- Email: [email protected]
Nondispersive Infrared Photometer Based on a Rotating Interference Filter for Investigation of Climacteric Fruit Ripening
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 168 (2016) 1223 – 1226
30th Eurosensors Conference, EUROSENSORS 2016
Nondispersive infrared photometer based on a rotating interference filter for investigation of climacteric fruit ripening André Eberhardta, *, Katrin Schmittb, Sven Rademacherb, Jochen Huberb, Marie-Luise Bauersfeldb, Jürgen Wöllensteina, b a
Department of Microsystems Engineering - IMTEK, University of Freiburg, Freiburg, Germany b Fraunhofer Institute for Physical Measurement Techniques IPM, Freiburg, Germany
Abstract In the supply chain of perishable goods, like fruits and vegetables, the measurement of ethylene is important to ensure constant high quality and prevent fast maturation and decay. Ethylene sensors must therefore be sensitive down to the ppb-range, reliable, robust and cheap enough for the use inside a container or storage location. We present a self-referencing single channel nondispersive infrared photometer with focus on system stability for measurement of the ethylene content at 10.53 μm. The selfreferencing is achieved using a rotating narrow-band interference filter swept across the ߥ absorption band. This allows the determination of a baseline for referencing. With our laboratory setup, we could show the proof-of-principle and a good agreement with simulations. A detection limit of 50 ppm was achieved with an optical path of 124.8 cm. Based on these results, simulations showed that resolutions down to ppb-range can be achieved with an improved optical long path cell and optimized integrated electronics for signal noise reduction. ©2016 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: gassenor; infrared; photometer; NDIR; tilting filter; ethylene; fruit storage
1. Introduction The monitoring of the ethylene concentration during transport and storage of fruits is very important for their quality [1]. Ethylene (C2H4) is a gaseous phytohormone and besides carbon dioxide (CO2) and oxygen (O2) an
* Corresponding author. Tel.: +49 761 8857 580; fax: +49 761 8857 224. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.423
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André Eberhardt et al. / Procedia Engineering 168 (2016) 1223 – 1226
important variable in the ripening process of climacteric fruits [2]. The C2H4 emission depends strongly on the ripening phase and the species. Concentrations range from the lower ppb-range in the pre-climacteric phase (unripe fruits) to the higher ppm-range in the climacteric phase [3, 4]. In overripe fruit the C2H4-emission decreases, so that the C2H4 concentration is a good indicator for the ripening state. On the other side, C2H4 is used for ripening on demand [2]. Commonly used techniques for C2H4 sensing are gas chromatography and electrochemical sensors. Both technologies are able to detect concentrations down to the ppb-range, but require high-maintenance and are costintensive. In comparison to these chemical technologies, nondispersive (NDIR) optical sensors seem to be a robust and versatile alternative. They offer a very simple setup and can be built using low-cost components [5]. Fonollosa et al. reported a lower detection limit of 30 ppm using an optical long path cell in combination with a four-channel thermopile array [6]. Yet detector arrays have some disadvantages, e.g. the received irradiance power is divided by the number of measurement channels, and possible different long-term drift behavior between the channels. 2. Measurement principle Figure 1 (left) shows the scheme of the sensor system. It consists of a broadband radiation source (1), e.g. thermal emitter, an optical chopper (2), a rotating interference filter (3), a measurement chamber (4) to provide a defined optical path and a broadband detector (5) for conversion of the radiation into an electrical property. The signal generated by the sensor (ܷ௨௧ ) follows equation 1: ାஶ
ܷ௨௧ (ߠ) ି ןஶ ܧ (ߣ) ܶ ڄ௦ (ߣ) ܶ ڄ௧ (ߣ, ߠ) ܴ ڄௗ௧ (ߣ, ܶ)݀ߣ,
(1)
It is proportional to the total incident radiation within the bandwidth of the detector, which is the result of the multiplication of the emission spectrum of the radiation source (ܧ ), the transmission spectrum of the gas in the measurement chamber, which itself depends on the concentration, optical path, temperature and pressure, the filters transmission spectrum ൫ܶ௧ ൯ and the response of the detector(ܴௗ௧ ). The transmission profile of the interference filter depends on the rotation angle in the optical path. It behaves like a thin Fabry-Pérot etalon [7]. The peak transmission wavelength (ߣ ) follows equation 2: ߣ (ߠ) =
ఒ,బ
ଶ െ ݊݅ݏଶ ߠ. ට݊
(2)
The wavelength shift depends on the effective refractive index ൫݊ ൯ and wavelength under normal incidence ൫ߣ, ൯. The center wavelength is shifted to smaller wavelengths with increasing rotation angles (ߠ). Figure 1 (right) 500 ppm, 124.8cm, NTP
1.0
1
2
4 3
transmission
0.8 C2H4
0.6
1.35° 31.35° 51.35° 71.35°
0.4 0.2 0.0 9
10 11 wavelength /μm
12
Fig 1. (left) Scheme of the basic setup of the implemented NDIR photometer – 1 radiation source, 2 optical chopper, 3 rotating interference filter, 4 measurement chamber and 5 detector. (right) FTIR-measurements of the interference filter at different angles in comparison to the transmission spectrum of C2H4 for 5000 ppm and an optical path of 124.8 cm under normal conditions (NTP).
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André Eberhardt et al. / Procedia Engineering 168 (2016) 1223 – 1226
/
4.5
raw signal baseline
0.3
4.0
absorption
measurement signal /V
5.0
3.5 3.0
0.2 0.1 0.0
2.5 2.0
-0.1 -50
-25
0 25 rotation angle /°
50
-50
-25
0 25 rotation angle /°
50
Fig. 2. (left) Baseline fit algorithm for an example spectrum at 5000 ppm. (right) Calculated absorption spectrum for the presented example. The indicated areas show the analyzed part of the absorption spectrum.
depicts the measured transmission profiles, at 1.35 °, 30.35 °, 50.35 ° and 70.35 ° angle of incidence, in comparison to a simulated spectrum of C2H4 (500 ppm, 124.8 cm, NTP) using a Bruker IFS 66 v/S Fourier Transform Infrared Spectrometer. In addition to the wavelength shift of up to 750 nm (70 °), the peak transmission decreases and the spectral width increases with the rotation angle, which was reported by Lissberger [8, 9]. The ߥ -band of C2H4 is well suited for use of rotating filters, because there is a strong peak in the center of the band at 10.53 μm, which is about three times stronger than the surrounding lines. The measured spectrum results of the radiation intensity and detector responsivity weighted convolution of the transmission spectrum with the filter transmission peak. An example measurement for 5000 ppm C2H4 in nitrogen (NTP, 124.8 cm) is presented in Figure 2 (left). The signal is increasing with increasing absolute values of the rotation angle (negative wavelength shift) due to the increasing intensity of the black body source. The main absorption peaks at 10.54 μm can be recognized at r32.4 °. In order to gain the absorption signal, a virtual baseline was fitted to the raw signal, as it is known from tunable diode laser spectrometers [10]. The indicated points at the lateral borders of the absorption peaks where used for the fit a polynomial (4th order). The mean value of the integral of the indicated areas of the absorption spectrum on the right side of Figure 2 corresponds to the gas concentration. 3. Measurement setup and results In order to evaluate the presented concept, a laboratory setup was built. The photograph in figure Figure 3 shows the optical part of the setup. It consists of a broadband thermal source (1), Hawkeye Technologies IR-30, with an emission area of 1.8 x 1.8 mm². Due to the slow thermal time constant, an optical chopper (MC2000B, Thorlabs), was used to modulate the signal with 3000 Hz. The radiation is projected on the input aperture of the optical long path cell (2) with an adapted optical path of 124.8 cm [11]. The signal was afterwards guided through the filter rotation unit (3) to the detector (4). The signal of the Detector (PVM - 2TE) is amplified using a voltage amplifier (DHPVA-100, FEMTO) and forwarded to a lock in amplifier (7265, EG&G Instruments). A measurement rack (Ni PXI-1042Q with Ni PXI 4461 cards) was used to collect the output signal of the lock-in amplifier. Measurements were performed using 5000 ppm C2H4 calibration gas in a gas mixing station. The temperature was kept constant at 24 °C during all measurements. Concentrations from 5000 ppm to 1000 ppm in 500 ppm steps and from 1000 ppm to 100 ppm in 100 ppm steps, alternating with pure N2 steps, were analyzed. The cycle was repeated after 12 hours. Figure 3 presents the results. The main goal of the low signal drift, 1.2 %±0.7 %, which corresponds to 4.6 ppm, and a detection limit of 50 ppm (1V) was achieved.
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André Eberhardt et al. / Procedia Engineering 168 (2016) 1223 – 1226 cycle 1 cycle 2
60
absorption
50 40 30 20 10 0 0
1000
2000 3000 4000 concentration /ppm
5000
Fig. 3. (left) Photography of the measurement systems components – 1 IR-emitter with optical chopper, 2 optical long path cell, 3 filter rotation unit and 4 thermoelectrically cooled MCT-detector. The grey line on the right side of the detector indicates a radiation blocker. (right) Results of a measurement of C2H4 in N2 in concentrations from 5000 ppm to 100 ppm. Two measurement cycles with a break of 12 hours are compared.
4. Conclusion and discussion We presented a self-referencing NDIR photometer for detection of C2H4 based on a rotating interference filter. The rotation causes a center wavelength tuning of the filter. A filter with a center wavelength of 10.778 μm, a bandwidth of 88 nm and an effective refractive index of 2.56, a tuning range of 750 nm was used for characterization of the main absorption peak at 10.53 μm. We used a virtual baseline to obtain the absorption signal from the measured raw signal. Lab measurements with the binary gas mixture C2H4/N2 revealed a good drift resistance, under lab conditions and a detection limit down to 50 ppm. For the first prototype a new measurement cell with an increased optical path of 4 m and a larger input aperture of 4.0 x 4.0 mm2 has been designed. Optimized integrated electronics and improvements in the signal processing are planned to reach the goal of a long-term stability with a resolution below 10 ppm. Acknowledgements This work was supported by a grant of the Georg H. Endress Fundation for investigation of the sustainable food production within the InnoSens project. References [1] J. Gustavsson, et al., Global Food Losses and Food Waste, 2011, ISBN 978-92-5-107205-9 [2] M.E. Saltveit, Effect of ethylene on quality of fresh fruits and vegetables, Postharvest Biol. Tec., 15 (1999) 279-292 [3] S. Janssen, et al., Ethylene detection in fruit supply chains, Phil. Trans., R. Soc. A, 372 (2014), 20130311 [4] R. Jedermann, et al., Applying autonomous sensor systems in logistics – Combining sensor networks, RFIDs and software agents, Sens. Actuators, A, 132 (2006) 370-375 [5] J. Hodgkinson, R.P. Tatam, Optical gas sensing: a review, Meas. Sci. Technol., 24 (2013) 012004 (59pp) [6] J. Fonollosa, et al., Ethylene optical spectrometer for apple ripening monitoring in controlled atmosphere store-houses, Sens. Actuators, B, 136 (2009) 546-554 [7] X. Baillard, et al., Interference-filter-stabilized external-cavity diode lasers, Opt. Commun., 266 (2006) 609-613 [8] P.H. Lissberger, Properties of All-Dielectric Interference Filters. I. A New Method of Calculation, J. Opt. Soc. Am., 49 (1959) 121-125 [9] P.H. Lissberger, W.L. Wilcock, Properties of All-Dielectric Interference Filters. II. Filters in Parallel Beams of Light Incident Obliquely and in Convergent Beams, J. Opt. Soc. Am., 49 (1959) 126-130 [10] R. Claps, et al., Ammonia detection by use of near-infrared diode-laser-based overtone spectroscopy, Appl. Opt., 40 (2001) 4387-4394 [11] J. Hildenbrand, et al., A compact optical multichannel system for ethylene monitoring, microsyst. Technol., 14 (2008), 637-64
ScienceDirect Procedia Engineering 168 (2016) 1223 – 1226
30th Eurosensors Conference, EUROSENSORS 2016
Nondispersive infrared photometer based on a rotating interference filter for investigation of climacteric fruit ripening André Eberhardta, *, Katrin Schmittb, Sven Rademacherb, Jochen Huberb, Marie-Luise Bauersfeldb, Jürgen Wöllensteina, b a
Department of Microsystems Engineering - IMTEK, University of Freiburg, Freiburg, Germany b Fraunhofer Institute for Physical Measurement Techniques IPM, Freiburg, Germany
Abstract In the supply chain of perishable goods, like fruits and vegetables, the measurement of ethylene is important to ensure constant high quality and prevent fast maturation and decay. Ethylene sensors must therefore be sensitive down to the ppb-range, reliable, robust and cheap enough for the use inside a container or storage location. We present a self-referencing single channel nondispersive infrared photometer with focus on system stability for measurement of the ethylene content at 10.53 μm. The selfreferencing is achieved using a rotating narrow-band interference filter swept across the ߥ absorption band. This allows the determination of a baseline for referencing. With our laboratory setup, we could show the proof-of-principle and a good agreement with simulations. A detection limit of 50 ppm was achieved with an optical path of 124.8 cm. Based on these results, simulations showed that resolutions down to ppb-range can be achieved with an improved optical long path cell and optimized integrated electronics for signal noise reduction. ©2016 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license © Published by Elsevier Ltd. This (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: gassenor; infrared; photometer; NDIR; tilting filter; ethylene; fruit storage
1. Introduction The monitoring of the ethylene concentration during transport and storage of fruits is very important for their quality [1]. Ethylene (C2H4) is a gaseous phytohormone and besides carbon dioxide (CO2) and oxygen (O2) an
* Corresponding author. Tel.: +49 761 8857 580; fax: +49 761 8857 224. E-mail address: [email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.423
1224
André Eberhardt et al. / Procedia Engineering 168 (2016) 1223 – 1226
important variable in the ripening process of climacteric fruits [2]. The C2H4 emission depends strongly on the ripening phase and the species. Concentrations range from the lower ppb-range in the pre-climacteric phase (unripe fruits) to the higher ppm-range in the climacteric phase [3, 4]. In overripe fruit the C2H4-emission decreases, so that the C2H4 concentration is a good indicator for the ripening state. On the other side, C2H4 is used for ripening on demand [2]. Commonly used techniques for C2H4 sensing are gas chromatography and electrochemical sensors. Both technologies are able to detect concentrations down to the ppb-range, but require high-maintenance and are costintensive. In comparison to these chemical technologies, nondispersive (NDIR) optical sensors seem to be a robust and versatile alternative. They offer a very simple setup and can be built using low-cost components [5]. Fonollosa et al. reported a lower detection limit of 30 ppm using an optical long path cell in combination with a four-channel thermopile array [6]. Yet detector arrays have some disadvantages, e.g. the received irradiance power is divided by the number of measurement channels, and possible different long-term drift behavior between the channels. 2. Measurement principle Figure 1 (left) shows the scheme of the sensor system. It consists of a broadband radiation source (1), e.g. thermal emitter, an optical chopper (2), a rotating interference filter (3), a measurement chamber (4) to provide a defined optical path and a broadband detector (5) for conversion of the radiation into an electrical property. The signal generated by the sensor (ܷ௨௧ ) follows equation 1: ାஶ
ܷ௨௧ (ߠ) ି ןஶ ܧ (ߣ) ܶ ڄ௦ (ߣ) ܶ ڄ௧ (ߣ, ߠ) ܴ ڄௗ௧ (ߣ, ܶ)݀ߣ,
(1)
It is proportional to the total incident radiation within the bandwidth of the detector, which is the result of the multiplication of the emission spectrum of the radiation source (ܧ ), the transmission spectrum of the gas in the measurement chamber, which itself depends on the concentration, optical path, temperature and pressure, the filters transmission spectrum ൫ܶ௧ ൯ and the response of the detector(ܴௗ௧ ). The transmission profile of the interference filter depends on the rotation angle in the optical path. It behaves like a thin Fabry-Pérot etalon [7]. The peak transmission wavelength (ߣ ) follows equation 2: ߣ (ߠ) =
ఒ,బ
ଶ െ ݊݅ݏଶ ߠ. ට݊
(2)
The wavelength shift depends on the effective refractive index ൫݊ ൯ and wavelength under normal incidence ൫ߣ, ൯. The center wavelength is shifted to smaller wavelengths with increasing rotation angles (ߠ). Figure 1 (right) 500 ppm, 124.8cm, NTP
1.0
1
2
4 3
transmission
0.8 C2H4
0.6
1.35° 31.35° 51.35° 71.35°
0.4 0.2 0.0 9
10 11 wavelength /μm
12
Fig 1. (left) Scheme of the basic setup of the implemented NDIR photometer – 1 radiation source, 2 optical chopper, 3 rotating interference filter, 4 measurement chamber and 5 detector. (right) FTIR-measurements of the interference filter at different angles in comparison to the transmission spectrum of C2H4 for 5000 ppm and an optical path of 124.8 cm under normal conditions (NTP).
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André Eberhardt et al. / Procedia Engineering 168 (2016) 1223 – 1226
/
4.5
raw signal baseline
0.3
4.0
absorption
measurement signal /V
5.0
3.5 3.0
0.2 0.1 0.0
2.5 2.0
-0.1 -50
-25
0 25 rotation angle /°
50
-50
-25
0 25 rotation angle /°
50
Fig. 2. (left) Baseline fit algorithm for an example spectrum at 5000 ppm. (right) Calculated absorption spectrum for the presented example. The indicated areas show the analyzed part of the absorption spectrum.
depicts the measured transmission profiles, at 1.35 °, 30.35 °, 50.35 ° and 70.35 ° angle of incidence, in comparison to a simulated spectrum of C2H4 (500 ppm, 124.8 cm, NTP) using a Bruker IFS 66 v/S Fourier Transform Infrared Spectrometer. In addition to the wavelength shift of up to 750 nm (70 °), the peak transmission decreases and the spectral width increases with the rotation angle, which was reported by Lissberger [8, 9]. The ߥ -band of C2H4 is well suited for use of rotating filters, because there is a strong peak in the center of the band at 10.53 μm, which is about three times stronger than the surrounding lines. The measured spectrum results of the radiation intensity and detector responsivity weighted convolution of the transmission spectrum with the filter transmission peak. An example measurement for 5000 ppm C2H4 in nitrogen (NTP, 124.8 cm) is presented in Figure 2 (left). The signal is increasing with increasing absolute values of the rotation angle (negative wavelength shift) due to the increasing intensity of the black body source. The main absorption peaks at 10.54 μm can be recognized at r32.4 °. In order to gain the absorption signal, a virtual baseline was fitted to the raw signal, as it is known from tunable diode laser spectrometers [10]. The indicated points at the lateral borders of the absorption peaks where used for the fit a polynomial (4th order). The mean value of the integral of the indicated areas of the absorption spectrum on the right side of Figure 2 corresponds to the gas concentration. 3. Measurement setup and results In order to evaluate the presented concept, a laboratory setup was built. The photograph in figure Figure 3 shows the optical part of the setup. It consists of a broadband thermal source (1), Hawkeye Technologies IR-30, with an emission area of 1.8 x 1.8 mm². Due to the slow thermal time constant, an optical chopper (MC2000B, Thorlabs), was used to modulate the signal with 3000 Hz. The radiation is projected on the input aperture of the optical long path cell (2) with an adapted optical path of 124.8 cm [11]. The signal was afterwards guided through the filter rotation unit (3) to the detector (4). The signal of the Detector (PVM - 2TE) is amplified using a voltage amplifier (DHPVA-100, FEMTO) and forwarded to a lock in amplifier (7265, EG&G Instruments). A measurement rack (Ni PXI-1042Q with Ni PXI 4461 cards) was used to collect the output signal of the lock-in amplifier. Measurements were performed using 5000 ppm C2H4 calibration gas in a gas mixing station. The temperature was kept constant at 24 °C during all measurements. Concentrations from 5000 ppm to 1000 ppm in 500 ppm steps and from 1000 ppm to 100 ppm in 100 ppm steps, alternating with pure N2 steps, were analyzed. The cycle was repeated after 12 hours. Figure 3 presents the results. The main goal of the low signal drift, 1.2 %±0.7 %, which corresponds to 4.6 ppm, and a detection limit of 50 ppm (1V) was achieved.
1226
André Eberhardt et al. / Procedia Engineering 168 (2016) 1223 – 1226 cycle 1 cycle 2
60
absorption
50 40 30 20 10 0 0
1000
2000 3000 4000 concentration /ppm
5000
Fig. 3. (left) Photography of the measurement systems components – 1 IR-emitter with optical chopper, 2 optical long path cell, 3 filter rotation unit and 4 thermoelectrically cooled MCT-detector. The grey line on the right side of the detector indicates a radiation blocker. (right) Results of a measurement of C2H4 in N2 in concentrations from 5000 ppm to 100 ppm. Two measurement cycles with a break of 12 hours are compared.
4. Conclusion and discussion We presented a self-referencing NDIR photometer for detection of C2H4 based on a rotating interference filter. The rotation causes a center wavelength tuning of the filter. A filter with a center wavelength of 10.778 μm, a bandwidth of 88 nm and an effective refractive index of 2.56, a tuning range of 750 nm was used for characterization of the main absorption peak at 10.53 μm. We used a virtual baseline to obtain the absorption signal from the measured raw signal. Lab measurements with the binary gas mixture C2H4/N2 revealed a good drift resistance, under lab conditions and a detection limit down to 50 ppm. For the first prototype a new measurement cell with an increased optical path of 4 m and a larger input aperture of 4.0 x 4.0 mm2 has been designed. Optimized integrated electronics and improvements in the signal processing are planned to reach the goal of a long-term stability with a resolution below 10 ppm. Acknowledgements This work was supported by a grant of the Georg H. Endress Fundation for investigation of the sustainable food production within the InnoSens project. References [1] J. Gustavsson, et al., Global Food Losses and Food Waste, 2011, ISBN 978-92-5-107205-9 [2] M.E. Saltveit, Effect of ethylene on quality of fresh fruits and vegetables, Postharvest Biol. Tec., 15 (1999) 279-292 [3] S. Janssen, et al., Ethylene detection in fruit supply chains, Phil. Trans., R. Soc. A, 372 (2014), 20130311 [4] R. Jedermann, et al., Applying autonomous sensor systems in logistics – Combining sensor networks, RFIDs and software agents, Sens. Actuators, A, 132 (2006) 370-375 [5] J. Hodgkinson, R.P. Tatam, Optical gas sensing: a review, Meas. Sci. Technol., 24 (2013) 012004 (59pp) [6] J. Fonollosa, et al., Ethylene optical spectrometer for apple ripening monitoring in controlled atmosphere store-houses, Sens. Actuators, B, 136 (2009) 546-554 [7] X. Baillard, et al., Interference-filter-stabilized external-cavity diode lasers, Opt. Commun., 266 (2006) 609-613 [8] P.H. Lissberger, Properties of All-Dielectric Interference Filters. I. A New Method of Calculation, J. Opt. Soc. Am., 49 (1959) 121-125 [9] P.H. Lissberger, W.L. Wilcock, Properties of All-Dielectric Interference Filters. II. Filters in Parallel Beams of Light Incident Obliquely and in Convergent Beams, J. Opt. Soc. Am., 49 (1959) 126-130 [10] R. Claps, et al., Ammonia detection by use of near-infrared diode-laser-based overtone spectroscopy, Appl. Opt., 40 (2001) 4387-4394 [11] J. Hildenbrand, et al., A compact optical multichannel system for ethylene monitoring, microsyst. Technol., 14 (2008), 637-64