Measurement (TX-2046)

1. Colour and Colour Measurement (TX-2046) Dr Huw Owens

2. Introduction • Aims and Objectives • Syllabus • Course structure • Colour in an industrial context • Reading List and Resources

3. Aims • To give an appreciation of the electromagnetic basis of colour • To provide an understanding of additive and subtractive colour mixing and the related applications • A basic introduction to the Human Visual System (HVS) • An understanding of the different methods of colour specification and colour difference. • A basic knowledge of a range of colour issues faced in industry • An ability to problem solve with respect to specific colour issues • An ability to measure colour using the correct technique

4. Syllabus • Light and colour • Colour mixing • Colour vision • The CIE system • Colour atlases • Colour difference • Psychology of colour • Reflectance • Absorption • Reflection

5. Course Structure • 12 hours of lectures • 24 hours of laboratory work • Assessment method and relative weightings • Unseen examination questions - 50% • Assessed Laboratory reports – 50% • Module worth 10 credits

6. Reading List and Resources (1) • WYSZECKI, Günter, and W. S. STILES. 1967. Color science: concepts and methods, quantitative data and formulas (New York: John Wiley & Sons). 2nd ed. 1982. • HUNT, R. W. G. 1987. Measuring colour (Chichester, England: Ellis Horwood). 2nd ed. 1991. • HUNT, R. W. G. 1988. The reproduction of colour in photography, printing and television (Tolworth, England: Fountain Press). • WRIGHT, William David. 1944. The measurement of color (London: Adam Hilger). 2nd ed. (New York: Macmillan, 1958). 3rd ed. (Princeton, New Jersey: Van Nostrand, 1964). 4th ed. 1969.

7. Reading List and Resources (2) • WANDELL B, 1995, Foundations of Vision, Sinauer Associates. • BOYNTON and McLeod, Colour Vision, Optical Society of America. • Berns RS, 2000, Billmeyer and Saltzman’s Principles of Color Technology, 3rd Edition, John Wiley & Sons, New York. • MACDONALD R(editor), 1997, Colour Physics for Industry, Society of Dyers and Colorists, Staples printers Rochester Ltd, 0-901956-70-8. • NASSAU K, 2001, The physics and chemistry of colour, The fifteen causes of colour 2nd Edition, Wiley Series in Pure and Applied optics, John Wiley & Sons, ISBN 0-471-39106-9.

8. Reading List and Resources (3) • Internet : • http://www.cie.co.at/cie/ • http://colorpro.com/info/ • http://ziggy.derby.ac.uk/ • http://www.colourware.co.uk • http://www.poynton.com • http://www.colourtech.org • http://www.cis.rit.edu/mcsl

9. Light and Colour - Summary • The Electromagnetic Spectrum • Visible spectrum/monochromators • Light Sources • Absorption and Reflection • Metamerism and colour constancy • Fluorescence

10. Light and Colour - Definitions • Light – That aspect of radiant energy of which the human observer is aware, through visual sensations arising from stimulation of the retina by the radiant energy. • Colour – That aspect of visual perception dependent on the spectral composition of observed radiant energy. • Colour – “Color is the aspect of appearance of objects and lights which depends upon the spectral composition of the radiant energy reaching the retina of the eye and upon its temporal and spatial distribution thereon.” (Judd, DB)

11. What is Light? • Greeks • Pythogorean school assumed every visible object emits a steady stream of particles • Aristotle concluded that light travels something like waves • Ancient Greeks knew that light travels in straight lines • Hero – discovered the angle of incidence and the angle of reflection are always equal • 1621 Snell tried to explain why a straight pole stuck in the water at an angle no longer appears straight to the observer. He named the bending of the light through a medium refraction.

12. What is light? • Snell failed to discover why the light bends. • 1678 Huygens suggested the answer – he suggested that the refractive index of any material is determined by the speed with which light travels through it. The greater the refractive index the slower light would travel through the medium. (Light as a wave.) • Newton (1666) (Light as a particle) • Young (Light as a wave) • Maxwell (mid 19th century) – identified light as part of a vast continuous spectrum of electromagnetic radiation. (Light as a wave) • Einstein (1905) applies Planck’s quantum theory and postulates light may have wave and particle properties.

13. Light and Colour – The Electromagnetic Spectrum • The spectrum and coloured light – Newton (1666)

14. Light and Colour – Prism Monochromator • Blue light refracted more, Red light refracted less • Newton’s experiments

15. Light and Colour – Diffraction Grating • Where θ is the angle of diffraction, λ is the wavelength of the incident light, d is the groove spacing (eg 300 lines per mm gives spectrum 10mm in length) and n is the integer defining the spectral order.

16. Light and Colour – The Rainbow • Descartes (1637)

17. Light and Colour – The Rainbow (An explanation) Sunlight Secondary Rainbow Sunlight Primary Rainbow

18. Light and Colour- Spectral Power Distributions • Light sources are specified by their spectral power curves. This is a graph that plots power (or energy) against wavelength.

19. Light and Colour – Light Sources • White light sources can be divided into two categories: • 1. Continuous • These are usually incandescent light emitted by hot bodies such as the sun, flames or heated metallic filaments such as tungsten. The most important light source is the sun. • 2. Line

20. Fraunhofer Lines • The spectrum of light was carefully examined by Joseph von Fraunhofer in 1814. Using a similar setup to Newton (except he used a narrow slit to define a ray of sunlight). • He found the solar spectrum contained a number of sharp dark lines where certain colours of the spectrum were missing. • Two of these lines (D1 and D2)correspond to the yellow sodium lines.

21. Colour Temperature – Blackbody Radiators • Max Planck (1900) – If a body is heated, like a black iron poker placed in a fire, it will become a barely perceptible dull red when it reaches about 700° C. • More visible light is produced as the temperature rises (shifts from red to orange and then to yellow). This gave rise to the term COLOUR TEMPERATURE. • Leads to colloquial terms such as “red hot” but unlike psychological colours red is a cold colour and blue is a hot colour.

22. Colour Temperature

23. Line Sources • Low pressure gas discharge lamps: pass electric current through a vapour of various elements. • SODIUM vapour gives sodium ‘D’ lines at 589nm and 589.6nm (almost monochromatic), used for street lighting • MERCURY vapour is used in fluorescent lamps, 4 peaks in the visible, one in the UV. The phosphors coating the glass spreads the light to other wavelengths including red. • TRIPHOSPHOR lamps – Phillips TL84, F11. Have three main peaks one in each of the red, green and blue. • NEON lights – red line is prominent. Used in advertising.

24. Fluorescent Lights • Fluorescent lights are low pressure mercury lamps with a coating of phosphors on the inside of a glass tube that spreads the light, and in particular enhances the light at the red end of the spectrum. SPD of Fluorescent Lamp SPD of Triphosphor Lamp (TL84) Mercury lines

25. Reflectance • The colour of a surface depends on how much is absorbed by the material and how much is scattered, transmitted and reflected. • The reflectance properties of a surface are specified by its surface spectral reflectance (SSR) curve. • The SSR curve is a plot of the percentage (or proportion) of reflectance against wavelength across the visible spectrum. Note the shapes and heights of the SSRs. Yellow and red do not peak like green and blue. Freshly fallen snow is close to 97% reflectance. Black velvet fabric is approx 0.5%. Perfect white And perfect black are theoretical. Our perception Of the grey scale (even visual steps between Black and white) is non-linear. A mid-grey has a Reflectance of ~20% at all wavelengths.

26. Surface Spectral Reflectance (SSR) • The SSR of a surface does not change when the light source is changed. • If two samples have identical SSR curves then they will be a visual match under different light sources. • Therefore a SSR curve may be thought of as the “fingerprint” of a colour.

27. Colour Constancy • Definition: This is when a coloured surface appears not to change when the light source is changed. • The converse is called colour inconstancy. • Colour standards should be colour constant.

28. Metamerism • Metamerism is the phenomenon of two colours that match under one set of conditions but fail to match under a different set. • Four types of metamerism are recognised:- • Illuminant • Observer (matches for one person and not another) • Field size (2° to 10°) • Geometric (eg Metallic paints)

29. Metamerism (Illuminant) • When a standard and a batch match under one light source but do not match under a different light source they are said to be metameric and exhibit metamerism. • A perfect non-metameric match can only be achieved when the SSR curve of the batch is identical to that of the standard. • To achieve a metameric match the SSR curves of the standard and the batch must cross at least three times across the visible spectrum (at least once in each third of the spectrum).

30. Metamerism