Retardation
Definition
The path difference (in nm) introduced between the fast and slow ray components as they traverse an anisotropic specimen of given thickness and birefringence. Read as an interference colour from the Michel-Lévy chart.
- Field
- Polarised light microscopy
- Unit
- Nanometres (nm)
- Depends on
- Birefringence and specimen thickness
- Read via
- Michel-Levy interference colour chart
Common questions
How is retardation calculated from birefringence?+
Retardation equals birefringence multiplied by the specimen's thickness, so the same fibre or crystal can show different interference colours depending on how thick the mounted section or fibre is at the point of measurement.
Why does retardation matter for fibre comparison in trace evidence?+
The interference colour and its corresponding retardation value are characteristic of a fibre's polymer type and can help distinguish or associate questioned and known fibres, alongside other optical properties such as sign of elongation.
Can two different materials produce the same retardation value?+
Yes, retardation alone is not unique to one material because different combinations of birefringence and thickness can yield the same path difference, so it is used alongside other optical and physical tests rather than in isolation.
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