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9. Calcite Grades

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Shown in the table below is the system of quality grades used for our calcite and polarizers, regarding material inhomogeneities and degree of internal scattering.

Calcite Optical Quality Grades
		Scattering
Striae	Grades	A	B	C
	I	Scatter-free and schlieren-free, suitable for use with narrow-beam lasers and high-precision interferometry	Schlieren-free but scatters uniformly over the aperture, suitable for use in interferometry	Schlieren-free but presents many visible scattering centers, for use in general optical instrumentation and in some interferometry settings
	II	Scatter-free but presents some schlieren, suitable for use with lasers and interferometry	Some schlieren and scattering uniformly distributed over the aperture, suitable for use in laser-based polarization instruments	Some schlieren and markedly visible scattering centers, suitable for use in experimental setups
	III	Scatter-free but presents visible inhomogeneities, suitable for use with lasers	Scattering uniformly distributed over the aperture, visible schlieren. Suitable for use in visual polarization instruments	High scatter and inhomogeneities. Suitable for educational demonstrations

The striae classification is based on the magnitude of the wavefront deformation that will result from the inhomogeneities. Occurrence of striae is classified as follows:

Grade I: Wavefront deformations of less than l/10 peak-to-peak over all the useful prism aperture. Totally free of inhomogeneities when observed using the Schlieren method.

Grade II: Wavefront deformations of less than l/4 peak-to-peak over all the useful prism aperture. Inhomogeneities are barely visible with the Schlieren method.

Grade III: Wavefront deformations of less than l peak-to-peak over all the useful prism aperture. Regions of inhomogeneity may be observed with the unaided eye.

The classification pertaining to scattering used the following grades:

Grade A: Completely free of scattering centers when observed visually with illumination by a narrow beam of a low power He-Ne laser.

Grade B: Presents slight and homogeneous scattering over all the aperture area when observed visually with illumination by a narrow beam of a low power He-Ne laser.

Grade C: Many conspicuous scattering centers when observed visually with illumination by a narrow beam of a low power He-Ne laser.

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