Chroma Class 101
R.A. Mateer, R.A. Beck
There are hundreds of applications where color analysis and the quantification of colors or appearances are critically important. Some of these include: foods, labeling, pharmaceuticals, paints and stains, forensic evidence, building materials, plastics, masonry composites, fabrics, pavement materials, inks, and antiquarian restorations.
In some cases, the appearance of substances can be correlated directly to other analytical parameters thereby simplifying seemingly unrelated analytical issues that are critically important.
For example, the ratio of black-brown coloration displayed by ripe bananas is quantitatively related to free sugars in the banana crop as a whole. Sugar analysis of any crop is time consuming and complex but color analysis takes only seconds and it provides an immediate answer.
Color and appearance matching is especially important for the precise matching of two or more colors as they appear on the same or different materials. Beyond this, color and appearance measurements serve as a key strategy for quantifying aging effects shown by coatings and materials as they are effected by light, weather elements, oxidation, radiation, corrosion and so on.
In yet other cases, color matching can have important values for matching unique colors of a competitors product or for applications where color may be a factor in dealing with forensic factors.
The color appearance of liquids or solids as perceived by the eye can be described in terms of measurable wavelengths of transmitted or reflected light. Wavelengths are ordinarily measured in terms of nanometers (nm) of visible spectral wavelengths (i.e. 400Ð700 nm).
Since it is nearly impossible to unbiasedly observe the color or chromatic appearance of any substance perceived by the eye of one individual, and then accurately report this color observation to another individual, instrumental methods of color analysis can have critical importance. Furthermore, many food products, ingredients, pharmaceuticals, consumer or industrial products rely on accurate quantitative characterizations of product color-attributes to ensure quality control and assurance profiles.
In many areas of pharmaceutical control, food product ingredients, colorization technologies, chemical processing, brand designations and so on, reliable documentations of color and appearance profiles can reduce countless hours of expense and analytical work that ordinarily ensure uniform manufacturing processes from one batch operation to another. Whether light interacts with liquids or surfaces, its transmitted or reflected wavelengths can be analyzed, transformed and documented in terms of uniform color analysis scales.
One of the most useful scales routinely applied to industry, commerce and technology is the concept of the opponent-color scale devised by the pioneering work of Richard Hunter in 1942. According to this color scale, optical sensors that intercept transmitted or reflected light, produce detectable voltages that correlate to the classical color appearance scale defined by the Commission Internationale de L’Eclairage or CIE-based tristimulus color system. Fundamental CIE-color definitions expressed in terms of x, y and z color values undergo transformation into a uniform three dimensional color coordinate L, ± a, ± b scale expression. This tri-coordinate color scale corresponds to black–white (L), red–green (± a), and yellow–blue (± b). Extreme values on the ± a and ± b scales correspond to saturated red (+a), green (Ðb), yellow (+b) and blue (Ðb), respectively.
The lightness term (L) is a non-linear function such as the square or cube root of the percentage of light reflectance or transmittance.
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