SCHOTT solutions no. 1/2016 > Infrared glasses

Infrared glasses
The chalcogenide glass family from SCHOTT delivers excellent transmission over wide ranges of the IR spectrum. Photo: SCHOTT/J. Stevens

Making the Invisible Visible

Through intensive R&D work and close contact with customers, the developers at SCHOTT constantly work on optimizing infrared glasses and tailoring these for use in industrial applications.

Dr. Barbara Stumpp

We cannot see infrared radiation, but we can certainly feel it as heat on the skin. Infrared (IR) or thermal radiation was scientifically proven for the first time around 1800 by the astronomer Friedrich Wilhelm Herschel. He divided sunlight into its spectral components by using a prism. Above the red, which is the longest wavelength range of visible light, he found invisible, yet warm radiation. While visible light has a wavelength of 400 nanometers (blue) to 780 nanometers (red), a distinction is made between near IR (780 nanometers to about 3 µm in wavelength), medium-wave IR (3.5 to 5 µm in wavelength) and thermal or long-wave IR (8 µm to about 14 µm in wavelength).

IR applications require high-quality optics

To make IR radiation visible, measurable and technically exploitable, whether it’s in night vision devices, thermal imaging cameras, motion control systems, pyrometers or diagnostic equipment, the optical materials used in the systems must meet very special requirements. Common soda-lime glass as well as many special glasses are opaque in the middle and thermal IR regions and are therefore unsuitable. This is caused by the absorption of IR radiation by molecular vibrations of the glass matrix (silicon-oxygen bond, etc.). Special IR glasses are perfectly usable, such as those that for years have been developed and constantly optimized in SCHOTT’s laboratories at its Duryea, Pennsylvania, site in the United States. ”Disturbing” silicon is replaced with arsenic, germanium, antimony or gallium; oxygen is replaced with sulfur, selenium or tellurium. This produces what are called chalcogenide glasses. These glasses offer the necessary excellent transmission in the short-, medium- and long-wave IR ranges, low temperature dependence on the refractive indices and low dispersion. These high-quality chalcogenide glasses can also be combined with other glasses from the series or other IR materials. ”We thus offer solutions and support optical designers in developing thermally resistant and powerful optical infrared systems,” explains Dr. Nathan Carlie, Research and Technology Development, SCHOTT North America. The optically excellent, yet sensitive glass must be separated from the operational environment by a protective window. The window must be made of a durable material that is transparent over the full operational range of the optic. For this purpose, SCHOTT scientist Dr. Keith Rozenburg has developed IRC-1, a ceramic process for producing polycrystalline zinc sulfide. IRC-1 avoids the pitfalls of zinc sulfide produced through the expensive chemical vapor deposition process.

”Our research in close proximity to manufacturing enables us to continuously optimize
the quality of the glasses – i.e. transmission and uniformity – while reducing the costs through improved melting efficiency and new manufacturing processes such as precision molding.”

Dr. Nathan Carlie, Research and Technology Development, SCHOTT North America

Infrared glasses
SCHOTT’s laboratories in Duryea, Pennsylvania, have been developing and optimizing IR glasses for many years. Photo: SCHOTT/J. Stevens

Concepts for the future

For several years, infrared image processing has been developing from classic into increasingly complex, compact and cost-effective multispectral applications. ”Multispectral” means that the visible wavelength range is detected up to long-wave IR. The range of the wavelength is therefore more than 10 times greater than with optics in the visible range. Applications possibilities include, among others, multispectral surveillance of the climate by satellites. While still visionary, IR cameras in smartphones and other mobile devices that will not only simplify, but will also in the future quite considerably expand the application fields of infrared technology in health care, agriculture, housing and the automotive sector are already being discussed.

The multispectral IR materials available thus far, such as zinc sulfide and zinc selenide produced through chemical vapor deposition, are crystalline, difficult to produce and expensive to process. Dr. Carlie explains, ”Glasses on the other hand can be melted in high volumes and molded into shape at a low cost.”
Infrared glasses
IR products like this lens are an economical alternative to costly manufactured multispectral IR materials, such as crystalline zinc sulfide or zinc selenide. Photo: SCHOTT/J. Stevens
The real enabler for this new technology is the ability to tailor the optical properties of glasses to suit innovative applications. SCHOTT has already developed new, optimized glass compositions that provide the required targeted dispersion and precise control of the thermal effects for the next generation of optical IR systems. ”We’ve just introduced multispectral chalcogenide IRG-X glasses to the market that are transparent from the visible range through the entire long-wave IR,” Dr. Carlie says.

The IR technology should soon receive another boost: SCHOTT is working on materials for graded-index lenses to enable miniaturized infrared imaging solutions. With gradient-index optics, refraction in the glass is changed by a continuously changing material variation, whereby complete lens groups can be replaced. IR image recognition systems can therefore be produced more easily in more compact form and thus more cost- effectively in the future. <
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