Transcript The presentation template - University of the West of England

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 (490435BC) 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
power
power
Where is the mixing
taking place?
wavelength
wavelength
wavelength
Additive Colour Mixing
Univariance
2x
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
CRT
LED
LED
Mobile
displays
Mobile
displays
plasma
plasma
Thesame
sameRGB
RGBvalues
valuesdisplayed
displayedon
onthese
thesedevices
devices
The
wouldmost
mostlikely
likelyresult
resultinindifferent
differentcolours
colours
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 very
dark and dull.
wavelength
reflectance
Besides, dyes that behave
like this so not exist!!
wavelength
reflectance
Subtractive Mixing
reflectance
wavelength
reflectance
wavelength
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
reflectance
wavelength
wavelength
The cyan, yellow and
magenta dyes control the
red, blue and green light
reflected respectively.
reflectance
wavelength
reflectance
reflectance
Subtractive Block Primaries
wavelength
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 C19th)
Alizarin crimsom (late C19th)
Violet red quinacridone (mid
C20th)
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 – www.handprint.com
Questions?
Thanks for listening ;).