COLOR THEORY

Color is defined a s the net response of an observer to visual physical

phenomena involving visible radiant energy of varying intensities over the

wavelength range 400 to 700 nanometers (nm). The net color seen by the

observer is dependent on integration of three factors:

 (l) the nature of the light source,

(2) the light absorption properties of the object observed, and

(3) the response of the eye to the 1i ght refl ected from the object.

 The relative intensities of the various wavelengths of visible

1ight observed by the eye are translated by the mind of the observer

resulting in the perception of color. In color measurement, the human eye

is replaced by a photocell which detects the light energy present at various visible wavelengths. Visible 1ight is a narrow band of electromagnetic radiation from 400 to 700 nm (1 nm equals 10-9 meters) detected by the human eye. Radiation falling below 400 nm is ultraviolet radiation, and that falling above 700 nm is infrared radiation; both are unseen by the human eye. If pure lightof a given wavelength is observed, it will have a color corresponding to that wavelength. Pure wavelengths of light are seen when white light is refracted by a prism into a "rainbow" spectrum of continuous color. Light sources such as sunlight, incandescent light, and fluorescent light are continuums of various wavelengths of light with the relative amounts of the various wavelengths of light being dependent on the overall intensity and type of light source. Sunlight at noon has very nearly the same intensityof each wavelength of 1ight throughout the visible spectrum, whereas at dusk sunlight is of lower intensity and has greater quantities of the longer, red wavelengths than of shorter, blue wavelengths. Fluorescent lights generally contain large amounts of shorter, blue wavelengths, while incandescent tungsten lights contain a large component of longer, red wavelengths compared to noon sunlight. Differences in intensity and wavelength distribution between light sources has a profound effect on the color observed for a dyed textile, since the textile can absorb and reflect only that 1ight available to it from the source. When a dyed fabric appears different in color or shade under two different light sources, the phenomenon is referred to as "flare." When two fabrics dyed with different dyes or dye combinations match under one light source but not under another, the

effect is called "metamerism." When 1ight from a source strikes a dyed textile surface, different portions of the light of the various wavelengths are absorbed by the dye, depending of the structure and light absorption characteristics of the dye. Light not absorbed by the dye on the textile is reflected from the surface as diffuse 1ight, and the observer sees the colors shown in Table 17-1. The color seen is a composite of all the wavelengths reflected from the fabric. If significant direct reflectance of light from the fabric occurs, the fabric exhibits a degree of a gloss. If little or no light throughout the visible range is absorbed by the fabric and the majority of 1ight is reflected, the fabric appears white. If the fabric absorbs all of the light striking it, the fabric is black. If uniform light absorption and reflectance across the visible wavelengths occurs at some intermediate level, the fabric will be a shade of grey.

Table 17-1. Colors After Absorption/Reflectance

Wavelength of Light Absorbed (nm)	Light Absorbed by Dyed Textile	Color Seen by the Observer
400-435	Violet	Yellow-green
435-480	Blue	Yellow
480-490	Green-blue	Orange
490-500	Blue-green	Red
500-560	Green	Purple
560-580	Yellow-green	Violet
580-595	Yellow	Blue
595-605	Orange	Green-blue
605-700	Red	Blue-green

The dye absorbs di screte packages or quanta of 1i ght, and the dye

molecule is excited to a higher energy state. This energy is normally

harmlessly dissipated through increased vibration within the dye molecule

as heat, and the dye is then ready to absorb another quantum of light. If

the dye cannot effectively dissipate this energy, the dye will undergo

chemical attack and color fading or color change will occur or the energy

will be transferred to the fiber causing chemical damage.