Interference colors are the colors that emerge when an anisotropic mineral is being illuminated with polychromatic light (Piegari and Flory 397). These colors appear as a result of the unequal transmission of slow and fast rays as well as their interference. Therefore, wavelengths of the white light rays that pass through a crystal substantially differ from their initial value. It happens because of the retardation of waves that are caused by the multitude of perpendicular planes forming a crystal structure (Piegari and Flory 397).
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Moreover, interference is correlated with the retardation inside an anisotropic mineral and corresponds with the number of waves passing it. Anisotropic minerals and plastics under the influence of mechanical stress are said to be birefringent; optical properties of such materials can be described by quantifying the difference between the maximum and minimum refractive indices (Piegari and Flory 397). The interference colors are often presented with the help of the Michel-Levy chromatic scale that allows the most efficient interpretation of color groups. It was developed by a French geologist Auguste Michel-Levy in 1888 (Houck and Siegel 85).
He was concerned with studying the question of the microscopic structure of eruptive materials. Michel-Levy’s position as a director of the Geological Survey of France allowed him to conduct extensive research on the origins of minerals and other birefringent materials which has led to the development of diameter, birefringence, and retardation scale that was called after him (Houck and Siegel 85). The chart is used for calculation of an unknown parameter of a birefringent specimen if the two other measurable factors are known. The colors in the Michel-Levy chromatic chart are classified into different orders in successive multiples of landas (Piegari and Flory 397).
The first order is made up of black, gray, yellow, orange, and red according to their retardations. Violet, blue and green are grouped in the second-order and correlate with one landa retardation of colors that complement them (Piegari and Flory 397). The same is true for two lands of yellow, orange, and red. All successive orders are composed of violet, blue, green, yellow, orange, and red in progressive multiples of landas (Piegari and Flory 397).
Interference color chart that is sometimes referred to as the Michel-Levy Table of Birefringence is widely applied in modern analytical microscopy; therefore, the majority of microscope manufacturers provide it with their products (Piegari and Flory 400). In the past, it was mainly used by mineralogists and petrologists for the identification and classification of minerals. It should be noted that Michel-Levy was the first scientist who applied a polarizing microscope in the field of mineralogy (Piegari and Flory 400).
Gabriel Lippman was another scientist who studied interference and even was awarded the Nobel Prize in Physics for the invention of interference color photography in 1908; however, due to the substantial difficulties with the production of photographs, this method has not become popular (Piegari and Flory 400). Nowadays, the Michel-Levy Table of Birefringence is being used for the process of identifying synthetic fibers, food ingredients, biological agents, drugs, catalysts, ores, fertilizers, and chemicals among others (Piegari and Flory 400).
Interference colors are also being deployed for the anti-counterfeiting purposes on banknotes, credit cards, trademarks, and other documents of significant value (Piegari and Flory 400). Their use is justified by the inability of modern copying equipment such as printers and cameras to reproduce complicated geometric and line patterns created by interference-color figures printed at varying screen frequencies (Piegari and Flory 400).
Houck, Max, and Jay Siegel. Fundamentals of Forensic Science. Amsterdam: Academic Press, 2011. Print.
Piegari, Angela, and Francois Flory. Optical Thin Films and Coatings. Cambridge: Woodhead Publishing Ltd, 2013. Print.