INTRODUCTION TO COLOR
THEORY
Bruce M. Mulholland
Hoechst Technical Polymers
8040 Dixie Highway
Florence, KY 41042
Abstract
The very basics of our
visual color perception process and instrumental color assessment will be
discussed in this brief overview.
Introduction
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 Observer
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.
The Object
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.
Summary
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:
・L: Lightness
・a: Red/Green
・b: Yellow/Blue
・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.”
References
1. W. Billmeyer, Jr. and
M. Saltzman, Principles of Color Technology, 2nd ed., John Wiley & Sons,
New York, (1981)
Key Words
・Color perception
・Color theory
・Color assessment
Figure
1
Figure 3
