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A new hyperspectral Raman imaging lidar system has been developed by researchers, allowing for the remote detection and identification of various plastics. This advancement could significantly aid in combatting the pervasive issue of plastic pollution in oceans by enhancing monitoring and analytical capabilities.
“The threat of plastic pollution is immense, impacting marine ecosystems and livelihoods connected to industries such as fisheries, tourism, and shipping,” explained Toshihiro Somekawa, the leader of the research team at the Institute for Laser Technology in Japan. “Effective management and protection of the marine environment require a comprehensive assessment of plastic debris in terms of size, concentration, and distribution. Unfortunately, traditional lab-based methods are often cumbersome, requiring extensive time and resources.”
Detailed in the journal Optics Letters, the researchers highlight that their innovative lidar system is compact and designed for low energy consumption, making it ideal for deployment on drones. The technology can successfully identify plastics situated 6 meters away, boasting a significant field of view of 1 mm x 150 mm.
“By deploying a drone outfitted with our lidar sensor, we could effectively survey marine plastic pollution both on land and offshore, which could lead to more efficient cleanup efforts and preventive strategies,” noted Somekawa. “Moreover, this system may have other applications, such as detecting leaks from hazardous gases.”
Achieving remote detection
Previously, the team showcased a flash Raman lidar approach that involved using bandpass filters tailored to each target for a stepwise detection process. However, this method is unsuitable for identifying marine plastics since the filter-switching process would complicate real-time 3D detection capabilities.
While other researchers have investigated hyperspectral Raman imaging to tackle plastic monitoring, conventional methods typically require the target to be in proximity to the equipment. This presents limitations in extensive environmental assessments.
To surmount this challenge, the researchers integrated lidar technology, which measures distances, with hyperspectral Raman spectroscopy. They constructed a prototype that utilized a pulsed 532-nm green laser for lidar and a 2D imaging spectrometer, paired with a gated intensified CCD (ICCD). This setup allows for capturing the Raman signal returning from distant targets as a vertical signal, with horizontal recording offering hyperspectral insights.
Range-resolved Raman imaging
“Our design enables simultaneous acquisition of imaging and spectral data,” Somekawa stated. “Given that each plastic type exhibits a distinct Raman spectrum, the imaging can reveal the spatial distribution of plastic debris while facilitating hyperspectral analysis for distant targets through range-resolved measurements enabled by the pulsed laser.”
The efficacy of the prototype was tested on a sample comprising a polyethylene sheet positioned above a polypropylene sheet. At a distance of 6 meters, the system successfully identified the unique spectra of both plastics and generated images reflecting their vertical distribution. The imaging pixel size of 0.29 millimeters at this stand-off distance suggests that even small bits of plastic debris can be effectively analyzed using the hyperspectral Raman imaging lidar system.
Looking ahead, the researchers aim to adapt their technology for monitoring microplastics both at the surface and below the waterline. Given that laser light at 532 nm penetrates well through water, this next phase holds promise for enhanced detection in aquatic environments.
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