NIR and SWIR spectroscopy

The Near Infrared Spectroscopy (NIR or SWIR in the US) is commonly used for the investigation of chemical compounds. Radiation in the wavelength range of the infrared region of the electromagnetic spectrum is used for the excitation of molecular vibrational modes. In the NIR region from 0.78 µm - 3 µm, especially overtone and combination modes of C-H, O-H and N-H chemical bonds are observed. Therefore, NIR-spectroscopy is a versatile tool for the detection and investigation of polymers, coatings, food and feed as well as water or hydroxyl containing minerals.

The detected data (the spectrum recorded) represents the intensity of the light which was transmitted through or reflected by the sample as a function of the wavelength. The spectra in the NIR-region are characterized by broad absorption bands. Therefore, mostly chemometric methods are used for qualitative or quantitative data analysis.

The NIR spectroscopy is a very fast and contact-free measurement technique in reflection mode, enabling a detection of moving objects. Therefore, this method is suitable for process analytical technologies (PAT), chemical imaging, optical sorting and many more applications. The field of application for modern NIR spectrometers is extremely broad. Control of chemical reactions, polymer production, sorting of plastics, paper and debris in recycling, quality control of thin films and coatings or usage in food and mineral industry - NIR spectrometer and cameras can be found everywhere.

  Wavelength range (µm)
Near Infrared (NIR) / IR-A 0.78 ... 1.4
Short wavelength Infrared (SWIR) / IR-B 1.4 ... 3.0
Mid-wave Infrared (MIR) 3.0 ... 50
Far Infrared (FIR) 50 ... 1000

Process analytical technology (PAT)

For sorting and process analytical applications, preferably NIR spectrometers / spectrographs with a diffraction grating are used. IR sensor arrays or IR line sensors detect the complete NIR spectrum for each measurement spot simultaneously. In case a 2D-sensorarry is used, the complete NIR-spectrum for a line shaped measurement plane is detected simultaneously. For the NIR wavelength range, the InGaAs sensors are particularly suitable due to their fast time response and high detectivity. The InGaAs sensor types are cooled by Peltier elements and allow high speed measurements at up to 1000 frames per second. Depending on the application, different sensor types are available for the NIR (SWIR) range: InGaAs standard up to 1.7 µm; InGaAs extended up to 2.2 µm, MCT up to 2.5 µm.

In addition to fast readout line spectrometers and imaging spectrometers, FT-NIR spectrometers are available. These FT-NIR spectrometers are working in a scanning mode. Due to their interferometer geometry and slower frequencies of measurement, these FT-NIR spectrometers are suitable rather for laboratory applications. Whether a NIR-line spectrometer or a hyperspectral imaging spectrometer is the first choice for an application depends on the application parameters. Spatial and spectral resolution, frequencies of measurement and dynamic range should be considered.

UV-VIS spectroscopy

Ultraviolet-visible (UV-VIS) spectroscopy utilises electromagnetic radiation in the visible or UV wavelength region to excite and detect electronic transitions in sample materials. The method is limited to the excitation of valence electrons.
If these electronic transitions are generated by radiation in the visible wavelength range (380 nm to 780 nm), then the material appears coloured to the human eye in the complementary colour.

The recorded UV-VIS spectroscopic data (spectra) plot the intensity of samples transmitted or reflected light versus the corresponding wavelength. Due to the spectral features in the data, the materials can be identified and several material properties can be determined. Due to their characteristically broad absorptive features, the UV-VIS spectra are usually analysed by automated analysis routines such as chemometrical methods.

The UV-VIS spectroscopy can be utilised as a fast and non-contact measurement method. Modern hyperspectral imaging cameras in the UV-VIS wavelength range such as uniSPEC0.9HSI enable the detection of fast moving objects as well as the preservation of spatial information. This technology is ideally suitable for real-time detection of material streams, enabling new sorting and online analysis applications which are not accessible by common NIR hyperspectral imaging cameras.

Fluorescence spectroscopy

Fluorescence is spontaneous emission of light (radiation in the VIS range) after excitation by ultraviolet radiation. The emission is caused by transition between electronic states of identical spin.

Due to the spin conservation, the lifetime of the fluorescence is shorter than the lifetime of other luminescence phenomena such as phosphorescence. Several materials such as the mineral fluorite exhibit a natural fluorescence which can be used for material identification. Fluorescence spectroscopy on biological systems such as haemoglobin or chlorophyll reveals information regarding biological activity of organisms.

Due to the high spatial resolution in combination with a high detection speed, the UV-VIS hyperspectral imaging camera is particularly suitable for online fluorescence measurement. The fluorescence analysis results therefore can be utilised for sensor-based sorting or online quality control applications.

Detection of color variations

For many products, the color impression of a surface plays an important role. In industrial production, the evaluation of the optical color impression for the producer has become an indispensable part of its quality assurance in order to continuously ensure high product quality. By imaging UV-VIS spectrometer uniSPEC0.9HSI surfaces can be examined in the ultraviolet and visible spectral range and color variations are detected objectively.

Wide range of applications in industry and research are conceivable, as the following examples:
■ Plastics industry
■ Paper and printing industry (e.g. magazines, packaging)
■ Cosmetics and pharmaceutical industries (e.g. tablet coatings)
■ Furniture industry (e.g. veneers, decorative films, decorative papers)
■ Automotive and aerospace industry (e.g. paints and coatings)

RGB / HSV colour value determination

HSV model
HSV model

RGB line scan cameras analyse the colour and determine RGB values by the RGB (Red Green Blue) colour space. The RGB values are transferred into the HSV (Hue Saturation Value) colour space which is enabling a more customer-friendly device operation and setup.

For colour determination by uniScanRGB camera, illumination units in the visible wavelength range are utilised for sample analysis. A material specific part of the light is absorbed by the sample, the other part is reflected. The reflected part of the light has therefore a different spectral distribution compared to the initial light of the illumination unit. These differences are utilised for the evaluation of the RGB values.

Due to the high spatial resolution in combination with high frame rates, the RGB line scan camera uniScanRGB is ideally suitable for online colour determination and sensor-based sorting applications.

Imaging NIR-line spectrometer

Imaging NIR spectrometer based on a linear sensorarray are successfully used in the sorting industry for more than 15 years.

LLA Instruments GmbH manufactures NIR spectrometer with holographic concave gratings. In the focal plane of the grating, a spectrum is generated. The spectrum is recorded by a camera including a linear sensor, which is especially designed for NIR spectroscopy. The measurement results are transferred to and analysed by a computer. The spectrometer possesses a single entrance slit and can therefore record only one measurement point at once. However, many measurement points or probes are required for process measurements.

For process measurements, an optical multiplexer precedes the spectrometer. The task of the multiplexer is it to switch the single probes consecutively to the entrance slit of the spectrometer. Applying NIR fibre cables, the probes can be included into a probe line or installed separately many meters apart from each other.

LLA Instrument NIR-Technology including linear sensors:
■ uniSPEC2.2USB / uniSPEC2.2S and uniSPEC2.2P (no longer available!)

Illustration and principle of a NIR-line spectrometer

Illustration and principle of a NIR-line spectrometer

NIR-spectroscopy using hyperspectral imaging cameras

Illustration high performance hyperspectral camera
Illustration high performance hyperspectral camera

Imaging or spatial NIR spectroscopy is a new generation of highly productive measurement technology. The typical information of the line spectrometers (intensity as a function of the wavelength) is extended in this new generation of spectrometers by a further dimension: location.

The optical layout of these imaging spectrometers is specifically calculated. For this layout, the spatial information of the measurement area which is projected on the entrance slit is preserved via the entrance slit height. The projection in the area of image surface consists of a spatial component and a vertically spread spectral component. Using a sensor array, both parameters are recorded simultaneously. For each image spot along a line-shaped measurement area, a complete NIR spectrum is recorded.

The reflectance spectrometers manufactured by LLA feature distortion-free optical components. In this case, the curvature of the line-shaped measurement area is negligible and the spectra of spatially closely adjacent objects do not mix. The high spectral and spatial resolution captures very fine object structures and enables the distinction of fine absorption band structures. NIR-corrected lenses with a short focal length allow the detection of a large field of view at short working distances. High frame rates enable the detection of extremely fast moving objects.

LLA Instruments NIR-technology including sensor arrays:

X-Ray Fluorescence spectroscopy (XRF)

Energy dispersive spectra
Energy dispersive spectra

X-ray fluorescence (XRF) is the emission of characteristic (fluorescent) X-rays from a material that has been excited by high-energy X-rays or gamma rays. XRF is widely used for the qualitative and quantitative elemental (chemical) analysis of e.g. metals, glass, ceramics and building materials. Due to its non destructiveness it is a commonly used tool for research in geochemistry, forensic science, archaeology and the study of art objects.

When materials are exposed to X-rays (or gamma rays), ionization of their component atoms may occur, if the atom is exposed to radiation with an energy greater than its ionization potential. The removal of an electron from an inner orbital makes the electronic structure of the atom unstable, and electrons from higher orbital’s "fall" into the lower orbital to fill the hole. The energy difference between the two orbital’s involved is released in the form of a photon. The energy of this photon is characteristic of the emitting atom (element) and the involved orbital’s (fluorescence line).

The fluorescence radiation is detected either by Energy-Dispersive (ED) or by Wavelength Dispersive (WD) detectors.
Wavelength dispersive systems have excellent energy resolution but are very time consuming, since the whole wavelength range has to be analysed step by step according to the desired resolution (limited only by the intrinsic resolution of the analyser crystal).
Energy dispersive systems are compared to WD-systems very fast, since a broad range of energies is detected simultaneously, but the energy resolution is much worse than with WD-systems.

In both cases the intensity of the characteristic lines is proportional to the amount of the according element in the material.

Due to the short measurement time in process analysis only ED-systems can be used. The XRF Line Camera manufactured by LLA Instruments GmbH therefore uses state-of-the art energy dispersive detectors. Those Silicon Drift Detectors (SDD´s) from KETEK GmbH are characterised by a high count rate capability (>1 Mcps) an excellent peak-to-background ratio, a good energy resolution (in the order of 130 eV) and a good quantum efficiency for a broad energy range (0.2 keV to 30 keV).