New advancements in technology now allow for the remote identification of various plastic types, providing an essential resource for monitoring and analyzing plastic pollution in oceans. A novel hyperspectral Raman imaging lidar system can detect and categorize different plastics from a distance, offering improved tools for tackling the pressing issue of oceanic plastic waste.
A team of researchers has introduced an innovative hyperspectral Raman imaging lidar system that can remotely identify different types of plastics. This breakthrough technology could significantly contribute to addressing the urgent problem of plastic pollution in the ocean by enhancing monitoring and analysis capabilities.
“Plastic waste is a major danger to marine environments and human livelihoods, impacting sectors such as fisheries, tourism, and shipping,” stated Toshihiro Somekawa, the leader of the research team from the Institute for Laser Technology in Japan. “In order to manage and conserve the marine ecosystem, it’s crucial to evaluate plastic debris in terms of size, concentration, and distribution; however, standard laboratory techniques are often slow, labor-intensive, and costly.”
In a publication within the Optica Publishing Group journal Optics Letters, the researchers outline their newly developed compact system that is designed for low energy consumption, making it suitable for drone deployment. They demonstrated that the system can recognize plastics from a distance of 6 meters with a broad field of view measuring 1 mm x 150 mm.
“A drone fitted with our lidar sensor could be utilized to evaluate plastic waste in marine or terrestrial environments, facilitating more focused cleanup and prevention initiatives,” Somekawa explained. “Moreover, this system could also serve other monitoring purposes, such as identifying hazardous gas leaks.”
Enabling remote detection
The research team previously showcased a monitoring method involving a flash Raman lidar technique, which utilized bandpass filters tailored to each target during successive measurements. However, this method is impractical for marine plastic detection since the need to change filters would disrupt immediate 3D measurements and detection.
Various research teams have investigated the use of hyperspectral Raman imaging for tracking plastic pollution. This method combines Raman spectroscopy and imaging to gather spatially detailed chemical data across a sample, resulting in comprehensive maps of molecular structure and composition. Nonetheless, traditional hyperspectral Raman imaging can only detect nearby targets.
To enable remote detection, the researchers integrated lidar for distance measurements with hyperspectral Raman spectroscopy. They developed a prototype system that included a pulsed 532-nm green laser for lidar readings and a 2D imaging spectrometer with a gated intensified CCD (ICCD). The Raman signal reflected from a faraway target appeared as a vertical line, with the hyperspectral data recorded horizontally. Utilizing an ICCD camera that operates on the nanosecond time scale was crucial for achieving precise Raman lidar measurements.
Distance-sensitive Raman imaging
“Our system was designed to capture both images and spectroscopic data simultaneously,” Somekawa noted. “Each type of plastic has a unique Raman spectrum, allowing us to use imaging data to determine the spatial distribution and species of plastic waste, while the pulsed laser permits range-resolved measurements from any distance.”
The researchers tested their prototype with a plastic sample that featured a polyethylene sheet on top and a polypropylene sheet below. From a distance of 6 meters, the system successfully collected the distinct spectra of each plastic type and generated images that depicted their vertical distribution. The imaging pixel size of 0.29 millimeters with the ICCD camera at a 6-meter distance indicates that the hyperspectral Raman imaging lidar system could also analyze smaller plastic debris.
Moving forward, the researchers intend to utilize their system to monitor microplastics that may be floating or submerged in water. This is expected to be feasible, as laser light at around 532 nm penetrates water efficiently, enhancing detection in aquatic environments.
