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Color perception. 1. Light as the source of color

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Color perception

1. Light as the source of color

“That light is the source of color was first demonstrated in 1666 by Isaac Newton, who passed a beam of sunlight through a glass prism, producing the rainbow of hues of the visible spectrum. This phenomenon had often been observed before, but it had always been related to latent color that was said to exist in the glass of the prism. Newton, however, took this simple experiment a step further. He passed his miniature rainbow through a second prism that reconstituted the original white beam of light. His conclusion was revolutionary: color is in the light, not in the glass, and the light people see as white is a mixture of all the colors of the visible spectrum.” [SynthLight]

2. Color

“Color is a phenomenon of perception not an objective component or characteristic of a substance. Color is a psychophysical response consisting of the physical reaction of the eye and the automatic interpretive response of the brain to wavelength characteristics of light above a certain brightness level (at lower levels the eye senses brightness differences but is unable to make color discriminations).” [SynthLight]

So, color is a visual sensation which depends on the characteristic of the object but also on the conditions of observation and on the lighting conditions.

3. Colorimetry

Colorimetrytry to quantify and describephysically human color perceptions.

Sources:

http://www2.konicaminolta.eu/eu/Measuring/pcc/en/part2/06.html

http://tpearcenciel.files.wordpress.com/2010/01/newton2021.jpg

http://www.fond-d-ecran.com/ciel/arc-en-ciel/ordinateur-87.html

Source : SynthLightHandbook, http://new-learn.info/learn/packages/synthlight/handbook/index.html

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Color perception

1. Light as the source of colors

In 1676, Newton showed in experiments that white solar light can be decomposed, using a prism, into a spectrum of colors. Using a second prism, he showed that white light could be recomposed from this spectrum of colors and paved the ways for additive color synthesis.

1.1. Additive color synthesis

Additive color synthesis is the superposition of several luminous radiations, each one having its own color. It is a manner to create various colors mixing, usually, the three primary colors (red, green and blue) in different amounts. Result of an additive synthesis is always brighter than primary colors and the addition of the three primary colors results in white light.

Two colors whose the addition produces white light are complementary colors (cyan, yellow and magenta).

1.2. Subtractive color synthesis

Subtractive color synthesis creates colors by removing some wavelengths of the light source. Mixing primary color pigments (cyan, yellow and magenta), in different quantities creates various colors. Each pigment of the mixture absorbing part of the spectrum of incidental radiation and reflecting some others wavelengths. Colors obtained by subtractive synthesis are always darker than the source colors.

Two pigments whose the subtraction produces dark color are complementary colors (red, green and blue).

Notice that additive primary colors correspond to complementary colors of the subtractive addition and vice versa.

www.votreopticien.com/generalites/les_couleurs/les_couleurs.html

Addition

Subtraction

http://www.astro-rennes.com/initiation/premices.php

Source : http://couleursdesophie.free.fr/physique.htm

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Color perception

1.3. Interaction between color of the source and color of the object.

Cyan pigment lit by a white light source absorbs red component of light and reflect blue and green components. That is why it is perceived by human beings as cyan.

Magenta pigment lit by a white light source absorbs green component of light and reflect blue and red components. That is why it is perceived by human beings as magenta.

Mixing, in same quantities, cyan and magenta pigments create blue color because the mixture absorb red and green components and reflect blue component of the white light source.

If a yellow filter is placed between the source and the object to create a yellow light (a mix of green and red light), the blue appears black because the mixture absorb red and green wavelengths of the yellow light and does not reflect the blue component which is stopped by the yellow filter.

http://college.ceret.free.fr/physique/optique/filtres.swf

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Color perception

1.4. Spectrum repartition

Artificial and natural sources of light present different spectrum.

Sun has a continuous spectrum as well as incandescent lamps. But contrary to artificial sources of light, natural light varies according to the climate, season and hour of the day.

Four significant spectral distribution can be distinguished:

Monochromatic radiation (laser, …)

Near monochromatic radiation (LED, …)

Continuous spectra (sunlight, incandescent light, …)

Line and band spectra (gas discharge lamps, …)

These different spectral distribution have an influence on our human perception of the environment. The variation of the spectrum of natural light also strongly influence our perception of building and create dynamism.

Different spectra :

Illustrations : http://tecfa.unige.ch/perso/lombardf/CPTIC/couleurs/couleur_ERAG/Pages/ch2p1.htm

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Color perception

2. Color

2.1. Color as a visual sensation

Color is a subjective visual sensation. Human eye analyses a stimulus (luminous radiation) in the physical world (objective field) and the brain interprets it and appreciates it as a color sensation (subjective field).

Color sensation (hue, saturation, lightness) should not be confused with real world stimulus (XYZ) even if it is correlated with this one.

2.2. Color as a neurophysiologic signal

Retina is the eye’s light sensor and is composed of two kind of light-sensitive cells which are cones and rods. Cones are the cells which allow vision of colors. There are three different kinds of cones (S, M, L) which are sensitive to different wavelengths (short/blue, medium/green and large/red wavelengths). They absorb luminous radiations and convert it in electrical signals transmitted to the brain though the optic nerve. Comparison of the signal emitted by each kind of cones entertain the vision of color.

http://misclab.umeoce.maine.edu/boss/classes/SMS_491_2003/Week_10.htm

http://www.physics.uc.edu/~sitko/LightColor/18-EyeStructure/Eye.htm

http://www.webexhibits.org/causesofcolor/1G.html

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Color perception

2.3. Color depending on conditions of observation

There is no univocal relation between the stimulus and the color sensation. A same stimulus can produce different sensations and different stimuli can produce an identical color sensation. It depends on the conditions of observation.

There is no cones of type S at the center of the fovea. So, the vision of very small objects is bivariant (M, L) and not trivariant (S, M, L) and different stimuli can be perceived in a same manner.

Sometimes, chromatic adaptation of the eye entertains the apparition of phantomcolors. This effectis due to the tiredness of a kind of coneswhichemit a tooweak signal after a too hard contribution.

It is thus possible to link a stimulus to a color perception only if conditions of observation are specified.

2.4. Color depending on lighting conditions

There are two kinds of reflections of light on an object: surface reflections which generally reflects color of the source, and material reflections which is characteristic of the object.

The colorimeter which determines (X,Y,Z) stimulus will be influenced by these two kinds of reflections. And contrary to the conditions of observation, lighting conditions which belong to physical world will influence stimuli as well as color sensations. This phenomenon is taken into account in artificial lighting through the color rendering index. It reflects the ability of an artificial source to render colors and gives information about the influence of the illuminant spectrum on the chromatic aspect of objects.

http://www.rabagnac.com/03_formation/couleur/CoulRem.html

http://tetedelart.hautetfort.com/archive/2009/07/19/1.html

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Color perception

3. Colorimetry

Colorimetryis the science of quantifying and describingphysically the human color perception, similarly to the spectrophometry.

It is difficult to quantify a color because brain interprets the initial stimuli. The eyecandistinguish more than 350 000 colours. It isthusnecessary to determine a classification.

There isthree main manners to classifycolors:

A visualapproach (Munsell (Hue, Value, Chroma), NCS,…)

A physicalapproach (CIE RGB, CIE XYZ,…)

A physical and psychometricapproach (CIELAB, CIELUV,…)

http://boards.core77.com/viewtopic.php?f=20&t=21520

http://www.coilinter.de/en/products/colours.shtml#ncs

http://www2.konicaminolta.eu/eu/Measuring/pcc/en/part1/09.html

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Color perception

1. Additive synthesis: LED monitor and other displays

2. Subtractive synthesis : printing, painting, ...

3. Urbanlighting : Yann Kersalé

LED monitor

Printer ink

Yann Kersalé, Allumouette, Paris, 2006

http://www.ykersale.com/index.php?rubrique=2&soussections=req2&idprojet=98

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Color perception

To teach students or to learn more :

http://www2.konicaminolta.eu/eu/Measuring/pcc/en/part1/index.html

http://prn1.univ-lemans.fr/prn1/siteheberge/PublisCours-OPI/OPI_fr_M07_C02/co/OPI_fr_M07_C02_web.html

Precise color communication: color control from perception to instrumentation, Konica Minolta, 2007

Available for free on

http://www.konicaminolta.com/instruments/knowledge/color/pdf/color_communication.pdf