Thomine Stolberg-Rohr

Spectral design of temperature-invariant narrow bandpass filters for the mid-infrared

The ability of narrow bandpass filters to discriminate wavelengths between closely-separated gas absorption lines is crucial in many areas of infrared spectroscopy. As improvements to the sensitivity of infrared detectors enables operation in uncontrolled high-temperature environments, this imposes demands on the explicit bandpass design to provide temperature-invariant behavior. The unique negative temperature coefficient (dn/dT<0) of Lead-based (Pb) salts, in combination with dielectric materials enable bandpass filters with exclusive immunity to shifts in wavelength with temperature. This paper presents the results of an investigation into the interdependence between multilayer bandpass design and optical materials together with a review on invariance at elevated temperatures.

NDIR humidity measurement

Danfoss IXA is developing a multi gas sensor that utilizes NDIR spectroscopy to simultaneously measure various gasses such as CO2 and humidity. NDIR spectroscopy is vastly ignored in commercial humidity sensing although humidity is absorbing radiation in large parts of the infrared spectrum, seemingly making it an obvious measurand for NDIR spectroscopy. The present work elaborates on the reason for this contradiction and explains why NDIR, despite all, is suitable for humidity measurements and specifically beneficial in aggressive environments where an optical approach allows for non-contact measurement which might enhance the long term stability. It is shown that the performance of the developed NDIR spectroscopy-based prototype can compete with commercially available humidity sensors in the ranges of interest for indoor climate control.

Determination of the embedded thermo-optical expansion coefficients of PbTe and ZnSe thin film infrared multilayers

This paper reports the first derived thermo-optical properties for vacuum deposited infrared thin films embedded in multilayers. These properties were extracted from the temperature-dependence of manufactured narrow bandpass filters across the 4-17 µm mid-infrared wavelength region. Using a repository of spaceflight multi-cavity bandpass filters, the thermo-optical expansion coefficients of PbTe and ZnSe were determined across an elevated temperature range 20-160 °C. Embedded ZnSe films showed thermo-optical properties similar to reported bulk values, whilst the embedded PbTe films of lower optical density, deviate from reference literature sources. Detailed knowledge of derived coefficients is essential to the multilayer design of temperature-invariant narrow bandpass filters for use in non-cooled infrared detection systems. We further present manufacture of the first reported temperature-invariant multi-cavity narrow bandpass filter utilizing PbS chalcogenide layer material.