Black DiamondBlack Diamond-6 (BD-6), a chalcogenide made of an amorphous mixture of arsenic (40%) and selenium (60%), has several advantages over germanium, which is traditionally used to fabricate IR optics. BD-6's thermally stable refractive index (see the Refractive Index tab) and low coefficient of thermal expansion (22.5 x 10-6 / °C) result in a smaller change in focal length as a function of temperature than for germanium. Additionally, germanium suffers from transmission loss as temperature increases, while BD-6 aspheric lenses can be used in environments up to 110 °C. This material performs particularly well over the 2 - 12 µm spectral range (click here for the transmission of an uncoated 1 mm thick sample).

Infrared lensglasses

Thorlabs offers Thermal Imaging Lenses that are ideal for focusing long-wave infrared (LWIR) sources onto detectors or image sensors. The lenses, manufactured by LightPath Technologies, are mounted in threaded, black anodized aluminum housings engraved with the focal length and f/# (see the table below for details). Designed using Black Diamond-6 glass, their aspheric geometry allows them to focus light without introducing spherical aberration. The TIL4 and TIL8 lenses feature singlet designs, while the TIL15, TIL20, and TIL24 are doublets.

Infraredlenses for photography

Infrared lensmaterial

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The refractive index, n, of Black Diamond-6 (BD-6) as a function of wavelength, shown above, was calculated using the Herzberger Equation, an infrared-specific analog of the Sellmeier Equation. The Herzberger coefficients for BD-6 are given in the table to the right.