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Calomel, Hg2Cl2, is a mercurous chloride crystalline material with a transmission range from visible to the LWIR region. It is a unique material with many exceptional and unique properties:

  • Transmission range from 0.38µm to 17µm.
  • Strong birefringence of Δn=0.683.
  • Optically positive, ne > no.
  • High extinction ratio between 2.5 to 17µm, ER > 1:15000.
  • High figure of merit, M2, for acousto-optical devices.

The properties listed above make Calomel an ideal material choice for infrared polarisation and acousto-optical components, and is one of the only materials available which can be used in LWIR applications which require these parameters.
Currently Calomel has been used in the following optical components:

  • Glan-Foucault polariser.
  • Wollaston polariser
  • Polarising beamsplitters.
  • Acousto-optical tunable filters.

Custom components can also be manufactured including:

  • Prisms
  • Windows
  • Cubes
  • Blocks
  • Wedges

Below is a summary of Calomel’s optical properties:

Transmission Range

0.38 to 15µm

Refractive Index

no 2.122, ne 3.190, Δn=+1.068 @ 380 nm
no 1.950, ne 2.548, Δn=+0.598 @ 700 nm
no 1.900, ne 2.449, Δn=+0.549 @ 3500 nm
no 1.890, ne 2.445, Δn=+0.547 @ 10 000 nm


7.19 g/cm3

Melting Point



1.5 Mohs

Operating Temperature

-5°C to 60°C


With our in-house state-of-the-art metrology laboratory you can be confident that your optical components will meet your requirements. You can find out more about our testing facilities here.

Additional Information

  • The first single crystal of calomel was produced in 1969 in the Czech Republic.
  • The birefringence it exhibits is the highest known for a crystalline material, almost four times higher than calcite.
  • The extinction ratio it exhibits in the IR region is >1:15000.
  • Calomel’s safety data sheet is available here.

Typical Applications

  • Infrared spectroscopy
  • MWIR polarisers and waveplates
  • Acousto-optical devices
  • Laser systems as has a CW laser damage resistance.

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