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     Calcite
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Calcite is a natural crystal that has high birefringence, wide spectral transmission and availability in reasonably sized rhombs. It's a negative uniaxial crystals and mostly used as visible and near IR polarizers.

 

Physical and Optical Properties
Transparency Range 350~2300nm
Partical Shape Crystalline rhombihedral
Density 2.7 g/cm3
Hygroscopic Susceptibility Low
Mohs Hardness 3
Thermal Expansion Coefficient a a=24.39x10 -6 /k; a c=5.68x10 -6 /k
Crystal Class Negative uniaxial with no=na=nb, ne=nc
Refractive index, birefringence(?=ne-no) and walk-off angle at 45°C (?) n o =1.6557, n e =1.4852
?n=-0.1705, ?= 6.20°@ 630nm
n o =1.6629, n e =1.4885
?n =-0.1705, ?= 6.20°@ 1300nm
Sellmeier Equation n o 2 =2.69705+0.0192064/(?2 -0.01820)-0.0151624?2
n e 2 =2.18438+0.0087309/(?2 -0.01018)-0.0024411?2

Capabilities:
1) Diameter : max 30~40mm
2) Length: max 25~35mm
3) Surface Quality: better than 20/10 scratch/dig Per MIL-0-13830A
4) Beam Deviation: <3 arc min
5) Optical Axis Orientation: +/-0.5°
6) Flatness: 7) Transimission Wavfront Distortion: < l /2 @633nm
8) Coating: upon customer's specification

Notes
As the susceptibility is low so that it's easy to moisture,it's difficult to achieve excellent optical quality due to low hardness.