Emerging Applications Signal New Opportunities for Spectroscopy
Faster, smaller spectroscopy instruments are probing the boundaries of conventional applications to expand and explore new end markets.
JOHN GILMORE, HAMAMATSU
Advancements in spectrometer components and modules are converging with system-level designs to enable more compact, cost-efficient, and robust instruments that are extending the benefits of spectroscopy to current and new end markets. From inline measurement of paint thickness to analysis of produce in real time, to automating the capture of key health indicators, new spectrometer designs are helping to bring laboratory-style analysis to samples on the factory floor, in the orchard, and in the home. Combining the speed and portable convenience of these new instruments with cloud-based applications and data storage could not only revolutionize the spectroscopy market, it could also have a significant impact on our daily lives.
Thin-film measurement
Prepainted metals find application for products such as highway vehicles, farm machinery, and filing cabinets. Coated metals further apply to the food and beverage packaging industry. Depending on the metal composition, coatings comprising several materials may be applied to create a desired finish. Fabrication of these products and applications often relies on efficient, high-volume coil coating processes, in which large metal spools weighing several tons are fed into an industrial machine that applies and cures coatings before rewinding the coated material as a finished product. However, with the advantages of volume production come challenges in inspection and quality control.
Understanding film properties is important in the coil coating industry. The common practice is to take samples of coated materials offline to measure coating thickness in the laboratory. While this approach confirms quality to some degree, it provides limited value for process control and quality monitoring compared to real-time inline measurement of coating thickness. Performing real-time, in-line thin-film measurements could potentially save significant material costs and provide important quality control records.
With this goal in mind, manufacturers are exploring the application of optical interference patterns associated with thin-film spectroscopy techniques to measure the thickness of coating layers (Figure 1). Such applications only require a broadband source, such as a halogen lamp or a laser-driven light source. Either option provides near-infrared (NIR) emissions, as well as good lifetime and stability.
Figure 1. In optical thin-film measurements, material thickness is calculated using the phase difference of the incident and reflected waves from a broadband light source (a). The wavelength of the illumination source is selected based on rough approximations of material thickness. The number of signals increases as the film becomes thick. The signal intervals in the short-wavelength range appear more often than those in the long-wavelength range (b). Courtesy of Hamamatsu Corp.
Some of the light incident on the surface of the coating will reflect back, but a portion transmits through the top coating layer to the next material junction —
either the next coating layer or the substrate — before returning to the detector. The lightwaves reflected by either junction create interference with each other. This interference is constructive when the delay introduced by material thickness matches an integer multiple of the incident wavelength. A half-wave delay, conversely, creates destructive interference, thereby decreasing the resulting amplitude of the signal. One additional factor is the index of refraction of the material because it will introduce a phase shift: The wavelength of the incoming light will depend on the materials and desired thickness.
Analyzing the qualities of the reflected light with an optical spectrometer permits calculation of the thickness of the coating layer. These calculations can provide important feedback on the performance of upstream coating equipment, which can be adjusted either by a human operator or by a factory automation system. The wavelength of the light depends on the material composition and thickness.
In general, longer wavelengths are needed to measure thinner coating layers. This is driving interest in spectroscopy systems capable of high-speed data capture in the NIR.
Digging deep
As world population continues to increase, the pressure is increasing on farmers to better manage their finite resources in the face of a changing climate to optimize crop yields. You cannot manage what you cannot measure, however, and improving the speed and scope of soil analysis will prove instrumental to success in future agricultural methods.
Soil analysis has historically been limited to sample testing, whereby farmers send soil to a laboratory that is capable of extracting important information about the density of nutrients and other substances important to aiding the harvest. Sample testing provides limited data with regard to the distribution of soil components over broad acreage, and it takes significant time to receive laboratory results.
Field-portable reflectance spectroscopy operating in the NIR range between 900 and 1700 nm is helping to make a difference by enabling real-time soil analysis.
One approach embeds a compact NIR spectrometer and a broadband halogen lamp behind a window inside a 25-in. coulter disc and hitches the system behind a tractor. As a tractor pulls the rotating disc through a field, light from the lamp reflects off the soil and is captured by the spectrometer. An encoder tracks the disc rotation to provide soil data to depths between 1 and 10 in. In addition, a GPS device yields precise information on the tractor’s location.
Combining the spectrometer, encoder, and GPS data allows farmers to create a unique map of soil nutrients at various depths. Field trials have demonstrated the spectrometer instrument’s durability and performance, and ongoing efforts continue to improve its ruggedization, which is the most likely obstacle to broader market acceptance.
Such innovations bring laboratory-quality, real-time NIR spectroscopy technology to farming operations already underway and allow farmers to map the distribution and density of key soil constituents, such as nitrogen, phosphorus, potassium, organic matter, pH, moisture, clay, and sand — all in a cost-effective and rapid manner (Figure 2).
Figure 2. Embedding a spectrometer in a coulter disc and dragging it through cultivated fields provides in situ multiconstituent analysis of soil components. EC: electrical conductivity; OM: organic material. Courtesy of SoilReader.
The agricultural industry is not the only sector benefiting from real-time spectrometric soil analysis. Forensic researchers and environmental scientists are also digging deep into the technology. Researchers have demonstrated the unique ability of field-portable NIR spectroscopy to detect residual traces of fatty acids in soil to locate human remains, and the technique is now moving into the commercial sector to support forensic investigations.
Under hydraulic pressure, a pointed metal probe is inserted into the soil up to depths of 1 m. Equipped with a ruggedized fiber optic probe, the device provides an illumination and collection path for reflectance spectroscopy analysis. Above ground, the field-portable instrument houses a broadband halogen light source and an NIR spectrometer, covering the wavelength range between 1100 and 2500 nm.
Significant progress has been made with both a traditional grating-based indium gallium arsenide (InGaAs) spectrometer and, more recently, a compact MEMS-based Fourier transform IR (FTIR) spectrometer. In a grating-based spectrometer, incoming light is separated into its spectral components and distributed onto a linear image sensor. Using a simple wavelength calibration to convert the linear image sensor pixel data into wavelength, spectral data from the probe can easily be measured. Grating-based
solutions offer significant speed advantages compared with MEMS FTIR (or FT-NIR), due to the parallel nature of spectral interrogation.
In a MEMS FTIR spectrometer, input light is fed into a Michelson interferometer that consists of a stationary and a moving mirror. The motion of the moving mirror creates constructive and destructive optical interference that can be measured by a single-element photodiode. The resulting interferogram is digitalized and converted into spectral data using fast Fourier transform techniques. MEMS FTIR spectrometers are significantly less costly than their grating-based counterparts. They are also very compact and offer excellent signal-to-noise ratio in the 1700- to 2500-nm region, but they sacrifice measurement speed.
Both novel techniques, however, are suitable for forensics applications, where they will increasingly expedite the precise location of human remains and facilitate targeted excavation efforts. Detectives will no longer be “digging blind,” because spectroscopy provides a chemical map of investigation sites rendered in three-dimensional coordinates.
Finding the sweet spot
The United Nations reports that one-third of the world’s food is wasted annually. Reducing waste through the use of spectroscopic techniques would have the net effect of increasing food production. Spectroscopy is also contributing to the quality and safety of food, thanks in part to advancements in instruments operating in the NIR and shortwave IR (SWIR) spectral windows.
Fruits, vegetables, and other crops comprise various biocompounds, including carbohydrates, sugars, starches, amino acids, proteins, pigments, and minerals. The composition of these compounds — as well as their water content — can change over time due factors such as drought, disease, or just the process of ripening. Spectroscopy provides a nondestructive means to measure the spectral signature of these compounds by detecting how certain wavelengths of light are transmitted, absorbed, or reflected.
Previously, much of this data could only be derived by testing random samples in a laboratory setting, and the tests were often destructive to the product. Hand-held reflectance-based spectrometers are changing this dynamic on the farm, and similar produce scanners are expected to make their way into grocery stores within the next five years (Figure 3). Combined with cloud-based data-gathering applications, hand-held spectrometers could improve the availability of valuable nutritional information. The potential downside: These instruments require instrument calibration to ensure proper readings, which could slow broader adoption.
Figure 3. Hand-held reflectance-based spectrometers are changing the dynamics of sample testing on the farm. These produce scanners are expected to make their way into grocery stores within the next five years. Courtesy of Felix Instruments.
The means to an end
It can be a challenge to encourage people afflicted with chronic illnesses to regularly track their health metrics. To that end, spectroscopy is offering a solution by supporting the development of smart toilets capable of tracking critical health measurements as part of our daily routine.
Unlike wearables, where daily use often fades over time, there is no escape from regular use of the bathroom. Bodily waste contains valuable health information that we flush away every day. Through automated analysis of waste, patients can effortlessly collect key health indicators and share the data with a networked health care professional to prevent or track chronic illness.
Current smart toilet designs leverage a combination of broadband light and passband filters to selectively capture relevant spectral data about key health indictors, such as the levels of glucose, ketone, sodium, creatinine, albumin, bilirubin, and urobilinogen (Figure 4).
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Technology
• NIR Spectrometer
• UV-VIS Spectrometer
Inputs Targeted
• Specific Gravity of Urine
• Glucose Level
• Ketone Level
• Sodium Level
• Albumin Level
• Albumin/Creatinine Level
• Urea Level
• Bilirubin Level
• Urobilinogen Level
|
Figure 4. Current smart toilet designs leverage a combination of broadband light and passband filters to automatically capture relevant spectral data about selective health indictors in urine. Courtesy of Medic.Life.
Smart toilet manufacturers are leveraging advancements in mobile spectroscopy to capitalize on more compact, cost-effective instruments with broad spectral coverage. Today’s microspectrometers are perfect candidates for embedding ultraviolet (UV) and visible spectral analysis in smart toilets, while MEMS-FTIR spectrometers cover the remainder of the spectrum out to 2500 nm. Combining these technologies with traditional halogen lamps and light-emitting diodes (LEDs) creates a future that is flush with potential for smart toilets.
This technological breakthrough may one day make patients’ annual comprehensive metabolic panels a thing of the past because health care professionals would already have much of the information needed.
Outlook
Companies actively engaging in these emerging applications will need to find the balance between technological advancements and data. The importance
of sharing data across platforms and
collaboratively working to improve
spectroscopy precision will have unforetold benefits to society. The evolution of miniature spectroscopy solutions will also enable and expand new opportunities, helping to further improve quality of life.
Meet the author
John Gilmore is a business development manager at Hamamatsu, with knowledge of experimental photonics-based measurements of all types.
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