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 ( 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 wavelength power wavelength power wavelength power What is being mixed? Where is the mixing taking place?
Additive Colour Mixing
Univariance M 2x
Rod vision has no colour!!
Trichromacy LMS
Metamerism Large L and M response Small S response Small L and M response Large S response However, additive colour-mixing occurs because when an object reflects more than a single wavelength the cone responses are not unique - metamerism occurs. The cone response to the mixture of red and green light is the same as for monochromatic yellow Each wavelength produces a unique ratio of LMS responses.
Additive Colour Gamut CIE x CIE y Colour primaries cannot be matched by mixing together other colours!! Colour primaries are (somehow) more pure than other colours!!
RGB is not standard CRT LED plasma Mobile displays The same RGB values displayed on these devices would most likely result in different colours CRT LED plasma Mobile displays The same RGB values displayed on these devices would most likely result in different colours (unless we have very good colour management!!)
Subtractive Mixing wavelength reflectance wavelength reflectance wavelength reflectance The process of mixing would be the same as for additive colour mixing. However, since the dyes are absorbing, not emitting, the mixtures would be very very dark and dull. Besides, dyes that behave like this so not exist!!
Subtractive Mixing wavelength reflectance wavelength reflectance wavelength reflectance Mixing together such broadband red and green dyes (for example) would again result in a very dark colour (black in theory, brown in practice).
Subtractive Block Primaries wavelength reflectance wavelength reflectance wavelength reflectance The cyan, yellow and magenta dyes control the red, blue and green light reflected respectively.
Subtractive Block Primaries wavelength reflectance wavelength reflectance wavelength reflectance 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! CMY (Murray 1934)
Broadband absorption spectra Ground state of dye Excited state of dye Absorption of energy In general, the larger (more conjugated) the organic molecule the less energy is required to enable the transition Packets of energy are small Packets of energy are large
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 C19 th ) Alizarin crimsom (late C19 th ) Violet red quinacridone (mid C20 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 McEvoy –
Questions? Thanks for listening ;).