Near-infrared spectroscopy

Near-infrared spectroscopy

© Fraunhofer IPMS
Assembled optical bench of the scanning mirror micro spectrometer.
© Fraunhofer IPMS
Miniaturized NIR spectrometer developed by Fraunhofer IPMS.

Compact and ultra-compact systems for nearinfrared (NIR) spectral analysis enabling the integration into portable devices or even mobile phones may contribute significantly to the development of future systems for sensing and testing applications. Besides size, more and more technological aspects become relevant for high volume fabrication. Competitive production needs reduced complexity of the core components, simple electronics and low requirements for the assembly and integration process.

The scanning mirror micro spectrometer is based on a simple 1D-scanner mirror. A fixed grating is illuminated and diffracts the light. A small wavelength interval is reflected back onto the scanner mirror plate. After passing the refocusing mirror and the exit slit the intensity is captured by either a single detector or an arrangement of multiple detectors. In comparison to the well-known scanning grating technology, the main advantage is the double spectral range which is captured for a defined MEMS defl ection range. Either half the deflection enables the same spectral range or double the range can be addressed with the same deflection. Furthermore the grating can be selected and adjusted for modular spectral range, resolution or blaze.

Miniaturised MEMS based spectroscopy systems

© Fraunhofer IPMS
MEMS grating spectrometer: As small as a sugar cube.
© Fraunhofer IPMS
Design of a miniaturised MEMS spectrometer.

Fraunhofer IPMS develops miniaturised spectrometer systems for mobile use in a wide range of applications, e.g. in the agricultural and food sector, in pharmacy, biotechnology and medicine as well as in the environmental and recycling sector.

With a volume of only 2.1 cm³, the miniaturised grating spectrometer developed is about 30 % smaller than an ordinary piece of sugar cube and is currently the smallest NIR spectrometer in the world. Due to its compact design, a weight of only 17g and a low power consumption in the mW range, it is ideal for integration into mobile analysis devices for on-site measurements. In the realised design, it addresses the NIR spectral range of 950-1900 nm with a spectral resolution of 10 nm.

Whether freshness testing of foodstuffs, sorting of plastic objects in recycling, determination of the type and concentration of starting materials in pharmacy or measurement of the composition of gases and liquids in the combustion tract of vehicles: in each case, the aim is to qualitatively and quantitatively determine the type and concentration of the materials involved in the most time-saving way possible. Spectroscopy, in which substances are illuminated and the intensity and wavelength of the reflected light are analysed, is predestined for all these applications. This is because measurement by means of electromagnetic radiation is contact-free, leaves the sample undamaged and is equally suitable for solid, liquid or gaseous substances.

The spectrometer uses a special discrete-time measuring principle which makes it possible to scan a spectrum with a single highly sensitive detector only by the rotational movement of the integrated MEMS grating. The central element of the spectrometer is a micro-electro-mechanical system (MEMS) measuring only (9.5 × 5.3 × 0.5) mm³ developed at Fraunhofer IPMS. 

Interactive Showroom

Functionality

© Fraunhofer IPMS
Setup of the near-infrared spectrometer.

The incident radiation from the entrance slit passes the collimation mirror on the scanner mirror before it is diffracted by the fixed grating in the optical path. Changes of the scanner mirror defl ection adjust the angles α and β in the spectrometer. Due to the grating equation n λ = g ( sinβ - sinα ) diffracted light of a selected wavelength interval meets the MEMS mirror plate again towards the refocusing mirror and finally the detector behind the exit slit. The detector signals are amplifi ed and converted in the approved way. The spectrum becomes available for the target application.

The new SMMS system features the same spectral range from 950 nm to 1900 nm using an uncooled InGaAs detector. The system volume remains at 2.1 cm³. The spectral resolution of 10 nm comes along with half the MEMS deflection of ±5° (mech.) only.

Applications

© Fraunhofer IPMS
Application example of the NIR: analysis of organic material in the field of food using a smartphone.

NIR spectral analysis is favorable for the evaluation of organic matter. This includes food, cloth, plastic material but also living cells. Application may be found in the field of nutrition, life science, health or medical but also in materials processing or quality monitoring for plastics, petrochemical, recycling or other measurement tasks which benefit from mobile, fast and reliable analysis in the field.

 

Future developments

Applying multiple detectors with adjusted spectral sensitivity each behind a mating exit slit will enable broad band spectrometers from visible to nearinfrared range. Some examples are shown in the table below. Compact systems collect more spectral information and open new applications for portable spectral measurement systems.

 

  DETECTOR 1 [nm] DETECTOR 2 [nm] deflection Detectors
Option 1 950 - 1900   < 5° Ext. InGaAs
Option 2 400 - 800 800 - 1600 < 9° Si & InGaAs
Option 3 475 - 950 950 - 1900 < 10° Si & ext. InGaAs

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Wie funktioniert die Nahinfrarotspektroskopie?