In a study recently published in the journal Optics Letters , researchers from the Institute of Laser Technology have developed a new hyperspectral Raman imaging lidar system that can remotely detect and identify different types of plastic. The technology could help solve the growing problem of plastic pollution in the oceans by providing improved monitoring and analysis tools.
Plastic pollution poses a serious threat to marine ecosystems and human livelihoods, affecting industries such as fishing, tourism and shipping. Assessing the size, concentration and distribution of plastic debris is essential to manage and protect the marine environment, but traditional laboratory methods are often time-consuming, laborious and expensive .
Toshihiro Somekawa, Research Team Leader, Institute for Laser Technology
It is small, energy-efficient, and suitable for use on drones. They demonstrated that the system can detect plastic up to 6 m away with a relatively wide field of view of 1 mm × 150 mm.
Drones equipped with our LIDAR sensors can be used to assess marine plastic debris on land or in the ocean, paving the way for more targeted cleanup and prevention efforts. The system can also be used for other monitoring applications such as toxic gas leak detection .
Toshihiro Somekawa, Research Team Leader, Institute for Laser Technology
Achieve remote detection
In the researchers’ previous demonstration of a monitoring system based on flash Raman lidar, a bandpass filter was adapted to each target in sequence for detection, but changing the filter prevented instantaneous 3D distance measurement and detection, making this method impractical for detecting marine plastics.
Other research groups have investigated hyperspectral Raman imaging as a means to monitor plastic pollution. This method combines imaging and Raman spectroscopy to obtain spatially resolved chemical information, producing detailed maps of a sample’s molecular composition and structure. However, traditional hyperspectral Raman imaging can only detect targets close to the device.
The researchers used a combination of hyperspectral Raman spectroscopy and LIDAR for remote sensing. They did this by building a prototype system with a 2D imaging spectrometer with a gated intensified charge-coupled device (ICCD) and a pulsed 532 nm blue laser for the LIDAR measurements.
The hyperspectral information contained in each point is recorded horizontally, while the Raman signal reflected from distant targets is detected as a vertical line.To achieve Raman lidar measurements with good range resolution, it is necessary to use an ICCD camera that can be controlled on the nanosecond time scale.
Distance-resolved Raman imaging
We designed a system to simultaneously collect images and spectral measurements. Because Raman spectra vary for different plastics, the image information can be used to understand the spatial distribution and type of plastic debris, while distance-resolved measurements with a pulsed laser allow us to obtain hyperspectral information from targets at any distance .
Toshihiro Somekawa, Research Team Leader, Institute for Laser Technology
The researchers tested the prototype system using plastic samples with a polyethylene sheet on top and a polypropylene sheet on the bottom. The system captured the characteristic spectrum of each plastic from a distance of six meters, producing an image showing the vertical distribution of the plastics.
According to the researchers, with an image pixel size of 0.29mm from an ICCD camera at a distance of 6 metres, their hyperspectral Raman imaging lidar system shows that it is possible to measure and analyse small plastic debris.
The researchers intended to use the system to track microplastics as they float or sink in water. 532 nm laser light penetrates water well, improving detection capabilities in aquatic environments.
Journal References:
Somekawa , T. et al . (2024) Remote detection and identification of plastics using hyperspectral Raman imaging LIDAR. Optics Letters . Citation: doi.org/10.1364/ol.544096.
