Chapter 1
Physical Properties of Colors

A. WHAT THIS BOOK IS ABOUT?

This is a book about color, colorants, the coloring of materials including measurement and control, and reproducing the color of materials through imaging.

Color can mean many things. In this book, color may mean a certain kind of light, its effect on the human eye, or - most important of all - the result of this effect in the mind of the viewer. We describe each of these aspects of color, and relate them to one another.

Colorants, on the other hand, are purely physical things. They are the dyes and pigments used in the process of coloring materials.

Coloring is a physical process: that of applying dyes to textiles or incorporating, by dispersion, pigments into paints, inks, and plastics. A part of this book is devoted to describing these physical substances and processes.

But color is much more than something physical. Color is what we see-and we repeat this many times-it is the result of the physical modification of light by colorants as detected by the human eye (called a response process) and interpreted in the brain (called a perceptual process, which introduces psychology). This is an enormously complicated train of events. To describe color and coloring, we must understand something of each aspect of it. A large portion of the book deals with this problem.

With an understanding of color in this broad sense, we can approach some commercial problems involving color. These problems are concerned with answering such questions as, Does this sample have the same color as the one I made yesterday, or last week, or last year? Does this batch of material have the same color as a standard? Does this reproduced image match the original? How much of what colorants do I use to produce a color just like this one? How can I choose colorants that will perform satisfactorily in a certain application?

Historically, most of these questions have had only subjective answers, based on the skill and memory of the trained color matcher or press operator. Fortunately, through the application of the principles of color technology and the use of color measurement, we can provide objective answers. We consider the industrial application of color technology largely in this objective vein.

In summary, we provide a brief résumé of the present state of the art of color, color control, coloring, color reproduction, and colorants-a very complex field. To simplify, we have omitted much. Among our omissions are conflicting points of view: we tend to present our best current opinion rather than a studied evaluation of all sides of any question. Some topics that are important to include are still evolving; for these cases, we will present the general concepts rather than focus on a particular solution. We hope our readers will be stimulated to seek more detailed and more varied information on many of the subjects we touch upon only briefly.

To this end we provide-and consider of major importance-an annotated bibliography in which we identify those citations among all of our references that we consider key to the body of knowledge comprising color technology. We also provide an annotated list of recommended books to establish a color-science library. We hope that our readers will recognize with us that this book can be no more than a beginning and that they will make use of its bibliography and book recommendations as a guide to the extensive and often complex literature on color.

This book is not a "how to" manual for any process or industry. It does not tell you the best way to make a beige shade in vinyl plastic at the lowest cost. Nor does it provide a detailed study of what ink amounts in a multi-ink printer are necessary to reproduce the beige plastic. It does tell you in principle how to avoid having that beige go off-shade in tungsten light; it does tell why different combinations of inks can match the beige.

This book is not an instrument manual or a catalog of instruments; it does not tell you how to operate any specific color-measuring instrument-designed for a single color or many colors simultaneously-to measure samples of a given material. It does tell you what types of instruments are available and for what purposes they can or cannot be used. It does tell you how to make the best use of these instruments.

This book does not attempt to give the "best" ways to use color, the "best" ways to use colorants, or the "best" colorants to use for any application. These are important practical questions, but to answer them would require much more detail than can be put into this book. For these subjects, as for others we do not discuss, there are references to the literature.

B. THE SPECTRUM AND WAVE THEORY

To describe color, we must talk about physical actions, such as producing a stimulus in the form of light, both directly and indirectly by interacting with a material, and subjective results, such as receiving and interpreting this stimulus in the eye and the brain or visual system. (Throughout the book, important terms will be set in italics the first time they are introduced.) This is depicted in Figure 1.1, a figure we will show throughout this book. Since color exists only in the mind of the viewer, these latter effects are the more important to us. To aid in understanding them, we first consider the visible spectrum.

Figure 1.1 Color results from the interaction of a light source, an object, and the eye and brain, or visual system.

Visible radiation is a form of energy, part of the family that includes radio waves and X-rays, as well as ultraviolet and infrared radiation. Radiation we can see is called light. Light can be described by its wavelength, for which the nanometer () is a convenient unit of length, shown in Figure 1.2. One nanometer is 1/1 000 000 000 m.

Figure 1.2 Radiation can be described as a wave. The distance from peak to peak is called its wavelength.

The relation of light to the other members of its family is shown in Figure 1.3. The relative insensitivity of the eye limits the visible part of the spectrum to a narrow band of wavelengths between about 380 and 780 nm. The hue we recognize as blue lies below about 480 nm; green, roughly between 480 and 560 nm; yellow, between 560 and 590 nm; orange, between 590 and 630 nm; and red at wavelengths longer than 630 nm. Magenta, which is produced by mixing red and blue light from the extremes of the spectrum, is one common hue not found in the spectrum.

Figure 1.3 A rendition of the visible spectrum and its relationship to other kinds of radiation (not to scale).

The vast majority of colored stimuli are composed of many wavelengths, shown as graphs where radiation, in the case of lighting, or reflection, in the case of an opaque material, is plotted as a function of wavelength. Such graphs are shown in Figures 1.4 and 1.5. Newton (1730) and others (see Hunt 2000) showed many years ago, by using a prism to disperse light into a spectrum, that white light is normally made up of all the visible wavelengths, shown in Figure 1.6.

Figure 1.4 The spectral irradiance (defined in Chapter 6) of a solid-state white light.

Figure 1.5 The spectral reflectance factor (defined in Chapter 6) of a yellow paint.

Figure 1.6 Dispersing white light into a spectrum. The color names are somewhat arbitrary; these were used by Newton (1730).

C. LIGHT SOURCES

Many of the objects we think of as sources of light emit light that is white or nearly white-the sun, hot metals like the filaments of light bulbs, and solid-state lamps, among others. The light from any source can be described in four ways.

The first is irradiance, the amount of light received on a surface per unit area, often defined by watts per unit area expressed in meters squared (W/m2) and the letter "E."

The second is radiance, the amount of light emanating from or falling on a surface per unit projected area, often defined by watts per unit area per solid angle expressed in meters squared steradians (W/m2 Sr) and the letter "L." We can think of solid angle as a measure of the size of an object relative to a fixed position. An object that is close to us subtends a larger solid angle than the same object viewed from a distance. Instruments that measure irradiance have diffusers while instruments that measure radiance have lenses, described in more detail in Chapter 6.

The third is to normalize the spectrum relative to a specific wavelength, and the fourth is to normalize to the same intensity ("brightness"). Daylight and a solid-state light are plotted the last three ways in Figure 1.7. Because solid-state lights can be narrow-band, such as shown in Figure 1.7, plotting multiple sources normalized to the same intensity is the most descriptive way to compare spectra.

Figure 1.7 Daylight (red lines) and a...