Polarized light microscopy

Polarized light microscopy uses controlled polarization of illumination and detection to reveal optical properties of a specimen. A polarizer before the specimen sets the incident polarization; a second polarizer, called the analyzer, selects which outgoing component reaches the image. With the two pass axes perpendicular, an ideal empty optical path is dark. A specimen can alter this result through birefringence or polarization-dependent absorption.^[ref]

For an ideal nonabsorbing birefringent plate between crossed polarizers, the transmitted light intensity iswhere $I_0$ is the intensity after the first polarizer, $\theta$ is the angle of a plate axis to its pass axis, $d$ is thickness, $\Delta n$ the birefringence, and $\lambda$ the vacuum wavelength. The phase retardation $\delta$ comes from different optical path lengths. With $\delta =\pi$, the plate is bright at $\theta =45^{\circ }$ and dark at $\theta =0$: rotating a specimen changes contrast without removing material.^[ref]

This sensitivity is useful for oriented biological materials. A2018 ivory study combined polarized-light observations with X-ray scattering and scanning electron microscopy to investigate mineralized collagen architecture. Brightness alone does not uniquely identify a substance: orientation, thickness, wavelength and optical properties all contribute.^[ref]