The prototype technology combines a lidar system for range-resolved measurements with a Raman imaging spectrometer for material analysis. [Image: Toshihiro Somekawa, Institute for Laser Technology]
Researchers in Japan have developed an optical system that enables different types of plastic to be detected and identified from distances of up to six meters (Opt. Lett., doi: 10.1364/OL.544096). The prototype system combines lidar for range-resolved measurements with Raman techniques for compositional analysis, which could pave the way for drone-based platforms for quantifying the size and distribution of plastic fragments in the ocean.
Mapping plastic fragments
“Traditional lab-based methods are often time-consuming, labor-intensive and expensive,” said team leader Toshihiro Somekawa from the Institute for Laser Technology. “A drone equipped with our lidar sensor could be used to assess plastic debris on land or in the sea, paving the way for more targeted clean-up and prevention efforts.”
Samples of plastic pollutants are often analyzed in the lab using hyperspectral Raman imaging. This methodology acquires images and spectroscopic data at the same time, allowing each type of plastic to be identified from its unique Raman spectrum. The imaging information is then used to map the size and spatial distribution of the plastic fragments.
Going remote
From six meters away, the system successfully acquired the characteristic spectra of each plastic and visualized their vertical distributions.
Somekawa and colleagues have now found a way to combine this analytical technique with the remote monitoring capabilities of lidar. The light emitted by a pulsed green laser is first widened to produce a line-shaped illumination profile. At the detector, an entrance slit captures just a narrow line of the backscattered Raman signal, yielding a field-of-view of 1 mm × 150 mm at a distance of six meters. Hyperspectral information is then recorded at each point along this line, with accurate distance measurements obtained using an imaging camera that can be time-gated with nanosecond precision.
The researchers tested their prototype using sheets of two common plastics, polyethylene and polypropylene, positioned one above the other. From six meters away, the system successfully acquired the characteristic spectra of each plastic and visualized their vertical distributions. By measuring the signal-to-noise ratio at different length scales, the scientists estimated the detection limit to be around 0.06 mm.
Somekawa and colleagues are confident that the system would be able to detect submerged microplastics, since water does not interfere with the Raman signal or cause significant attenuation of the green laser light. For drone-based operation, they believe that the prototype could be made more compact and power efficient through the use of single-photon lidar.