I recently attended a Konica Minolta seminar on color and color instrumentation, held at the Columbus Museum of Art. What a great idea, discussing the science of color in a building where color is critical and has been used to express so many concepts, ideas, and ideals. Konica Minolta holds these seminars around the country, and this one in Columbus had 50+ people in attendance.
The seminar was three-hours long, and there were two main takeaways:
- Color is a very complex topic.
- There is a big difference between color and appearance.
Many attending this seminar were from the automotive industry. (Honda has a large assembly plant not far from Columbus.) They routinely deal with textured plastic that is painted, or faux leather that has a grain, or cloth with its woven texture. For these colored materials, it is all about appearance, and, of course, color is one element in the overall appearance. The way light bounces off a textured surface also influences the appearance of a colored product, so it becomes a challenge when discussing “color” of a material with a pronounced texture. By contrast, the prepainted metal industry has only a limited number of textured products. The metal is always fabricated, of course, but the space between corrugations, for example, is smooth. In this sense, coil coatings are somewhat easier to measure.
But I don’t want to suggest that it is simple! Color is not simple. As an example, at the seminar there was a lot of confusion when the speaker was attempting to elucidate the difference between “specular included” and “specular excluded” geometry of sphere-based color instruments. I was sympathetic to everyone’s frustration. Many of us in the coil coating industry have struggled with this difference and especially understanding the purpose and value of each set-up. Perhaps the following may help.
When light strikes an object, some of that light bounces off the surface at the same angle as the incident light beam. We measure gloss in this fashion. The light strikes the surface at a 60-degree incoming angle, and bounces off at a 60-degree outgoing angle. This is the definition of gloss: specular (mirror-like) reflection. The percent difference between the amount of light striking the object and the amount of light bouncing off the surface is the gloss. But where did the other incident light go? It is called diffuse reflectance. (Diffuse means to spread over a wide area.) When measuring gloss, we don’t care about diffuse reflectance, only specular reflectance. Now onto color!!!
Many spectrophotometers—not just those used for color measurement—utilize a sphere to collect all the energy that bounces off a sample. When light strikes a painted metal panel, it bounces off that panel in all directions, and capturing all those reflections is the sphere’s job. When all the reflections are captured—both the specular and diffuse reflectances—the designation for the machine set-up is “specular component included” (SPIN). If you choose to exclude the specular component (why you may wish to do so is explained below), the equipment can let the specular component escape from the sphere. This set-up is designated “specular component excluded” (SPEX). So now you know what SPIN and SPEX mean, but the real question is, “Why do I care?”
Imagine a glossy saturated color such as bright red or a shiny jet-black surface. Now imagine flattening these 80%-gloss surfaces down to, say, 30% gloss. You can perhaps visualize the change in the appearance of the lower-gloss panel. It appears to be lighter than the color of the high-gloss panel. Going in the opposite direction is a similar scenario. Apply Armor All® to your car tires and they suddenly look blacker. In these examples, the color of the sample has not changed, but its appearance certainly has. For some applications (using color matching software, for example), it is critically important to understand a panel’s color. You can only do this by including and factoring in all the reflectances, both diffuse and specular components. Another way to say this is: When you are truly interested in the color, and are unconcerned how the specular component of color may have changed the appearance of the object, then you need to include all the reflectances, both specular and diffuse components.
In many cases—frankly in most instances—we are interested in whether the appearance has changed. A good example of this is when measuring the color change (fade) of a weathered panel. The pigments used in coil coatings rarely fade when exposed to sunlight, unlike dyes used to color cloth or the inexpensive red pigment that fades so badly on interstate highway signs. Upon exposure, the pigments’ color remains constant, but the binder system may degrade, which causes the gloss of the panel to drop. This gloss drop has the effect of making the color look lighter. The appearance has changed, even though that actual color has not. If your interest is truly to measure color change, then—as described above—you need to set-up the spectrophotometer to include the specular component (SPIN). But if you are most interested in the change in appearance, then you need to…wait for it!…exclude the specular component. A building owner only cares about the change in appearance, so don’t attempt to persuade him with an esoteric explanation that the color of his building has not actually changed. You may find yourself being chased off his property…and for good reason.
What about the common 0/45 instruments that we routinely use in our industry? The “0” and the “45” mean that light strikes the panel head-on (a 0° incident angle), and the detector that receives the light is at 45°. In other words, only diffuse reflectance is measured and only at a 45° diffuse reflectance angle. Specular reflectance, therefore, is automatically excluded. (NOTE: A 45/0 instrument simply means the light strikes the panel at 45° and the detector picks-up the diffuse reflectance only at 0°. Same concept; different configuration.) The prevalence of 0/45 (or 45/0) instruments in our industry is perfectly sensible. We are most concerned that appearance from run-to-run is consistent, and that the change in appearance of products weathered by sunlight fall within a certain limit.
We use the term color when we really mean appearance. Although this color topic is admittedly complex, here’s the one thing to remember. The human eye is the best spectrophotometer, and “color” and color change (i.e., appearance change) are best measured with whatever machine set-up agrees with what we and our customers see. This is usually specular component excluded, and therefore we don’t need to change a thing that we are doing.
– David Cocuzzi, NCCA Technical Director