Color and Shade Matching

The ability to perceive color is an ability controlled by human genetic make-up. Color vision is part of human vision but is not totally understood. Basically, it is believed that color vision is due to light energy, electromagnetic radiation, striking nerve endings located in the retina of the eye causing complex photochemical reactions to occur. This stimulus sends a signal through the optic nerve to the brain. This signal produces the information that the brain interprets as color vision. However, this is a very complex process. Color vision also seems to be affected by the emotional state of the person. People use the perception of color to describe emotional situations.

Most everyone has used terms such as ‘a gray day’, ‘blue Monday’, ‘black Friday’, and ‘I was green with envy. Therefore, the ability to perceive color can be described as a psychological- physical phenomenon.

Some people confuse certain colors. This condition is known as color blindness. The most common form of color blindness is when people confuse reds and greens. Although well known, very few people have the colorblind condition of confusing blues and yellows. Even fewer people are completely color blind, that is, they only perceive black, white, and various shades of gray. Proper color vision testing is the only way to accurately determine color perception capability. People who are not tested as color normal should not be involved with the shade matching of dyed textiles.

Color Perception and Light Energy

Based on the previous description of color vision, it is obvious that without light energy there is no perception of color.   Described in a different way, if all light is removed from a room, all the objects in the room become the same. That is, if there is no light, there is no color.

Visible light is energy that causes stimulation of the nerve endings in the human eyes to produce the sensation of vision and color vision. Light energy is a narrow band of the broad spectrum of electromagnetic radiation produced by stars. This includes lethal radiations such as cosmic rays or x-rays. It also includes harmful radiation such as ultraviolet or microwave energy. Additionally, it includes harmless radiation such as radio waves. People have learned to produce certain radiation, such as microwaves or radio waves, and develop useful devices such as microwave ovens.

The current scientific description of electromagnetic radiation states that all of these energies move at the same speed, the speed of light. All of these energies move through space in the form of waves described as ‘sine’ waves. Radiations types are separated by their individual wavelength.   Wavelength is the measured distance from the top of one sine wave to the top of the adjacent wave.    Cosmic rays, which are lethal, have very short wavelengths while radio waves, which are harmless to humans, have very long wavelengths. For uniformity, the wavelengths are normally stated in units of meters.   The range of wavelengths of visible light which is also harmless is from 380-780 X 10-9 meters. This unit is converted to a new unit called nanometers.

Figure 12 shows that different groups of visible light wavelengths produce different color sensations. When these groups are spread out so that these wavelength bands can be seen, we call this array the visible color spectrum. The combination of all of these wavelengths produces white light energy. Wavelengths shorter than 380 nanometers are not visible and fall into the ultraviolet radiation range. Wavelengths longer than 780 nanometers are also not visible and fall into the infrared radiation region. Colors produced by these individual wavebands of energy are known as spectral colors. Colors that are produced by mixtures of these wavelengths are known as non-spectral colors. The special case of black, white, or gray sensations is known as achromatic colors.

Figure 12 – Hues of Monochromatic Light

Color Perception and Objects (Textile Substrates)

Textile substrates produce no color of their own, unlike sources of light energy. Textile substrates require an external light source or illuminant to produce a color sensation. This fact is illustrated in Figure 13.

From this schematic, when incoming light energy strikes a textile fabric (substrate) various results can occur. First, all of the light energy can be reflected, similar to a mirror. Nearly all textile fabrics reflect only a portion of the incoming light. Alternatively, all of the light energy could be absorbed by the fabric.

Textile fabrics only absorb a portion of the light. Only a black hole in space completely absorbs all light energy. In general, a portion of the light energy is reflected and the remainder of the light is absorbed. However, especially with sheer fabrics or substrates, there is the possibility of light energy being transmitted through the fabric. In this case, the fabric acts as a light filter.

Sheer fabrics are important textile products; however, in terms of color evaluation, the color of the fabric can be misread when transmitted light from the color of the background is viewed through the fabric. For color evaluation, sheer fabrics should be folded until no light can be transmitted through the fabric so that only reflection and absorption can occur.

 

Basically, it is the interaction of the light energy, striking the fabric, with the dyes that are in the fibers of the fabric or the pigments glued on the surface of the fabric that produce the color sensation. Incoming white light energy contains all the color wavelengths. When this white light strikes the fabric, the dyes absorb certain color wavelengths and reflect others. The color of the textile fabric is due to the combination of wavelengths reflected. When several dyes are mixed together, the color of the textile substrate will result from the combination of color wavelengths reflected.

The structural nature of the fiber and its cross-sectional shape, as well as, the geometric structure of the textile substrate have a direct influence on the final color of the product. For example, the color of trilobal nylon 66 fiber will appear to be somewhat different from the color of round nylon 66 fiber when they are dyed together. The color of 18/1’s ring-spun 100% cotton yarn will appear to be different from that of 36/2’s ring-spun 100% cotton yarn.

The color of the cut pile carpet will appear to change as the direction of the pile is changed. All of these color differences are due to the different angles of light reflection from the different surface geometries.     Additionally, cotton and rayon are both pure cellulose-based fibers and will dye with the same dyes.   However, when textile substrates composed of the two fibers are dyed together the color produced on each substrate will be similar in shade but not match. This color difference is due to the different internal molecular configurations of the two fibers. Similar results are seen with other natural fibers and also with synthetic fibers.

Color Mixing and Shade Matching

Today, color is produced by placing dyes and pigments on textile substrates. Color is also produced by colored light sources. In either case, single or primary colors are mixed to produce a wide variety of colors known as a color gamut or range. However, the specific mixing technologies are quite different between colored light and dyes. In the light mixing system, the primary colors are red, blue, and green light.

When red and blue light are mixed, magenta or violet is produced. When blue and green light is mixed, cyan or turquoise is seen. When red and green light is mixed, yellow is produced. The colors produced by mixing, magenta cyan, and yellow, are brighter than the primary colors because light energy has been added together.    This is known as the ‘additive color’ mixing system.    When all the primary colors are mixed together in the proper proportion, white light is produced.   The sensation of black is produced by the total absence of light energy.

For the dye (pigment) mixing system, the primary colors are red (magenta), blue (cyan), and yellow. Magenta and cyan are the scientifically true primary colors; however, modern dyehouses use red and blue dyes which are used for most of their production shades as their color primaries.   When red and blue dyes are mixed, purple is produced. When red and yellow dyes are mixed, orange is produced. When blue and yellow dyes are mixed, green is produced.

The mixture colors, purple, orange, and green are duller than the primary dyes because they subtract more light energy from an illuminating light source than the primary dyes. Therefore, this system is known as the ‘subtractive color’ system. Black is produced by mixing the proper portion of each primary dye. Interestingly, black is known as the ultimate recovery color for textile products. Virtually, any off-quality dyeing can be overdyed into a first quality black shade. However, the quality of the black shade is highly dependent on the skill of the dyeing company. There are no white dyes. White on textile substrates is produced by the absence of any dye.

On most textile fibers, white is produced by bleaching which destroys any color-producing material in the fiber. The resulting white sensation is due to the light-scattering properties of the textile fiber. However, white is produced by pigment whitening agents such as titanium oxide, TiO2. Pigment whites are often used in printing. TiO2 is extruded with certain synthetic polymers to produce white fibers. Otherwise, these fibers would be clear.

The purpose of dyeing is to produce color in textile substrates. However, the specific shades produced are not an accident of processing. Textile substrates are dyed with a specific dye formulation to match a color standard. Color standards are a part of the overall specifications of the textile end product.    Color standards are generally agreed to by both supplier and customer. The details of color standards development and the specifics of shade matching are beyond the scope of this document. However, certain basic facts should be understood.

Visual shade matching is still the most widely used method for textile substrates. Professional shade matches should be routinely tested for color blindness and properly trained in shade matching techniques. They should understand the various properties of the dyes and textile substrates with which they are working. Shade matching should take place using standard light sources and employing a neutral gray background. The specific geometry of textile substrates along with any influence of surrounding colors on the color match should be known and taken into account. Visual shade matching is a complex operation that requires extensive knowledge and a high degree of skill.

In recent years, color measuring instruments or color computer systems have proven to be highly valuable. There are shade matching programs such as ‘Ecmc‘ which have produced reliable results. The typical way these systems work is to illuminate a colored substrate with a standard light source. The amount of reflected light energy from the substrate is measured wavelength by wavelength across the visible spectrum. This reflection data is used to calculate a color difference or ‘E’.

For most textile products E= 1.0 is the typical pass-fail boundary for shade matching.   However, this boundary can be higher or lower depending on the fabric construction and fiber content of the end product. Color instruments only measure one color at a time and have difficulty with multicolor designs such as stripes or plaids. They also do not automatically account for substrate texture.    Therefore, it must be kept in mind that color computers measure data and calculate color.   People see color.   These two are not the same. It should also be noted that the limits of an acceptable shade match are often business-dependent. When business is good, shade tolerances for many textile products are wider. When business is not good, these tolerances can be tighter.

Color computers, when programmed with the proper software, can use reflectance data to calculate predicted dye formulas.   They can also be a useful tool to store specific information concerning dye properties, dye costs, mixing procedures, and dye combination issues. These instruments can monitor overall dyeing quality. Many companies use color computer systems because the systems allow colors to be matched at lower production costs. The use of these instruments also helps improve overall process productivity.