Diffuse reflectance infrared spectroscopy has gained traction in many industrial applications in the recent years due to the emergence of new generation of low cost handheld spectrometers that did not exist a decade ago. Real-time monitoring puts a limit on the sample preparation process especially with inhomogeneous samples in the food industry, like grains, hay, wheat and corn. The heterogeneity of the samples and the pseudo-random spatial arrangement of the grains in front of the optical interface, leads to prediction errors. The spatial variations depend also on the spot size of the diffuse-reflected scattered light from the sample that is collected by the spectrometer. A larger spot size leads to simultaneous averaging of a larger amount of spectrospatial information from different locations on the sample, leading to better repeatability and better prediction accuracy. Up to date, the Microelectromechanical (MEMS) based spectrometers reported in the literature have limited optical spot size, usually smaller than 3 mm in diameter. We report MEMS based FTIR spectral sensors with optical spot sizes of 6 mm, 10 mm and 20 mm working across the spectral range of 1350 nm to 2500 nm. The core spectral engine comprises monolithic MEMS chip, micro-optics for light coupling and a single photodetector in a tiny package. The optical head combines several miniaturized filament- based lamps and reflective optics for illumination. The sensors are compared and the 10-mm sensor gives an optimal performance with a Signal to Noise Ratio (SNR) of 4000:1 and spectrospatial photometric repeatability down to 0.02 absorbance units.
Air pollution is used to refer to the release of pollutants into the air, where these pollutants are harmful to the human health and our planet. The main source of these pollutants comes from energy production and consumption that release Volatile Organic Compounds (VOCs) such as BTEX and Aldehydes group. Real time monitoring of these VOCs in factories, stations, homes and in the street is important for analysis of the pollution sources fingerprint and for alerting, when exceeding the harmful limits. In this work we report the use of a MEMS FTIR spectrometer in the mid-infrared for this purpose. The spectrometer works in the wavelength range of 1.6 μm - 4.9 μm with a resolution down to 33 cm-1. This covers the absorption spectrum of water vapour, BTEX, Aldehydes and CO2 around 2.65 μm, 3.27 μm, 3.6 μm and 4.3 μm, respectively. The spectra of Toluene with different concentrations are measured, using a multipass gas cell with a physical length of 50 cm and an optical path length of 20 m, showing excellent sensor linearity. The minimum concentration measured is 350 ppb limited by the interference of the side lobes of the strong absorption of water vapour, which can be overcome in the future by humidity compensation. The SNR is measured and found to be 5000:1, corresponding to a detection limit of about 90 ppb. The achieved results open the door for a compact and low-cost solution targeting air pollution monitoring.
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