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.