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.
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