Evolution of NIR Spectroscopy: Past, Present and Future

 

Fri, 11/14/2014 - 4:04pm

Joe Siddall, TI DLP Embedded Products Program Manager

 

Typical NIR spectroscopy applications. Images: Texas Instruments Near-infrared (NIR) spectrometers have been around for over 60 years, yet only a small fraction of the population is familiar with these dependable tools. It’s astounding that NIR spectroscopy does so much for so many people who have never heard the word “spectrometer.” NIR spectrometers help a diverse set of users make decisions in their daily jobs.

Multiple industries including food, agriculture, forensics, arson detection, pharmaceuticals, petroleum and medical depend on critical information provided by spectrometers. NIR spectrometers measure energy reflected from, or transmitted through, a material sample. This technique allows people to understand the types of molecules within the sample. In a sense, NIR spectrometers look for the molecular “fingerprint” of a substance.

NIR spectroscopy: The past
Early spectrometers were often large, bulky, sensitive instruments with moving parts. Illumination sources were dispersed into their subcomponent wavelengths by prisms or gratings. The gratings rotated in small increments under hand control for each measurement point on the desired spectrum. Data was constructed into a spectral plot for each sample measured. Then, comparisons to references and other samples were made by hand. These early spectrometers remained stationary in a laboratory, and were rarely moved once installed. 

The 1970s heralded the advent of the microprocessor in spectroscopy, both for controlling the spectrometer and for processing the resulting data. The semiconductor industry picked up its pace from the 1970s through the 2000s. This revolutionized microprocessors and computers to better control spectrometers and process spectrum data. The advent of analog-to-digital converters allowed sampling of spectrum data under processor control.

The present

 

Rotating grating (above) and DMD (below) architectures 

Today, there are six architectures commonly used for NIR spectrometers:

• Fixed filters: These instruments measure at a fixed number of wavelengths, each corresponding to an optical filter located on a wheel. Readings are taken as the wheel steps from filter to filter. 

• Rotating grating/prism: These instruments use a motor to move a grating’s dispersed output (wavelengths) across a single point detector. 

• FT-NIR: These instruments use the Fourier transform properties of the Michelson interferometer technique. They have moving mirrors to create interference patterns which can be mathematically related to a spectrum.       

• Linear array detector: These instruments use a grating to disperse wavelengths across a 1-D linear pixel array, which can have upwards of 256 or 512 elements.
• Linear variable filter: These instruments use a variable filter element in front of a linear array detector which allows specific wavelengths to impact each pixel element.

• DLP technology: The digital micromirror device (DMD) directs light dispersed on it by a grating to a single pixel detector. The DMD is programmable and flexible, allowing wavelengths to be sampled in any order or at different resolutions. This can be done within a single scan.

Current NIR spectrometers are far ahead of their predecessors, leveraging microprocessor control, precision A/D sampling and computerized spectrum calculation with statistical analysis. Use models can vary across architectures:

• Laboratory use: These are typically large, high-precision, general-purpose instruments. The computers processing spectral data can be internal or remotely located and connected via Ethernet or USB. They process enormous amounts of data and make comparisons to a distributed reference library in seconds. 

• Portable use: Portable NIR spectrometers look similar to small laboratory units, which are moveable and usually run off an AC 110-V supply or a 12-V supply with inverter. Often they’re bigger than a lunchbox and can sit on the tailgate of a truck for use in the field or industrial settings, like a farm or mine.

• In-line use: These specialized units monitor factory environments and are typically use specific. A factory install can contain multiple spectrometers on an assembly line linked via Ethernet or wirelessly to a main control facility. 

• Handheld use: There is a big focus on making handheld spectrometers which are truly mobile and user friendly. Current examples can be battery operated and are about the size of a large hand drill. The benefit is that they are truly portable and run off a built-in power supply for remote use.

DLP technology.The future

The future of NIR spectrometry looks very bright. Traditionally, instruments have been large and expensive, housed in controlled laboratory environments and not typically available to the general public.  Use models have been determined mostly by the size and cost of this typical laboratory equipment. Technical improvements enabling size and cost reductions have made more recent generations of NIR spectrometers more portable and suitable for use in the field.

Thanks to the development of new detector and DLP technologies, the miniaturization supplied by the semiconductor industry and the advent of cloud-based computing, it’s easy to envision a future with mobile NIR spectrometers. Small, affordable, effective and user-friendly solutions can bring spectrometers to the public.

Newer technologies can enable disruptive spectrometer businesses. The massive popularity of smartphones now puts incredible computing power in the palm of your hand. NIR spectrometers can theoretically follow a similar size and cost curve, eventually moving measurements to the home or “personal use” model. It may not be called an “NIR spectrometer,” but future personal measurement devices may someday help families evaluate foods for ripeness, detect food allergens, confirm purity of expensive olive oils, assist in medical monitoring or check automotive fluids. The inevitable drive towards smaller, more powerful NIR spectrometers truly opens up a vast spectrum of future applications.

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