A Powell lens is a unique optic that projects a line output profile from a collimated laser with its output characteristics dictated by the input beam and geometry of the lens. The goal of this work was to demonstrate that a Powell lens can be used to shape the output of the broadband source to be complimentary to the input of a passive line scanning hyperspectral spectrometer. In this paper the use of a Powell lens with a broadband white light source is demonstrated to produce a similar output when compared to a monochromatic source. Output from a tungsten halogen lamp was collimated and transmitted through a Powell lens with input diameters ranging from 400 to 5000 μm. The output from the lens was characterized by a fiber spectrometer at three distances from the lens creating a profile of the output beam. Complementary experiments were then performed with a 532 nm laser to provide a direct comparison to the broadband source and mimic typical usage of the lens. The results show that while the resultant output does produce a line of white light, the intensity of the output is reduced when compared to the source. In addition, the input diameter of the source can be greater than specified limit of the lens, depending on the application as imperfections in the lens surface can cause aberrations in the output.
Artificial illumination is required for a line scanning passive hyperspectral spectrometer when operating a system of this type in non-daylight conditions. While in general more photons will yield a larger reflectance signal return to the sensor, a source that outputs a large number of photons is unlikely to be compatible with a compact hyperspectral spectrometer on a small aircraft or using in a handheld manner. Therefore, in this paper we investigate a small tungsten halogen source coupled with off-the-shelf optics to create a compact artificial illumination source to provide photons for the spectrometer. After characterizing the compact halogen source and comparing its output characteristics to larger sources currently in use, several optical trains were designed to focus the sources output to the instruments’ field-of-view. The results detailed herein show that a compact source can allow for a hyperspectral spectrometer to operate with a compact artificial illumination source with a minimal decrease in performance.
Building on our previous development of a compact, portable, and low SWaP gas analyzer (11” x 6.7” x 5.1”, 7.8 lbs) based on photoacoustic spectroscopy and using broadband quantum cascade laser arrays, we demonstrate here compositional analysis of airborne aerosols using this instrument. With an integration time of 330-ms per laser, and ~70 seconds for a spectrum covering 950-1500 cm-1, our instrument showed a detection sensitivity at the mg/m3 level for solid and liquid-loaded solid aerosols. Additionally, Malathion-loaded aerosols can be discriminated from pure Syloid aerosols based on their absorption features. The preliminary results show a potential path for developments of a portable real-time aerosol composition analyzer.
KEYWORDS: Raman spectroscopy, Explosives, Luminescence, Signal to noise ratio, Chemical analysis, Sensors, Prototyping, Near infrared, Optical engineering, Hazard analysis
Handheld Raman systems have become powerful analytical tools for the detection and identification of hazardous chemical materials that are now commonly used by both the civilian and military communities. Due to the availability of compact lasers and sensitive detectors, systems typically operate at 785 nm. However, the Raman return at this wavelength can still be obscured by fluorescent impurities in the targeted materials or their matrices. To potentially mitigate this shortcoming, a prototype dual-wavelength Raman incorporating both 785- and 1064-nm excitations was developed and assessed at the Edgewood Chemical Biological Center. The results of that evaluation are discussed here.
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