Michel-Levy interference color chart

J.G. Delly, The Michel-Levy Interference Color Chart—Microscopy s Magical Color Key. (2003) Available at http //www.modemmicroscopy. com. Accessed October 2006. 

In white light, anisotropic structures may appear brightly colored when viewed in crossed (or parallel) polars. These polarization or interference colors depend on the retardation (Section 3.3). An estimate of sample retardation can be made from the standard sequence of colors, published as the Michel-Levy chart in many texts [4, 7, 9, 17, 18]. Color can also be used to find the sign of birefringence when a first order red plate is inserted as a compensator in white light. 

Interference colors coupled to fiber diameter can be used to obtain birefringence without resorting to the use of different mounting media. This is accomplished with a Michel-Levy chart, shown in the color insert, and microscope accessories called compensators or wave plates. Recall that a birefringent material breaks polarized light into a fast and a slow component. The thicker the sample, the greater is the offeet. Suppose a fiber is rotated such that interference colors are at their most vivid and... 

Compensators are used in this context when the difference between fast and slow components is relatively small. On the Michel-Levy chart, there is a large band to the left where the interference colors are whiti and not very informative. A compensator adds a constant offset to the small offset created by the fiber. The labels on the bottom of the chart correspond to fixed ofifeets that can be added to increase the contrast. A compensator is a crystal that is inserted in the light path after the sample, but before the polarizer. Compensators are labeled by the amount of offset they add. A full wave-plate compensator adds 550 nm, for example, corresponding to the x-axis labels on the Michel-Levy chart. A quarter wave plate would add 137.5 nm. A full wave-plate compensator is also called a first-order red compensator, because of the red color it imparts to the background under crossed polars. 

---