A revised Michel-Levy interference colour chart based on first-principles calculations

Two revisions of the original Michel-Levy interference colour chart are presented, and discussed here. Compared to older charts these give a more precise match to the actual observations in the modern optical microscope. As an example, interference colour transitions on wedge shaped olivine grain boundaries toward epoxy match accurately the interference colour in the new charts. The differences in the colour between older charts and the new are most pronounced in the second order, where the saturated green is now replaced by greenish white and turquoise. In addition, the pinks are shown to cover a larger range than previously thought. The revised charts make it easier to determine interference colour and order, and hence also to determine the birefringence of the common silicate minerals. Because of the first-principle origin of the charts it is easy to adjust the charts for different types of optics and illumination. Also the improved understanding of interference colour opens up new possibilities in image analysis of transmitted-light images involving interference colour.