Written and maintained by CASRAI Editorial Board
Last updated
Polarized optical microscopy (also called polarizing microscopy or polarized light microscopy) is a contrast technique that images only materials that are birefringent, meaning optically anisotropic. A specimen is lit with linearly polarized light and viewed through a second, crossed polarizer, so isotropic background stays dark while ordered structures such as crystals, fibers and stressed plastics appear bright.
Short answer
Polarized optical microscopy places a polarizer below the specimen and an analyzer above it, set perpendicular to each other. With nothing on the stage the field is completely dark. A birefringent specimen splits the light into two rays and lets some of it through the analyzer, so it shows up against the dark field without any stain or label.
Definition and scope
Nikon’s MicroscopyU defines birefringence as “the double refraction of light in a transparent, molecularly ordered material”, and expresses it as the difference between the two refractive indices of the material, B = |ne − no|. Leica Microsystems makes the practical point: to detect birefringence, linearly polarized light must be used for illumination, which is why two polarizing filters are built into the light path.
The technique therefore answers a narrower question than most light microscopy. It does not report on size, color or chemistry. It reports on whether, and how strongly, a material is molecularly ordered. Leica notes it is routinely used in the material and earth sciences to identify materials and minerals, and in biology to image birefringent structures such as cellulose in plant cell walls and starch grains.
How it works in practice
Polarizer, analyzer and the dark field
Per Nikon, a dichroic polarizer transmits light whose electric vectors lie along one direction and absorbs the perpendicular component. The second polarizer, after the specimen, is the analyzer; it passes only the light components parallel to its transmission azimuth. When the two are crossed (vibration directions perpendicular), no light reaches the eyepieces or camera, a point Leica states in the same terms: with the filters in the crossed position the image is dark.
Extinction and rotation
A birefringent crystal whose optical axis lies parallel to the polarizer’s transmission direction looks nearly invisible between crossed polarizers; Nikon calls this extinction and notes it is used as a reference when measuring refractive indices. Because brightness depends on orientation, many polarization microscopes have a rotating stage so the sample’s polarization plane can be aligned with that of the microscope (Leica).
Compensators and conoscopic viewing
For quantitative work, Leica notes that a retardation plate or compensator is useful. Nikon describes a calibrated plate such as a full-wave retardation plate or first-order red compensator, whose roughly 550 nm region is the most sensitive to small changes in retardation. A Bertrand lens is used for conoscopic observation of interference patterns (Leica).
Types of birefringence you may see
Nikon groups the sources of birefringence in specimens into three families, each with examples from its own page:
| Source | What creates it | Examples named by the source |
|---|---|---|
| Structural | Ordered molecular assemblies | Chromosomes, muscle fibers, microtubules, liquid crystalline DNA, fibrous protein structures |
| Stress or strain | Mechanical deformation of materials that are not naturally birefringent | Stretched films and fibers, deformed glass and plastic lenses, stressed polymer castings |
| Flow | Alignment of rod-like or plate-like molecules in flowing solution | High molecular weight DNA and detergents |
What you see and what it means
Use this table to interpret what appears between crossed polarizers.
| Observation between crossed polarizers | What it indicates |
|---|---|
| Dark field, no specimen | Polarizer and analyzer are crossed (Nikon, Leica) |
| Specimen stays dark | Not birefringent, or its optical axis currently lies along the polarizer direction (extinction, Nikon) |
| Specimen bright against dark field | Birefringent material splitting the light into two rays (Leica) |
| Interference colors with a first-order red plate | Retardation changes are being made visible; this region is the most sensitive (Nikon) |
Where it matters for labs and equipment decisions
A lab deciding whether an existing microscope can do polarization work can check it against the components the two sources name. This is a checklist of what each source describes, not a ranking of products.
| Component | Role | Source |
|---|---|---|
| Polarizer (below specimen) | Produces linearly polarized illumination | Nikon, Leica |
| Analyzer (above specimen) | Passes only components parallel to its azimuth | Nikon, Leica |
| Rotating stage | Aligns the sample’s polarization plane with the microscope’s | Leica |
| Compensator or retardation plate | Quantitative analysis of birefringence | Leica, Nikon |
| Bertrand lens | Conoscopic observation of interference patterns | Leica |
Related CASRAI guides cover neighbouring decisions: microscope objective selection, phase contrast microscopy setup, and inverted microscopy. For a staining readout that relies on polarization, see Congo red staining, and for the materials side see what is mineralogy.
Common misunderstandings
- It is not a stain or a fluorescence method. The contrast comes from the optical anisotropy of the material itself; isotropic material is dark between crossed polarizers.
- A dark specimen is not necessarily isotropic. At extinction a birefringent crystal looks nearly invisible (Nikon), which is why the stage is rotated.
- The three names mean the same technique. Polarized optical microscopy, polarizing microscopy and polarized light microscopy all describe imaging with crossed polarizers.
FAQ
What is polarized optical microscopy used for?
Leica lists identifying materials and minerals in the material and earth sciences, and imaging cellulose in plant cell walls and starch grains in biology. Nikon adds chromosomes, muscle fibers and microtubules as structurally birefringent examples.
What does a polarizing microscope need beyond a standard one?
A polarizer and an analyzer in the light path, usually a rotating stage, and for quantitative work a compensator or retardation plate. A Bertrand lens is used for conoscopic observation.
Why is the background dark?
The polarizer and analyzer are crossed, so with no birefringent material in the path no light passes to the eyepieces or camera.
Sources
- Nikon MicroscopyU, Principles of Birefringence: https://www.microscopyu.com/techniques/polarized-light/principles-of-birefringence (checked 2026-10-29).
- Leica Microsystems, Birefringence is the key to polarization microscopy: https://www.leica-microsystems.com/science-lab/polarization-contrast/ (checked 2026-10-29).
