MULTISPECTRAL AND HYPERSPECTRAL IMAGING SYSTEMS Improving the way we recycle, by Geralyn Miller, Teledyne DALSA
According to the World Bank, humans generated 1.3 billion tons of garbage in 2013. And while much of that went to landfill sites or incinerators, the amount of plastic, glass, paper and metal recycled from waste has been steadily rising for decades. In 2013 the global leaders in recycling municipal waste were Germany (65%), South Korea (59%), Slovenia and Austria (both at 58%) and Belgium (55%), according to the OECD. This is a big change from even a decade ago, yet most countries are still a long way from reaching their recycling targets. A new way to waste less waste In most waste treatment facilities, the amount of material that can be recycled is limited by the sorting technology. This matters because increasing the purity of a recycled material by even a few per cent can double its value, and extracting more recyclable material means disposing of less waste.
After basic sorting is completed, some recycling centers are using hyperspectral or multispectral imaging to achieve near-perfect purity. It’s an ideal technology for plastics, which are difficult to sort because of their different chemical compositions but similar weights, colors and specific densities. Shortwave infrared (SWIR) cameras and linear arrays are the most common type of multispectral systems used in plastic recycling, and most are based on InGaAs (a semiconductor alloy of gallium and indium arsenide) sensors, which work well at room temperature—meaning they don’t require expensive cooling systems. Because they are so sensitive, SWIR multispectral systems are used near the end of the plastic sorting process, and are typically run twice or more to achieve up to 99% purity.
Seeing inside black plastics A new generation of multispectral and hyperspectral Conventional optical sorting systems operate within cameras is increasing the purity of many recycled the near-infrared range, which works well for most materials to close to 100%. These cameras plastics but fails for black plastics: the soot that gives divide light into hundreds of narrow bands over them their color absorbs a continuous range that most of the signal, so the spans the electromagnetic system can’t “see” them. spectrum. The cameras SWIR multispectral cameras INCREASING THE collect hundreds of data detect the spectral fingerprints points per pixel and combine PURITY OF A RECYCLED of every kind of plastic, these to generate a unique MATERIAL BY EVEN A regardless of pigments or spectral signature (also other additives. FEW PER CENT CAN called a fingerprint) for each material based on its Thanks to an innovation DOUBLE ITS VALUE chemical composition. from Germany’s Fraunhofer Here’s how these systems are being used to improve the sorting of two of the most common and difficult-tosort waste materials—plastics and papers. Finding the plastic fingerprint Most recycling centers use a range of mechanical sorting methods, such as revolving drums that sort light from heavy objects, flotation tanks for sorting even lighter weight materials (like Styrofoam), and detectors and magnetic conveyors to sort metals.
Institute for High Frequency Physics and Radar Techniques, SWIR cameras are no longer the only option for InGaAs-based black plastic sorting. The Institutes’ new high-volume system uses a radar camera and sophisticated algorithms to detect even minute differences in spectra, and can achieve near-100% purity. The system is self-learning, so its precision increases over time, and its cost is comparable to hyperspectral cameras. A better way to shuffle paper