*2.1.1.2 Dispersive NIR spectrometers*

In conventional dispersive spectrometer designs, broadband light is passed through the sample and into an entrance slit to create a narrow line of light which is imaged onto a detector. A dispersive grating is inserted in the path causing the image of the narrow line to be spread out into a spectrum where the light is separated into its various wavelengths. These are then focused onto an array detector. **Figure 1** shows a basic dispersive spectrometer based on the Czerny-Turner design which uses mirrors to minimize the overall size of the design.

One major disadvantage of the design shown in **Figure 1** is the need for an array InGaAs detector which can be expensive. Alternative designs requiring only a single-element detector have been developed to help mitigate this expense. For example, instead of collecting all wavelengths simultaneously, spectrometers based on the Fabry-Perot interferometer design shown in **Figure 2** collect the wavelengths sequentially. This design features one fixed and one moveable half-silvered mirror aligned along the same optical axis. As the light bounces between the mirrors, constructive and destructive interference determines the spectrum of light that passes through the other side of the moveable mirror and onto the detector. When the spacing between the fixed and moveable mirrors equals an integer number of half wavelengths, maximum constructive interference occurs leading to a peak in the output spectrum at that wavelength. As with the Michelson interferometer, the spectral range of interest is thus examined by translating the moveable mirror over a specific spatial extent. An example of a compact instrument based on the Fabry-Perot design is Spectral Engines' MEMS Fabry-Perot spectrometer, the NIRONE [18].

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*DOI: http://dx.doi.org/10.5772/intechopen.97624*

**Figure 2.**

**Figure 3.**

*Fabry-Perot spectrometer design [17].*

*2.1.1.3 Linear variable filter (LVF) NIR spectrometers*

a 128-pixel InGaAs array [21].

*2.1.1.4 Hadamard spectrometers*

One problem common to all dispersive designs like the Czerny-Turner is that the light must be allowed to disperse over a given spatial extent such that the wavelengths can separate before reaching the detector. This causes limitations in designing for compactness [19]. One way to surmount this problem is with the use of LVFs which are generally formed from wedge-shaped optics and behave much like a Fabry-Perot interferometer but scan by lateral position along the filter instead of by movement of a mirror along the optical axis [12]. The LVF can be applied directly to a detector array, leading to a simple and compact mechanical design with no moving parts (see **Figure 3**). Viavi's MicroNIR OnSite features an LVF applied to

*Diagram showing the operating principle behind the LVF used in the MicroNIR OnSite [20].*

The Hadamard spectrometer design has a couple of key advantages over the conventional dispersive design. First, it overcomes the slow scanning process of dispersive techniques where individual wavelengths must be collected one after another. This is often referred to as the multiplexing or Fellget advantage. Additionally, Hadamard spectrometers tend to be more sensitive and the sensors themselves have a higher optical throughput, resulting in what is termed the Jacquinot advantage [22]. **Figure 4** shows the basic layout for this type of spectrometer, the key component of which is the mask positioned just before the focusing lens. This mask blocks out a certain portion (usually ~50%) of the diffracted light at a time. The blocking elements are moved in discrete steps to form a binary matrix where the elements

**Figure 1.** *Dispersive spectrometer based on the Czerny-Turner design [16].*

*Advanced Optical Technologies in Food Quality and Waste Management DOI: http://dx.doi.org/10.5772/intechopen.97624*

**Figure 3.**

*Innovation in the Food Sector Through the Valorization of Food and Agro-Food By-Products*

on a variety of designs.

NIRONE [18].

*2.1.1.2 Dispersive NIR spectrometers*

specificity and applicability to a broad range of sample types has made NIR spectroscopy a popular approach for food analysis [11, 15]. These advantages have in turn encouraged the development of numerous portable NIR spectrometers based

In conventional dispersive spectrometer designs, broadband light is passed through the sample and into an entrance slit to create a narrow line of light which is imaged onto a detector. A dispersive grating is inserted in the path causing the image of the narrow line to be spread out into a spectrum where the light is separated into its various wavelengths. These are then focused onto an array detector. **Figure 1** shows a basic dispersive spectrometer based on the Czerny-Turner design

One major disadvantage of the design shown in **Figure 1** is the need for an array InGaAs detector which can be expensive. Alternative designs requiring only a single-element detector have been developed to help mitigate this expense. For example, instead of collecting all wavelengths simultaneously, spectrometers based on the Fabry-Perot interferometer design shown in **Figure 2** collect the wavelengths sequentially. This design features one fixed and one moveable half-silvered mirror aligned along the same optical axis. As the light bounces between the mirrors, constructive and destructive interference determines the spectrum of light that passes through the other side of the moveable mirror and onto the detector. When the spacing between the fixed and moveable mirrors equals an integer number of half wavelengths, maximum constructive interference occurs leading to a peak in the output spectrum at that wavelength. As with the Michelson interferometer, the spectral range of interest is thus examined by translating the moveable mirror over a specific spatial extent. An example of a compact instrument based on the Fabry-Perot design is Spectral Engines' MEMS Fabry-Perot spectrometer, the

which uses mirrors to minimize the overall size of the design.

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**Figure 1.**

*Dispersive spectrometer based on the Czerny-Turner design [16].*

*Diagram showing the operating principle behind the LVF used in the MicroNIR OnSite [20].*

### *2.1.1.3 Linear variable filter (LVF) NIR spectrometers*

One problem common to all dispersive designs like the Czerny-Turner is that the light must be allowed to disperse over a given spatial extent such that the wavelengths can separate before reaching the detector. This causes limitations in designing for compactness [19]. One way to surmount this problem is with the use of LVFs which are generally formed from wedge-shaped optics and behave much like a Fabry-Perot interferometer but scan by lateral position along the filter instead of by movement of a mirror along the optical axis [12]. The LVF can be applied directly to a detector array, leading to a simple and compact mechanical design with no moving parts (see **Figure 3**). Viavi's MicroNIR OnSite features an LVF applied to a 128-pixel InGaAs array [21].

#### *2.1.1.4 Hadamard spectrometers*

The Hadamard spectrometer design has a couple of key advantages over the conventional dispersive design. First, it overcomes the slow scanning process of dispersive techniques where individual wavelengths must be collected one after another. This is often referred to as the multiplexing or Fellget advantage. Additionally, Hadamard spectrometers tend to be more sensitive and the sensors themselves have a higher optical throughput, resulting in what is termed the Jacquinot advantage [22]. **Figure 4** shows the basic layout for this type of spectrometer, the key component of which is the mask positioned just before the focusing lens. This mask blocks out a certain portion (usually ~50%) of the diffracted light at a time. The blocking elements are moved in discrete steps to form a binary matrix where the elements

**Figure 4.** *Basic design layout for a Hadamard spectrometer [23].*

#### **Figure 5.**

*Hadamard spectrometer designs and devices. (a) a coil and magnet [23] mask design; (b) layout of Texas Instruments' DLP® DMD-based NIRscan Nano optical engine [12].*

of each successive row are shifted by one position to the left. The detector readings are recorded with each shift and pieced together to form a data vector. A Hadamard transformation using the binary matrix is applied to the data matrix to yield the measured spectrum [12].

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*Advanced Optical Technologies in Food Quality and Waste Management*

flipped in sequence to form the Hadamard mask [12].

*2.1.1.5 Applications of handheld NIR spectrometers for food analysis*

The Hadamard mask itself can be implemented in a variety of ways. Early designs used a coil and magnet to move the mask linearly in front of a separate entrance slit (see **Figure 5(a)**). The microPHAZIR NIR spectrometer by Thermo Fisher Scientific uses a programmable MEMS diffraction grating as the Hadamard mask. Texas Instruments offers two NIR Hadamard spectrometer devices based on its Digital Light Projection (DLP®) digital micromirror device (DMD) technology, the DLP NIRscan and the DLP NIRscan Nano (see **Figure 5(b)**) [12]. The DMD contains an array of individually pivotable micromirrors which can be aligned and

Advancement in MEMS technology and LVFs has led to the rapid miniaturization of NIR spectrometers, thus enabling the development of portable NIR spectrometers. Portable NIR spectrometers have been used to evaluate the quality of fruits and vegetables primarily during the pre-harvest stage while on the vine/tree. Such analyses focus on maturity parameters to enable determination of optimal harvest dates and include measurements of soluble solids content (SSC), titratable acidity, pH, weight, size, firmness, juice content, juice weight, pericarp thickness,

NIR analyses of meat and fish are typically performed for shelf-life estimation and freshness evaluation. Examples include traceability analysis of pasture-fed lambs and stall-fed lambs, authenticity testing for pork and pork fat in veal sausages, moisture, protein, and fat analysis in Iberian pork muscles, fat characterization in Iberian ham, freshness evaluation in beef sirloin and beef eye of round, shelf life estimation of pork meat, and monitoring and control of the drying process in

Portable NIR spectrometers have also been used to measure quality factors in milk and beverages. Components such as fat, protein, lactose, and moisture percentages have been measured to determine milk quality [15], and NIR spectral differences have been exploited to distinguish between organic and non-organic milk products [24]. Quality of rice wine, tea drinks, and beers have been evaluated via NIR measurement of alcohol, nitrogen, apparent extract, and non-sugar solids percentages, polyphenol and free amino acid concentrations, and bitterness and

The mid-infrared (MIR) spectrum covers a range of wavelengths from ~2500 nm to ~5000 nm and contains many of the fundamental absorption bands of organic components. Spectra in this range are very sensitive to chemical composition, leading to high specificity. Furthermore, organic functional groups produce well-delineated absorption bands in this region, a feature that can be exploited to individually separate different components present in a mixture by their unique fingerprints in the absorption spectrum [14]. Given the high cost of InGaAs detectors and the need for cooling to lower noise to a manageable level, MIR spectrometers generally feature single-element detector designs. Most exploit a technique based on

One subset of FTIR spectrometers is based on the Michelson interferometer design that was used for Michelson and Morley's speed of light measurements.

*DOI: http://dx.doi.org/10.5772/intechopen.97624*

and others [15].

fermented sausages [13].

beer distinction factors [15].

the Fourier transform.

*2.2.1 Fourier transform infrared spectrometer (FTIR)*

**2.2 Mid-infrared spectroscopy**

*Advanced Optical Technologies in Food Quality and Waste Management DOI: http://dx.doi.org/10.5772/intechopen.97624*

*Innovation in the Food Sector Through the Valorization of Food and Agro-Food By-Products*

of each successive row are shifted by one position to the left. The detector readings are recorded with each shift and pieced together to form a data vector. A Hadamard transformation using the binary matrix is applied to the data matrix to yield the

*Hadamard spectrometer designs and devices. (a) a coil and magnet [23] mask design; (b) layout of Texas* 

*Instruments' DLP® DMD-based NIRscan Nano optical engine [12].*

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**Figure 5.**

**Figure 4.**

*Basic design layout for a Hadamard spectrometer [23].*

measured spectrum [12].

The Hadamard mask itself can be implemented in a variety of ways. Early designs used a coil and magnet to move the mask linearly in front of a separate entrance slit (see **Figure 5(a)**). The microPHAZIR NIR spectrometer by Thermo Fisher Scientific uses a programmable MEMS diffraction grating as the Hadamard mask. Texas Instruments offers two NIR Hadamard spectrometer devices based on its Digital Light Projection (DLP®) digital micromirror device (DMD) technology, the DLP NIRscan and the DLP NIRscan Nano (see **Figure 5(b)**) [12]. The DMD contains an array of individually pivotable micromirrors which can be aligned and flipped in sequence to form the Hadamard mask [12].

#### *2.1.1.5 Applications of handheld NIR spectrometers for food analysis*

Advancement in MEMS technology and LVFs has led to the rapid miniaturization of NIR spectrometers, thus enabling the development of portable NIR spectrometers. Portable NIR spectrometers have been used to evaluate the quality of fruits and vegetables primarily during the pre-harvest stage while on the vine/tree. Such analyses focus on maturity parameters to enable determination of optimal harvest dates and include measurements of soluble solids content (SSC), titratable acidity, pH, weight, size, firmness, juice content, juice weight, pericarp thickness, and others [15].

NIR analyses of meat and fish are typically performed for shelf-life estimation and freshness evaluation. Examples include traceability analysis of pasture-fed lambs and stall-fed lambs, authenticity testing for pork and pork fat in veal sausages, moisture, protein, and fat analysis in Iberian pork muscles, fat characterization in Iberian ham, freshness evaluation in beef sirloin and beef eye of round, shelf life estimation of pork meat, and monitoring and control of the drying process in fermented sausages [13].

Portable NIR spectrometers have also been used to measure quality factors in milk and beverages. Components such as fat, protein, lactose, and moisture percentages have been measured to determine milk quality [15], and NIR spectral differences have been exploited to distinguish between organic and non-organic milk products [24]. Quality of rice wine, tea drinks, and beers have been evaluated via NIR measurement of alcohol, nitrogen, apparent extract, and non-sugar solids percentages, polyphenol and free amino acid concentrations, and bitterness and beer distinction factors [15].

#### **2.2 Mid-infrared spectroscopy**

The mid-infrared (MIR) spectrum covers a range of wavelengths from ~2500 nm to ~5000 nm and contains many of the fundamental absorption bands of organic components. Spectra in this range are very sensitive to chemical composition, leading to high specificity. Furthermore, organic functional groups produce well-delineated absorption bands in this region, a feature that can be exploited to individually separate different components present in a mixture by their unique fingerprints in the absorption spectrum [14]. Given the high cost of InGaAs detectors and the need for cooling to lower noise to a manageable level, MIR spectrometers generally feature single-element detector designs. Most exploit a technique based on the Fourier transform.

#### *2.2.1 Fourier transform infrared spectrometer (FTIR)*

One subset of FTIR spectrometers is based on the Michelson interferometer design that was used for Michelson and Morley's speed of light measurements.

**Figure 6.** *Diagram of an FTIR spectrometer based on the Michelson interferometer [25].*

This interferometer consists of two optical pathways oriented perpendicular to one another (see **Figure 6**). A collimated broadband light source enters from the left and strikes a half-silvered mirror (i.e., beam-splitter) oriented at 45°. Half of the beam then passes through this mirror and strikes a stationary mirror at the end of the pathway where it is reflected back toward the beam-splitter. The other half of the incident beam is directed toward a mirror that is allowed to move back and forth along this pathway. Upon reflection from this mirror and arrival at the beamsplitter, the two reflected beams produce an interference pattern which is focused on a single-element detector. The sample is typically inserted between the beamsplitter and the detector. After the moving mirror is swept through its full range of motion and the full interferogram recorded, these patterns are processed to produce the spectrum. Processing in this case is done with a Fourier transform which converts the sensor response as a function of spatial mirror position to a function of frequency. The Fourier transform accomplishes this by determining the optimal mixture of sine and cosine functions that can replicate the sensor response.

Like Hadamard spectrometry, FTIR spectrometry has several advantages over dispersive spectrometry such as that used in most NIR spectrometers. It enjoys both the multiplexing (Fellget) advantage and the throughput (Jacquinot) advantage. This latter characteristic serves to significantly reduce the noise in the sensor output. As this design includes only one moving component, the mirror in the path with the variable length, FTIR instruments have a mechanical design that it is highly robust to breakdown. Finally, many FTIR instruments include a HeNe laser that acts as an internal calibration standard, eliminating the need for calibration during operation (Connes advantage) [26]. SiWare Systems' NeoSpectra-Scanner is an FTIR NIR spectrometer with a MEMS-based Michelson interferometer design [27].

Another popular design for FTIR spectrometers is based on the property of attenuated total reflection (ATR). As shown in **Figure 7**, broadband infrared light is directed into a high refractive index crystal typically made of germanium, silicon, zinc sulfide, or diamond [28]. The ends of this crystal are cut such that the angle of incidence for the light will result in total internal reflection through the crystal. Although the light wave does not propagate outside of the crystal, an evanescent wave can still pass through the top of the crystal where the sample is placed. This evanescent wave interacts with the sample and absorbs portions of the infrared light, resulting in an attenuation of the light that reaches the detector. One of the primary advantages of this technique is that the light does not have to travel through the entire sample as it does for other designs, which often results in severe

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*Advanced Optical Technologies in Food Quality and Waste Management*

attenuation and loss of signal. An example of a handheld FTIR spectrometer based on the ATR design is the Ocean MZ5, a miniature ATR-FTIR spectrometer produced

MIR spectrometers have long been used for food analysis, but most have been conducted in a laboratory setting. Examples include detection of food spoilage bacteria in meat and dairy produce, brand authentication of a range of Trappist beers, and adulteration of milk and of beef burgers [10]. More recently, portable MIR devices have been used for the simultaneous analysis of sugar and amino acid concentrations in raw potato tubers, the measurement of quality factors in tomato juices, and the measurement of fatty acid content of marine oil dietary

Raman spectroscopy is often seen as complimentary to infrared spectroscopy given the relative nature of the phenomena involved. While infrared spectroscopy measures the absorption of energy, Raman spectroscopy measures the exchange of energy with radiation provided by a monochromatic light source (usually a laser with a wavelength in the ultraviolet to NIR range). This exchange causes a shift in the source's wavelength. Molecules are infrared active only if the vibration induced by the source results in a change to the dipole moment, whereas the Raman shift is caused by changes in the molecules' polarization [10]. Thus, these two methods provide mutually exclusive information. Raman peaks tend to be much sharper than infrared peaks and data collection tends to be faster, but the Raman effect is inherently weaker. Furthermore, Raman spectrometers tend to be more expensive

**Figure 8** shows an example design for a Raman spectrometer. Light from the laser is directed to the sample and the output is passed through a notch filter to separate out all but the Raman scattered light. A spectrograph grating then disperses this light into its constituent wavelengths and onto a detector. Metrohm's Mira M-1 is an example of a portable Raman spectrograph with a 785 nm laser [32]. Laser wavelengths for other Raman spectrometers can range from the ultraviolet (UV) to the NIR bands. Since spectral sensitivity and resolution increase with decreasing laser wavelength,

UV lasers tend to be optimal for applications featuring bio-molecules [33].

*2.2.2 Applications of handheld MIR spectrometers for food analysis*

*DOI: http://dx.doi.org/10.5772/intechopen.97624*

by Ocean Optics [29].

*Diagram illustrating the ATR concept [28].*

**Figure 7.**

supplements [30].

**2.3 Raman spectroscopy**

*2.3.1 Raman spectrometers*

to manufacture than their infrared counterparts.

*Advanced Optical Technologies in Food Quality and Waste Management DOI: http://dx.doi.org/10.5772/intechopen.97624*

**Figure 7.** *Diagram illustrating the ATR concept [28].*

*Innovation in the Food Sector Through the Valorization of Food and Agro-Food By-Products*

This interferometer consists of two optical pathways oriented perpendicular to one another (see **Figure 6**). A collimated broadband light source enters from the left and strikes a half-silvered mirror (i.e., beam-splitter) oriented at 45°. Half of the beam then passes through this mirror and strikes a stationary mirror at the end of the pathway where it is reflected back toward the beam-splitter. The other half of the incident beam is directed toward a mirror that is allowed to move back and forth along this pathway. Upon reflection from this mirror and arrival at the beamsplitter, the two reflected beams produce an interference pattern which is focused on a single-element detector. The sample is typically inserted between the beamsplitter and the detector. After the moving mirror is swept through its full range of motion and the full interferogram recorded, these patterns are processed to produce

*Diagram of an FTIR spectrometer based on the Michelson interferometer [25].*

the spectrum. Processing in this case is done with a Fourier transform which converts the sensor response as a function of spatial mirror position to a function of frequency. The Fourier transform accomplishes this by determining the optimal

mixture of sine and cosine functions that can replicate the sensor response.

Like Hadamard spectrometry, FTIR spectrometry has several advantages over dispersive spectrometry such as that used in most NIR spectrometers. It enjoys both the multiplexing (Fellget) advantage and the throughput (Jacquinot) advantage. This latter characteristic serves to significantly reduce the noise in the sensor output. As this design includes only one moving component, the mirror in the path with the variable length, FTIR instruments have a mechanical design that it is highly robust to breakdown. Finally, many FTIR instruments include a HeNe laser that acts as an internal calibration standard, eliminating the need for calibration during operation (Connes advantage) [26]. SiWare Systems' NeoSpectra-Scanner is an FTIR NIR spectrometer with a MEMS-based Michelson interferometer

Another popular design for FTIR spectrometers is based on the property of attenuated total reflection (ATR). As shown in **Figure 7**, broadband infrared light is directed into a high refractive index crystal typically made of germanium, silicon, zinc sulfide, or diamond [28]. The ends of this crystal are cut such that the angle of incidence for the light will result in total internal reflection through the crystal. Although the light wave does not propagate outside of the crystal, an evanescent wave can still pass through the top of the crystal where the sample is placed. This evanescent wave interacts with the sample and absorbs portions of the infrared light, resulting in an attenuation of the light that reaches the detector. One of the primary advantages of this technique is that the light does not have to travel through the entire sample as it does for other designs, which often results in severe

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design [27].

**Figure 6.**

attenuation and loss of signal. An example of a handheld FTIR spectrometer based on the ATR design is the Ocean MZ5, a miniature ATR-FTIR spectrometer produced by Ocean Optics [29].
