Hyperspectral Imaging Using Laser Excitation for Fast Raman and Fluorescence Hyperspectral Imaging for Sorting and Quality Control Applications

A hyperspectral measurement system for the fast and large area measurement of Raman and fluorescence signals was developed, characterized and tested. This laser hyperspectral imaging system (Laser-HSI) can be used for sorting tasks and for continuous quality monitoring. The system uses a 532 nm Nd:YAG laser and a standard pushbroom HSI camera. Depending on the lens selected, it is possible to cover large areas (e.g., field of view (FOV) = 386 mm) or to achieve high spatial resolutions (e.g., 0.02 mm). The developed Laser-HSI was used for four exemplary experiments: (a) the measurement and classification of a mixture of sulphur and naphthalene; (b) the measurement of carotenoid distribution in a carrot slice; (c) the classification of black polymer particles; and, (d) the localization of impurities on a lead zirconate titanate (PZT) piezoelectric actuator. It could be shown that the measurement data obtained were in good agreement with reference measurements taken with a high-resolution Raman microscope. Furthermore, the suitability of the measurements for classification using machine learning algorithms was also demonstrated. The developed Laser-HSI could be used in the future for complex quality control or sorting tasks where conventional HSI systems fail.

For industrial applications, hyperspectral imaging methods are mainly used in the ultraviolet (UV), visible (VIS) and near-infrared (NIR) spectral ranges, because they are reasonably inexpensive, readily available, and ideally suited and proven for on-line or in-line quality control [11,12].These methods allow a fast and extensive inspection of samples or continuous processes.Common HSI methods use broadband lighting for a sufficient excitation in the whole spectral range of interest, while other methods, such as laser-induced fluorescence (LIF) and Raman his, are not very common in the field of macroscopic HSI, although they are widely used for microscopic imaging [13][14][15].
LIF is a spectroscopic technology with high sensitivity, a wide dynamic range, and low detection limits [16].It can be utilized, for example, for live cell microscopy of cells, using the high spectral resolution to increase the number of fluorophores that can be measured simultaneously [17].
Raman spectroscopy (RS) is a highly selective technology providing narrow and non-overlapping peaks of the measured sample.In contrast to near infrared spectroscopy, the signals are not interfered with by carbon or water absorption and nearly no sample pre-treatment is necessary.The main drawback of RS is the low fraction of scattered photons (typically < 0.0001%), which leads to low signal-to-noise ratios and long acquisition times.Some applications are, for example label-free imaging of cells [18] or high-resolution imaging of single-walled carbon nanotubes [19].
Fluorescence and Raman scattering are two competing effects and often occur together.Therefore, it is sometimes difficult to detect the weak Raman signals due to the high fluorescence background [20].
The aim of this paper is to use the simultaneously appearing fluorescence and Raman signals after laser excitation for chemical imaging of large surfaces in short periods of time.Because HSI for evaluation or classification tasks requires the highest possible variance in the underlying spectral data, the exact knowledge of the origin of the signals is of secondary importance.A distinction between fluorescence and Raman events is desirable but not absolutely necessary.The idea is that the technology, from here on called Laser-HSI, can be used for various applications in the field of process and food monitoring or sorting.Previous work has mainly focused on Raman imaging using NIR lasers to reduce the fluorescence background [21,22], while the focus of this work is the combination of Raman and fluorescence imaging.This is reflected mainly in the choice of the excitation laser and the imaging spectrometer used.
In this paper, the design of the Laser-HSI system is described, and the system is calibrated and characterized.Furthermore, some sample measurements are presented to illustrate possible applications in different areas.

System Design and Software
The developed Laser-HSI system is schematically illustrated in Figure 1.The illumination is accomplished using a frequency-doubled Nd:YAG laser with a wavelength of 5322 nm and a maximum output power of 300 mW (GLK 32XX TS, LASOS Lasertechnik, Jena, Germany).The spectral linewidth of the laser is ≤ 1 MHz, the beam quality is M2 < 1.2, and the power stability is stated by the manufacturer as ≤±2%.The laser beam is guided through a laser clean-up filter (LL01-532, Semrock, Rochester, NY, USA), which is used to remove spontaneous emission noise.The beam then impinges a scanning mirror (dynAxis S, Scanlab, Puchheim, Germany) with variable scanning length.The scanning rate can be adjusted to the measurement task.The scanning mirror reflects the beam again and spreads the laser to form a divergent line.After passing an achromatic collimation lens with a focal length of f = 300 mm (AC508-300-A-ML, Thorlabs, Newton, MA, USA), the line passes a dichroic mirror (LPD02-532RU, Semrock, USA) with a length of 6 cm, which reflects light with a wavelength below 537 nm and allows the transmittance of light coming back from the sample with a wavelength greater than 537 nm.The dichroitic mirror is mounted at 45 • to the laser plane and projects the beam to the sample surface, creating an excitation line of variable length, ranging from 0.5 cm up to 10 cm.

Spectral Calibration and Spectral Resolution
The aim of the spectral calibration of the Laser-HSI is to assign correct absolute wavelength or relative Raman shifts to the rows of the detector array.For the absolute wavelength calibration, a neon lamp calibration source with characteristic emission lines between 550 nm and 750 nm (Renishaw, Wotton-under-Edge, UK) is used.The calibration source is placed underneath the camera and the spectrum is averaged 200 times using slit widths of 25, 40 and 60 µm, the Cinegon lens and no binning.For each measurement, the positions of the measured lines are used to fit a linear function between the known wavelengths of the calibration source and the row indices of the detector array.The calibration can be done automatically using an own-written Matlab ® script.
To obtain the spectral resolution of the imaging system, two sharp peaks of the neon lamp (583.4 nm and 701.6 nm) were used.In theory, these peaks should cover only one pixel of the detector.Due to the point spread function of the imaging system, the measured peaks are broadened.The resolution of the imaging system can therefore be calculated using the full width at half maximum (FWHM) of the measured peaks.

Spatial Resolution
The spatial resolution perpendicular to the measurement line depends on the speed of the moving stage and the frame-rate of the camera, and can, therefore, be easily controlled and adjusted to achieve squared pixels.The spatial resolution parallel to the measurement line depends on several factors, namely the working distance, the focal length of the used lens, and the number of detector pixels in the spatial dimension of the camera.The fixed structure of the spectrograph and camera allows the spatial resolution required for the application to be adjusted with a lens with an appropriate focal length, which simultaneously results in a certain field of view (FOV).To measure the spatial resolution, a calibration target (R2L2S1P1, Thorlabs, Newton, MA, USA) with black and white lines is placed underneath the camera and illuminated with halogen light.One line of the target is measured 200 times and averaged.The measured black and white transitions of the sample at a wavelength of 600 nm are differentiated and the FWHM of the obtained peaks is taken as the spatial resolution.The spatial resolution for the three different lenses was determined with working distances of 330 mm and 82.7 mm and FOVs of 386 mm, 104 mm and 4 mm.A slit width of 40 µm was used for the measurements.

Example Measurements
To evaluate the developed Laser-HSI, four samples from quite different fields of application were measured: (a) a mixture of sulphur and naphthalene; (b) a carrot slice; (c) black polymer particles; and, (d) a lead zirconate titanate (PZT) piezoelectric actuator with contaminations.The specific experimental conditions are shown in Table 1.For referencing the experiments (a) and (d), a Raman microscope (inVia, Renishaw GmbH, Wotton-under-Edge, UK) with a 514.5 nm argon ion laser source was used.In case of (a), a 100× magnification lens was used and the spectra were accumulated for 10 s and measured between 150 cm −1 and 3500 cm −1 .
In case of (d), areas with suspected contaminations were mapped using a 50× magnification lens and a spatial resolution of approximately 5 µm.The integration time was 0.1 s and the spectra were measured between 150 cm −1 and 2000 cm −1 .
For referencing the experiment (b), the same Raman microscope with a 785 nm diode laser (to reduce fluorescence) and 50× magnification lens was used.The spectra were measured between 800 cm −1 and 1800 cm −1 and accumulated ten times for 1 s.

Classification Experiments
To show that the system can be used for process monitoring and sorting tasks, classification experiments were performed for the experiments (a), (c), and (d).Therefore, 500 spectra per class were selected at random and a random forest [23] model was trained using the fitcensemble function of Matlab ® using default hyperparameters.Prior to the training, a principal component analysis was conducted and only the scores of the first five principal components were used.To estimate the accuracy of the trained model, 10-fold cross validation was used and the cross-validated accuracy was used as a measure of the quality of the model.Finally, the trained model was applied to all spectra in the hyperspectral measurement to generate a classification image of the sample.

System Design
The individual components of the Laser-HSI were selected and optimized with regard to the specific application requirements.With a wavelength of 532 nm, the frequency-doubled Nd:YAG laser used offers a good compromise between the expected intensity of the Raman scattering and the fluorescence intensity.Furthermore, the wavelength of the light emitted by the samples is in the range of the optimum quantum efficiency of the used silicon detector.If Raman measurements are made, this wavelength is particularly suitable for inorganic or weakly fluorescent samples.A near-infrared laser (e.g., 785 nm) would be more suitable for organic or highly fluorescent samples, but this would also further decrease the Raman intensity.Compared to argon ion lasers, which emit a similar wavelength, the Nd:YAG laser offers low power consumption, easy handling and a compact design, which facilitates industrial use (e.g., in recycling plants).
To generate the laser line, a scanning mirror was used.Alternatively, one could use a cylindrical or Powell lens [24] to generate the excitation line.Both approaches have advantages and disadvantages.If a cylindrical lens is used, no moving parts are required and a compact design can be achieved.On the other hand, a cylinder lens produces a non-uniform intensity distribution along the excitation line.This can be reduced by using a Powell lens, but these lenses must be optimized for a certain beam diameter and fan angle, which reduces the flexibility of the system.A disadvantage of the scanning mirror is an increased intensity at the reversal points of the mirror (at the edges of the excitation line) and the limited scanning speed, which can lead to problems using short integration times for the detector.Because the setup presented here should be as flexible as possible, a scanning mirror was used.For later industrial applications, the use of cylindrical or Powell lenses can be considered.
The used imaging spectrometer is optimized for spectroscopy in the visible and near-infrared wavelength range (400-1000 nm).Therefore, the spectral resolution is quite low for Raman spectroscopy (see Section 3.2).This has a negative effect on the usability of the device for tasks where a high spectral resolution is required.On the other hand, it is possible to measure a much larger wavelength range than with a Raman spectrometer.This could be useful for some questions; for example, when fluorescence signals are to be measured, which often extend over a wider wavelength range (see experiment (c)).In addition, the used imaging spectrometer is cheaper and more readily available then a Raman imaging spectrometer.
The EMCCD detector used enables a reduction of the readout noise due to the built-in amplification and thus enables the measurement of weak Raman and fluorescence signals.
The disadvantage is the relatively low frame rate of ~12 Hz using the full detector, which can, however, be increased using binning.
The C-mount connection allows flexibility regarding the FOV and the spatial resolution of the measurement.In summary, due to the modular design of the Laser-HSI system an individual adaptation to the respectively specific measuring application is possible.

Spectral Calibration and Spectral and Spatial Resolution
The results of the spectral calibration for different slit widths are shown in Table 2.For all three slit widths, a similar covered wavelength range of approximately 330 nm to just over 1100 nm is obtained.A wavelength interval of about 0.8 nm per pixel is determined for all three slits.The quality of the spectral calibration can be determined from the mean absolute error of the line positions to the reference line positions.The mean absolute error is relatively small for all three measurements, but increases with wider slits.The reason for this is the decreasing spectral resolution, which leads to broader lines and makes the automated peak selection more difficult, resulting in larger calibration errors.The spectral resolution of the system was measured for the peaks at 583.4 nm and 701.6 nm, and ranges from about 2 nm for 25 µm slit width, to over 4 nm for 60 µm slit width.It can also be seen that the spectral resolution decreases slightly for larger slit widths.Because the used spectrometer is optimized for visible and near-infrared hyperspectral imaging, the spectral resolution (in nanometers) decreases inherently due to the equidistant wavelength-resolution of the detector.In addition, it is possible to cover a much wider spectral range, from 535 nm up to 1000 nm, compared to a classic Raman spectrometer.Because of the low spectral resolution of the system, the typical calibration for Raman spectrometer, using reference substances like sulphur or naphthalene, could not be applied.Sometimes it was not possible to clearly assign the measured signals to the bands of the reference spectra.Therefore, the typical calibration could not be performed and the Raman shifts in the measurements were only calculated using the wavelengths obtained by the absolute calibration with the neon lamp calibration source.
The results for the determination of the spatial resolution for different lenses and FOVs are shown in Table 3.The measured resolutions are 1.31 mm for the Cinegon lens, 0.41 mm for the Xenoplan and 17 µm for the Sill telecentric lens with FOVs of 386 mm, 104 mm, and 4 mm, respectively.This shows the versatility of the developed laser-hyperspectral imager in terms of resolution and measurement area.It can be used to measure small samples with high resolution or to measure larger samples; e.g., in a conveyer belt application for sorting and recycling tasks.The FOV and the achievable resolution mostly depend on the used optics.Nevertheless, it has to be taken into account that with increasing FOV and working distance, more laser power is needed to maintain the signal intensity.A mixture of sulphur and naphthalene was measured using the Laser-HSI and the results are summarized in Figure 2. A picture of the sample (Figure 2a,c) shows a false color image of the measured intensity at ~180 cm −1 .At this wavenumber there is a slight overlap between the peaks of the two materials, but the intensity of sulphur is much higher than the intensity of naphthalene, so the position of the naphthalene can be clearly identified in the false color image.Despite the low resolution of the spectrograph, the characteristic peaks of both sulphur and naphthalene are clearly visible and the spectra show good agreement with reference spectra (Figure 2b).Fine spectral features like the sulphur peaks at 154 cm −1 and 219 cm −1 are not resolved.In addition, a precise determination of the peak position is not possible due to the low spectral resolution.Nonetheless, it is possible to determine the spatial distribution of the two materials in the sample.The measurement of the sample area of ~16 cm 2 only took around 20 s with a spatial resolution of approximately 0.4 mm.The acquired spectra were used to train a classification model to discriminate between sulphur and naphthalene.The resulting model has a high cross-validation accuracy of 99.9%.The classification image is shown in Figure 2d.A mixture of sulphur and naphthalene was measured using the Laser-HSI and the results are summarized in Figure 2. A picture of the sample (Figure 2a,c) shows a false color image of the measured intensity at ~180 cm −1 .At this wavenumber there is a slight overlap between the peaks of the two materials, but the intensity of sulphur is much higher than the intensity of naphthalene, so the position of the naphthalene can be clearly identified in the false color image.Despite the low resolution of the spectrograph, the characteristic peaks of both sulphur and naphthalene are clearly visible and the spectra show good agreement with reference spectra (Figure 2b).Fine spectral features like the sulphur peaks at 154 cm −1 and 219 cm −1 are not resolved.In addition, a precise determination of the peak position is not possible due to the low spectral resolution.Nonetheless, it is possible to determine the spatial distribution of the two materials in the sample.The measurement of the sample area of ~16 cm 2 only took around 20 s with a spatial resolution of approximately 0.4 mm.The acquired spectra were used to train a classification model to discriminate between sulphur and naphthalene.The resulting model has a high cross-validation accuracy of 99.9%.The classification image is shown in Figure 2d.A carrot slice was measured with the Laser-HSI and the results are shown in Figure 3.A picture of the sample (Figure 3a,c) shows a false color image of the measured intensity at ~1000 cm −1 before any pre-processing.This image, therefore, shows the fluorescence of the carrot slice.Figure 3b shows an example spectrum of the carrot slice scan after fluorescence background removal with two characteristic carotenoid peaks at 1156 cm −1 and 1521 cm −1 , which originate from C=C stretching vibrations of the carotenoids [25].The background was removed fitting a polynomial of fourth degree to the spectra using a non-quadratic cost function [26].The peaks are also in good agreement with the peaks from the reference spectra acquired with the Raman microscope.The intensity of the peaks is proportional to the carotenoid concentration.The band at 1521 cm −1 was used to calculate the distribution of the relative carotenoid concentration (Figure 3d).The image shows an uneven distribution of the carotenoid in the carrot slice.The concentration in the xylem is relatively low and increases towards the cambium.Towards the outer areas of the slice (phloem), the concentration decreases again.This is in good agreement with previous findings and shows the potential of the Laser-HSI in the area of food surveillance, quality control, and research [25].When comparing images Figure 3c and d, it can be seen that there is a correlation between the distribution of the fluorescence and the carotenoid distribution in the inner part of the slice.However, at the edge of the slice, the carotenoid concentration is very low, but a strong fluorescence can be observed.This shows that it can be useful to measure Raman and fluorescence signals at the same time to get the information with only one measurement, which could be especially useful for time-critical tasks.
acquired with the Raman microscope.(c) False color image of the measured intensity at ~180 cm −1 (sulphur).(d) Results of the Laser-HSI classification (blue = sulphur; red = naphthalene).

Carotenoid Distribution in a Carrot Slice (b)
A carrot slice was measured with the Laser-HSI and the results are shown in Figure 3.A picture of the sample (Figure 3a,c) shows a false color image of the measured intensity at ~1000 cm −1 before any pre-processing.This image, therefore, shows the fluorescence of the carrot slice.Figure 3b shows an example spectrum of the carrot slice scan after fluorescence background removal with two characteristic carotenoid peaks at 1156 cm −1 and 1521 cm −1 , which originate from C=C stretching vibrations of the carotenoids [25].The background was removed fitting a polynomial of fourth degree to the spectra using a non-quadratic cost function [26].The peaks are also in good agreement with the peaks from the reference spectra acquired with the Raman microscope.The intensity of the peaks is proportional to the carotenoid concentration.The band at 1521 cm −1 was used to calculate the distribution of the relative carotenoid concentration (Figure 3d).The image shows an uneven distribution of the carotenoid in the carrot slice.The concentration in the xylem is relatively low and increases towards the cambium.Towards the outer areas of the slice (phloem), the concentration decreases again.This is in good agreement with previous findings and shows the potential of the Laser-HSI in the area of food surveillance, quality control, and research [25].When comparing images Figure 3c and 3d, it can be seen that there is a correlation between the distribution of the fluorescence and the carotenoid distribution in the inner part of the slice.However, at the edge of the slice, the carotenoid concentration is very low, but a strong fluorescence can be observed.This shows that it can be useful to measure Raman and fluorescence signals at the same time to get the information with only one measurement, which could be especially useful for time-critical tasks.

Black Polymer Sorting (c)
The appearance of black polymers is achieved by adding filling materials like carbon black.The recycling of black polymers to high purity recycled materials is difficult because sorting techniques fail due to the low recognition rates of common sensor techniques like near-infrared spectroscopy.Laser-HSI is a promising technology to solve this problem because many black polymers show characteristic fluorescence signals.Therefore, three different kind of black polymer particles, namely polyamide 6 (PA6), polyamide 66 (PA66), and thermoplastic polyurethane (TPU) were imaged using the Laser-HSI.A picture of the three black polymers (Figure 4a,b) shows a false color image of the measured intensity at 580 nm. Figure 4d shows some Laser-HSI example spectra of the polymers.It is evident that there are clear differences in the spectral signatures which may be used to classify and, in consequence, sort the polymer particles.
The obtained classification model for the polymers shows a cross validation accuracy of 99.6%, which is very promising for the use of the Laser-HSI for sorting black plastics for recycling.The trained model was applied to all spectra in the hyperspectral measurement to generate a classification image of the sample (Figure 4c).False color image of the measured intensity at ~1000 cm −1 before any pre-processing of the spectra.(d) Color-coded relative intensity of the carotenoid peak at 1521 cm −1 .

Black Polymer Sorting (c)
The appearance of black polymers is achieved by adding filling materials like carbon black.The recycling of black polymers to high purity recycled materials is difficult because sorting techniques fail due to the low recognition rates of common sensor techniques like near-infrared spectroscopy.Laser-HSI is a promising technology to solve this problem because many black polymers show characteristic fluorescence signals.Therefore, three different kind of black polymer particles, namely polyamide 6 (PA6), polyamide 66 (PA66), and thermoplastic polyurethane (TPU) were imaged using the Laser-HSI.A picture of the three black polymers (Figure 4a,b) shows a false color image of the measured intensity at 580 nm. Figure 4d shows some Laser-HSI example spectra of the polymers.It is evident that there are clear differences in the spectral signatures which may be used to classify and, in consequence, sort the polymer particles.
The obtained classification model for the polymers shows a cross validation accuracy of 99.6%, which is very promising for the use of the Laser-HSI for sorting black plastics for recycling.The trained model was applied to all spectra in the hyperspectral measurement to generate a classification image of the sample (Figure 4c).

Contaminations on PZT Piezoelectric Actuators (d)
For the last experiment, a PZT piezoelectric actuator was investigated with a high-resolution Laser-HSI measurement.The actuator is contaminated with organic substances of unknown origin and the aim of the experiment is to locate these impurities, using the Laser-HSI.Figure 5a shows a picture of the PZT piezoelectric actuator and Figure 5b shows a section of a measurement of a contaminated sample acquired with the Laser-HSI.Figure 5c shows a section of the same area, which was measured with the Raman microscope.Both figures show a false color representation of the Raman intensity at 230 cm −1 .Figure 5d shows spectra of the PZT ceramic and the contamination, measured with the Laser-HSI and the Raman microscope.The PZT spectra shows the typical of the PZT ceramic [27].In the areas of contamination, strong fluorescence caused by the organic impurities can be seen.The shape and dimensions of the contamination show a good agreement for both measurements.The trained classification model shows a cross validation accuracy of 100% and the classification image is shown in Figure 5e.If needed, the peaks of the PZT ceramic bands could be used to find, for example, defects in the layers of the piezoelectric actuator.
This experiment shows that the developed Laser-HSI provides equivalent results to a Raman microscope for this task.While the Laser-HSI measurement takes about three min per sample, a complete mapping with the Raman microscope would take several hours.Therefore, the Laser-HSI could be used as a fast tool for the high-resolution quality control of small components.

Contaminations on PZT Piezoelectric Actuators (d)
For the last experiment, a PZT piezoelectric actuator was investigated with a high-resolution Laser-HSI measurement.The actuator is contaminated with organic substances of unknown origin and the aim of the experiment is to locate these impurities, using the Laser-HSI.Figure 5a shows a picture of the PZT piezoelectric actuator and Figure 5b shows a section of a measurement of a contaminated sample acquired with the Laser-HSI.Figure 5c shows a section of the same area, which was measured with the Raman microscope.Both figures show a false color representation of the Raman intensity at 230 cm −1 .Figure 5d shows spectra of the PZT ceramic and the contamination, measured with the Laser-HSI and the Raman microscope.The PZT spectra shows the typical bands of the PZT ceramic [27].In the areas of contamination, strong fluorescence caused by the organic impurities can be seen.The shape and dimensions of the contamination show a good agreement for both measurements.The trained classification model shows a cross validation accuracy of 100% and the classification image is shown in Figure 5e.If needed, the peaks of the PZT ceramic bands could be used to find, for example, defects in the layers of the piezoelectric actuator.
This experiment shows that the developed Laser-HSI provides equivalent results to a Raman microscope for this task.While the Laser-HSI measurement takes about three min per sample, a complete mapping with the Raman microscope would take several hours.Therefore, the Laser-HSI could be used as a fast tool for the high-resolution quality control of small components.

Conclusions
In this paper a fast, line-scanning hyperspectral imaging system with laser excitation using fluorescence or/and Raman scattered light for imaging analysis was introduced.
Four different application examples demonstrate the wide range of possible applications.The experiments carried out show that the described Laser-HSI system makes it possible to measure the Raman or fluorescence spectra of relatively large sample areas, with a high spatial resolution and in a relatively short amount of time.This opens up interesting application possibilities in many areas, from quality control in the food industry to surface inspection and recycling.The modular design of the system makes it possible to adapt the measuring range and spatial resolution to the problem at hand.In the experiments, measuring widths between 38 cm and 4 mm, with resolutions between 1.3 mm and 20 mm, were achieved.Furthermore, the system was spectrally calibrated and it could be shown that the spectra obtained for a number of test substances are in good agreement with literature or reference measurements.The minimal spectral resolution of approximately 1.8 nm at 583 nm is below the resolution of a conventional Raman spectrometer.On the other hand, the spectral range covered is much larger.This will be used in future attempts to obtain further information about the samples; for example, by measuring reflective properties in the spectral ranges in which no Raman or fluorescence signals are expected.Future developments will focus, in particular, on the investigation of the application for the recycling of black polymers.In addition, it will be investigated how measurements can be further accelerated.Other hyperspectral cameras and stronger lasers with different wavelengths could be used for this purpose.

Patents
In reference to the work presented here, two patents with the patent numbers DE102018210019.5 and DE102018210015.2have been filed.

Figure 1 .
Figure 1.Schematic illustration of the developed laser hyperspectral imaging system (Laser-HSI) system.

Figure 1 .
Figure 1.Schematic illustration of the developed laser hyperspectral imaging system (Laser-HSI) system.

Figure 2 .
Figure 2. Laser-HSI of mixture of sulphur and naphthalene.(a) Picture of the sample with laser excitation line.(b) Normalized example spectra of sulphur and naphthalene with reference spectra

Figure 2 .
Figure 2. Laser-HSI of mixture of sulphur and naphthalene.(a) Picture of the sample with laser excitation line.(b) Normalized example spectra of sulphur and naphthalene with reference spectra acquired with the Raman microscope.(c) False color image of the measured intensity at ~180 cm −1 (sulphur).(d) Results of the Laser-HSI classification (blue = sulphur; red = naphthalene).

2 .
Carotenoid Distribution in a Carrot Slice (b)

Figure 3 .Figure 3 .
Figure 3. Laser-HSI of the carrot slice.(a) Picture of the carrot slice.(b) Normalized example spectra of the carrot slice from the phloem and reference spectra acquired with the Raman microscope.(c) Figure 3. Laser-HSI of the carrot slice.(a) Picture of the carrot slice.(b) Normalized example spectra of the carrot slice from the phloem and reference spectra acquired with the Raman microscope.(c) False color image of the measured intensity at ~1000 cm −1 before any pre-processing of the spectra.(d) Color-coded relative intensity of the carotenoid peak at 1521 cm −1 .

Figure 4 .
Figure 4. Laser-HSI measurement of the black polymer samples.(a) Picture of the sample showing the different kind of black polymer particles.(b) False color image of the measured intensity at 580 nm.(c) Classification result of the Laser-HSI measurement (green = PA66 (polyamide 66); blue = PA6

Figure 4 .
Figure 4. Laser-HSI measurement of the black polymer samples.(a) Picture of the sample showing the different kind of black polymer particles.(b) False color image of the measured intensity at 580 nm.(c) Classification result of the Laser-HSI measurement (green = PA66 (polyamide 66); blue = PA6 (polyamide 6); red = TPU (thermoplastic polyurethane).(d) Example spectra of the three black polymers.

Figure 5 .
Figure 5. Results of the fourth experiment.(a) Picture of the measured PZT piezoelectric actuator.(b) False color image of the measured intensity at ~230 cm −1 measured with the Laser-HSI.(c) False color image of the measured intensity at ~230 cm −1 measured with the Raman microscope.(d) Normalized example spectra of the PZT ceramic and the contamination and reference spectra acquired with the Raman microscope.(e) Classification result of the Laser-HSI measurement (blue = contamination; red = PZT ceramic).

Figure 5 .
Figure 5. Results of the fourth experiment.(a) Picture of the measured PZT piezoelectric actuator.(b) False color image of the measured intensity at ~230 cm −1 measured with the Laser-HSI.(c) False color image of the measured intensity at ~230 cm −1 measured with the Raman microscope.(d) Normalized example spectra of the PZT ceramic and the contamination and reference spectra acquired with the Raman microscope.(e) Classification result of the Laser-HSI measurement (blue = contamination; red = PZT ceramic).

Table 2 .
Results of the spectral calibration of the Laser-HSI.

Table 3 .
Results of the spatial calibration of the Laser-HSI.

Table 3 .
Results of the spatial calibration of the Laser-HSI.