WASHINGTON — Researchers have developed and tested an infrared spectroscopy system that can rapidly detect chemical aerosols from a distance by using the light reflected from common surfaces such as traffic signs, tree trunks or painted surfaces. The new method could make it possible to detect hazards without complicated instruments, helping improve safety and easing operations at industrial sites, public venues and other high-risk locations.
"Previously, many remote chemical detection systems have relied on placing a mirror or other highly reflective target in the field to bounce the laser signal back to the detector, which isn't practical in many real-world situations," said research team leader Tim Johnson from Pacific Northwest National Laboratory . "Our approach eliminates that requirement by using reflections from ordinary surfaces, allowing us to detect aerosolized chemicals from a distance without installing specialized equipment at the target location."
In the Optica Publishing Group journal Applied Optics , the researchers report results from laboratory tests using reflected infrared laser light from different surfaces. They showed that many non-metallic surfaces could be used for aerosol and vapor detection at standoff distances of up to 11 meters.
"The ability to detect aerosols or vapors from tens of meters away without complex equipment could improve safety in a wide range of settings, from protecting workers and military personnel to detecting chemical spills and monitoring facility perimeters," said Johnson. "For example, it could be used at large sporting events or concerts to help identify an accidental or intentional chemical release before it reaches the crowd."
Creating a practical spectral system
When hazardous chemicals are released into the environment, they often exist as aerosols — tiny liquid droplets or solid particles suspended in the air. Although infrared spectroscopy can be used to identify the chemical composition of aerosols, this analytical technique is difficult to implement outside the laboratory.
One of the biggest obstacles to using infrared spectroscopy in the field is the reliance on mirrors to reflect the light back to the instrument, thus enabling detection. Even when there is access to position a mirror, maintaining stable optical alignment is challenging, and mirror surfaces can quickly become scratched or dirty, degrading performance.
The researchers developed a method that eliminates the need for such mirrors by allowing the laser to be pointed at everyday surfaces. The reduced reflectivity is partially compensated for by using high-power lasers and software that parses noise in the data to ultimately detect the reflected light at the detection system. Another key innovation was an infrared laser/telescope system that automatically adjusts the outgoing laser beam for different standoff distances while efficiently collecting the returning light, maximizing the amount of signal that reaches the detector.
"Even if they aren't 100% reflective like a new mirror, many surfaces offer substantial reflectivity," said Johnson. "Our approach makes it possible to use these surfaces to generate enough optical signal for the spectrometer to detect an aerosol chemical plume in the path of the laser beam."
As part of the work, the researchers characterized the reflectivity of almost 50 common materials such as siding, wallboard, traffic signs, tree bark and car panels. They found that certain sidings and traffic signs offered higher reflectivity, compared to other materials tested.
Aerosol chamber testing
To evaluate the new method, the researchers constructed an aerosol chamber where they could generate aerosols with well-characterized particle sizes and chemical compositions, including chemical mixtures.
They used the chamber to test their new method with a subset of the reflective surfaces they previously characterized. They analyzed several different aerosols, including diethyl sebacate, a stand-in for hazardous chemicals, and calcium carbonate, which is found in rocks like chalk and mineral limestone. In each case, the system successfully detected and identified the aerosols despite differences in chemical composition and surface reflectivity.
Next, the researchers plan to evaluate the method using aerosols with broader distributions of particle sizes and increasingly complex chemical mixtures. These more realistic scenarios produce more complicated infrared spectra, making chemical identification more challenging. The team expects that artificial intelligence and machine learning may help analyze these complex spectral signatures and improve detection performance.
This work is part of the Pursuing Intelligent Complex Aerosols for Rapid Detection (PICARD) program, an initiative by the Intelligence Advanced Research Projects Activity (IARPA) to develop advanced sensing platforms that rapidly detect dangerous chemical aerosols in challenging, real-world environments.
Paper: T. Johnson, D. Cancino, J.D. Erickson, Z.M. Comstock, T.L. Myers, C. A. Kendziora, R. Furstenberg, S. B. Pilkington, R. M. Levine, S. H. Pullins, M. D. Melendez, A. R. Luxon, J. M. Mueller, D. E. Hunka, D. S. Alburty, D. J. Radke, "Aerosol and Vapor Detection via Infrared Laser Reflectance from Common Surfaces: All That Shimmers is Not Gold," Applied Optics, 65
