Terminologies

Birefringence (δ_max): double refraction (splitting the light beam into two perpendicularly polarised rays). The difference in the refractive indices (n values) of the fast (n_f) and slow (n_s) ray is birefringent (the value between the minimum and maximum refractive index). The birefringent leads to interference colours. Depending on the orientation and thickness variable, colours occur.

δ_max = (n_s - n_f)

Dextrogyre extinction lines: Extinction lines are curved lines that bend to the right (clockwise).

Extinction angle: The angle between an extinction direction and a crystallographic direction. The extinction angle of 'parallel extinction' is 0°. The extinction angle of 'inclined Extinction' (symmetrical extinction) is 45°. The extinction angle of 'oblique Extinction' is between 1° and 89° but not 45°. The extinction angles may assist in the identification of nannofossils.

Extinction lines (cross-polarised light dims): In cross-polarized light, If the C-axis of the segments are oriented parallel to the analyzer or polariser, no light passes the analyzer, and the segment appears extinct.

Extinction: Nannofossil or its segments are extinct If the optical axis (C-axis) is aligned with the microscopical axis or with the vibration directions of the polarizer and analyzer.

Inclined extinction: In cross-polarised light, a segment becomes extinct when its length is aligned at 1-89° because the crystallographic axes are not at 90° (not perpendicular to one another). Inclined extinction can be symmetrical or oblique.

Interference colour or polarisation colours: Colours observed under cross-polarized light only are called interference colours. They are formed due to birefringence (=double refraction) by the difference in speed of the two rays. The difference in speed gives different interference colours. When the rays emerge from the segment, they combine to produce interference colours. Interference colours are dependent on the orientation and the thickness of the segments. High birefringence produces high interference colours, whereas low birefringence produces low.

Laevogyre extinction lines: Extinction lines are curved lines that bend to the left (counterclockwise).

Oblique extinction: In cross-polarised light, a segment becomes extinct when the length of the segments is not aligned at 0°, 45° and 90°. The crystallographic axes are not at 90° (not perpendicular to one another).

Parallel extinction: In cross-polarised light, a segment becomes extinct (dark) when the length of the segment is aligned to the north-south and east-west (the optical axis is parallel to the vibration of the analyzer or polariser). It only occurs when the crystallographic axes are at 90° to one another.

Retardation (Δ): Depends on velocities/refracting indices (n values) of slow (n_s) and fast (n_f) rays and the thickness (d) and measured in nanometers (nm). In summary, retardation is equivalent to the multiplication of the birefringence and the thickness of the segment. The birefringent colour we observe results from retardation because of the thickness of the nannofossils.

Δ = d (n_s - n_f)

Symmetrical extinction: In cross-polarised light, a segment becomes extinct when its length is aligned at 45° (NW, SE directions) because the crystallographic axes are not at 90° (not perpendicular to one another).