Color Vs. Appearance: Choose The Appropriate Instrument Geometry For Your Color Measuring Instrument

With the number of color measuring instruments on the market, how do you choose the most appropriate one? It helps to understand the difference between measuring color and measuring appearance.

When light strikes an opaque object the total amount of reflected light is characterized as two distinctly different light reflections from the surface: specular reflection and diffuse reflection.

Specular reflection is light that is directed at an angle opposite to the incident light and is perceived by the observer to be glare caused by the shininess or glossiness of the sample. To see the apparent color of the sample, observers must move their eyes away from the glare (specular) and concentrate on examining the diffuse (scattered) reflectance from the sample. These viewing conditions have led to standardized test methods such as ASTM D 1729, "Standard Practice for Evaluation of Color Differences of Opaque Materials."

When the smoothness of a surface changes (lower gloss or increased texture, for example), the light-reflection characteristics from that surface become much more complicated. For a mediumgloss sample, the amount of specular reflectance decreases in one direction, then spreads out and becomes mixed with the diffuse reflected light. A matte or heavily textured specimen scatters almost all the specular light that mixes with the normally scattered diffuse light, causing an apparent dilution of the perceived color. Since the specular light is normally white, the appearance is a lighter color with less chroma when it is mixed with the colored diffuse light. Therefore, the appearance of the color has been affected by the scatter of the specular light. To the average observer, a high-gloss specimen would appear to have more chroma and be a darker color than an identically pigmented speciment with lower gloss or increased surface texture.