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Michel—Lévy chart (birefringence chart, interference colour chart) A chart of standard colours used in measuring birefringence. In crossed-polarized light, thin sections of anisotropic minerals with a standard thickness of 0.03 mm (30 μm) will give a series of interference colours, depending on the amount of birefringence (double refraction). A number of sections are examined in different orientations and the maximum interference colour matched with the equivalent colour on the chart. From the point representing 30 μm thickness on the appropriate colour a radiating line is followed to the edge of the chart to give a reading for birefringence. The accessory quartz wedge may also be used to determine birefringence.