Hyperspectral imaging lidar system achieves remote plastic identification

Researchers have developed a new hyperspectral Raman imaging lidar system that can remotely detect and identify various types of plastics. This technology could help address the critical issue of plastic pollution in the ocean by providing better tools for monitoring and analysis.

"Plastic pollution poses a serious threat to marine ecosystems and human livelihoods, affecting industries like fisheries, tourism and shipping," said research team leader Toshihiro Somekawa from the Institute for Laser Technology in Japan."To manage and protect the marine environment, it's essential to assess the size, concentration and distribution of plastic debris, but traditional lab-based methods are often time-consuming, labor-intensive and expensive."

In the Optica Publishing Group journal Optics Letters, the researchers describe their new system, which is compact and optimized for low energy consumption, making it suitable for use aboard a drone. They show that the system can identify plastics that are 6 meters away with a relatively wide field of view of 1 mm x 150 mm.

"A drone equipped with our lidar sensor could be used to assess marine plastic debris on land or in the sea, paving the way for more targeted cleanup and prevention efforts," said Somekawa. "The system could also be used for other monitoring applications, such as detecting hazardous gas leaks."

Achieving remote detection

The researchers previously demonstrated a monitoring system based on a flash Raman lidar technique in which bandpass filters were matched to each measurement target for detection in a successive manner. This technique, however, isn't practical for detecting marine plastics because switching the filters would hinder instantaneous 3D ranging and detection.

Other research groups have explored using hyperspectral Raman imaging to monitor plastic pollution. This technique combines Raman spectroscopy with imaging to capture spatially resolved chemical information across a sample, producing detailed maps of molecular composition and structure. However, conventional hyperspectral Raman imaging can only detect targets that are close to the instrument.

For remote detection, the researchers combined lidar for distance measurement with hyperspectral Raman spectroscopy. They did this by building a prototype systemthatincluded a pulsed 532- nm green laser for lidar measurements and a 2D imaging spectrometer equipped with a gated intensified CCD (ICCD). The Raman signal backscattered from a distant target was detected as a vertical line, and the hyperspectral information contained in each point recorded horizontally. Using an ICCD camera that can be gated on a nanosecond time scale was essential for achieving the Raman lidar measurement with fine range resolutions.

Range-resolved Raman imaging

"We designed our system to acquire images and spectroscopic measurements simultaneously," said Somekawa. "Since the Raman spectrum is unique for each plastic type, the imaging information can be used to understand the spatial distribution and type of plastic debris and hyperspectral information can be obtained from targets at any distance due to the pulsed laser enabling range-resolved measurements."

The researchers tested their prototype system on a plastic sample consisting of a polyethylene sheet in the upper position and a polypropylene sheet in the lower position. From 6 meters away, the system was able to acquire the characteristic spectra of each plastic and produce images showing the vertical distribution of the plastics. The researchers say that the imaging pixel size of 0.29 millimeters with the ICCD camera at the stand-off distance of 6 meters implies that small plastic debris could be measured and analyzed using the hyperspectral Raman imaging lidar system.

Next, the researchers plan to use their system to monitor microplastics that are floating or submerged in water. This should be feasible since laser light around 532 nm transmits effectively through water, enabling better detection in aquatic environments.