Hyperspectral imaging for identification of irregular-shaped microplastics in water.

In this article, we demonstrate detection and identification of ten microplastic types directly in a water sample using an identification table derived from microplastic hyperspectral images. We selected a total of fourteen wavelengths which can be used to distinguish these ten microplastic types. We enhanced the visibility of these wavelengths by computationally removing water and baseline correcting with reflectance at 1550 nm. This method avoids, prevents, and eases most of the laborious sample preparation mandatory prior to analysis with robust techniques such as Raman spectroscopy and Fourier transform infrared spectroscopy. The ten different plastics were studied in water, first separately and then in a mixture. The microplastic concentrations varied depending on microplastic type and were kept <12 mg/ml per type. Finally, detection and identification were confirmed pixel-wise in a hyperspectral image of a realistic water matrix simulant including mixtures of only a few microplastic particles. All measurements have been performed with microplastics of different sizes and irregular shapes made in-house by milling commercial pellets and sheets. It enabled the establishment of a procedure for the identification of these vicious particles in real water samples. The present measurement setup of hyperspectral imaging and method of data analysis of a mixture of microplastics directly from a water-based sample may open a path towards fast, reliable, and on-site detection.

Determination of the refractive index of microparticles by utilizing light dispersion properties of the particle and an immersion liquid.

The knowledge of the refractive index of a particle is important in sensing and imaging applications, e.g., in biology, medicine and process industry. The refractive index of tiny solid particles such as microsize particles can be determined by the so-called liquid immersion technique. This study deals with three different types of interrogation methods to get the refractive index of a particle in a liquid matrix. These methods utilize thermo-optical properties and wavelength-dependent refractive index of the particle and the immersion liquids, as well as, the classical method using a set of in advance prepared set of immersion liquids with different refractive indices. The emphasis is on a method to get especially the wavelength-dependent refractive index of microparticles and exploiting different wavelength-dependences of immersion liquid and a solid particle because identification of a particle is more reliable if the refractive index of the particle is known at several wavelengths. In this study glycerol-water mixtures served as immersion liquids to obtain the refractive index of CaF2 at several discrete wavelengths in the spectral range 200-500 nm. The idea is to find the maximum value of light transmission of suspension by scanning the wavelength of a commercial spectrophotometer. The light dispersion-based method is suggested as a relatively easy, economic and fast method to determine the refractive index of a particle by a spectrophotometer at several wavelengths of light. The accuracy of the detection of the refractive index is suggested to be better than ± 0.005 refractive index units.

Broadening of a THz pulse as a measure of the porosity of pharmaceutical tablets.

THz pulse time delay was measured from ethylcellulose tablets having nominal porosities 13%, 22% and 30%. It is suggested that the observed pulse broadening of sample THz pulse is due to scattering and dispersion of the THz wave in a porous tablet. A parameter related to the pulse broadening is suggested as a sensitive measure for the porosity and porosity change of the tablet.

Measurement of the refractive index of highly turbid media: comment.

We comment on the recent Letter by Calhoun et al. [Opt. Lett. 35, 1224 (2010)]. We wish to correct their statement about the order of the magnitude of the absorption coefficient of inks that we have we studied. In addition, we consider the inconsistency of their theoretical model with the experimental data obtained for milk and milk-cream samples.

Using modified Kramers-Kronig relations to test transmission spectra of porous media in THz-TDS.

We show that modified Kramers-Kronig relations provide a useful tool to test the validity of the complex refractive index extracted from transmission terahertz spectra of porous matrices containing pharmaceutical materials. The role of scattering of terahertz radiation is qualitatively considered as a reason for the observed discrepancy between experimental data and the values extracted from the inverted complex refractive index. As an example we present an analysis of the terahertz spectra of carbamazepine and lactose alpha-monohydrate.

On reflectometric measurement of a refractive index of milk.

The refractive index of milk samples was measured with a reflectometer. Fresnel's formula for intensity reflectance and the concept of critical angle were applied to measured data. Milk samples were also measured with surface plasmon resonance sensor for refractive index assessment. The experiments were carried with commercial milk that had fat volume concentrations of 0.004, 1.53, and 3.55%. We observed that simultaneously quantitative information about the refractive index and absorption of milk, as a function of fat concentration, could be obtained with both devices.

On-line measurement of the thickness and optical quality of float glass with a sensor based on a diffractive element.

An on-line apparatus for accurate measurement of float-glass thickness and edge distortion was built and tested in industrial environments. The intelligent part of the sensor is a diffractive optical element. The theory and construction of the apparatus are described, and data from on-line measurements are presented.

Estimation of the wavelength-dependent effective refractive index of spherical plastic pigments in a liquid matrix.

Refractive-index data are important for the prediction of light scattering from spherical pigments. Reflectance from a slurry that contains plastic pigments was studied with the aid of a reflectometer. The effective refractive index of spherical plastic pigments in a slurry was determined by use of reflectance data and a phase-retrieval procedure based on the maximum-entropy method. This method provides a simple way to estimate the effective refractive index of pigments in a liquid matrix.

Optical surface roughness study of starch acetate compacts.

Optical surface roughness of starch acetate compacts was investigated by specular reflection of a laser beam as a function of angle of incidence. The intensity data was fitted using the model of Gaussian and Lorentzian curves to solve numerical values for optical surface roughness which was observed to be order of 1 microm with the present samples.

Computer simulation of the scatter plate interferometer by scalar diffraction theory.

Detailed computer simulations of the scatter plate interferometer with random scatterers in the scatter plate are performed, for the first time to our knowledge, by use of the scalar diffraction theory in the paraxial domain. It is shown that the computer simulations produce output image patterns of the expected qualitative characteristics. A qualitative comparison of the computed pattern with the experimentally observed pattern is presented. The effects of translation of the scatter plate and distortion and tilting of the test object are also successfully simulated by the computer.

Fluorescence and phase-conjugated signals of organic materials.

The role of fluorescence has not always been fully understood in optical engineering experiments that are devoted to the generation of phase-conjugated waves from organic materials. It is pointed out that fluorescence can appear if the material absorbs at the wavelength of the excitation. Therefore proper experimental precautions should be taken in the detection of the intrinsic phase-conjugated signal.

Comparison between the optical constants obtained by the Kramers-Kronig analysis and the maximum entropy method: infrared optical properties of orthorhombic sulfur.

The optical constants of orthorhombic sulfur computed by the Kramers-Kronig phase-shift analysis were recently reported by Fuller et al. The results obtained by Fuller et al. are compared with the ones obtained by a novel maximum entropy procedure.

Durability investigation of a KCI:Li F(A)-center optical memory at room temperature.

The reorientation of F(A)(II) centers in electrolytically colored KCl:Li crystals under ruby laser exposure was studied at room temperature. A prolonged series of laser pulses was applied over a period of three days to check the durability and possible fatiguing of the crystals.