FUNDAMENTALS OF COLOR

CHAPTER-2

By Granch Berhe

2.1 Introduction

Color is a visual sensation arising from simulation of retina of the eye.

Thus color is define as psychophysical i.e. a psychological response to a physical stimulus.

Color is perceived when the wavelengths constituting white light are absorbed, reflected, refracted, scattered, or diffracted by matter on their way to our eyes.

Color may be different meanings to different people.

To chemist: it is a chemical compound (dye or pigment)

To physicist: it is a scattering and absorption of light or reflectance spectra of an object

To artists and others it is the means to create sensation to the mind, for example red and yellow treats the sustentation of warmness and green and blue associated with feeelingness of coolness.

?......?.....?....... DyeIs a benzenoid aromatic compound which

makes materials coloured.Possess unsaturated groups called chromophores

and substituent groups called auxochromesChromophores:- Provide coulorAuxochromes:- Intensify and deepen colour -Impart colour to the substrate

Examples of Chromophores or its combination chromogen

N=N Azo Chr.C=O Antraquinone Chr.NO Nitrso Chr,NO2 Nitro Group Chr.C= Triarylmethane Chr.

Examples of AuxochromesNH2NHMeNMe2 as Cations E.g +NMeCl2SO3H, OH, COOH as anions E.g O-, SO3-

Molecule is coloured but Useless as a dye

Coloured Molecule and used as a dye

?......?......?....Light

An energy producing a sensation of brightness that makes seeing possible propagating in the form of electromagnetic waves from 1nm to kilometers of wavelength .

The different wavelength are identified by different names as γ-ray X-ray, Ultra-violet, Visible light, Infra-red…..

Visible Light is distinguished from the other by its ability to be detected in the retina of human eye.

Violet400-435nm

Blue435-480nm

Indigo480-500nm

Green500-560nm

Yellow560-595nm

Orange

595-605nm

Red605-750nm

y-rays X-rays Ultra-violet

Visible Light

Infra-red

When white light falls on a substanceIf the light totally reflected – The

substance appears White If the light totally Absorbed- The

substance appears blackIf certain Portion of the light is absorbed

and the rest are reflected-The substance has colour of reflected light

If only single band is absorbed-The substance has the complementary colour of the absorbed band

Coloured absorbed Visible (Complementary)

Violet Yellow-GreenBlue YellowGreen-Blue Orange Blue-Green RedGreen PurpleYellow-green VioletYellow BlueOrange Green-BlueRed Blue=Green

The relationship b/n absorption and wavelength The relationship b/n absorption and wavelength is given by a combination of Lambert’s and is given by a combination of Lambert’s and Beer’s Law(Beer-Lambert LawBeer’s Law(Beer-Lambert Law))Lambert's Law states that equal amount of absorption occur Lambert's Law states that equal amount of absorption occur when light passes in equal thickness of materialwhen light passes in equal thickness of materialBeer’s Law states that equal amount of absorption occurs Beer’s Law states that equal amount of absorption occurs when light passes through equal amount of material (i.e. when light passes through equal amount of material (i.e. Concentration)Concentration)

II00= Intensity of incident light= Intensity of incident lightIItt = Intensity of transmitted light = Intensity of transmitted light ε = moral absorbanceε = moral absorbancec = Proportionality constant (absorption coefficient)c = Proportionality constant (absorption coefficient)d = the path length through which the light passesd = the path length through which the light passes

Beer-Lambert Law is written as Beer-Lambert Law is written as I Itt = I = I00*10*10εcdεcd

The colour of dye is mainly determined by the wavelength of maximum absorption and intensity of the colour depends mainly on the value of maximum absorption.

The phenomenon that wavelength of maximum absorption (λmax) is shifted to longer wave length(red shift) is called Bathochromic effect and shift to shorter wavelength(blue shift) is called hypsochromic effect.

An increase in the value of maximum moral absorbance (εmax) is termed as hyperchromic effect and its reverse is hypochromic effect.

SIGNIFICANCE OF COLORS

EVERY COLOR HAS A MEANING!

Hot, Dangerous!

Environmental, ProductiveCool,

Supernatural

Pure

Perception of color involves a series of events which are interdisplinary in nature. To perception colour these things should be fulfilled.

How is color perceived? 2.2 Perception of Colour

The Human Eye

How Materials Modify By Light?

Apple illuminated under whitelight appears normal (red).

Apple illuminated under greenlight appears dark or black.

This kind of effect is called Rendition

The illuminating radiations are modified by physical process of object such asScatteringAbsorption reflectionTransmission

Object

The relative proportions of these processes depend on the characteristics of the material

Scattering of light is travelling of light in many directions other than incident directions. The colour presented by a group of colorants depends on their scattering and absorption efficiencies. Molar absorbance coefficient (i.e. ε) depends on the property of dye and incident light wavelength. The colour of the dye depends mainly by λmax and intensity (deepness) on the value of (εmax)

Reflectance of light is divided into two types:Defuse reflection:-scattered or reflection in multi directionSpecular reflection:-reflection in one direction.

Transmitte

d

The common detector of light and color is eye, nerve system and brain. The eye focuses the image of object on retina. The photosensitive detector on retina is called as rods and cons from their shape.

Rods are used to detect the light and cons detect the color.

To humans, color sensation is a matter of subjective perception resulting from the effect of light on the cones of the eyes. There are three types of cone receptors in the retina, each one with a particular sensitivity to different wavelengths.

Red sensitive conesGreen sensitive conesBlue sensitive cones

Observer

This decomposition of light into three color components is called the tristimulus theory of color and is the basis for the RGB color model.The light reflected from pigmented object stimulates the three types of colors sensitive cones in the eye and brain interprets the color as cumulative effect of the signals received from cones. The mechanism of the perception of colour is based on additive color mixing.Equal simulation of the tree cones gives the sensation of gray to white. If blue and red cone receptors are simultaneously existed, the sensation of purple is created. How yellow impression created? R+G

Additive Color MixingAdditive color mixing occurs when two or more lights are added by focusing them on white screen.

Applied to lightPrimaries are red, green, and blue (RGB)Colors are produced by adding various amounts of each primaryMixing additive primaries produces white

2.3 Color Mixing Laws premier color scan.EXE

Subtractive Color Mixing

Applied to pigments and dyesPrimaries are cyan, magenta, and yellow (CMY)Light is subtracted through the process of selective absorptionMixing additive primaries produces black

Subtractive Color Mixing

Applied to pigments and dyesPrimaries are cyan, magenta, and yellow (CMY)Light is subtracted through the process of selective absorptionMixing additive primaries produces black

Subtractive color mixing occurs also when one or more spectral components are removed from incident light, by absorption and scattering. Subtraction made by absorption only, it is said to be simple subtractive mixing. On the other hand when the light is removed by scattering and absorption, it is said to be complex subtractive mixing. The most commonly used primaries in subtractive color mixing are; yellow, magenta and cyan. The subtractive primaries are obtained by removing blue, green and red light from white light. White-blue=yellow White-green=magenta White-red=cyan

This is achieved by using broadband filters known as subtractive filters, which absorb a particular wavelength of color.

Magenta filter Absorb greenCyan filter absorb red and yellow filters absorb blue radiations.

Simple filtersM C Y

R RRG G GB B B

Combined filtersM C Y

MYC

G G GR RB B R B

For technological application the most widely approximate equation for complex subtractive color mixing is given by, Kubellka Munk equation.

F(R) =k/s= (1-R) 2/2R Where,

k=absorption coefficient, s=scattering coefficient, R=reflectance and F(R)=Function of reflectance factor.

This equation is basic for color matching for industrial products (e.g. paint, plastics, textiles/ leathers, papers, etc.).