Orthoscopic observation

Orthoscopic examination with crossed polars is carried out first of all to determine the isotropism or the anisotropism of a sample. The polarization colors, the defects and variation in molecular orientation, and the orientation pattern or texture of liquid crystals are observed in this examination. With a heating stage the temperature of phase transition is also determined. In addition, with use of a compensator, the determination of vibration directions of the ordinary and extraordinary rays, the determination of relative retardation and birefringence are possible. In this section, the optical basics for orthoscopic observations are briefly outlined. The description of textures frequently observed for polymeric liquid crystals is given in Section 4.1.4. 

In a uniaxial liquid crystal, the ordinary and extraordinary rays are both polarized. The following rules determine the vibration directions of the two  

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In studies of the optical properties of the liquid crystal phase the microscope may be arranged for orthoscopic observations between crossed polars, or it may be arranged for conoscopic observations. These two arrangements are shown in Figure 4.4. 

In orthoscopic observations parallel rays are required because the optical property of liquid crystals is direction-dependent. Often a low-power, low aperture objective lens is used so that most of the light will travel through the sample in the same direction or nearly so. Otherwise the substage iris (D in Figure 4.4) is closed accordingly so that a sufficient approximation of the parallel condition is achieved. 

Orthoscopic observation n. The normal way of viewing an object microscopically (cf, conoscopic observation). With Kohler illumination the field diaphragm and the ocular front focal plane as well as the specimen will be in focus. 

Figure 4.4. Polarizing microscope for orthoscopic (a) and conoscopic (b) observations.

Conoscopic observation n. The study of the back focal plane of the objective by removing the eyepiece, by inserting a Bertrand lens, by examining the image at the eye-point above the eyepiece with a magnifier or by using a phase telescope is called conoscopic because the observations are associated with the cone of light furnished by the condenser and viewed by the objective (cf., orthoscopic). 

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