Color and appearance: III

Color and the interrelated topic of gloss are important to the decorative a spects of the use of coatings and, sometimes, to the functional aspects of their use. We have all dealt with color since we were babies, but most peop le have little understanding of color. Many technical people think of it as an aspect of physics dealing with the distribution of visible light. While that is a factor, color is a psychophysical phenomenon. The difficulty of understanding color can be seen by considering the most rigorous definition of color that has been prepared: Color is that characteristic of light by which an observer may distinguish between two structure-free fields of view of the same size and shape. Tn effect, it says that color is what is left to distinguish between two objects when all the other variables are removed . Not a very satisfying definition. Color has three components: an observer, a light source, and an object. (Th e single exception is when the light source is the object being viewed.) Th ere is no color on an uninhabited island. This is not just a semantic state ment; color requires an observer. There is no color in the absence of light ; in a completely darkened room there is no color, not because you cannot s ee it, but because it is not there. There must be an object; if you look ou t the window of a spaceship without looking at a planet or star, there is n o color-there is an observer, there is light, but there is no object. Another major factor affecting appearance is surface roughness. If a surfac e is very smooth, it has a high gloss; if it is rough on a scale below the ability of the eye to resolve the roughness, it has a low gloss. If, howeve r, the roughness can be resolved visually, a film may exhibit brush marks, orange peel, texture, and so forth. Furthermore, there can be a combination of small scale and larger scale roughness, so films can,for example, have a low gloss and brush marks or high gloss and orange peel. The eye can reso lve irregularities in surface smoothness of approximately 25 mm, depending on the distance from the object. Adding to the complications, color and glo ss interact, changing either changes the other.