May 2010 Does subtractive colour mixing exist Stephen

May 2010 Does subtractive colour mixing exist? Stephen Westland Professor of Colour Science and Technology School of Design University of Leeds

Overview To explain the nature of additive colour mixing To make explicit the relationship between additive and subtractive colour mixing Say some interesting things about colour primaries and colour gamuts All colour is created by the mixture of three "primary" colours. The three primary colours of paint are red, blue and yellow You cannot mix a primary red, yellow or blue using any other colors.

Dogma and Doctrine air (sky) water (sea) fire (sun) earth (earth) Empedocles (490435 BC) believed that everything that is permanent is four-fold. The elements fire, water, air and earth were the roots of all things Greek thought became obsessed with the number four and thus the four-colour doctrine was born: Aristotle – warm, dry, damp cold Hippoocrates – black bile, blood, yellow bile, phlegm Four ‘humors’ – melencholic, sanguine, choleric, phlegmatic Four seasons – winter, sporing, summer autumn Four ages of man – child, youth, man, greybeard

Additive Colour Mixing What is being mixed? power Where is the mixing taking place? wavelength

Additive Colour Mixing

Univariance 2 x M

Rod vision has no colour!!

Trichromacy S M L

Metamerism Each wavelength produces a unique ratio of LMS responses. Small L and M response Large S response Large L and M response Small S response The cone response to the mixture of red and green light is the same as for monochromatic yellow However, additive colour-mixing occurs because when an object reflects more than a single wavelength the cone responses are not unique - metamerism occurs.

Additive Colour Gamut Colour primaries cannot be matched by mixing together other colours!! Colour primaries are (somehow) more pure than other colours!! CIE y CIE x

RGB is not standard CRT LED Mobile displays plasma Thesame. RGBvaluesdisplayedononthesedevices The wouldmostlikelyresultinindifferentcolours would (unless we have very good colour management!!)

reflectance Subtractive Mixing The process of mixing would be the same as for additive colour mixing. wavelength reflectance However, since the dyes are absorbing, not emitting, the mixtures would be very dark and dull. wavelength reflectance Besides, dyes that behave like this so not exist!! wavelength

reflectance Subtractive Mixing reflectance wavelength Mixing together such broadband red and green dyes (for example) would again result in a very dark colour (black in theory, brown in practice).

reflectance Subtractive Block Primaries reflectance wavelength The cyan, yellow and magenta dyes control the red, blue and green light reflected respectively.

reflectance wavelength reflectance Subtractive Block Primaries wavelength CMY (Murray 1934) Green results from a mixture of yellow and cyan, but the amount of green light present is actually controlled by the amount of magenta dye!

Broadband absorption spectra In general, the larger (more conjugated) the organic molecule the less energy is required to enable the transition Excited state of dye Absorption of energy Packets of energy are large Packets of energy are small Ground state of dye

Realistic dye reflectance curves Adobe RGB (1998)

Additive/Subtractive Gamuts The gamut of a device is the range of colours that it can reproduce Display gamut Print gamut

Additive vs Subtractive

Why RYB? Vermillion or a lake of cochineal or madder (early C 19 th) Alizarin crimsom (late C 19 th) Violet red quinacridone (mid C 20 th) Cobalt violet?

Why RYB? Thanks to modern intense and lightfast pigments, we can choose much more effective paints than were available to artists of the past, and as a result the traditional primary triad — red, yellow, and blue — is obsolete and should not be taught. Bruce Mc. Evoy – www. handprint. com

Questions? Thanks for listening ; ).