@article{AdebayoHashimHassetal.2017,
  author    = {Adebayo, Segun Emmanuel and Hashim, Norhashila and Hass, Roland and Reich, Oliver and Regen, Christian and M{\"u}nzberg, Marvin and Abdan, Khalina and Hanafi, Marsyita and Zude-Sasse, Manuela},
  title     = {Using absorption and reduced scattering coefficients for non-destructive analyses of fruit flesh firmness and soluble solids content in pear},
  series = {Postharvest Biology and Technology},
  volume    = {130},
  journal   = {Postharvest Biology and Technology},
  publisher = {Elsevier},
  address   = {Amsterdam},
  issn      = {0925-5214},
  doi       = {10.1016/j.postharvbio.2017.04.004},
  pages     = {56 -- 63},
  year      = {2017},
  abstract  = {Quality attributes of fruit determine its acceptability by the retailer and consumer. The objective of this work was to investigate the potential of absorption (μa) and reduced scattering (μs') coefficients of European pear to analyze its fruit flesh firmness and soluble solids content (SSC). The absolute reference values, μa* (cm-1) and μs'* (cm-1), of pear were invasively measured, employing multi-spectral photon density wave (PDW) spectroscopy at preselected wavelengths of 515, 690, and 940 nm considering two batches of unripe and overripe fruit. On eight measuring dates during fruit development, μa and μs' were analyzed non-destructively by means of laser light backscattering imaging (LLBI) at similar wavelengths of 532, 660, and 830 nm by means of fitting according to Farrell's diffusion theory, using fix reference values of either μa* or μs'*. Both, the μa* and the μa as well as μs'* and μs' showed similar trends. Considering the non-destructively measured data during fruit development, μa at 660 nm decreased 91 till 141 days after full bloom (dafb) from 1.49 cm-1 to 0.74 cm-1 due to chlorophyll degradation. At 830 nm, μa only slightly decreased from 0.41 cm-1 to 0.35 cm-1. The μs' at all wavelengths revealed a decreasing trend as the fruit developed. The difference measured at 532 nm was most pronounced decreasing from 24 cm-1 to 10 cm-1, while at 660 nm and 830 nm values decreased from 15 cm-1 to 13 cm-1 and from 10 cm-1 to 8 cm-1, respectively. When building calibration models with partial least-squares regression analysis on the optical properties for non-destructive analysis of the fruit SSC, μa at 532 nm and 830 nm resulted in a correlation coefficient of R = 0.66, however, showing high measuring uncertainty. The combination of all three wavelengths gave an enhanced, encouraging R = 0.89 for firmness analysis using μs' in the freshly picked fruit.},
  language  = {en}
}
@article{LoehmannsroebenSchaelReichetal.2000,
  author    = {L{\"o}hmannsr{\"o}ben, Hans-Gerd and Schael, Frank and Reich, Oliver and Lemke, Matthias and Schober, Lars},
  title     = {Untersuchung von organischer Bodensubstanz und Bodengasen mit laser-spektroskopischer in-situ Meßtechnik},
  year      = {2000},
  language  = {de}
}
@incollection{AscheBoeckmannLaueetal.2000,
  author    = {Asche, Hartmut and B{\"o}ckmann, Christine and Laue, Steffen and L{\"o}hmannsr{\"o}ben, Hans-Gerd and Lemke, Matthias and Schober, Lars and Reich, Oliver and L{\"u}ck, Erika and Sch{\"u}tte, Marc and Domsch, Horst and Makower, Alexander and Scheller, Frieder W. and St{\"o}cklein, Wolfgang and Wollenberger, Ursula and Schultze, Rainer and Hengstermann, Theo and Schael, Frank},
  title     = {Umweltforschung f{\"u}r das Land Brandenburg : Projekt Umweltanalytik / Umweltmeßtechnik / Informationssysteme},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-3862},
  publisher      = {Universit{\"a}t Potsdam},
  pages     = {176 -- 227},
  year      = {2000},
  language  = {de}
}
@article{BresselReich2014,
  author    = {Bressel, Lena and Reich, Oliver},
  title     = {Theoretical and experimental study of the diffuse transmission of light through highly concentrated absorbing and scattering materials Part I: Monte-Carlo simulations},
  series = {Journal of quantitative spectroscopy \& radiative transfer},
  volume    = {146},
  journal   = {Journal of quantitative spectroscopy \& radiative transfer},
  publisher = {Elsevier},
  address   = {Oxford},
  issn      = {0022-4073},
  doi       = {10.1016/j.jqsrt.2014.01.007},
  pages     = {190 -- 198},
  year      = {2014},
  abstract  = {In many technical materials and commercial products like sunscreen or paint high particle and absorber concentrations are present. An important parameter for slabs of these materials is the diffuse transmission of light, which quantifies the total amount of directly and diffusely transmitted light. Due to the high content of scattering particles not only multiple scattering but also additional dependent scattering occurs. Hence, simple analytical models cannot be applied to calculate the diffuse transmission. In this work a Monte-Carlo program for the calculation of the diffuse transmission of light through dispersions in slab-like geometry containing high concentrations of scattering particles and absorbers is presented and discussed in detail. Mie theory is applied for the calculation of the scattering properties of the samples. Additionally, dependent scattering is considered in two different models, the well-known hard sphere model in the Percus-Yevick approximation (HSPYA) and the Yukawa model in the Mean Spherical Approximation (YMSA). Comparative experiments will show the accurateness of the program as well as its applicability to real samples [1]. (C) 2014 Elsevier Ltd. All rights reserved.},
  language  = {en}
}
@misc{WernerMuenzbergHassetal.2016,
  author    = {Werner, Peter and M{\"u}nzberg, Marvin and Hass, Roland and Reich, Oliver},
  title     = {Process analytical approaches for the coil-to-globule transition of poly(N-isopropylacrylamide) in a concentrated aqueous suspension},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {944},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-43116},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-431162},
  pages     = {807 -- 819},
  year      = {2016},
  abstract  = {The coil-to-globule transition of poly(N-isopropylacrylamide) (PNIPAM) microgel particles suspended in water has been investigated in situ as a function of heating and cooling rate with four optical process analytical technologies (PAT), sensitive to structural changes of the polymer. Photon Density Wave (PDW) spectroscopy, Focused Beam Reflectance Measurements (FBRM), turbidity measurements, and Particle Vision Microscope (PVM) measurements are found to be powerful tools for the monitoring of the temperature-dependent transition of such thermo-responsive polymers. These in-line technologies allow for monitoring of either the reduced scattering coefficient and the absorption coefficient, the chord length distribution, the reflected intensities, or the relative backscatter index via in-process imaging, respectively. Varying heating and cooling rates result in rate-dependent lower critical solution temperatures (LCST), with different impact of cooling and heating. Particularly, the data obtained by PDW spectroscopy can be used to estimate the thermodynamic transition temperature of PNIPAM for infinitesimal heating or cooling rates. In addition, an inverse hysteresis and a reversible building of micrometer-sized agglomerates are observed for the PNIPAM transition process.},
  language  = {en}
}
@misc{HassSandmannReich2017,
  author    = {Hass, Roland and Sandmann, Michael and Reich, Oliver},
  title     = {Photonic sensing in highly concentrated biotechnical processes by photon density wave spectroscopy},
  series = {Proceedings SPIE 10323, 25th International Conference on Optical Fiber Sensors},
  volume    = {10323},
  journal   = {Proceedings SPIE 10323, 25th International Conference on Optical Fiber Sensors},
  publisher = {IEEE},
  address   = {New York},
  isbn      = {978-1-5090-4850-2},
  issn      = {0277-786X},
  doi       = {10.1117/12.2263617},
  pages     = {4},
  year      = {2017},
  abstract  = {Photon Density Wave (PDW) spectroscopy is introduced as a new approach for photonic sensing in highly concentrated biotechnical processes. It independently quantifies the absorption and reduced scattering coefficient calibration-free and as a function of time, thus describing the optical properties in the vis/NIR range of the biomaterial during their processing. As examples of industrial relevance, enzymatic milk coagulation, beer mashing, and algae cultivation in photo bioreactors are discussed.},
  language  = {en}
}
@article{HassReich2011,
  author    = {Hass, Roland and Reich, Oliver},
  title     = {Photon density wave spectroscopy for dilution-free sizing of highly concentrated nanoparticles during starved-feed polymerization},
  series = {ChemPhysChem : a European journal of chemical physics and physical chemistry},
  volume    = {12},
  journal   = {ChemPhysChem : a European journal of chemical physics and physical chemistry},
  number    = {14},
  publisher = {Wiley-Blackwell},
  address   = {Malden},
  issn      = {1439-4235},
  doi       = {10.1002/cphc.201100323},
  pages     = {2572 -- 2575},
  year      = {2011},
  language  = {en}
}
@article{SchaelEngelhardReich2001,
  author    = {Schael, Frank and Engelhard, Sonja and Reich, Oliver},
  title     = {Optische Sensorik f{\"u}r die in-situ-Analytik von Brauprozessen},
  year      = {2001},
  language  = {de}
}
@misc{ReichLoehmannsroebenSchael2003,
  author    = {Reich, Oliver and L{\"o}hmannsr{\"o}ben, Hans-Gerd and Schael, Frank},
  title     = {Optical sensing with photon density waves: investigation of model media},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-13147},
  year      = {2003},
  abstract  = {Investigations with frequency domain photon density waves allow elucidation of absorption and scattering properties of turbid media. The temporal and spatial propagation of intensity modulated light with frequencies up to more than 1 GHz can be described by the P1 approximation to the Boltzmann transport equation. In this study, we establish requirements for the appropriate choice of turbid model media and characterize mixtures of isosulfan blue as absorber and polystyrene beads as scatterer. For these model media, the independent determination of absorption and reduced scattering coefficients over large absorber and scatterer concentration ranges is demonstrated with a frequency domain photon density wave spectrometer employing intensity and phase measurements at various modulation frequencies.},
  language  = {en}
}
@article{RuizHassReich2012,
  author    = {Ruiz, Salome Vargas and Hass, Roland and Reich, Oliver},
  title     = {Optical monitoring of milk fat phase transition within homogenized fresh milk by Photon Density Wave spectroscopy},
  series = {International dairy journal},
  volume    = {26},
  journal   = {International dairy journal},
  number    = {2},
  publisher = {Elsevier},
  address   = {Oxford},
  issn      = {0958-6946},
  doi       = {10.1016/j.idairyj.2012.03.012},
  pages     = {120 -- 126},
  year      = {2012},
  abstract  = {Photon Density Wave (PDW) spectroscopy was applied for temperature dependent monitoring of melting and crystallization of milk fat within homogenized fresh milk. As an in-line process analytical technique, PDW spectroscopy quantifies continuously the optical properties of turbid material, providing an insight into its structural processes. Here, the measured absorption coefficients reflect temperature as well as fat content of milk and the reduced scattering coefficients probe physical changes of the light scattering fat droplets and casein micelles. Thermal processing reveals breakpoints within the temperature trend of the reduced scattering coefficient of fat containing milk. Found at 16 degrees C and 24 degrees C while cooling and heating, respectively, they are associated to the phase transitions of milk fat. Continuous isothermal measurement of the optical coefficients showed that the crystallization process requires several hours. The strongly changing reduced scattering coefficient implies that the thermal history of milk will have a major impact on any method based on light scattering as quantitative analytical technique.},
  language  = {en}
}
@article{HassMunzkeRuizetal.2015,
  author    = {Hass, Roland and Munzke, Dorit and Ruiz, Salome Vargas and Tippmann, Johannes and Reich, Oliver},
  title     = {Optical monitoring of chemical processes in turbid biogenic liquid dispersions by Photon Density Wave spectroscopy},
  series = {Analytical \& bioanalytical chemistry},
  volume    = {407},
  journal   = {Analytical \& bioanalytical chemistry},
  number    = {10},
  publisher = {Springer},
  address   = {Heidelberg},
  issn      = {1618-2642},
  doi       = {10.1007/s00216-015-8513-9},
  pages     = {2791 -- 2802},
  year      = {2015},
  abstract  = {In turbid biogenic liquid material, like blood or milk, quantitative optical analysis is often strongly hindered by multiple light scattering resulting from cells, particles, or droplets. Here, optical attenuation is caused by losses due to absorption as well as scattering of light. Fiber-based Photon Density Wave (PDW) spectroscopy is a very promising method for the precise measurement of the optical properties of such materials. They are expressed as absorption and reduced scattering coefficients (mu (a) and mu (s)', respectively) and are linked to the chemical composition and physical properties of the sample. As a process analytical technology, PDW spectroscopy can sense chemical and/or physical processes within such turbid biogenic liquids, providing new scientific insight and process understanding. Here, for the first time, several bioprocesses are analyzed by PDW spectroscopy and the resulting optical coefficients are discussed with respect to established mechanistic models of the chosen processes. As model systems, enzymatic casein coagulation in milk, temperature-induced starch hydrolysis in beer mash, and oxy- as well as deoxygenation of human donor blood were investigated by PDW spectroscopy. The findings indicate that also for very complex biomaterials (i.e., not well-defined model materials like monodisperse polymer dispersions), obtained optical coefficients allow for the assessment of a structure/process relationship and thus for a new analytical access to biogenic liquid material. This is of special relevance as PDW spectroscopy data are obtained without any dilution or calibration, as often found in conventional spectroscopic approaches.},
  language  = {en}
}
@misc{LoehmannsroebenKantorKumkeetal.2005,
  author    = {L{\"o}hmannsr{\"o}ben, Hans-Gerd and Kantor, Zoltan and Kumke, Michael Uwe and Schm{\"a}lzlin, Elmar and Reich, Oliver},
  title     = {OPQS - optische Prozess- und Qualit{\"a}ts-Sensorik},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-13205},
  year      = {2005},
  abstract  = {Im vorliegenden Beitrag wird an Hand dreier Beispiele der Einsatz von optischer Sensorik zur Produktcharakterisierung dargestellt, n{\"a}mlich Untersuchungen zum O2-Gehalt in Fruchts{\"a}ften, zur Isotopiesignatur von CO2 in Mineralw{\"a}ssern und zu Lichtstreueigenschaften eines Sonnenschutzmittels. Inhalt: Bestimmung von O2 mit Lumineszenzsonden Isotopenselektive Bestimmung von CO2 mit TDLAS Optische Charakterisierung stark streuender Materialien mit Photonendichtewellen},
  language  = {de}
}
@article{SchaelReichLoehmannsroeben2000,
  author    = {Schael, Frank and Reich, Oliver and L{\"o}hmannsr{\"o}ben, Hans-Gerd},
  title     = {Near infrared diode laser spectroscopy of organic compounds in turbid aqueous solutions},
  isbn      = {0-9528666-1-7},
  year      = {2000},
  abstract  = {Diffuse reflectance measurements and photon migration studies with near infrared (NIR) diode lasers were employed to elucidate experimental methods for determining absorption and scattering coefficients and species concentrations in highly scattering solutions. Applicability of theoretical approaches were established by investigating model systems with absorbing (e.g. ink, malachite green) and scattering (e.g. milk powder, caolinit) species in aqueous solution. While diffuse reflectance measurements practically requires calibration procedures, photon migration studies allow quantitative determination of absorption and scattering coefficients of turbid solutions consistent with absorptions coefficients obtained from Lambert-Beer's law. Furthermore, NIR absorption spectra of water, chlorinated hydrocarbons (chloroform, 1,2-dichloroethane, trichloroethene) and of various sugars (\$alpha\$-D-glucose, sucrose, maltose) are discussed. Spectral variations of NIR water absorption with temperature and solvents are exammined. Exemplary, NIR diode laser detection of water in acetone/water mixtures is performed.},
  language  = {en}
}
@misc{BresselHerzogReich2019,
  author    = {Bressel, Lena and Herzog, Bernd and Reich, Oliver},
  title     = {Monte-Carlo simulations of light transport in dense materials},
  series = {Diffuse Optical Spectroscopy and Imaging},
  volume    = {11074},
  journal   = {Diffuse Optical Spectroscopy and Imaging},
  publisher = {SPIE},
  address   = {Bellingham},
  isbn      = {978-1-5106-2841-0},
  issn      = {0277-786X},
  doi       = {10.1117/12.2527076},
  pages     = {3},
  year      = {2019},
  abstract  = {Monte-Carlo calculations are carried out to simulate the light transport in dense materials. Focus lies on the calculation of diffuse light transmission through films of scattering and absorbing media considering additionally the effect of dependent scattering. Different influences like interaction type between particles, particle size, composition etc. can be studied by this program. Simulations in this study show major influences on the diffuse transmission. Further simulations are carried out to model a sunscreen film and study best compositions of this film and will be presented.},
  language  = {en}
}
@article{MunzkeSaundersOmranietal.2012,
  author    = {Munzke, Dorit and Saunders, John and Omrani, Hengameh and Reich, Oliver and Loock, Hans-Peter},
  title     = {Modeling of fiber-optic fluorescence probes for strongly absorbing samples},
  series = {Applied optics},
  volume    = {51},
  journal   = {Applied optics},
  number    = {26},
  publisher = {Optical Society of America},
  address   = {Washington},
  issn      = {1559-128X},
  doi       = {10.1364/AO.51.006343},
  pages     = {6343 -- 6351},
  year      = {2012},
  abstract  = {The dynamic range of fiber-optic fluorescent probes such as single fibers and fiber bundles is calculated for strongly absorbing samples, such as process liquids, foodstuffs, and lubricants. The model assumes an excitation beam profile based on a Lambertian light source and uses analytical forms of the collection efficiency, followed by an Abel transformation and numerical integration. It is found that the effect of primary absorption of the excitation light and secondary absorption of the fluorescence is profound. For fiber bundles and bifurcated fiber probes, the upper accessible concentration limit is roughly given by the absorption length of the primary and secondary absorption. Fluorescence detectors that are placed at right angles to the excitation beam axis or collinear to the beam axis are equally strongly affected by secondary absorption. A probe in which the same fiber is used for excitation and for collection of the fluorescence emerges as the fiber probe with the largest accessible concentration range.},
  language  = {en}
}
@misc{MuenzbergHassKhanhetal.2016,
  author    = {M{\"u}nzberg, Marvin and Hass, Roland and Khanh, Ninh Dinh Duc and Reich, Oliver},
  title     = {Limitations of turbidity process probes and formazine as their calibration standard},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch Naturwissenschaftliche Reihe},
  number    = {831},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-42826},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-428263},
  pages     = {12},
  year      = {2016},
  abstract  = {Turbidity measurements are frequently implemented for the monitoring of heterogeneous chemical, physical, or biotechnological processes. However, for quantitative measurements, turbidity probes need calibration, as is requested and regulated by the ISO 7027:1999. Accordingly, a formazine suspension has to be produced. Despite this regulatory demand, no scientific publication on the stability and reproducibility of this polymerization process is available. In addition, no characterization of the optical properties of this calibration material with other optical methods had been achieved so far. Thus, in this contribution, process conditions such as temperature and concentration have been systematically investigated by turbidity probe measurements and Photon Density Wave (PDW) spectroscopy, revealing an influence on the temporal formazine formation onset. In contrast, different reaction temperatures do not lead to different scattering properties for the final formazine suspensions, but give an access to the activation energy for this condensation reaction. Based on PDW spectroscopy data, the synthesis of formazine is reproducible. However, very strong influences of the ambient conditions on the measurements of the turbidity probe have been observed, limiting its applicability. The restrictions of the turbidity probe with respect to scatterer concentration are examined on the basis of formazine and polystyrene suspensions. Compared to PDW spectroscopy data, signal saturation is observed at already low reduced scattering coefficients.},
  language  = {en}
}
@article{ZakrevskyyCywinskiCywinskaetal.2014,
  author    = {Zakrevskyy, Yuriy and Cywinski, Piotr and Cywinska, Magdalena and Paasche, Jens and Lomadze, Nino and Reich, Oliver and L{\"o}hmannsr{\"o}ben, Hans-Gerd and Santer, Svetlana},
  title     = {Interaction of photosensitive surfactant with DNA and poly acrylic acid},
  series = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr},
  volume    = {140},
  journal   = {The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr},
  number    = {4},
  publisher = {American Institute of Physics},
  address   = {Melville},
  issn      = {0021-9606},
  doi       = {10.1063/1.4862679},
  pages     = {8},
  year      = {2014},
  language  = {en}
}
@article{HassMunzkeReich2010,
  author    = {Hass, Roland and Munzke, Dorit and Reich, Oliver},
  title     = {Inline-Partikelgroeßenmesstechniken fuer Suspensionen und Emulsionen},
  issn      = {0009-286X},
  doi       = {10.1002/cite.200900172},
  year      = {2010},
  abstract  = {Die Inline-Bestimmung von Teilchengroeßen in Emulsionen und Suspensionen stellt besondere Anforderungen an die Messtechnik, da auch bei sehr hohen Teilchenkonzentrationen im Prozess verduennungsfreie Analytik betrieben werden soll. Neben einer Klaerung der Begriffe atline, online und in-line gibt der Beitrag eine Einfuehrung in die mathematische Beschreibung von Groeßenverteilungen. Als Inline-Techniken werden Photonendichtewellen-Spektroskopie, Focused Beam Reflectance Measurement und Ultraschallextinktion-Spektroskopie diskutiert und ihre sehr unterschiedlichen physikalischen Messprinzipien erlaeutert. Auch wird kurz erklaert, wie Teilchengroeßen aus den Messresultaten erhalten werden. Die wesentlichen Charakteristika dieser drei Methoden werden abschließend im ueberblick dargestellt.},
  language  = {de}
}
@article{SandmannMuenzbergBresseletal.2022,
  author    = {Sandmann, Michael and M{\"u}nzberg, Marvin and Bressel, Lena and Reich, Oliver and Hass, Roland},
  title     = {Inline monitoring of high cell density cultivation of Scenedesmus rubescens in a mesh ultra-thin layer photobioreactor by photon density wave spectroscopy},
  series = {BMC Research Notes / Biomed Central},
  volume    = {15},
  journal   = {BMC Research Notes / Biomed Central},
  number    = {1},
  publisher = {Biomed Central (London)},
  address   = {London},
  issn      = {1756-0500},
  doi       = {10.1186/s13104-022-05943-2},
  pages     = {7},
  year      = {2022},
  abstract  = {Objective Due to multiple light scattering that occurs inside and between cells, quantitative optical spectroscopy in turbid biological suspensions is still a major challenge. This includes also optical inline determination of biomass in bioprocessing. Photon Density Wave (PDW) spectroscopy, a technique based on multiple light scattering, enables the independent and absolute determination of optical key parameters of concentrated cell suspensions, which allow to determine biomass during cultivation. Results A unique reactor type, called "mesh ultra-thin layer photobioreactor" was used to create a highly concentrated algal suspension. PDW spectroscopy measurements were carried out continuously in the reactor without any need of sampling or sample preparation, over 3 weeks, and with 10-min time resolution. Conventional dry matter content and coulter counter measurements have been employed as established offline reference analysis. The PBR allowed peak cell dry weight (CDW) of 33.4 g L-1. It is shown that the reduced scattering coefficient determined by PDW spectroscopy is strongly correlated with the biomass concentration in suspension and is thus suitable for process understanding. The reactor in combination with the fiber-optical measurement approach will lead to a better process management.},
  language  = {en}
}
@article{HassMuenzbergBresseletal.2013,
  author    = {Hass, Roland and M{\"u}nzberg, Marvin and Bressel, Lena and Reich, Oliver},
  title     = {Industrial applications of photon density wave spectroscopy for in-line particle sizing [Invited]},
  series = {Applied optics},
  volume    = {52},
  journal   = {Applied optics},
  number    = {7},
  publisher = {Optical Society of America},
  address   = {Washington},
  issn      = {1559-128X},
  doi       = {10.1364/AO.52.001423},
  pages     = {1423 -- 1431},
  year      = {2013},
  abstract  = {Optical spectroscopy in highly turbid liquid material is often restricted by simultaneous occurrence of absorption and scattering of light. Photon Density Wave (PDW) spectroscopy is one of the very few, yet widely unknown, technologies for the independent quantification of these two optical processes. Here, a concise overview about modern PDW spectroscopy is given, including all necessary equations concerning the optical description of the investigated material, dependent light scattering, particle sizing, and PDW spectroscopy itself. Additionally, it is shown how the ambiguity in particle sizing, arising from Mie theory, can be correctly solved. Due to its high temporal resolution, its applicability to highest particle concentrations, and its purely fiber-optical probe, PDW spectroscopy possesses all fundamental characteristics for optical in-line process analysis. Several application examples from the chemical industry are presented. (C) 2013 Optical Society of America},
  language  = {en}
}
@article{SchaelReich2001,
  author    = {Schael, Frank and Reich, Oliver},
  title     = {In-situ-Analytik heterogener Medien mit frequenz- und leistungsmodulierten Diodenlasern},
  year      = {2001},
  abstract  = {Electronic intensity and frequency modulation of diode lasers enables new perspectives for highly sensitive analytical in-situ techniques. For analyis of "heterogeneous" media, which may consist of coexisting gaseous, liquid, and solid phases and often show multiple light scattering, non-invasive techniques with outstanding performance characteristics can be developed. Analysis of photon density waves launched by intensity-modulated lasers provide an elegant way for the determination of absorption and scattering coefficients of multiple scattering media. The absorption coefficient can be used for qualitative and quantitative analysis of chemical constituents (which absorb in the range of the laser wavelengths). The scattering coefficient allows characterization of physical and morphological properties of the sample. In the current work, applicability of appropriate radiation transport models was investigated with simple light scattering solutions and compared with results obtained from the treatment according to the theory of Kubelka and Munk. Measurements of human blood samples are discussed. A new method for determination of water vapor partial pressure in a polyurethane foam with a frequency-modulated external cavity diode laser is discussed.},
  language  = {de}
}
@article{AndrewsRossMunzkeetal.2016,
  author    = {Andrews, Nicholas L. P. and Ross, Rachel and Munzke, Dorit and van Hoorn, Camiel and Brzezinski, Andrew and Barnes, Jack A. and Reich, Oliver and Loock, Hans-Peter},
  title     = {In-fiber Mach-Zehnder interferometer for gas refractive index measurements based on a hollow-core photonic crystal fiber},
  series = {Optics express : the international electronic journal of optics},
  volume    = {24},
  journal   = {Optics express : the international electronic journal of optics},
  publisher = {Optical Society of America},
  address   = {Washington},
  issn      = {1094-4087},
  doi       = {10.1364/OE.24.014086},
  pages     = {14086 -- 14099},
  year      = {2016},
  abstract  = {We describe an in-fiber interferometer based on a gas-filled hollow-core photonic crystal fiber. Expressions for the sensitivity, figure of merit and refractive index resolution are derived, and values are experimentally measured and theoretically validated using mode field calculations. The refractive indices of nine monoatomic and molecular gases are measured with a resolution of delta(ns) < 10(-6). (C)2016 Optical Society of America},
  language  = {en}
}
@article{AndrewsFergusonRangaswamyetal.2017,
  author    = {Andrews, N. L. P. and Ferguson, T. and Rangaswamy, A. M. M. and Bernicky, A. R. and Henning, N. and Dudelzak, A. and Reich, Oliver and Barnes, Jack A. and Loock, Hans-Peter},
  title     = {Hadamard-Transform Fluorescence Excitation-Emission-Matrix Spectroscopy},
  series = {Analytical chemistry},
  volume    = {89},
  journal   = {Analytical chemistry},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {0003-2700},
  doi       = {10.1021/acs.analchem.7b02400},
  pages     = {8554 -- 8564},
  year      = {2017},
  abstract  = {We present a fluorescence excitation-emission-matrix spectrometer with superior data acquisition rates over previous instruments. Light from a white light emitting diode (LED) source is dispersed onto a digital micromirror array (DMA) and encoded using binary n-size Walsh functions ("barcodes"). The encoded excitation light is used to irradiate the liquid sample and its fluorescence is dispersed and detected using a conventional array spectrometer. After exposure to excitation light encoded in n different ways, the 2-dimensional excitation-emission-matrix (EEM) spectrum is obtained by inverse Hadamard transformation. Using this technique we examined the kinetics of the fluorescence of rhodamine B as a function of temperature and the acid-driven demetalation of chlorophyll into pheophytin-a. For these experiments, EEM spectra with 31 excitation channels and 2048 emission channels were recorded every 15 s. In total, data from over 3000 EEM spectra were included in this report. It is shown that the increase in data acquisition rate can be as high as [{n(n + 1)}/2]-fold over conventional EEM spectrometers. Spectral acquisition rates of more than two spectra per second were demonstrated.},
  language  = {en}
}
@article{MunzkeBoehmReich2015,
  author    = {Munzke, Dorit and B{\"o}hm, Michael and Reich, Oliver},
  title     = {Gaseous Oxygen Detection Using Hollow-Core Fiber-Based Linear Cavity Ring-Down Spectroscopy},
  series = {Journal of lightwave technology},
  volume    = {33},
  journal   = {Journal of lightwave technology},
  number    = {12},
  publisher = {Inst. of Electr. and Electronics Engineers},
  address   = {Piscataway},
  issn      = {0733-8724},
  doi       = {10.1109/JLT.2015.2397177},
  pages     = {2524 -- 2529},
  year      = {2015},
  abstract  = {We demonstrate a method for the calibration-free and quantitative analysis of small volumes of gaseous samples. A 10 m hollow-core photonic bandgap fiber is used as the sample cell (volume = 0.44 mu L) and is placed inside a linear resonator setup. The application of cavity ring-down spectroscopy and in consideration of rather small coupling losses, this leads to an increased effective optical path length of up to 70 m. This implies a volume per optical interaction path length of 6.3 nL.m(-1). We used tunable diode laser spectroscopy at 760 nm and scanned the absorption for oxygen sensing. The optical loss due to sample absorption is obtained by measuring the ring-down time of light propagating inside the cavity. The resultant absorption coefficient shows a discrepancy of only 5.1\% comparing to the HITRAN database. This approach is applicable for sensitive measurements if only submicroliter sample volumes are available.},
  language  = {en}
}
@article{BeschererMunzkeReichetal.2013,
  author    = {Bescherer, Klaus and Munzke, Dorit and Reich, Oliver and Loock, Hans-Peter},
  title     = {Fabrication and modeling of multimode fiber lenses},
  series = {Applied optics},
  volume    = {52},
  journal   = {Applied optics},
  number    = {4},
  publisher = {Optical Society of America},
  address   = {Washington},
  issn      = {1559-128X},
  doi       = {10.1364/AO.52.000B40},
  pages     = {B40 -- B45},
  year      = {2013},
  abstract  = {We report on the fabrication, modeling, and experimental verification of the emission of fiber lenses fabricated on multimode fibers in different media. Concave fiber lenses with a radius of 150 mu m were fabricated onto a multimode silica fiber (100 mu m core) by grinding and polishing against a ruby sphere template. In our theoretical model we assume that the fiber guides light from a Lambertian light source and that the emission cone is governed solely by the range of permitted emission angles. We investigate concave and convex lenses at 532 nm with different radii and in a variety of surrounding media from air (n(0) = 1.00) to sapphire (n(0) = 1.77). It was found that noticeable focusing or defocusing effects of a silica fiber lens in ethanol (n(0) = 1.36) and dimethyl sulfoxide (DMSO) (n(0) = 1.48) are only observed when the fiber lens radius was less than the fiber diameter.},
  language  = {en}
}
@article{BresselMuellerLeseretal.2020,
  author    = {Bressel, Katharina and M{\"u}ller, Wenke and Leser, Martin Erwin and Reich, Oliver and Hass, Roland and Wooster, Tim J.},
  title     = {Depletion-induced flocculation of concentrated emulsions probed by photon density wave spectroscopy},
  series = {Langmuir},
  volume    = {36},
  journal   = {Langmuir},
  number    = {13},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {0743-7463},
  doi       = {10.1021/acs.langmuir.9b03642},
  pages     = {3504 -- 3513},
  year      = {2020},
  abstract  = {Stable, creaming-free oil in water emulsions with high volume fractions of oil (phi = 0.05-0.40, density matched to water) and polysorbate 80 as an emulsifier were characterized without dilution by Photon Density Wave spectroscopy measuring light absorption and scattering behavior, the latter serving as the basis for droplet size distribution analysis. The emulsion with phi = 0.10 was used to investigate flocculation processes induced by xanthan as a semi-flexible linear nonabsorbing polymer. Different time regimes in the development of the reduced scattering coefficient mu(s)' could be identified. First, a rapid, temperature-dependent change in mu(s)' during the depletion process was observed. Second, the further decrease of mu(s)' follows a power law in analogy to a spinodal demixing behavior, as described by the Cahn-Hilliard theory.},
  language  = {en}
}
@article{SalffnerBoehmReichetal.2014,
  author    = {Salffner, Katharina and Boehm, Michael and Reich, Oliver and L{\"o}hmannsr{\"o}ben, Hans-Gerd},
  title     = {A broadband cavity ring-down spectrometer based on an incoherent near infrared light source},
  series = {Applied physics : B, Lasers and optics},
  volume    = {116},
  journal   = {Applied physics : B, Lasers and optics},
  number    = {4},
  publisher = {Springer},
  address   = {New York},
  issn      = {0946-2171},
  doi       = {10.1007/s00340-014-5762-9},
  pages     = {785 -- 792},
  year      = {2014},
  language  = {en}
}