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Savart Plates
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The Savart plate comprises two beam splitting plates with their optic axes at 45° to the surface normal, rotated 90° with respect to each other. An incident beam propagating through Plate 1 is resolved into ordinary and extraordinary beams which are displaced from each other in the 1st principal plane. Upon entering Plate 2, the ordinary beam becomes an extraordinary beam, and vice-versa. Splitting again takes place in the 2nd principal plane, which is orthogonal to the 1st. The result is two emerging beams displaced along a diagonal. The displacement is root 2 times that of each plate. The optical path difference between the 2 beams is zero for normal incidence.
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Operation
When placed between parallel or crossed polarizers, a collimated input beam produces a set of parallel fringes. The Savart plate can be used to detect very weakly polarized light in the presence of mainly unpolarized light - typically starlight. The intensity of the interference bands is indicative of the degree of polarization.
When used with a diffraction grating, the plate functions as an achromatic retardation plate for the diffracted orders. Savart plates can be manufactured in both calcite and quartz, and are cemented together to transmit over a wide spectral range.
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Beam Displacement
The beam displacement produced by the Savart plate is 0·075 times the total thickness of both plates for calcite, and 0·0042 times the total thickness for quartz at 500nm. For any required displacement, this factor can be used to calculate the corresponding total thickness. When ordering a Savart plate, specify the aperture and total thickness.
For example, Model SPC20-10 will have a 20mm aperture with a combined thickness of 10mm, and will produce a displacement (shear) of 0.75mm (750μm). Note that the spectral dispersion of the displacement varies in the same fashion as that of beam displacers (q.v.)
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Mounts
Savart plates can be supplied unmounted as free components or mounted as shown. The mount comprises a black anodised aluminium ring with silicone rubber shock absorbing material between the ring and the plate. To specify mounted displacers, simply add suffix:
M3 for Type 3 mount (shown)
Mount dimensions are advised at the time of ordering.
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Specifications
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Model #
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Material
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Aperture (mm)
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Spectral Range (nm)
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Peak Trans. (%)
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Flatness @NaD
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Savart Plates - Calcite
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SPC10
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Calcite
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10
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350-2500
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90
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lambda/4
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SPC15
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Calcite
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15
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350-2500
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90
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lambda/4
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SPC25
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Calcite
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25
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350-2500
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90
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lambda/4
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SPQ38
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Calcite
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38
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350-2500
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90
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lambda/4
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SPC50
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Calcite
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50
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350-2500
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90
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lambda/4
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Savart Plates - Quartz
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SPQ10
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Quartz
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10
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350-2500
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90
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lambda/4
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SPQ15
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Quartz
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15
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350-2500
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90
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lambda/4
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SPQ25
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Quartz
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25
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350-2500
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90
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lambda/4
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SPQ38
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Quartz
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38
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350-2500
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90
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lambda/4
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SPQ50
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Quartz
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50
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350-2500
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90
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lambda/4
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