Wolfgang Reichl’s research while affiliated with E+E Elektronik and other places

We report on the design, simulation and experimental characterization of a simple germanium bolometer utilizing a Fabry–Perot absorbing structure, which can be used for monitoring, e.g., CO2 where no additional filter is required. CO2 absorbs IR-radiation in a major band centered around a vacuum wavelength of 4.26μm. The selectivity of the whole sensor-configuration is mainly accomplished by the wavelength-response associated with the Fabry–Perot structure. The analysis shows, that the designed bolometer has an adequate response-function for the measurement of CO2 concentration. Combining the analysis for the bolometer with ray tracing simulations for a connected sample chamber yields the response for an entire IR-absorption sensor system which is proven to be in good agreement with measurements.

Non-dispersive infrared (NDIR) gas sensors make use of the specific infrared absorption of particular gas molecules in order to measure their distinctive gas concentration. The main parts of such a NDIR gas sensor are: an IR-emitter, a chamber containing the sample-gas, and an IR-detector with a filter for the characteristic absorption wavelength. The effectiveness of the IR-source for the total system is characterized by its temperature and the emissivity (i.e., the difference to blackbody radiation) of the device surface. Due to the fact that conventional metal surfaces provide a rather low emissivity, their emitting temperature must be set very high to generate sufficient IR-radiation for this kind of sensors. We developed an IR-source consisting of a stack of thin films with a much higher emissivity. Its main part is a combination of two mirrors and a dielectric layer which represent a Fabry-Perot structure. The obtained emission of the Fabry-Perot structure and the consequences for the performance of the whole NDIR gas sensor system were simulated with the enhanced transmittance matrix approach and a 3D ray tracing model. As an example, CO2 was considered as sample gas where the major characteristic absorption occur around 4.26 mum. The theoretical results are validated by comparing them to experiments obtained with prototype devices.

Recently, Fabry–Perot IR-absorbing structures have been proven suitable as low cost detectors in non-dispersive infrared (NDIR) gas-sensors for measuring diluted gases (e.g., CO2) in a pure nitrogen (N2) atmosphere. To identify the monitoring capability of the devised prototype system in ambient air, the cross-sensitivity to other gases, mainly water vapor, has to be explored. The absorption coefficient associated with the individual infrared absorbing bands of water vapor is small compared to that of CO2; however, the atmospheric concentration of vapor much is higher. To improve the impact of CO2 absorption compared to other gases a new method using a normal glass plate as infrared filter is introduced. The presented theoretical and experimental analysis investigates the achievable response to CO2 as well as the resulting cross-sensitivity for absolute humidity, where also the dependence of the characteristics on the temperature was considered. In particular a model for the bolometer is combined with ray tracing simulations for a connected sample chamber yielding the response of the entire IR-absorption sensor system, which is compared with measurements.

Recently, Fabry–Perot IR-absorbing structures have been proven suitable as low cost detectors in non-dispersive infrared (NDIR) gas-sensors for measuring diluted gases (e.g., CO2) in a pure nitrogen (N2) atmosphere. To identify the monitoring capability of the devised prototype system in ambient air, the cross-sensitivity to other gases, mainly water vapor, has to be explored. The absorption coefficient associated with the individual infrared absorbing bands of water vapor is small compared to that of CO2; however, the atmospheric concentration of vapor is much higher. To reduce the impact of H2O absorption, a new method using a glass plate as infrared filter is introduced. The presented theoretical and experimental analysis outlines the resulting cross-sensitivity for absolute humidity and thus explores the associated limits of the systems applications. In particular, a model for the bolometer is combined with ray tracing simulations for a connected sample chamber yielding the response of the entire IR-absorption sensor system, which is compared with measurements.

Many gas molecules absorb electromagnetic radiation at characteristic wavelengths in the infrared region. This absorption can be used to identify defined substances like CO₂, ammoniac, and so forth. A lot of different types of gas sensors are based on the principle of infrared absorption like photoacoustic sensors (e.g., Golay cells), dispersive infrared instruments (e.g., utilizing a diffraction grating), or Fourier transform infrared (FTIR) spectrometers. However, the most commonly used types of infrared absorbing gas sensors are nondispersive infrared (NDIR) sensors. Such a NDIR gas sensor consists of some basic function blocks, i.e., an IR-source, the sensor cell or optical path containing the sample gas, a gas specific filter, and an IR-detector. One of the central issues in the design of this kind of sensors is the geometry of the sensor cell. In this paper we investigate the use of statistic ray tracing to predict the efficiency of 3-D cell geometries for NDIR gas sensors. We demonstrate the feasibility of the method and show examples on how to apply it on given 3-D sensor models where we illustrate the agreement between simulated and measured gas response data.

Every gas (e.g. CO2) absorbs IR-radiation at individual gas specific IR-wavelengths. Non-dispersive infrared (NDIR) gas sensors exploit this property for gas monitoring. Such sensors are used in various applications, e.g. for control of air quality in office buildings or cars. This is a big market for low cost sensors. A NDIR sensor consists basically of three components: an IR-emitter, a chamber containing the sample gas, and an IR-detector with a filter for the observed wavelength. Commercially available systems use broadband IR-emitters (e.g.: micro-lamps) in combination with thermopile or pyroelectric detectors fabricated with a narrowband gas-specific IR-filter, e.g., an interference filter. We devised a concept for a simple and cost-effective NDIR-gas sensor based on two non-symmetric Fabry-Perot absorberstructures as IR-emitter and as IR-detector where no additional interference filter is needed. The presented sensor combines thin layer technology with optical sensing techniques. The system was first analyzed using ray tracing models based on a Monte Carlo method in order to model the response function of the system's sample chamber. For our results, the sample gas is CO2 where the major absorption is centered around 4.26mum.

We report on the design of a simple germanium bolometer designed as a Fabry-Perot absorbing structure, which can be used for monitoring, e.g., CO2, where no additional filter is required. CO2 is absorbs IR radiation in a major band centered around 4.26μm. The selectivity of the whole sensor-configuration is mainly accomplished by the wavelength-response associated with the Fabry-Perot structure involved. The analysis shows, that the designed bolometer has an adequate response-function for the measurement of CO2 concentration. Combining this analysis for the bolometer with ray tracing simulations for a connected sample chamber yields the response for an entire IR-absorption sensor system, which is in good agreement with measurements.