THEORY AND TECHNIQUES 



yepiece 



Rotafable 

 analyzer 



Objective 



Opfic GxiSv^p 



1— Quarfer-wave plate 



Calcite plates — * 

 Specimen 



Half-wave plate— ^ 

 Calcite plates— I \f -y- — 



"Condenser 

 Polarizer 



piaie ~~ 7 / / ^ \ 

 tates- H f V^ 



Optic axis 



b. 



Mirror 



Fig. 8. The AO-Baker interference microscope, (a) The shearing system. The transmission axis o 

 the polarizer is set at 45° to the axis of the first calcite plate. The calcite plates are essentially cleavage 

 sections, similar to those found in nature. Each ray is split into two components as shown in Figure 2c. 

 The half-wave plate axis is parallel to the polarizer axis, as is the quarter-wave plate axis. Measurements 

 are made by rotating the analyzer, (b) The double-focus system. The only difference from the shearing 

 sj^stem is that here the axes of the calcite plates are parallel to the faces of the plates. 



encountered by the reference beam in the 

 region in which they were separated. 



In the double-focus system shown in Fig. 

 8b, the optic axis of the birefringent plate A 

 is parallel to the surfaces. For a set of object 

 rays passing through the specimen, the cor- 

 responding reference rays pass through an 

 area above the specimen. As before, the 

 phase difference between the two beams after 

 they are reunited represents the difference 

 in optical path encountered by the two 

 beams. 



In both systems the method for detecting 

 and measuring this phase difference is shown 

 in the upper part of Fig. 8a. A quarter-wave 

 plate is oriented with its effective optic axis 

 at 45° to the plane of polarization of the ob- 

 ject beam. The two beams are thereby con- 

 verted into circularh^ polarized light, one 

 rotating clockwise, the other counter-clock- 

 wise. The resultant of these two vibrations 



is simply plane polarized light. If the two 

 beams are in phase, the plane of polarization 

 is parallel to the axis of the quarter-wave 

 plate, which may be called zero azimuth. 

 Otherwise, the azimuth angle of the plane 

 of polarization is just half the phase differ- 

 ence between the object and reference 

 beams.* If the object beam is behind the 

 reference beam, as in the case of a phase re- 

 tarding specimen, the plane of polarization 

 is rotated counter-clockwise from the zero 

 azimuth. The opposite is true in case the 

 surround has a greater optical path than 

 the specimen. 



If an analyzer is oriented perpendicular 

 to the zero azimuth the background, where 

 the optical paths are equal, will appear dark 

 or black. Objects having greater or less op- 



* Phase difference is often measured in degrees, 

 360° corresponding to one wavelength of phase 

 difference. 



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