What is color ? 

Color is something which makes the object more appealing, attractive and gives the pleasure of observation. color is defined as visual sensation arising from the stimulation of the retina of eye. Thus it is defined as psychophysical (the psychological response to the physical stimulus). Color may have different meaning to the different people. 

To chemist – It may be a chemical compound (Dye or Pigment). 

To physicist – It is a scattering or absorption of light or reflectance spectra of an object. 

To physiologist – It is a measurable electrical activity of nerves. 

To psychologist – It is a complex process in brain of interpreting the signal of the nerve. 

To artist and others – It is the means to create the sensation in the mind of the observer. 

With the consideration of textiles, color is a very desirable parameter of a textile product. In case of technical textiles, color of the material is not necessary. Because it is only made with functional characteristics for high performance application. But in case of apparel production, color is a very crucial parameter. Because the consumers while buying the cloth in the market, they are highly attracted by the color of the material as well as the handle of the product. 

The color of the textile product may varies with person to person and also the source which is used to perceive the color. The main reason for the variation of color vision in human is body motors (physically balancing) and mentality (psychologically balancing) of the person. Then the color of the material varies under the different sources. It is called metameric behaviour of the textile material. Also the color of the sample should be matched with buyers sample was very difficult. There are numerous coloring material available for coloration of textiles. Now we shall learn about principle of perception of color in the human vision. 

Perception of color involves a series of events which are interdisciplinary in nature. Perception of color includes source of light, object that is illuminated and eye and brain that perceive the color. It is clearly shown in the following figure. 

Fig: Perception of color

The source of the light is characterized by its energy distributed at different wavelengths (i.e. spectral power distribution of the light source). The modification of the incident light depending upon the nature the colorant which is present in the object. Then the radiations reflected by the object is received by the eye. The radiant energy is absorbed by the photosensitive pigments in the retina of eye. This gives rise to nerve impulse which transmitted to the brain. The brain interprets the signal of the nerve depending on the informations (data’s) stored in the memory. We shall now consider each of these components of color viewing separately. 

Source of Light: 

Light is a form of energy and its propagates in the form of electromagnetic waves. Wavelength is important characteristic of electromagnetic wave. Therefore the the waves at the different ranges are identified by different names like gama rays, X rays, UV rays, visible rays, IR rays. Only a small part of the electromagnetic spectrum produces the sensation of vision. This part of the spectrum is called visible region. The wavelengths of the visible region varies from 380 to 750 nm. 

Fig: Electromagnetic spectrum

The source of the light is characterized by its relative power distribution at different wavelengths. The light sources may be incandescent lamp, fluorescent lamp and arc lamp. Spectral power distribution of sources like candle light, sunlight and incandescent lamps are continuous. 

The standard sources are recommended by CIE ( Commission International de L’Eclairage).This system quantify the colors interms of mathematical numbers. The sources are called as standard illuminants A,B and C. 

CIE illuminant A has color temperature of about 2854 K. its spectral power distribution is similar to that energy radiated by 100 watt tungston lamp. 

CIE illuminant B has color temperature of about 2854 K. its spectral power distribution is similar to that energy radiated by noon daylight. 

CIE illuminant C has color temperature of about 6500 K. its spectral power distribution is similar to that energy radiated by average daylight. 

CIE defines the above illuminants for visible range of wavelength about 380 to 780 nm. But with the development of fluorescent colors and fluorescent whiting agent, it is necessary to give standard illuminant for UV region. So that CIE defined the daylight illuminant D65. The color temperature of the D65 illuminant is 6500 K. The following figure will shows the relative power distribution of the CIE standard illuminants. 

With the development of source technology, variety of efficient fluorescent sources have come into the market and are widely used in the house and shops for lightings. These are mainly fluorescent sources having discontinuous spectral power distribution. The manufacturers of garments and textiles, they would like to asses their color of products under these illuminants. CIE also defines the fluorescent sources F1,F2 and F3 to enable to calculate matameric indices of any product. 

Object: 

The illuminating radiations are modified by the physical processes such as transmission, absorption,reflection and scattering. The relative proportions of these processes depends on the characteristics of the material. When the emerging light is same in the direction of incident light , it said to be transmission. When the material absorbs light it absorb all the spectral components except some color, so that it appears some color. The absorption occur in the material depends on the concentration of the coloring matter in the material. In addition to the absorption, scattering may occur in the material. In scattering process light travels in many direction other than the incident direction. When sufficient scattering occurs, it said to be diffuse reflection or transmission. The amount of light scatters depends relative refractive index and particle size of colorants. When the light is incident on the surface, some amount of light may be reflected from the surface. If the angle of incident is angle of reflection, then it said to be specular reflection. The reflection with varies angle is called diffused reflection. The amount of reflection depends on the nature of the colorants in the material and surface texture of the material. 

The appearance of the object is characterized by its spectral reflectance curve. The location of peaks spectral reflectance curve determines the hue of the object color. The object exhibits absorption peaks in the spectral reflectance curve at definite wavelength depending upon the color of the object. The spectral reflectance of achromatic colors( black and white) do not exhibits any peaks. The spectral reflectance curve of achromatic and chromatic objects are shown in the below figure. 

The reflectance of any object can be determined by using the instrument called spectrophotometer. It gives a reflectance curve (spectrum) of an any object. The reflectance value varies from 0 to 1. The corresponding reflection percentage lies between 0 to 100. 

Observer: 

The common detector of light and color is eye, nerve system and brain. The focuses the image of the object on retina. The photosensitive detectors in retina are called as rods and cones from their shape. The rods only detect the light but it have no ability to specify the color. The cones are responsible for color detection. There are three types of cones receptors in the retina. They are sensitive to light at different range of wavelengths. Three types of cones in the retina are

1.      Red sensitive cones

2.      Green sensitive cones

3.      Blue sensitive cones.

The ultimate sensation of colors depends on the degree of stimulation of these three color receptors. If all the three cones are equally stimulated, it gives sensation of grey to white depending on the degree of stimulation. 

The CIE recommends the standard observer data for color calculation. The standard observer is a person who is physically and psychologically balanced. A task of defining standard observer which may represent the normal color vision of average human population is difficult. CIE defined the standard observer data. There are two types of standard observer data called as 2º and 10º observer data. 2º observer data were prepared using fovea region of human eye. In practice in visual assessment of color, one uses larger are of retinal surface. The structure of fovea and the surroundings of retina are different. Therefore CIE defined another set of data is called 10º observer data. These data’s defined by using 10º field of view. 

Color Mixing Theories: 

Primary colors: 

Red, yellow and blue are called primary colors. These colors cannot be mixed or formed by any combination of other colors. All other colors are derived from the primary colors. 

Secondary colors: 

Green, orange and purple are called secondary colors. These colors are formed by mixing of the primary colors. 

Tertiary colors: 

Yellow-orange, red-orange, red-purple, blue-purple, blue-green and yellow-green are called tertiary colors. These colors are formed by mixing of primary and secondary colors. 

There are two types of color mixing namely additive and subtractive mixing. 

Additive color mixing:

Additive color mixing occurs when two or more lights are added by focusing them on the white screen. In additive color mixing, red, green and blue are used as primaries to produce different colors. The primary colors are selected are independent in the sense that mixture of two primary will not produce the other primary. The combination of different primaries produces different colors like following, 

Red + Green → yellow 

Green + Blue → cyan (blue-green) 

Blue + Red → purple (magenta) 

Red + Green + Blue → White 

Subtractive color mixing: 

Subtractive color mixing occurs when one or more spectral components are removed from the incident light. The removal of part of incident energy can occur by process of absorption or scattering. In absorption process, the light energy is converted into a heat. When the subtraction is made by absorption only, it said to be simple subtractive mixing. When the light is removed by scattering and absorption, it said to be complex subtractive color mixing. The subtractive primaries are obtained by removing blue, green and red light from the white light.
 

Conclusion : 

The following parameters should govern the perception of color. 

Relative spectral power ( defined by CIE) 

Standard observer data ( defined by CIE) 

Reflectance value for visible spectra ( given by spectrophotometer) 

By using these data’s we should find out the tristimulus values which may express the color interms of mathematical numbers. These values are used in the color matching of sample to the target( buyer’s sample).