Choosing the Best Optical Windows for Spectroscopy

The material of the optical window is a primary consideration. Each type has distinct transmission properties and is suited for different spectral ranges. Fused silica offers excellent UV to near-infrared (NIR) transmission, while sapphire is ideal for UV to mid-infrared (MIR) applications due to its high thermal and chemical resistance. For far-infrared (FIR) or specialized applications, materials like zinc selenide or germanium might be the best fit. Your choice should align with the wavelengths you'll be working with and the environmental conditions the window will face.

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Substrate Properties

Substrate properties and optical surface specifications are two attributes you will want to keep in mind as you select custom optics for your application.

The material properties of the substrate used will determine transmission, refractive index, and hardness. Our Potassium Bromide substrates, for instance, will transmit UV visible and infrared light. They have a density of 2.75 g/cm3 and an index of refraction of 1.527. Zinc Selenide, on the other hand, blocks UV as well as some visible light and transmits higher wavelength visible light and infrared. It has an index of refraction of 2.631. Fused silica has a density of 2.202g/cm and a index of refraction of which varies from 1.55 to 1.40.

The index of refraction quantifies how much the light is slowed down as it is transmitted through the substrate. It is calculated as the ratio of the speed of light in a vacuum to the speed of light through the substrate. For instance, the index of refraction of Zinc Selenide, 2.631, means that light travels through a vacuum 2.631 times faster than it does through ZnSe.

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The refractive index of our sapphire windows is 1.76-1.77. For optical windows, the refractive index is typically specified at 587.6 nm, the Helium d-line wavelength.  Optical glasses with a high index of refraction are sometimes called &#;flints&#;, while windows with low index of refraction are called &#;crowns&#;.

Another important specification, the Abbe Number, describes how the refractive index varies with wavelength.  The lower the Abbe number, the higher the dispersion. The Abbe number of sapphire is 72.24, and for BK7, 64.17.

MgF2 windows are hard and durable, with a refractive number of 1.378 and an Abbe number of 106.22. They have a high laser damage threshold, and very good broadband transmission from 120 nm to 8 μm. Our MgF2 windows are often used with UV radiation sources and receivers.

If your specifications application is weight-sensitive, you may need to pay attention to substrate density. Although the refractive index of optical materials tends to increase as density increases, this is not always the case, and the relationship is not always linear.

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