Hyperspectral imaging lidar system achieves remote plastic identification

“Plastic pollution poses a serious threat to marine ecosystems and human livelihoods, affecting industries such as fisheries, tourism and shipping,” said Toshihiro Somekawa, leader of the research team at the Japan Institute of Laser Technology. Managing and protecting the marine environment is crucial to assessing plastic debris. size, concentration, and distribution, but traditional laboratory methods are often time-consuming, laborious, and expensive.

In Optica Publishing Group Journals Optics Expressthe researchers describe their new system, which is compact and optimized for low energy consumption, making it suitable for use on drones. They showed that the system could identify plastic from 6 meters away, with a relatively wide field of view of 1mm x 150mm.

“Drones equipped with our lidar sensors can be used to assess marine plastic debris on land or at sea, paving the way for more targeted cleanup and prevention efforts,” Somekawa said. “The system can also be used for other monitoring Applications such as detecting hazardous gas leaks.”

Implement remote detection

The researchers previously demonstrated a monitoring system based on flash Raman lidar technology, in which bandpass filters were matched to each measurement target for detection in a continuous manner. However, this technique is not practical for detecting marine plastics because switching filters would prevent instantaneous 3D ranging and detection.

Other research groups have explored the use of hyperspectral Raman imaging to monitor plastic pollution. The technology combines Raman spectroscopy with imaging to capture spatially resolved chemical information in a sample, producing detailed maps of molecular composition and structure. However, traditional hyperspectral Raman imaging can only detect targets close to the instrument.

For far-end detection, the researchers combined lidar for distance measurement with hyperspectral Raman spectroscopy. They achieved this by building a prototype system that included a pulsed 532 nm green laser for lidar measurements and a 2D imaging spectrometer equipped with a gated intensified CCD (ICCD). Raman signals backscattered from distant targets are detected as vertical lines, and the hyperspectral information contained in each point is recorded horizontally. The use of ICCD cameras that can be gated on nanosecond time scales is critical to achieving Raman lidar measurements with fine range resolution.

Distance-resolved Raman imaging

“We designed the system to acquire images and spectral measurements at the same time,” Somekawa said. “Because the Raman spectrum of each plastic type is unique, the imaging information can be used to understand the spatial distribution and type of plastic debris, and because pulsed lasers are capable of range-resolved measurements, hyperspectral spectra can be obtained from targets at any distance. information.

The researchers tested their prototype system on a plastic sample consisting of an upper polyethylene sheet and a lower polypropylene sheet. The system can acquire the characteristic spectrum of each plastic from a distance of 6 meters and produce images showing the vertical distribution of the plastic. The researchers said that the ICCD camera has an imaging pixel size of 0.29 mm at a distance of 6 meters, which means that small plastic fragments can be measured and analyzed using a hyperspectral Raman imaging lidar system.

Next, the researchers plan to use their system to monitor microplastics floating or submerged in water. This should be possible because lasers around 532 nm can pass through water efficiently, allowing for better detection in aquatic environments.