From VIS to SWIR: a challenge for optical glass and IR materials

Abstract

More and more applications utilize the short wave infrared (SWIR) spectral range. The SWIR range is defined from about 0.9 to 3 μm. SWIR applications can be found for example in inspection processes of circuit boards, solar cells, bottles and food. The SWIR range is used in identification, sorting, surveillance, inspection and more. With SWIR applications characteristics can be visualized that normally would not be detectable with visible light, like rotten fruits in fruit sorting, fakes in paintings, content levels in visually non-transmitting bottles. For all these machine vision applications specific optics are used that in the ideal case have transmittance in the visual spectral range and in the SWIR range. Optical designs require materials that are transmittance in the visible and the SWIR range, sometimes even up to 4 μm. Most optical glasses have a good transmittance up to 2 μm, but at 2.5 μm the transmittance strongly decreases. There is also not much available information on the details of the transmittance curve in the SWIR range available of optical glass. This presentation will demonstrate SCHOTT optical glasses with good transmittance even up to 4 μm. If applications require transmittance at even larger wavelengths, it is possible to utilize IRG infrared material with transmittance up to 8 µm, that still can be used in the visible down to 0.6 μm. Other critical information for the optical designs are the dispersion and index change with temperature (dn/dT) characteristics in the SWIR range of these materials. This paper will discuss the availability of such data.