Selective plane illumination

Three other very different techniques that use light are becoming available. The simplest in concept forces the optical signal to come from a single plane in the optical microscope by using illumination at 90° that has been focused (by a cylindrical lens) into a sheet. This is called selective plane illumination microscopy (SPIM)... 

Because its contrast results from a unilateral displacement of beams, DIG provides directional contrast within the image the specimen appears as if illuminated from one side, and is perceived as if three-dimensional , an appearance that may genuinely represent the third dimension, or which may be an artifactual expression of differences of RI. DIG provides a high-resolution image that lacks the haloes of phase contrast, and has excellent optical-sectioning properties for imaging selected planes of focus. 

Polarization-based contrast (211 in which the specimen is illuminated normally with linearly polarized light and the analyzer is replaced by a small disc of Polaroid in the center of the objective back focal plane. This allows any intermediate contrast condition between bright field and dark field to be selected, and appears to be untried in the context of liquid crystals. 

For a local analysis of the crystal structure, i.e., for the study of individual grains or selected phases, a selected area aperture can be inserted. Only the sample area selected by the aperture contributes to the diffraction pattern (Fig. 1). A plane-wave illumination, i.e., the appearance of sharp diffraction spots, can thus be maintained (Fig. 4). This technique is called SAD.f The smallest area that can be selected is about 0.5 pm given by the smallest selected area aperture. 

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