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In The Spotlight: Calomel Infrared (IR) Polarisers

8th Feb 2023

Knight Optical, the leading supplier of on-spec, metrology-tested optics, is proud to announce that it now offers a new range of high-spec components, Calomel Infrared (IR) Polarisers. In this Spotlight blog, the global optics provider runs through everything you need to know about these highly birefringent crystal polarisers and uncovers some of their typical uses.


Following the release of a brand-new range of laser optics and improvements to our acrylic dome capabilities, we have once again answered the requirements of our customers by expanding our stock range of optics, with the introduction of Calomel IR Polarisers.


Here, we break down our latest offering, exploring the many substrate benefits and qualities of Calomel, its many uses, and the two different polariser varieties that are available from our wide-ranging catalogue.

optical components
Calomel (Mercurous Chloride [Hg2Cl2])

As a substrate, several factors make Calomel (Mercurous Chloride [Hg2Cl2]) an ideal option for polarisation applications and use as acousto-optical components, such as tunable filters. As well as its excellent extinction ratio for polarisers (>1:10000), Calomel is also highly birefringent (Δn=0.683), particularly in IR wavelengths. In fact, the birefringence it exhibits is the highest known for a crystalline material and is almost four times higher than that of calcite. It’s an ideal material for applications that require polarisation in the mid-IR (5 and 17µm), and it positively transmits from 0.38µm through to 17µm.


The first single crystal of Calomel dates back to 1969 when it was produced in the Czech Republic. Today, you can find Calomel, specifically IR Polarisers, across a broad spectrum of sectors. They’re employed for everything from spectroscopy and microscopy to thermal imaging and, due to their CW laser damage resistance, lasers. Moreover, thanks to their unique properties, Calomel is one of the only materials available that can be used in long wavelength infrared (LWIR) applications.


Here at Knight Optical, we offer two varieties of Calomel IR Polarisers from our stock catalogue, a Glan-Foucault and a Wollaston polariser.


Let’s take a look at the difference between the two.


Glan-Foucault Polarisers

The formation of a Glan-Foucault Polariser comprises two prisms spaced apart using an ‘air gap’, which is recognised as an ‘air-spaced design’. These polarisers are known for their transmission of the positively polarised ordinary ray (o-ray).


Sometimes referred to as ‘Glan-Air’ prisms, the Glan-Foucault Polariser’s insertion loss characteristics are slightly higher than typical cemented designs. This is because the interface incident angle sits close to Brewster’s angle for the o-ray, resulting in a relatively low insertion loss.


Available from our portfolio of over 3000 stock optical components, which is utilised by customers worldwide for projects that demand quick lead times, Knight Optical’s Glan-Foucault Polarisers are available uncoated in 10 x 10 x 10mm dimensions (length, width, thickness).


Wollaston Polarisers

Unlike the Glan-Foucault variety, Wollaston Polarisers are constructed of two prisms that are cemented together, rather than air spaced. Invented by English Chemist and Physicist William Hyde Wollaston, the Wollaston Polariser is typically used in two ways – either as a beamsplitter/combiner or, as the name suggests, a polariser that removes an angled orthogonally-polarised component of a beam. These polarisers have two orthogonally-polarised outputs (the e-ray and the o-ray) with a separation angle of 14°.


Again, similar to our Glan-Foucault components, our stock range of Wollaston Polarisers are available uncoated in 10 x 10 x 10mm dimensions (length, width, thickness).


For better handling and alignment, both our Glan-Foucault and Wollaston Polarisers are provided mounted in 360° rotational mounts.

Custom Glan-Foucault and Wollaston Polarisers

If our stock range of Calomel IR Polarisers doesn’t quite meet your application’s specific requirements, we can offer custom-made Glan-Foucault and Wollaston Polarisers. From coatings to dimensions, we can supply project-specific components if required.


To find out more about our bespoke capabilities or to make an order, do not hesitate to get in touch with a member of our team today.

Premium-Quality, Metrology-Tested Calomel IR Polarisers

When ordering Calomel IR Polarisers from Knight Optical, you can rest assured that they will arrive to your precise specification. Quality is of the utmost importance to us, and to help us achieve excellence with every order, we send all our optical components through a rigorous testing and quality process prior to dispatch.


Starting in our state-of-the-art Metrology Laboratory, your Glan-Foucault and Wollaston Polarisers with be meticulously tested by our fully trained team of Metrology Technicians to ensure they are fit for purpose. After passing through this testing regime, your optics will be handed over to our hawk-eyed QA department, which will visually inspect each component for surface imperfections.


Together, our Metrology and QA processes help us to maintain our long-standing leading status in the optical component sector, and it is thanks to our quality-first ethos and transparency that customers all around the world rely on our optical expertise and high-precision components.


Why Choose Knight Optical for your Application?

Discerning customers rely on Knight Optical not only for the premium quality of our output and in-house state-of-the-art Metrology Laboratory and QA Department’s capabilities but because – as well as a range of Stock Optics – we also offer our optics as Custom-Made Components.


In 2021, we celebrated 30 years in business. With over three decades of experience under our belt and a whole host of long-standing world-renowned customers on our books, we are proud to have worked on some of the most ground-breaking innovations.


If you are looking for premium-quality, stock, and bespoke optical components, please do not hesitate to get in touch with a member of the Technical Sales Team here.


Europe, UK, Asia & RoW:

[email protected]

+44 (0)1622 859444


USA & Canada:

[email protected]

+1 401-583-7846