Conoscopic Illumination
Definition
A convergent polarised beam mode in which a range of angles is passed through the specimen simultaneously, producing an interference figure (optical figure) in the back focal plane that reveals crystal symmetry and the optic axial angle (2V).
- Field
- Polarising microscopy
- Beam type
- Convergent polarised light
- Output
- Interference (optical) figure in the back focal plane
- Reveals
- Crystal symmetry and optic axial angle (2V)
Common questions
How does conoscopic illumination differ from ordinary (orthoscopic) polarised light examination?+
Orthoscopic examination passes light through the specimen at a single angle and shows features like colour and pleochroism in the plane of the sample. Conoscopic illumination passes a full cone of angles through the specimen simultaneously, producing an interference figure that reveals angular optical properties invisible under ordinary illumination.
Why does the optic axial angle matter for forensic mineral or fibre identification?+
The 2V angle is a diagnostic optical constant specific to a mineral's crystal structure, so measuring it from the interference figure helps distinguish between minerals or synthetic materials that otherwise look similar under standard polarised light, narrowing an unknown sample to a smaller set of candidate materials.
Related terms
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- Native fluorescence of endogenous biological molecules (collagen, elastin, NADH, flavins) that produces background signal in fluorescence microscopy, potentially mimicking specific probe signals....
- Birefringence
- The difference between the highest and lowest refractive indices of an anisotropic mineral, seen as a characteristic interference colour between crossed polars....
- Crossed Nicols
- The configuration in which the polariser (below the condenser) and the analyser (above the objective) are oriented at 90 degrees to each...
- DAPI
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- Dichroic Mirror
- A spectrally selective beam splitter that reflects shorter (excitation) wavelengths at 45 degrees and transmits longer (emission) wavelengths, forming the core element...
- Epi-Fluorescence Illumination
- Fluorescence excitation through the objective lens from above the specimen, with the same objective collecting the emission. A dichroic mirror directs excitation...
- Extinction Angle
- The angle, measured in degrees on the rotating stage of a petrographic microscope, between a crystallographic reference direction (cleavage trace or crystal...
- Michel-Lévy Chart
- A birefringence-thickness diagram correlating retardation values with the interference colour seen at that retardation under white-light crossed polars. Standard reference for identifying...
- Photobleaching
- The irreversible photochemical destruction of a fluorophore by repeated excitation. Limits the observation time for low-copy forensic fluorescence samples. Mitigated by antifade...
- Retardation
- The path difference (in nm) introduced between the fast and slow ray components as they traverse an anisotropic specimen of given thickness...
- Stokes Shift
- The wavelength difference between the excitation peak and the emission peak of a fluorophore. A positive Stokes shift (emission at longer wavelength...