Spath Blade Double Image Micrometer

Double-image micrometers generate two images from one incident light source. Instead of setting an external device - like in the filar micrometer - with respect to the object to be measured, the two images with equal brightness are oriented with respect to each other to measure position angle and distance.

The first construction was introduced by P. Muller:1 he used a birefringent quartz glass crystal. Figure 13.1 shows the orientation of the crystal axes, the incident

Figure 13.1. The

Muller double-image prism.

Figure 13.1. The

Muller double-image prism.

ray i and the ordinary and the extraordinary rays o and e, respectively. The quartz prism is shifted with the micrometer screw and hence the relative position of the images can be changed.

B. Lyot and H. Camichel2 suggested a modification with a rotatable calcite plate being the only optical device in the micrometer. Currently this type is the only commercially available double image micrometer made by Meca-Precis, France3 (Figure 13.2). This uses the spath blade prism (Figure 13.3).

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