From measuring air and water quality using spectroscopy to gauging wind speed with lidar, Greg Blackman investigates how photonics technology is gathering environmental data in the field
Maintaining the integrity of our waterways and wetlands requires a good deal of management; it’s certainly not a case of sitting back and letting nature take its course. Ecologists and water managers will monitor the water quality for nutrient levels, pollutants, turbidity and a myriad of other parameters that indicate how healthy or otherwise the wetland is. Water Insight, a company based in the Netherlands, has developed its WISP-3 product as a portable device that gives data on the material suspended in surface water.
WISP-3 compares inputs from three different Jaz spectrometers from Ocean Optics to make the water quality readings. It analyses the light returning at different wavelengths from the water to determine levels of chlorophyll, which is present in algae, suspended sediments, and organic matter. The device can also calculate data on water transparency.
‘There are two important parameters that are typically tested,’ says Marnix Laanen at Water Insight. ‘One is the transparency of the water, which is the remaining light penetrating the water once the sum amount of all light-absorbing particles has been subtracted. The other is the level of chlorophyll in the water, because that will indicate eutrophication in the water system. These are factors that the European Water Framework Directive requires water managers to monitor.’
Standard chlorophyll testing involves taking water samples back to the laboratory for analysis. There are some other similar ways to measure transparency with electronic devices, notes Laanen, but the majority of water managers use a Secchi disk, a black and white patterned disk that is lowered into the water until the analyst can no longer discriminate between the black and the white. This was developed in the mid-1800s and is still being used today.
The WISP-3 device incorporates three spectrometers from Ocean Optics. One measures spectral radiance from the sky to determine how much light is falling on the water at a certain angle. A second radiometer measures the amount of light reflected back from the water at the same angle. The third records the total amount of light coming into the scene and the amount reflected back. ‘By using these three radiometers, the system is independent of external light conditions,’ says Laanen.
‘Ocean Optics’ Jaz spectrometer was ideal for our needs,’ says Laanen. ‘It is a modular design so we could attach three spectrometers and operate them at the same time. Using the microcomputer on the Jaz, we could control the spectrometers and even program our water quality algorithms onboard. This still is quite a unique concept. You can buy other portable spectrometers, but not many are multichannel.’ The WISP-3 product was developed over two years, with Water Insight developing the algorithms to turn an optical spectrum into critical water quality parameters and Ocean Optics providing expertise on hardware and how to set up the spectrometers to take accurate readings. The device has recently won the Dutch Partners for Water Award, a Dutch award for innovative technology sold outside of The Netherlands.
‘Lots of people use spectrometers to measure water quality, but the WISP-3 is a complete system,’ adds Laanen. ‘The spectrometers operate simultaneously, they are already positioned at the right angles, and the software is onboard; as an operator you just have to press a button and the results are presented on the screen.’
Measuring air quality
Spectrometers can also be used to measure air quality to quantify pollutant concentrations and other chemical molecules in the atmosphere. Chalmers University of Technology in Sweden, for example, uses Ocean Optics’ spectrometers to monitor the plumes of volcanoes in order to give early warning of increased levels of activity. Hyperspectral cameras from Canadian company Telops are also used to detect and quantify greenhouse gases like carbon dioxide and nitrous oxides, along with other noxious gas species like sulphur oxides.
The Hyper-Cam cameras are based on Fourier transform infrared (FTIR) spectroscopy and Telops has developed a suite of gas identification and quantification algorithms for monitoring things like smokestack emissions and gas plumes from power plants.
The advantage with these portable devices is that they allow the scientist to take measurements in the field, rather than taking samples back to the lab. Marco Snikkers, sales and marketing director at Ocean Optics, says that over the last few years there have been advances in spectroscopy enabling the development of portable devices suitable for making environmental measurements. One of these has been an improvement in optical components such as gratings used in spectrometers, that in turn has reduced the amount of stray light. This means devices can now measure higher optical density absorption and lower concentrations, which is especially important in monitoring air quality, as gas species tend to be at very low concentrations.
Simulating optical paths within spectrometers can also be achieved much more accurately. ‘We can much better predict how light is diffracted by a grating,’ explains Snikkers. ‘We use special mirrors to ensure the light is more efficiently coupled to the detector. These components –together with careful design of the optical bench – reduce the total amount of stray light, increasing performance and signal-to-noise levels.’
Snikkers also draws attention to improvements made in the detectors, saying that the latest back-thinned silicon detectors have a much higher inherent sensitivity in the UV, which is where a lot of absorption peaks are found. Ocean Optics’ STS microspectrometer, designed for handheld applications like water, air and soil quality monitoring, was able to be built because of the advances in components.
Furthermore, in the field of Raman spectroscopy, the advent of more powerful lasers means that handheld versions of Raman spectrometers are now being developed. ‘Raman is quite a low signal measurement, so the devices do need high enough signal-to-noise ratio to get good data,’ comments Snikkers. He feels that handheld Raman spectroscopy could be a key measurement technique for a large number of applications. Ocean Optics has, together with its customers, developed application-specific handheld Raman systems that are used in the field.
When it comes to monitoring air or water quality, the other advantage with being able to make the measurements in the field is that a sample might change or degrade over time, so capturing a snapshot in situ is sometimes preferential to taking a sample back to the lab for further testing. The advances in optical design and engineering have allowed handheld spectrometers and instruments that are allowing scientists to make these measurements in the field.