Color Matching Tips

March 18th, 2010 · 1 Comment

How lighting affects color matching.

By Mike Henry, Manager PPG Color Print/ Electronic Tools, North America

Past editions of Repaint Reporter referenced details about OEM color variation, automotive paint pigments and color geometry. This edition will explore the impact that lighting has on our perception of color and how variations in lighting affect color match decision-making.

But first, let’s review the basics.Three things are necessary to see color: 1) a colored object, like a car, 2) a light source, such as the sun, and 3) an observer with normal color vision. Each factor plays a role in how color is perceived.

The Lighting Factor In the simplest terms, daylight can be equated to what most of us call white light. In the 1600s, Sir Isaac Newton used a prism to split white light into the spectral colors of the rainbow. Today we recognize those colors as being associated with bands of wavelengths on the electromagnetic spectrum. The wavelengths discernible to the human eye range from about 380 nanometers on the blue side to 740 nanometers on the red side.

Daylight is the best light source for observing automotive paint color. Daylight is composed of electromagnetic energy in sunlight, plus the blue light that is scattered by air molecules in the atmosphere and white light scattered by water and pollutants. It can be affected by weather conditions, time of day—even your latitude on the planet. That’s why a color judgment made in New York can be different than one made the following week in Sacramento.

Indoor lighting used for color matching should be as close to daylight as possible. Lighting companies offer a variety of bulbs that can be used to imitate daylight in fluorescent and incandescent light fixtures. The best artificial daylight bulbs will match up well to the average spectral power distribution of daylight, but none are perfect. Paint booths are usually a poor placein which to make color decisions. A typical paint booth design focuses on airflow and the filtration of airborne solvent vapors. It is not usually equipped with skylights, artificial daylight bulbs or strong directional light sources. For these reasons, the booth is generally not the best place from which to make variant formula selections.

The Color Factor The pigments used in today’s advanced automotive color formulations require strong directional light and broad spectral energy (daylight or something to simulate it). Here’s why… Light interacts differently with pigment in straight shade toners than with those found in metallic and pearlescent toners. Straight shade toners absorb most of the light that strikes them and reflect only a narrow band of color. For instance, a bright blue toner appears blue because it selectively reflects blue wavelengths of light. It absorbs most of the green, yellow and redwavelengths leaving only the blue wavelengths to be ref lected. Pearlescent pigments, on the other hand, produce their coloristic effects by refracting (bending) and reflecting light. The type and thickness of metal oxide coatings on pearl flakes determine their color.

As light strikes a blue pearl flake, some of the light is reflected off as white light, the rest passes through the coating and is refracted or bent to a specific degree to produce blue light. When that refracted or interference blue light encounters an edge between the flake and the paint binder, the blue light is ref lected up and out of the paint film. Weak complementary yellow wavelengths of light transmitted into the paint film and depending on the color formula, are absorbed or combined with the reflected light of other pigments on the flop. So it’s important to consider that with today’s pearlescent colors, what you see is strongly affected by the quality of your light source.

Metallic toners containing various types of aluminum f lake pigments absorb no light at all. They reflect and scatter all the light that hits them. Differences in flake particle size and surface treatment create the differences in appearance that we associate with fine, coarse and lenticular flakes. To fully appreciate the impact they have on color match you need strong directional lighting. Two different formulas might look the same or blendable in a paint booth or shop with fluorescent lights, but very different in direct daylight if one has lenticular aluminum flake.

The Human Factor It’s important to keep in mind that in the United States, about 7 percent of the male population—or about 10.5 million men—either cannot distinguish red from green, or see red and green differently.* The same applies to about 0.4 percent of the female population. To perceive a color correctly, it is essential that the observer has normal color vision, can recognize shifts in hue, vividness and lightness/darkness. Plus, a good grasp on the coloristic effects produced by metallic and pearlescent pigments is very helpful when dealing with modern automotive colors.

The Success Factor Being conscious of all of these lighting factors will help you pick the most accurate variant and achieve blendable matches. When evaluating color choices, consider shop lighting, the angle or direction of the light, the environment (air conditions, etc.) and your own ability to discern colors. Whenever possible, use direct sunlight—or a source that simulates it—to help you see all the color’s properties and make successful color decisions.

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