Michel-Levy chart

Polarizing colors are then dependent on two variables the birefringence of the substance and the thickness of the particle. There is a chart showing polarizing colors versus thickness and birefringences called a Michel-Levy chart 2.J]. Thickness is plotted on the ordinate, while polarizing colors are plotted along the abscissa. 

Michel-Levy chart n. A chart relating thickness, birefringence, and retardation so that any one of these variables can be determined when the other two are known. 

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. 

Although the Michel-Levy chart is easy to use, there are difficulties associated with it and its applications. What might those difficulties be In what other areas could this be an issue in forensic analysis ... 

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... 

Answer The two refractive indices provide the information necessary to obtain the birefringence value of 0.050 (the difference between the two). V th this information and the Michel-Levy chart, the vector from the birefringence value (on the right y-axis) is retraced until it intersects the vivid green band with a central value of 840 nm. Movit directly across to the left intersects at a thickness of 20 /tm or slightly less. Because the cross section is circular, the thickness is ffie same as the diameter. 

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. 

Hot stage Michel-Levy chart Sign of elongation... 

Birefringence The difference in the refractive indexes of an anisotropic material can be calculated by subtraction or with a Michel-Levy chart. 

Michel-Levy chart A chart showing interferenoe colors and relating them to birefringence and thickness shown in the color insert. 

Figure 13 The Michel-Levy chart used in polarized light microscopy. 

The Michel-Levy chart of these polarization colors [61,62,64] can be used to estimate the retardation of a specimen, within the range 200-1600 nm. For example, first order red is at 575 nm, third order green is at 1250nm. At... 

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