INTRODUCTION TO COLOR THEORY
Bruce M. Mulholland
Hoechst Technical Polymers
8040 Dixie Highway
Florence, KY 41042
The very basics of our visual color perception process and instrumental color assessment will be discussed in this brief overview.
People usually associate color with physical objects that are viewed in sharp contrast to their surroundings: be it a beautiful flower arrangement, or a colorful spinnaker on a sailboat against a dreary gray sky. In addition to colored objects, color is also associated with creating overall visual impressions which set certain moods, tell the time of day, or indicate certain climates or temperature. Furthermore, color can be used to create emotions or stimulate feelings such as artist’s colors in paintings and a designer’s choice of colors in decorating a room. But to us in the plastics industry, the coloring of an object is most important. The purpose of this paper is to provide a very brief overview of how objects are colored, and how we as observers perceive their color. What is presented herein not only applies to coloring plastic resin, but also applies to all colored media including textiles, paints, ceramics, inks and so on.
Visual Color Perception Process
In order to experience color, three things must be present: a light source, an object and an observer. The color perception process occurs as follows. A beam of light from the source reaches the surface of our object. A portion of the light is reflected due to the surface interface and is called the specular reflection or gloss component. The gloss is reflected in an angle equal in magnitude to the angle of incidence of the light beam, but opposite in direction. The gloss component contains all wavelengths of the light source. Therefore, if the light source is white light, the gloss component will also be reflected as white light.
The remainder of the light penetrates the surface of the object where it is modified through selective absorption, reflection, and scattering by the colorants, polymers and additives. Selective absorption and reflection by wavelength create color. For example, if an object absorbs all wavelengths from a white light source other than blue, nothing will happen to the
blue light component of the light source, and the blue light will be reflected or transmitted from the object. The observer will see this reflected or transmitted blue light and we say that the object appears blue.
To fully understand this process, it is important to discuss the three items needed for color more completely. All three -- the light source, object and observer, are governed by sciences applicable to their functioning. Light sources operate in the visible region of the electromagnetic spectrum and therefore can be described using physics. The object modifies the light using the chemistry of the colorants and other ingredients which absorb, reflect or scatter the light. The physiological construction of our eye determines our ability to respond to the stimulus from the object. Finally, psychology governs how the brain interprets the energy from the eye and transforms it into our perception of the color.
The Light Source
Obviously, the light source plays a major role in the color we perceive. Light is nothing more than electromagnetic radiation similar to radio waves and other energy. Light is different because our eyes happen to be sensitive to this particular energy and can be seen. All electromagnetic energy travels in waves. A wavelength is defined as the length of one wave unit measured from peak to peak. The wavelength of the electromagnetic radiation is an indication of the amount of energy
contained in it. The shorter the wavelength, the higher the energy content in the radiation.
Long wavelengths exhibit relatively low energy. Examples of these include radio and television transmissions which have a wavelength of over one meter. Shorter wavelengths of about one millimeter in length begin the infrared region. We know this is higher energy because heat is generated at these wavelengths. The visible light region is next, with wavelengths generally defined as 400 to 700 nanometers. The visible region will be discussed later in more detail. Energy continues to
increase as the wavelengths become shorter. Just shorter than the visible region are ultra-violet wavelengths. These show their higher energy by creating sunburn on skin or causing UV degradation of plastics. Shorter and shorter wavelengths of one angstrom and less create the highest energy sources of X-rays, gamma rays and cosmic rays.
The visible region contains all possible wavelengths of light described commonly by hue. The 700nm end is the color red. Orange appears between 590 and 630nm. Yellow is slightly shorter between 560 and 590nm. Green light occurs next between 480 and 560nm and blue is the shortest wavelength below about 480nm. The energy corollary holds in the visible region as well. Blue is shortest in wavelength and therefore should be the highest energy. If one thinks of fire, it is true that a blue flame is hotter than an orange or red flame, and thus exhibits higher energy.
In considering the visual color perception process, light sources that are comprised of all of the wavelengths of the visible region are most useful. These light sources will emit white light and include such sources as the sun, filaments of light bulbs, and fluorescent lamps. But we know from experience that colors can look different whether viewed under a 60W light bulb
or outdoors under the bright sun. This is called color rendition or the color rendering effect of the light source. Furthermore, there are other light sources such as mercury vapor lamps which appear nearly white, but do not contain all possible wavelengths. Therefore, it is important to know how much energy, if any, is present in the light source wavelength by wavelength. This can be measured and is called the spectral energy (or power) distribution of the light source.
We previously stated that our visual color perception is dependent on the object modifying the light from the source. Thus, the spectral energy distribution is important to understand and control in color matching. In that earlier example, we stated that the object would appear blue since no modification was done to the blue wavelengths and the object reflected blue light which was seen by the observer. However, if the light source was a red light which contained no blue wavelengths, then obviously the object could not reflect blue light and we could not perceive the color of it as what we call “blue”. Moreover, our object could potentially be perceived as being bluer in color viewed under a cool white fluorescent lamp or sunlight
compared to being viewed under a 60W bulb. Fluorescent light contains more blue light energy compared to the 60W bulb. Our object would then have this higher amount of energy in the blue region to reflect back to the observer.
This whole discussion stresses the importance of standardizing the light source for viewing color. Manufacturers of color viewing booths and light sources generally describe their light sources in terms of color temperature. Common sources include tungsten filament at 2854K (CIE Source A), cool white fluorescent at 4200K and north daylight at 6500K. Thinking back to our energy corollary, the higher the color temperature, the bluer the light source since blue wavelengths are the
highest energy. And while color temperature is used to describe sources, it is important to point out that the spectral energy distribution is really what is important for our color perception and that the color temperature does not explicitly define this. The spectral energy distribution is the amount of energy present in the light source measured at each wavelength in the visible light region. Examples of spectral energy distributions are shown in Figure 1. Two light sources may measure as 6500K, but
their spectral energy distributions could be different giving rise to differences in perceived color under each. What’s more, color measuring instrumentation generally report color difference under illuminant D (6500K) and others. These illuminants are mathematical descriptions of a light source defined by a numerical spectral energy distribution. D6500K, for example, is defined by the spectral energy distribution of average, natural daylight and can not be duplicated artificially. Therefore, 6500K in a light booth, and D6500K used to calculate color data by an instrument do not have the same spectral energy distribution so colors may “appear” somewhat different under each.
The most important color observer is the human eye. This is true in any industry that relies on a product which is colored, or is packaged or labeled with color. While sophisticated color measuring instruments are used in industries to control color, the ultimate consumer, you and me, uses his or her eyes only to judge and observe color.
We perceive color by our eye’s ability to detect the light reflected or transmitted from the object. The lens in the eye focuses the light on the retina at the back of the eye. The retina contains two types of light detectors -rods and cones. Rods are responsible for our night vision, but are not involved in our color perception. That is why, in very dim light, we can see but most objects appear black or various shades of gray.
Cone receptors are what give rise to color vision. Current thinking is that there are actually three types of cones: one type that is sensitive to red wavelengths, one sensitive to green, and the other sensitive to blue. The individual responses of thesecones are combined by the brain to generate the sensation which we describe as color. The mechanism of how these responses are combined is very complicated and is not critical to our understanding of color perception. It is important to realize, however, that the brain’s ability to interpret these responses can be influence by psychological factors, and thus alter the perceived color of the object. The old adage of seeing red when you are mad can be true and is an example of this. The object, light source, and your eye’s physiological construction have not changed. What has changed is the way the brain combines and interprets the responses from the cones, and you perceive things to be redder than they would normally appear.
The primary cause of differences of color perception between human observers generally is due to physiological differences in construction of the eye. These differences can be hereditary, or attributed to injury or the aging process. The aging process typically causes the macular fluid in the eye to yellow. The light reflected from the object travels through this fluid before it reaches the cones on the retina. Color bodies in the macular fluid may act as a filter of sorts, absorbing some of the reflected light, and can change our perceived color of an object over time.
Injuries to the eye directly or to the head can be catastrophic causing complete loss of vision. Less severe injuries could lead to detached or partially detached retinas or damaged cones which could alter our perceived color of objects.
By far the greatest factor in causing human observers to perceive colors differently is physiological differences which are hereditary or simply inborn. Knowing that there are two general types of cells – rods and cones, and three specific types of cone receptors on the retina, it becomes easy to rationalize that no two people can have the exact distribution of these receptors on their retinas. Therefore, all individuals must perceive color slightly different because of this.
People that exhibit gross differences in cone distribution or functioning are typically referred to as colorblind. This is a misnomer in that colorblind implies seeing no color and only shades of gray. While there are a very small fraction of the population who are actually colorblind by that definition, most people claiming they are colorblind are actually color deficient. These people with color-deficient vision see color, but their perceived color is not normal by definition. About 8% of all males, and only about ½% of all females, have some degree of color-deficient vision. The most common is called deuteranopia (or the less severe anomaly deuteranomaly) which is a red.... green deficiency. Up to 5% of all males have some degree of this. Deuteranomaly accounts for the majority of all female color deficiencies. Complete red blindness (protanopia) or the anomaly protanomaly (red weak) are the next most common deficiency occurring from 1 to 2% in males.
Yellow.... violet lacking tritanopia or the corresponding anomaly tritanomaly (blue weak) are the rarest forms for both sexes.
This discussion should make it clear to the reader that persons placed in a position of making visual color decisions must be tested to ensure they have normal color vision. This can be accomplished by having them perform at least one of the color aptitude tests commercially available today.
While the light source and observer discussions above are strictly generic to any industry, discussions of the object become very industry specific. For us in the plastics industry, the object is typically a formed plastic article. As stated earlier, the light from our source penetrates the surface of the object where it is modified through selective absorption, reflection, and scattering by the colorants, polymers and additives. For transparent or very translucent articles, reflection is either replaced by or supplemented with light transmission. For simplicity, we will assume opaque articles and just discuss reflectance.
Objects are typically colored by incorporating pigments or dyes (colorants) into the polymer matrix. Colorants act on the incident light by selectively absorbing certain wavelengths. This is determined by the specific chemical bonds within the colorant structure. Chemicals are useful as colorants as long as these bonds absorb electromagnetic energy within the visible region. Iron oxide is a useful colorant because the compound typically absorbs blue and green wavelengths of light. Red wavelengths are not modified; therefore, this colorant appears red due to the reflected red light. It is helpful to remember that the perceived color of a pigment or dye is due not to what the compound does to the wavelengths of that dominant hue, but rather to all of the other wavelengths. In our example, a red pigment acts on blue and green wavelengths, and not red.
For the most part, the colorants themselves do the majority of the absorbing of the incident light by wavelength to create color. A few polymer additives can impart a yellowish color by absorbing some blue light. The polymer matrix can also do this. Other additives, particularly mineral fillers, can impart a grayish or brownish color due to light absorption. Polymer matrices and additives can, however, contribute significant light scattering. Scattering occurs when the light beam contacts particles or regions with refractive indexes different from that of the polymer. If scattering occurs equally at all wavelengths with no absorption, the object will look white. The amount of scattering depends on the magnitude of difference in refractive index between the polymer and additive and the particle size of the scattering constituent. It is important to understand the mechanism of scattering because it can have a tremendous impact on the colorability of the polymer system. Significant light scattering in our polymer system will increase the amount of diffuse reflection (white light) which is mixed with the reflected colored light generated by our colorant interactions. This mixing will dilute the color strength and the color of our object will appear lighter and less bright to the observer.
The other limiting factor in coloring a specific polymer is the stability and suitability of the colorant for a particular polymer and application. A general rule of thumb is that as the recommended processing temperature for a resin increases, the number of colorants that can withstand those temperatures decreases. Most of the colorants that drop out are organic pigments that are typically used to achieve the bright, high chroma colors or the deep, dark colors. Furthermore, polymers with harsh chemical environments like PVC, acetal or nylon limit colorants based on chemical stability. Conversely, the chemistry of the colorant can also render the polymer unstable, making it unsuitable for use as well. Finally, end use requirements such as agency compliance (FDA) or UV stability will further restrict the number of colorants that can be used in a specific polymer. Thus the achievable color gamut for a resin system not only depends on the light scattering of the polymer but also on the types of colorants that can be utilized.
It has been shown that generally for color perception to occur, three things must be present: a light source, an object and an observer. But for us in industry, only meaningful color perception can occur when a controlled and characterized light source is used to illuminant a representative object being viewed by an observer with demonstrated normal color vision. Specifically, the light source spectral energy distribution must be defined and controlled to industry specifications, and there. can be no stray light from nearby light sources entering the viewing area. Other variables which should be controlled include the viewing distance from the light source, the surround color (neutral gray is best), and the angle of viewing if deemed necessary.
Instrumental Color Measurement Process
Similar to the visual color perception process, the instrumental process requires a light source, an object and an observer in order to mathematically describe color. This is accomplished by developing energy descriptions for each. We have already demonstrated that the light source can be described by a spectral energy distribution which is the amount of energy contained in the light source wavelength by wavelength over the entire visible region (Figure 1). In the instrumental process, the illuminant is easily described using a mathematical spectral energy distribution, which is simply energy versus wavelength. The most common illuminant used to calculate color data is Illuminant D65, or 6500K. Other mathematical illuminants exist and include Illuminant A (tungsten filament) and Illuminant F (cool white fluorescent). We discussed in the visual process how the object selectively absorbs and reflects light energy. And it is the reflected light which we detect and interpret to perceive its color. In color measurement then, spectrophotometers are used to measure this reflected (or transmitted) light energy wavelength by wavelength. The resulting data creates the spectral reflectance curve for the object, which again is energy versus wavelength. An example of a reflectance curve for a green color is shown in Figure 2. The primary variable of this measurement is the treatment of the specular or gloss component. Integrating sphere geometry instruments can either include or exclude the light energy resulting from the surface gloss. The preferred method is to always include the gloss component. Instrument geometries of 0/45 (light source at 0../detector at 45..) or 45/0 by design theoretically exclude the gloss.
The observer in the instrumental case was most difficult to quantify. Using experiments with people having known normal color vision, data was generated to quantify the response of the human eye versus wavelength. This data is expressed as a three component stimulus function defining the CIE standard observer. Data is expressed in terms of energy (response) versus wavelength as shown in Figure 3. The three individual functions could certainly be a result of having the three specific types of cone receptors in the eye. Each response curve could be thought of as the individual response curve for each type of cone cell. Color can be described numerically by combining these energy descriptions for the illuminant (spectral energy distribution), object (spectral reflectance curve), and the observer (three component CIE standard observer). The resulting three products labeled X,Y,Z are called tristimulus values of the color. In theory, these tristimulus values represent the amounts of imaginary primaries used to achieve the measured color. If two samples have identical tristimulus values,theoretically they are identical in color. If the two samples have different tristimulus values, only the lightness difference can be determined by comparing the Y values of the two colors. If Sample A has a lower Y value than Sample B, Sample A is darker than Sample B. No other color difference information can be obtained by comparing tristimulus values directly.
Tristimulus values are however important because they represent the fundamental mathematical description of color.
And while there may be problems in describing the standard observer, and problems using illuminants that don't physically exist (D6500 for example), tristimulus values are building blocks for more meaningful numerical color description and color difference characterization. Researchers over the years have applied non-linear transformations to tristimulus values with the goal of creating uniform color spaces which allow calculation of color difference magnitude and direction. The most common transformations used today are HunterLab, CIELab, and the CMC equation. These equations give us color coordinates that are widely used in industry such as:
・DL: Light(+) / Dark(-) difference
・Da: Red(+) / Green(-) difference
・Db: Yellow(+) / Blue(-) difference
・DE: total color difference
It is extremely important when reporting these numerical color values to explicitly define the parameters used to obtain the values. Reporting that the DE was 0.7 is meaningless unless all of these parameters are defined and understood. Therefore, all color data must be reported with the definition of the illuminant, standard observer, color space or units, and preferably with the geometry of the instrument. An example of the required information is “Color data calculated under Illuminant D65, 10...Observer, Sphere Geometry, Specular Included, Expressed in CIELab units.”
1. W. Billmeyer, Jr. and M. Saltzman, Principles of Color Technology, 2nd ed., John Wiley & Sons, New York, (1981)