MicrobeHunter.com Microscopy Forum

Recently I was viewing the spore capsules of a moss using phase contrast and 10x and 20x objectives. In particular I was interested in the peristomal teeth, structures which look like fangs and surround the opening in the tip of the capsule where the spores are released. I decided to add crossed polarizers to the phase contrast setup. With the polarizers crossed at 90 degrees the field becomes dark like darkfield. The image which is visible is due to light which has been rotated by the refractive qualities of the specimen. I don’t know if this technique is really useful but I find it very interesting. Anyone know if there is a term for this? Anyone else use this method?

Polarized light is rotated by asymmetric structures - both at the level of molecular structure, or the way the molecules are packed together within the object. The latter tends to give the strongest effect which is known as birefringence (refractive index differs along different axes of the structure). Because of the wavelength dependence of refraction, birefringent materials often reveal color fringes which tells you something about the thickness of the sample. The method is widely used in mineralogy where as additional "retarder" element is also put in the optical path as a reference to add to or subtract from the effect produced by the sample. We have used the method to look for microplastics in water samples. Many plastic materials show birefringence due to the alignment of polymer molecules within during the molding or extrusion process https://www.youtube.com/watch?v=iNNkdHyvhbs&t=1s.

Nice explanation. I I was wondering if anyone else uses crossed polarizers combined with phase contrast. I think that the phase contrast gives the polarized light image more dimensionality although the halo artifact is also noticeable. It might be better to stick with just one technique at a time but the combination seems to produce rather striking images.