Illumination
Transmitted Brightfield Crossed Polars

Crossed polarizing filters detect molecular anisotropy in objects: molecular alignment crystal structure, and stress. Objects that are bright are anisotropic. Anisotropic particles can be rotated into "extinction" position where their anistropy aligns with the linear polarizing filters. The blue fiber on the right is in an extinction position except for the stressed end, where it was cut. Mature cotton fiber has no extinction position as is shown by the large wavy cotton fiber in the middle of the field of view.