Integrating Sphere Setup Research Articles

Characterization of the Optical Properties of Photoluminescent Turbid Media Using an Integrating Sphere and Monte Carlo Simulations.

In this paper, we report on the measurement of the optical properties (absorption and scattering coefficients) of photoluminescent turbid media using a homemade integrating sphere setup equipped with a tunable monochromatic light source. The hemispherical reflectance and transmission data are analyzed with the radiative transfer equation using a Monte Carlo simulation-based lookup table to obtain the optical properties of the sample. The results are compared with the optical properties received from a classical integrating sphere setup equipped with a broadband white light source. The additional light of the photoluminescence generates artifacts within the optical properties, which are not present using a monochromatic light source. Additionally, a batch of samples with a broad range of scattering coefficients and dye concentrations were prepared and characterized with the aforementioned setup. The findings can help to generate a digital twin with the optical properties of the sample, which improves the physically based rendering and the design of, e.g., white-light LEDs. Dental restoration and photodynamic therapy also benefit from determination of the optical properties of photoluminescent turbid media.

Development of silicone-based phantoms for biomedical optics from 400 to 1550 nm.

This work describes the development of silicone-based evaluation phantoms for biomedical optics in the wavelength range from 400 to 1550 nm. The absorption coefficient μ a and the reduced scattering coefficient were determined using an integrating sphere setup. Zirconium dioxide pigments were used as scatterers and carbon black as absorbers. We developed an in-house manufacturing process using a Hauschild SpeedMixer to ensure reproducibility. A set of nine cubic phantoms with three different reduced scattering and absorption coefficients was produced. Prediction of the μ a and was done by using the weighted mass concentrations of the used materials. The average prediction accuracy over all wavelengths and phantoms is 1.0% for the reduced scattering coefficient and 3.5% for the absorption coefficient.

Reconstruction of Optical Properties in Turbid Media: Omitting the Need of the Collimated Transmission for an Integrating Sphere Setup.

Currently, the most reliable approach to reconstruct optical properties, namely absorption coefficient, reduced scattering coefficient, scattering coefficient and asymmetry factor, of turbid media is through inverse Monte Carlo simulation. To determine these optical properties, three measurements are required: total transmission, total reflection and collimated transmission. However, the accurate determination of the collimated transmission is very difficult. To overcome the difficulty of measuring the collimated transmission, it is proposed to measure the total transmission and total reflection of the same sample with two different thicknesses instead. To prove this alternative solution, machine learning is used to prove that the repeated measurement for two different thicknesses carries all the necessary information. As a result, all four optical properties can be measured with high accuracy, particularly for interpolation problems where the test data fall within the range of the training data. For extrapolation problems, high accuracy can be achieved in the determination of at least the absorption coefficient, reduced scattering coefficient and scattering coefficient. Hence, these results allow that in the future, an easier and therefore more precise reconstruction of the optical properties is possible, potentially even with inverse Monte Carlo simulations as the current standard.

Light matters: Quantifiable optical interaction between near-infrared laser radiation and nano-additivated polymers in all states of matter present in laser powder bed fusion

Comprehending the interaction between light and material is crucial to the success of various laser-based processes, including laser powder bed fusion of polymers. The use of the promising near-infrared laser wavelength in laser powder bed fusion of polymers is limited by the necessity for absorption-enhancing (nano-) additives and insufficient knowledge of the (nano-) particulate impact on the material and process behavior. The present study lays the groundwork for an improved understanding of the laser-material interaction at different states of matter in laser powder bed fusion by means of thermo-optical measurements with a double integrating sphere setup. Analyses and experiments were based on polyamide 12 and its nanocomposite powder surface-coated with carbon black nanoparticles by two distinct additivation methods. The attenuation coefficient and penetration depth of each material were calculated from the measured transmittance and reflectance in all relevant states of matter. The effects of the feedstock particle size, the nano-additivation method, and the nanoparticle quantity were evaluated. The optical results of resolidified samples were validated using a commercially available spectrophotometer. Finally, the thermo-optical results were qualitatively correlated with the depth of fusion of specimens processed by powder bed fusion using an 808 nm diode laser.

Investigations on correlations between changes of optical tissue properties and NMR relaxation times

BackgroundAccurate light dosimetry is a complex remaining challenge in interstitial photodynamic therapy (iPDT) for malignant gliomas. The light dosimetry should ideally be based on the tissue morphology and the individual optical tissue properties of each tissue type in the target region. First investigations are reported on using NMR information to estimate changes of individual optical tissue properties. MethodsPorcine brain tissue and optical tissue phantoms were investigated. To the porcine brain, supplements were added to simulate an edema or high blood content. The tissue phantoms were based on agar, Lipoveneous, ink, blood and gadobutrol (Gd-based MRI contrast agent). The concentrations of phantom ingredients and tissue additives are varied to compare concentration-dependent effects on optical and NMR properties. A 3-tesla whole-body MRI system was used to determine T1 and T2 relaxation times. Optical tissue properties, i.e., the spectrally resolved absorption and reduced scattering coefficient, were obtained using a single integrating sphere setup. The observed changes of NMR and optical properties were compared to each other. ResultsBy adjusting the NMR relaxation times and optical tissue properties of the tissue phantoms to literature values, recipes for human brain tumor, white matter and grey matter tissue phantoms were obtained that mimic these brain tissues simultaneously in both properties. For porcine brain tissue, it was observed that with increasing water concentration in the tissue, both NMR-relaxation times increased, while µa decreased and µs’ increased at 635 nm. The addition of blood to porcine brain samples showed a constant T1, while T2 shortened and the absorption coefficient at 635 nm increased. ConclusionsIn this investigation, by changing sample contents, notable changes of both NMR relaxation times and optical tissue properties have been observed and their relations examined. The developed dual NMR/optical tissue phantoms can be used in iPDT research, clinical training and demonstrations.

Temperature-Dependent Reflectance of Copper with Different Surface Conditions Measured at 1064 nm

AbstractIn order to fully understand and to eventually overcome the instabilities in laser welding of copper using 1064 nm wavelength lasers, it is important to get a deeper understanding about the interaction of the incident beam with the copper surface during the process. One of the main parameters determining this behavior is the temperature-dependent absorptance. However, the existing datasets only show little consistency between the different sources, do often not include the important absorptance step at melt temperature, or they are only calculated. This article thus presents an experimental approach to measure the reflectance of electro tough-pitch copper (Cu-ETP) from room temperature until beyond the melting point with an integrating sphere setup. The setup was developed with particular attention to measurement accuracy up to high temperatures. The reflectance using a 1064 nm laser under an incident angle of 12° has been measured for four different sample groups, which are untreated rolled copper, mechanically polished copper, oxide reduced copper and resolidified copper. The measurement results reveal significant differences in the temperature-dependent reflectance between the sample types and, for some types, also between the samples of the same type. This study investigates the origins of the most significant features in the reflectance curves and their potential impact on processing copper using NIR lasers.

Customized Integrating-Sphere System for Absolute Color Measurement of Silk Cocoon with Corrugated Microstructure.

Silk fiber, recognized as a versatile bioresource, holds wide-ranging significance in agriculture and the textile industry. During the breeding of silkworms to yield new varieties, optical sensing techniques have been employed to distinguish the colors of silk cocoons, aiming to assess their improved suitability across diverse industries. Despite visual comparison retaining its primary role in differentiating colors among a range of silk fibers, the presence of uneven surface texture leads to color distortion and inconsistent color perception at varying viewing angles. As a result, these distorted and inconsistent visual assessments contribute to unnecessary fiber wastage within the textile industry. To solve these issues, we have devised an optical system employing an integrating sphere to deliver consistent and uniform illumination from all orientations. Utilizing a ColorChecker, we calibrated the RGB values of silk cocoon images taken within the integrating sphere setup. This process accurately extracts the authentic RGB values of the silk cocoons. Our study not only helps in unraveling the intricate color of silk cocoons but also presents a unique approach applicable to various specimens with uneven surface textures.

Efficiency scale for scattering luminescent particles linked to fundamental and measurable spectroscopic properties

Comparing the performance of molecular and nanoscale luminophores and luminescent micro- and nanoparticles and estimating achievable signal amplitudes and limits of detection requires a standardizable intensity scale. This initiated the development of the relative MESF (number of molecules of equivalent soluble fluorochromes) and ERF (equivalent reference fluorophores) scales for flow cytometry and fluorescence microscopy. Both intensity scales rely on fluorescence intensity values assigned to fluorescent calibration beads by an intensity comparison to spectrally closely matching fluorophore solutions of known concentration using a spectrofluorometer. Alternatively, the luminophore or bead brightness (B) can be determined that equals the product of the absorption cross section (σa) at the excitation wavelength (σa(λex)) and the photoluminescence quantum yield (Φpl). Thereby, an absolute scale based on fundamental and measurable spectroscopic properties can be realized which is independent of particle size, material, and luminophore staining or labeling density and considers the sensitivity of the optical properties of luminophores to their environment. Aiming for establishing such a brightness scale for light-scattering dispersions of luminescent particles with sizes exceeding a few ten nanometers, we demonstrate how the brightness of quasi-monodisperse 25 nm, 100 nm, and 1 µm sized polystyrene particles (PSP), loaded with two different dyes in varying concentrations, can be obtained with a single custom-designed integrating sphere setup that enables the absolute determination of Φpl and transmittance and diffuse reflectance measurements. The resulting Φpl, σa(λex), imaginary parts of the refractive index, and calculated B values of these samples are given in dependence of the number of incorporated dye molecule per particle. Finally, a unitless luminescence efficiency (LE) is defined allowing for the direct comparison of luminescence efficiencies of particles with different sizes.

Ex Vivo Determination of Broadband Absorption and Effective Scattering Coefficients of Porcine Tissue

A novel approach for precise determination of the optical scattering and absorption properties of porcine tissue using an optimized integrating sphere setup was applied. Measurements on several sample types (skin, muscle, adipose tissue, bone, cartilage, brain) in the spectral range between 400 nm and 1400 nm were performed. Due to the heterogeneity of biological samples, measurements on different individual animals as well as on different sections for each sample type were carried out. For all samples, we used an index matching method to reduce surface roughness effects and to prevent dehydration. The derived absorption spectra were used to estimate the concentration of important tissue chromophores such as water, oxy- and deoxyhemoglobin, collagen and fat.

Determining the optical properties of apple tissue and their dependence on physiological and morphological characteristics during fruit maturation. Part 2: Mie's theory

Mie's theory was used to develop a theoretical model to describe light propagation in apple tissue based on its microstructural properties. Taking into account the size distributions and volume fractions of intercellular space and starch granules, which are known in detail from previous studies, the model predicted a decrease in the effective scattering coefficient μ′s of about 30 % during maturation. For the lateral change of μ′s within a ripe apple, the model predicted an increase up to 35 % from core to skin. In both cases, the relative changes agree well with the experimental results obtained with spatial frequency domain imaging (SFDI) and an integrating sphere setup. Based on the model, at least 70 % of total μ′s is attributable to intercellular space, which accordingly plays a dominant role in the temporal and lateral change of scattering. Using an extended Mie model for layered particles assuming a dense cell structure, a negligible effect on μ′s was observed for the cell wall. However, for cells separated from the surrounding tissue by a narrow air gap (e.g. dissolution of the middle lamella), μ′s was 20 % higher than for cells theoretically isolated in air. Lastly, it was shown that an increase in soluble solids content (SSC) during maturation by up to 15 % and the associated higher refractive index in the cells led to an increase in μ′s by about 3–5 % compared to the case without considering SSC. Overall, the application of Mie's theory proved suitable to investigate the influence of different morphological structures on light scattering in apple tissue.

Measuring the absorption coefficient of optical materials with arbitrary shape or distribution within an integrating sphere.

The absorption coefficient of a material is classically determined by measuring the transmittance of a homogeneous sample contained within flat optical faces and under collimated illumination. For arbitrary shapes this method is impracticable. The characterization of inhomogeneous or randomly distributed samples such as granules, powders or fibers suffers the same problem. Alternatively, an integrating cavity permits us to illuminate a sample under a homogenous and isotropic light field where the analysis simplifies. We revisit this strategy and present a new formal basis based on simple radiometric laws and principles. We introduce a new concept to describe the absorption: the optical form factor. We tackle a rigorous treatment of several regular forms, including full absorption range and the reflection at its surfaces. We also model and improve an integrating sphere setup to perform reliable measurements. Altogether, it permits achieving simple but general conclusions for samples with arbitrary shape or spatial distribution, from weak to highly absorbing, expanding the applicability of quantitative absorption spectroscopy. Finally, we validate it by measuring different sample formats made of PMMA: a cube, groups of granules and injection molding loose parts. The absorption coefficient of PMMA varies near three orders of magnitude in the explored range (380-1650 nm).

Revealing the internal luminescence quantum efficiency of perovskite films via accurate quantification of photon recycling

The internal luminescence quantum efficiency ($Q_\mathrm{i}^\mathrm{lum}$) provides an excellent assessment of the optoelectronic quality of semiconductors. To determine $Q_\mathrm{i}^\mathrm{lum}$ of perovskite films from the experimentally accessible external luminescence quantum efficiency ($Q_\mathrm{e}^\mathrm{lum}$) it is essential to account for photon recycling, and this requires knowledge of the photon escape probability ($p_\mathrm{e}$). Here, we establish an analysis procedure based on a curve fitting model that accurately determines $p_\mathrm{e}$ of perovskite films from photoluminescence (PL) spectra measured with a confocal microscope and an integrating sphere setup. We show that scattering-induced outcoupling of initially-trapped PL explains commonly observed red-shifted and broadened PL spectral shapes and leads to $p_\mathrm{e}$ being more than 10% higher in absolute terms compared to earlier assumptions. Applying our model to CH$_3$NH$_3$PbI$_3$ films with exceptionally high $Q_\mathrm{e}^\mathrm{lum}$ up to 47.4% sets a real benchmark for $Q_\mathrm{i}^\mathrm{lum}$ at $78.0 \pm 0.5\%$, revealing there is beyond a factor of two more scope for reducing non-radiative recombination than previously thought.

Evaluating the performance of a double integrating sphere in measurement of reflectance, transmittance, and albedo of coniferous needles

Leaf reflectance and transmittance spectra are essential information in many applications such as developing remote sensing methods, computing shortwave energy balance (albedo) of forest canopies, and monitoring health or stress of trees. Measurement of coniferous needle spectra has usually been carried out with single integrating spheres, which has involved a lot of tedious manual work. A small double integrating sphere would make the measurements considerably faster, because of its ease of operation and small sample sizes required. Here we applied a compact double integrating sphere setup, used previously for measurement of broad leaves, for measurement of coniferous needles. Test measurements with the double integrating sphere showed relative underestimation of needle albedo by 5–39% compared to a well-established single integrating sphere setup. A small part of the bias can be explained by the bias of the single sphere. Yet the observed bias is quite significant if absolute accuracy of measurements is required. For relative measurements, e.g. for monitoring development of needle spectra over time, the double sphere system provides notable improvement. Furthermore, it might be possible to reduce the bias by building an optimized measurement setup that minimizes absorption losses in the sample port. Our study indicates that double spheres, after some technical improvement, may provide a new and fast way to collect extensive spectral libraries of tree species.

Photoluminescence Quantum Yield of Fluorescent Silicon Carbide Determined by an Integrating Sphere Setup.

The excitation-dependent photoluminescence quantum yield (PL-QY) of strong n-type nitrogen–boron codoped 6H fluorescent silicon carbide (f-SiC) at room temperature is experimentally determined for the first time. The PL-QY measurements are realized by an integrating sphere system based on a classical two-measurement approach. In particular, in accordance to the difference between our in-lab setup and the standard setup of the two-measurement approach, we have technically modified the experimental design, the data processing algorithm, and the estimation of relative uncertainty. The measured highest PL-QY of f-SiC samples is found to reach above 30%. We compare the PL-QYs at a certain excitation power of all f-SiC samples by considering their intrinsic defect densities. Finally, the evolution of the excitation power-dependent PL-QY of f-SiC is attributed to both band-to-band and impurity-assisted Auger recombination.

Leaf-Level Spectral Fluorescence Measurements: Comparing Methodologies for Broadleaves and Needles

Successful measurements of chlorophyll fluorescence (ChlF) spectral properties (typically in the wavelength range of 650–850 nm) across plant species, environmental conditions, and stress levels are a first step towards establishing a quantitative link between solar-induced chlorophyll fluorescence (SIF), which can only be measured at discrete ChlF spectral bands, and photosynthetic functionality. Despite its importance and significance, the various methodologies for the estimation of leaf-level ChlF spectral properties have not yet been compared, especially when applied to leaves with complex morphology, such as needles. Here we present, to the best of our knowledge, a first comparison of protocols for measuring leaf-level ChlF spectra: a custom-made system designed to measure ChlF spectra at ambient and 77 K temperatures (optical chamber, OC), the widely used FluoWat leaf clip (FW), and an integrating sphere setup (IS). We test the three methods under low-light conditions, across two broadleaf species and one needle-like species. For the conifer, we characterize the effect of needle arrangements: one needle, three needles, and needle mats with as little gap fraction as technically possible. We also introduce a simple baseline correction method to account for non-fluorescence-related contributions to spectral measurements. Baseline correction was found especially useful in recovering the spectra nearby the filter cut-off. Results show that the shape of the leaf-level ChlF spectra remained largely unaffected by the measurement methodology and geometry in OC and FW methods. Substantially smaller red/far-red ratios were observed in the IS method. The comparison of needle arrangements indicated that needle mats could be a practical solution to investigate temporal changes in ChlF spectra of needle-like leaves as they produced more reproducible results and higher signals.

Reflectance matrix approach to absolute photoluminescence measurements with integrating spheres.

Absolute measurements of photoluminescence are commonly performed using an integrating sphere setup, as this allows the collection of all emitted photons independent of the spatial characteristics of the emission. However, such measurements are plagued by multiple reflection effects occurring within the integrating sphere that make the sample illumination and sphere throughput sample dependent. To address this problem, we developed a matrix theory for integrating spheres with photoluminescent surfaces. In conjunction with a bispectral luminescence data set, this model allows for multiple reflection effects to be fully accounted for. The bispectral data is obtained by mounting both the sample and a non-luminescent reference on the sphere and permuting their positions in order to compare direct and diffuse sample illumination conditions. Experimental measurements of a photoluminescent standard confirm the validity of the method.

Investigation of optical properties of dissected and homogenized biological tissue.

Knowledge of tissue optical properties, in particular the absorption μa and the reduced scattering coefficient μs', is required for diagnostic and therapeutic applications in which the light distribution during treatment has to be known. As it is generally very difficult to obtain this information with sufficient accuracy in vivo, optical properties are often approximately determined on ex vivo tissue samples. In this case, the obtained optical properties may strongly depend on the sample preparation. The extent of the expectable preparation-dependent differences was systematically investigated in comparative measurements on dissected and homogenized porcine tissue samples (liver, lung, brain, and muscle). These measurements were performed at wavelengths 520, 635, 660, and 785nm, using a dual-step reflectance device and at a spectral range of 515 to 800nm with an integrating sphere setup. In a third experiment, the density of tissue samples (dissected and homogenized) was investigated, as the characteristic of the packaging of internal tissue structures strongly influences the absorption and scattering. The standard errors of the obtained absorption and reduced scattering coefficients were found to be reduced in case of homogenized tissue. Homogenizing the tissues also allows a much easier and faster sample preparation, as macroscopic internal tissue structures are destroyed in the homogenized tissue so that a planar tissue sample with well-defined thickness can easily and accurately be prepared by filling the tissue paste into a cuvette. Consequently, a better reproducibility result was obtained when using homogenized samples. According to the density measurements accomplished for dissected and homogenized tissue samples, all types of tissues, except lung, showed a decrease in the density due to the homogenization process. The presented results are in good agreement for μs' regardless of the preparation procedure, whereas μa differs, probably influenced by blood content and dehydration. Because of faster and easier preparation and easier sample positioning, homogenization prior to measurement seems to be suitable for investigating the optical properties ex vivo. Additionally, by means of using the homogenization process, the sample size and thickness do not need to be particularly large, as is the case for most biopsies from the OR.

An In vitro Laboratory Investigation on Layer Thickness-Independent Prediction of the Hemoglobin Concentration

Abstract In the noninvasive determination of the hemoglobin concentration a main challenge is the "optical path". With sensors - fixed on human skin - the optical path cannot be exactly determined, as it is defined as the layer thickness in the Lambert Beer principle. The layer thickness is significantly involved in the optical interactions in the tissue. To circumvent this problem self-learning algorithms were evaluated which provide the hemoglobin concentration from reflection and transmission data without knowledge of the layer thickness. First various regression models were trained based on an high rate data set. To evaluate the six most promising models, a prediction dataset was measured in a prospective randomized and blinded study to guarantee integrity of the results. For both data sets, the transmission and reflection of diluted heparinized erythrocyte concentrate was determined with a double integrating sphere setup (laser diodes with 780 to 1310 nm). The evaluated hemoglobin concentrations ranged from 4 to 16 g/dl at a constant oxygen saturation above 97 %. Optical flow through cuvettes (1, 2, 3 mm) simulated different layer thicknesses of the blood. The evaluation of the predictions yielded that the layer thickness independent prediction of the hemoglobin concentration is feasible with the selected approaches. The mean absolute error (MAE) of the best regression model (GPRM - Matern 5/2) is 0.79 g/dl. In the clinically relevant tHb range of less than 8 g/dl the MAE was as low as 0.52 g/dl.

In Vitro Determination of Concentration-Dependent Optical Properties of Hemoglobin

Abstract The interest of this paper is the determination of the optical properties of oxygenated (saturation above 97 %) hemoglobin in clinical relevant concentrations (ranging from 5 to 15 g/dl), dependent on the layer thickness. Furthermore the generation of a high rate data set for training with machine learning approaches was intended. With a double integrating sphere setup (laser diodes from 780 to 1310 nm) - as a well referenced method - and flow through optical cuvettes ranging from 1 to 3 mm layer thickness, the transmission (𝑀𝑇) and reflection (𝑀𝑅) values of the samples were acquired. From those the layer thickness independent absorption (𝜇𝑎) and reduced scattering coefficients (𝜇𝑠’) were calculated by the means of the Inverse Adding Doubling (IAD) algorithm. For each sample the same coefficients should result correspondingly for all cuvette thicknesses in test. This relationship serves as an internal standard in the evaluation of the collected data sets. In parallel a spectrophotometer in the range from 690 to 1000 nm recorded transmission spectra for all samples as a second reference. First, the IAD algorithm provided optical coefficients (𝜇𝑎, 𝜇𝑠’) in all measurements, with few exceptions at low hemoglobin concentrations. The resulting coefficients match independently of the layer thickness. As a main second result, a high rate data set was generated which serves for further analysis - for example with machine learning approaches.

Light absorption and scattering of 40–170 nm gold nanoparticles on glass substrates

The localized surface plasmon resonance (LSPR) effect in metal nanoparticles is important for many applications ranging from detectors and sensors to photovoltaic devices. The LSPR wavelength is sensitive to the shape, size, surface condition, and surrounding environment. Therefore, it is important to compare the optical properties of metal nanoparticles of nominally similar dimensions and external conditions, but fabricated with different techniques. Here, a systematic study of the optical properties of large, periodic arrays (3 × 3 mm) of cylindrical, gold nanoparticles with diameters ranging from 39 ± 4 nm to 167 ± 5 nm and a height of 25 ± 1 nm is presented. The large arrays allow us to investigate the optical properties using an integrating sphere setup collecting the light scattered and absorbed by the nanoparticles. To the best of our knowledge, such a setup has not been used previously for electron beam lithography (EBL) fabricated samples mainly due the large sample area required. The authors compare our results with relevant literature and find a good agreement, which confirms the expected reproducibility of EBL. Further, the authors compare our absorption and scattering measurements with previous absorption and scattering measurements on large arrays of gold nanoparticles prepared on glass using hole-mask colloidal lithography. Finally, a comparison with simulations using a finite difference time domain software package (Lumerical, Inc.) is presented. The simulation results matches well with experimental results and are also supporting and detailing our comparison with published literature. The authors find a good agreement between the two fabrication methods. The small deviations found can be contributed to differences in the particle size and density distributions.