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By measuring the refractivity and the temperature of a gas, its pressure can be assessed from fundamental principles. The highest performing instruments are based on Fabry–Perot cavities where a laser is used to probe the frequency of a cavity mode, which is shifted in relation to the refractivity of the gas in the cavity. Recent activities have indicated that such systems can demonstrate an extended uncertainty in the 10 ppm (parts-per-million or 10−6) range. As a means to reduce the influence of various types of disturbances (primarily drifts and fluctuations) a methodology based on modulation, denoted gas modulation refractometry (GAMOR), has recently been developed. Systems based on this methodology are in general high-performance, e.g. they have demonstrated precision in the sub-ppm range, and they are sturdy. They can also be made autonomous, allowing for automated and unattended operation for virtually infinite periods of time. To a large degree, the development of such instruments depends on the access to modern photonic components, e.g. narrow line-width lasers, electro- and acousto-optic components, and various types of fiber components. This work highlights the role of such modern devices in GAMOR-based instrumentation and provides a review on the recent development of such instruments in Sweden that has been carried out in a close collaboration between a research institute and the Academy. It is shown that the use of state-of-the-art photonic devices allows sturdy, automated and miniaturized instrumentation that, for the benefit of industry, can serve as standards for pressure and provide fast, unattended, and calibration-free pressure assessments at a fraction of the present cost.

Specular reflectance of monochromatic linearly polarized light, reflected on the interface of a prism coupled to a transparent medium at the critical angle of the transition to total internal reflection, has been long utilized to accurately determine optical constants, such as the refractive index and the dielectric constant, by the so-called "derivative method". However, in its original formulation, the derivative method does not account for the imaginary part of the complex optical constants that chararacterize attenuating media and becomes increasingly inaccurate even for the real part when attenuation grows. We therefore devise a proper analytic extension of the derivative method for attenuating media. Reflectance and angle of incidence at the derivative maximum are the experimental input quantities that yield both parts of the complex dielectric constant for both linear polarizations.

Edge-illumination X-ray phase-contrast tomography (EIXPCT) is a promising imaging technology where partially opaque masks are utilized with laboratory-based X-ray sources to estimate the distribution of the complex-valued refractive index. EIXPCT resolution is mainly determined by the period of a sample mask, but can be significantly improved by a dithering technique. Here, dithering means that multiple images per tomographic view angle are acquired as the object is moved over sub-pixel distances. Drawbacks of dithering include increased data-acquisition times and radiation doses. Motivated by the flexibility in data-acquisition designs enabled by a recently developed joint reconstruction (JR) method, a novel partial-dithering strategy for EIXPCT data-acquisition is proposed. In this strategy, dithering is implemented at only a subset of the tomographic view angles. The strategy can result in spatial resolution comparable to that of the conventional full-dithering strategy, where dithering is performed at every view angle, but the acquisition time is substantially decreased. Here, the effect of dithering parameters on image resolution are explored.

Optical techniques used for the measurement of the optical properties of blood are of great interest in clinical diagnostics. Blood analysis is a routine procedure used in medical diagnostics to confirm a patient's condition. Measuring the optical properties of blood is difficult due to the non-homogenous nature of the blood itself. In addition, there is a lot of variation in the refractive indices reported in the literature. These are the reasons that motivated the researchers to develop a mathematical model that can be used to predict the refractive index of human blood as a function of concentration, temperature and wavelength. The experimental measurements were conducted on mimicking phantom hemoglobin samples using the Abbemat Refractometer. The results analysis revealed a linear relationship between the refractive index and concentration as well as temperature, and a non-linear relationship between refractive index and wavelength. These results are in agreement with those found in the literature. In addition, a new formula was developed based on empirical modelling which suggests that temperature and wavelength coefficients be added to the Barer formula. The verification of this correlation confirmed its ability to determine refractive index and/or blood hematocrit values with appropriate clinical accuracy.

Tomographic phase microscopy (TPM) facilitates three-dimensional imaging of live cells based on quantitative measurement of the distribution of the refractive index, but without the need for specific staining. However, the limited imaging speed and the anisotropic resolution of the reconstructed refractive index map remain major obstacles to the extension and further application of TPM. To address these obstacles, we first formulate a general measurement model that linearises the relationship between the measurement data and refractive index map based on a system matrix. In this way, the measurement system is interpreted as a linear system in a complete manner. Then we propose a reconstruction framework for retrieving the refractive index map from the measurement data with reduced angular sample frequency and limited angular coverage of illumination. The framework aims to transform the reconstruction task into an optimisation scheme based on total variation norm regularisation, followed by an efficient solution using the accelerated alternating direction method of multipliers algorithm. Using this method, only sparse angular illuminations need to be collected, thus speeding up the imaging process. We obtained experimental results from both cell-mimic phantom data and real measurement data, which showed that the proposed method can improve the imaging speed while still providing refractive index images with better quality compared with a conventional reconstruction method.

Optical non-linear multimodal tomography is a powerful diagnostic imaging tool to analyse human skin based on its autofluorescence and second-harmonic generation signals. Recently, the field of clinical non-linear imaging has been extended by adding coherent anti-Stokes Raman scattering (CARS)-a further optical sectioning method for the detection of non-fluorescent molecules. However, the heterogeneity of refractive indices of different substances in complex tissues like human skin can have a strong influence on CARS image formation and requires careful clinical interpretation of the detected signals. Interestingly, very regular patterns are present in the CARS images, which have no correspondence to the morphology revealed by autofluorescence at the same depth. The purpose of this paper is to clarify this phenomenon and to sensitize users for possible artefacts. A further part of this paper is the detailed comparison of CARS and autofluorescence images of healthy human skin in vivo covering the complete epidermis and part of the upper dermis by employing the flexible medical non-linear tomograph MPTflex CARS.

Recently, DNA molecules embedded with magnetite (Fe3O4) nanoparticles (MNPs) drew much attention for their wide range of potential usage. With specific intrinsic properties such as low optical loss, high transparency, large band gap, high dielectric constant, potential for molecular recognition, and their biodegradable nature, the DNA molecule can serve as an effective template or scaffold for various functionalized nanomaterials. With the aid of cetyltrimethylammonium (CTMA) surfactant, DNA can be used in organic-based applications as well as water-based ones. Here, DNA and CTMA-DNA thin films with various concentrations of MNPs fabricated by the drop-casting method have been characterized by optical absorption, refractive index, Raman, and cathodoluminescence measurements to understand the binding, dispersion, chemical identification/functional modes, and energy transfer mechanisms, respectively. In addition, magnetization was measured as a function of either applied magnetic field or temperature in field cooling and zero field cooling. Saturation magnetization and blocking temperature demonstrate the importance of MNPs in DNA and CTMA-DNA thin films. Finally, we examine the thermal stabilities of MNP-embedded DNA and CTMA-DNA thin films through thermogravimetric analysis, derivative thermogravimetry, and differential thermal analysis. The unique optical, magnetic, and thermal characteristics of MNP-embedded DNA and CTMA-DNA thin films will prove important to fields such as spintronics, biomedicine, and function-embedded sensors and devices.

The purpose of the present communication is to study the metrological compatibility of measurement results of mass fractions of sodium chloride in aqueous solutions, by different analytical techniques: refractometry and oscillation-type densimetry. In complement pH measurements were made in solutions of NaCl of 3,5 cg g−1 and in a synthetic seawater standard (OSIL). Previously, metrological compatibility of measurement results with refractometer and oscillation-type density meter was showed for aqueous solutions of glucose, from 2 cg g−1 to 42 cg g−1. Due to its interest for the food science and health science, sodium chloride mass fraction in aqueous solution between 0,5 cg g−1 and 7 cg g−1 and for brine systems were also measured. Comparison was displayed between potentiometric pH and pH deduced by salinity experimental value.

We report the preparation of a high-purity optical-quality four-component glass of composition 68TeO2 — 22WO3 — 8La2O3 — 2Bi2O3, containing (2.7±0.5)×10-5 mol % OH groups. Its refractive index has been determined in the range 0.9 — 5.45 μm using interference refractometry. The data are used to assess the dispersion and guidance characteristics of microstructured optical fibres potentially attractive for supercontinuum generation in the range 1 — 5 μm

The use of THz radiation as a potential tool for medical imaging is of increasing interest. In this paper three methods of analysis of THz spectroscopic information for diagnosis of tissue pathologies at THz frequencies are presented. The frequency-dependent absorption coefficients, refractive indices and Debye relaxation times of pure water and pure lipids were measured and used as prior knowledge in the different theoretical methods for the determination of concentration. Three concentration analysis methods were investigated: (a) linear spectral decomposition, (b) spectrally averaged dielectric coefficient method and (c) the Debye relaxation coefficient method. These methods were validated on water and lipid emulsions by determining the concentrations of phantom chromophores and comparing to the known composition. The accuracy and resolution of each method were determined to assess the potential of each method as a tool for medical diagnosis at THz frequencies.

Accurate measurement of various key parameters is indispensable for the characterization of spray droplets. Global rainbow refractometry shows great potential for its unique capability of measuring the refractive index (RI) of a droplet, but is still restricted to the laboratory. A portable design of a global rainbow refractometry instrument is implemented based on a new optical layout, with software developed as well. This instrument has four operation modes in its hardware design. Performance testing on the instrument consists of image quality, multi-working distance measurement, and measurement accuracy. The quality of the rainbow image is improved using the new system. The results of the multi-working distance measurement show that the measured refractive indices with working distances of 16, 25, and 30 cm are exact with a maximum deviation of about 2 × 10−4, while the deviation is as large as 1.2 × 10−3 at 50 cm, which is explained by the aberration from a long working distance (≤32–35 cm). The measurement accuracy is verified further by ethanol–water spray droplets with volume concentrations of 0%–100%. The mean error of the RI measurement is about 2 × 10−4 at different concentrations. The measured mean droplet diameter has an accuracy of 2 µm in the experiments. The rainbow refractometry instrument has great potential for in situ measurement of various sprays, i.e. spray combustion/pollutant removal in a large containment.

Near- and sub-wavelength photonic structures are used by numerous organisms (e.g. insects, cephalopods, fish, birds) to create vivid and often dynamically-tunable colors, as well as create, manipulate, or capture light for vision, communication, crypsis, photosynthesis, and defense. This review introduces the physics of moth eye (ME)-like, biomimetic nanostructures and discusses their application to reduce optical losses and improve efficiency of various optoelectronic devices, including photodetectors, photovoltaics, imagers, and light emitting diodes. Light-matter interactions at structured and heterogeneous surfaces over different length scales are discussed, as are the various methods used to create ME-inspired surfaces. Special interest is placed on a simple, scalable, and tunable method, namely colloidal lithography with plasma dry etching, to fabricate ME-inspired nanostructures in a vast suite of materials. Anti-reflective surfaces and coatings for IR devices and enhancing light extraction from visible light emitting diodes are highlighted.

Diffraction-enhanced imaging (DEI) is a new x-ray imaging modality that has been shown to enhance contrast between normal and cancerous breast tissues. In this study, diffraction-enhanced imaging in computed tomography (DEI-CT) mode was used to quantitatively characterize the refraction contrasts of the organized structures associated with invasive human breast cancer. Using a high-sensitivity Si (3 3 3) reflection, the individual features of breast cancer, including masses, calcifications and spiculations, were observed. DEI-CT yields 14, 5 and 7 times higher CT numbers and 10, 9 and 6 times higher signal-to-noise ratios (SNR) for masses, calcifications and spiculations, respectively, as compared to conventional CT of the same specimen performed using the same detector, x-ray energy and dose. Furthermore, DEI-CT at ten times lower dose yields better SNR than conventional CT. In light of the recent development of a compact DEI prototype using an x-ray tube as its source, these results, acquired at a clinically relevant x-ray energy for which a pre-clinical DEI prototype currently exists, suggest the potential of clinical implementation of mammography with DEI-CT to provide high-contrast, high-resolution images of breast cancer (Parham 2006 PhD Dissertation University of North Carolina at Chapel Hill).

A system often used for vacuum metrology purposes in order to calibrate vacuum gauges in the UHV region and to calibrate gas leak rates is the throughput system, employing the continuous (or dynamic) expansion method. An important component in such systems is the flowmeter, which has to deliver a pure and well-determined gas flow into the system. To determine the generated gas flow, a number of factors including the pressure inside the flowmeter have to be determined. However, it has turned out that the calibration uncertainty when measuring the pressure in the flowmeter gives a main contribution to the total uncertainty (of typically about 0.1%) for the generated flow, thereby limiting the accuracies of the generated vacuum pressure as well as gas leak rates in UHV metrology. A feasibility study is reported in this paper about the possibility of using laser refractometry to monitor dynamic gas density in situ in the flowmeter, as an alternative and possibly more accurate means of determining the generated gas flow, thereby potentially improving the calibration gas leak rates in the range 10?8?10?4 Pa m3 s?1.

As a new method, x-ray diffraction enhanced imaging (DEI) has extremely high sensitivity for weakly absorbing low-Z samples in medical and biological fields. Conventional performance parameters, such as spatial resolution and low-contrast resolution, are not enough to describe the characteristics of a DEI system. This paper focuses on refraction-angle resolution which describes the ability of a DEI system to differentiate the x-rays refracted by the sample. The analysis of refraction-angle resolution is composed of two parts: the analysis of the single DEI image measured in a certain position of the rocking curve and the analysis of the refraction-angle image calculated by extraction methods. A 2D computer simulation experiment is performed to prove the results of the analyses. The limitations and conclusions of refraction-angle resolution are described in the end.

Phase contrast (PC) imaging is one of the most exciting emerging x-ray imaging techniques, with the potential of removing some of the main limitations of conventional radiology. After extensive experimentation carried out particularly at synchrotron radiation (SR) facilities, the scientific community agrees that it is now time to translate these ideas towards the first clinical implementations. In this framework, a complete model, based on Fresnel/Kirchoff diffraction integrals, was devised. This model accounts for source dimensions, beam spectrum and divergence and detector point spread function (PSF), and can thus be applied to any x-ray imaging system. In particular, by accepting in input the above parameters along with the ones describing the sample, the model can be used to optimize the geometry of the set-up, i.e. to assess the source-to-sample and sample-to-detector distances which maximize feature detection. The model was evaluated by acquiring a range of images of different samples with a laboratory source, and a good agreement was found between simulated and experimental data in all cases. In order to maximize the generality of the results, all acquisitions were carried out using a polychromatic source and an energy-resolving detector; in this way, a range of monochromatic images could be obtained as well as polychromatic images, which can be created by integrating different parts of the acquired spectra. One of the most notable results obtained is that in many practical cases polychromatic PC imaging can provide the same image quality as its monochromatic counterpart. This is an important step in the wider application of PC using conventional sources.

In order to study the heat and mass transfers between a spray of droplets and the atmosphere in thermal-hydraulics conditions representative of a severe accident in a Pressurized Water Nuclear Reactor, the French Institute for Radioprotection and Nuclear Safety (IRSN) developed the TOSQAN facility. The present paper presents the development and the quantification of an optical diagnostic, global rainbow refractometry, in order to measure falling droplet temperature.

Near-infrared optical techniques for clinical breast cancer screening in humans are rapidly advancing. Based on the computational inversion of the photon diffusion process through the breast, these techniques rely on optical tissue models for accurate image reconstruction. Recent interest has surfaced regarding the effect of refractive index variations on these reconstructions. Although many data exist regarding the scattering and absorption properties of normal and diseased tissue, no measurements of refractive index appear in the literature. In this paper, we present near-infrared refractive index data acquired from N-methyl-N-nitrosourea-induced rat mammary tumours, which are similar in pathology and disease progression to human ductal carcinoma. Eight animals, including one control, were employed in this study, yielding data from 32 tumours as well as adjacent adipose and connective tissues.

The refractive index of human skin tissues is an important parameter in characterizing the optical response of the skin. We extended a previously developed method of coherent reflectance curve measurement to determine the in vitro values of the complex refractive indices of epidermal and dermal tissues from fresh human skin samples at eight wavelengths between 325 and 1557 nm. Based on these results, dispersion relations of the real refractive index have been obtained and compared in the same spectral region.

The results of development of two versions of the intracavity laser refractometry in reflection (ILRR) are reviewed in this paper. The ILRR method is based on the use of the properties of two-mode lasers with broken cavity and allows the precise refractive investigations of turbid media to be fulfilled in real time. The ILRR applications are primarily planed to biological liquids study. The results of blood refractometry are offered.

We analyse the effect on scattered photons of anomalous optical inclusions in a turbid slab with otherwise uniform properties. Our motivation for doing so is that inclusions affect scattering contrast used to quantify optical properties found from transmitted light intensity measured in transillumination experiments. The analysis is based on a lattice random walk formalism which takes into account effects of both positive and negative deviations of the scattering coefficient from that of the bulk. Our simulations indicate the existence of a qualitative difference between the effects of these two types of perturbations. In the case of positive perturbations the time delay is found to be proportional to the square of the size of the inclusion while for negative perturbations the time delay is a linear function of its volume.

In view of its potential for the intracavity laser refractometry, He-Ne laser with two orthogonally polarized modes at wavelength of 1.15 μ was studied. The amplitude and frequency characteristics of this laser were investigated theoretically and experimentally. It has been shown that stable two-mode lasing may be implemented for any split between modes upon applying the transverse magnetic field.

A review is given of how measurements of intensive thermodynamic properties of a gas, such as of refractivity and permittivity, can enable single-parameter monitoring of gas density. The last decade has seen the introduction of a number of new and improved optical and dielectric measurement techniques as well as significant advances in atomic structure calculations. Such advances may find application, for example, in the determination of primary gas mixtures and fundamental constants, buoyancy compensation in precision mass metrology and in dynamic pressure metrology.

A new method for the analysis of the dispersion behaviour of a transparent medium is presented: the refractive index n is obtained as a function of the wavelength λ: n = n(λ). The procedure is based on the analysis of the hybrid bi-dimensional fringe pattern obtained at the exit plane of a spectrometer which performs the spectral analysis of a white-light interferogram. The phase of the signal depends both on a spatial coordinate and on the chromatic variable wavenumber σ = λ−1. Taking advantage of the dispersion behaviour of the sample, the phase of the signal can be forced to become stationary at certain points of this hybrid plane. The line which joins the stationary phase points stores the parameters of a series expansion of the refractive index as a function of wavenumber. These parameters are experimentally obtained through an appropriate numerical fitting procedure.

Near-infrared (NIR) tomography is a technique used to measure light propagation through tissue and generate images of internal optical property distributions from boundary measurements. Most popular applications have concentrated on female breast imaging, neonatal and adult head imaging, as well as muscle and small animal studies. In most instances a highly scattering medium with a homogeneous refractive index is assumed throughout the imaging domain. Using these assumptions, it is possible to simplify the model to the diffusion approximation. However, biological tissue contains regions of varying optical absorption and scatter, as well as varying refractive index. In this work, we introduce an internal boundary constraint in the finite element method approach to modelling light propagation through tissue that accounts for regions of different refractive indices. We have compared the results to data from a Monte Carlo simulation and show that for a simple two-layered slab model of varying refractive index, the phase of the measured reflectance data is significantly altered by the variation in internal refractive index, whereas the amplitude data are affected only slightly.

We review the recent development of T-ray computed tomography, a terahertz imaging technique that allows the reconstruction of the three-dimensional refractive index profile of weakly scattering objects. Terahertz pulse imaging is used to obtain images of the target at multiple projection angles and the filtered backprojection algorithm enables the reconstruction of the object's frequency-dependent refractive index. The application of this technique to a biological bone sample and a plastic test structure is demonstrated. The structure of each target is accurately resolved and the frequency-dependent refractive index is determined. The frequency-dependent information may potentially be used to extract functional information from the target, to uniquely identify different materials or to diagnose medical conditions.

The far-infrared dielectric function of a wide range of organic molecules is dominated by vibrations involving a substantial fraction of the atoms forming the molecule and motion associated with intermolecular hydrogen bond vibrations. Due to their collective nature such modes are highly sensitive to the intra- and intermolecular structure and thus provide a unique fingerprint of the conformational state of the molecule and effects of its environment. We demonstrate the use of terahertz time-domain spectroscopy (THz-TDS) for recording the far-infrared (0.5-4.0 THz) dielectric function of the four nucleobases and corresponding nucleosides forming the building blocks of deoxyribose nucleic acid (DNA). We observe numerous distinct spectral features with large differences between the molecules in both frequency-dependent absorption coefficient and index of refraction. Assisted by results from density-functional calculations we interpret the origin of the observed resonances as vibrations of hydrogen bonds between the molecules.

We describe a new class of experiments involving applications of terahertz radiation to problems in biomedical imaging and diagnosis. These involve scale model measurements, in which information can be gained about pulse propagation in scattering media. Because of the scale invariance of Maxwell's equations, these experiments can provide insight for researchers working on similar problems at shorter wavelengths. As a first demonstration, we measure the propagation constants for pulses in a dense collection of spherical scatterers, and compare with the predictions of the quasi-crystalline approximation. Even though the fractional volume in our measurements exceeds the limit of validity of this model, we find that it still predicts certain features of the propagation with reasonable accuracy.

The comparison of mass standards in air generally requires air buoyancy corrections. To determine these corrections it is necessary to estimate the density of moist air inside the balance case during the weighing procedure. The relative combined standard uncertainty of the air density determination is about 10?4 when using the CIPM-81/91 formula. The aim of our present work is to reduce the air density uncertainty using other methods and the purpose of this paper is to compare the performance of the following three methods: (a) application of the CIPM-81/91 formula (absolute determination); (b) direct determination using air buoyancy artefacts having the same surface area but a large volume difference (absolute determination); (c) refractometry, exploiting the high correlation between air density and air index of refraction using a novel refractometer well adapted to our requirements (relative determination). So far, the coherence between the three methods is satisfactory; the response characteristics for the three determinations are comparable and the agreement between the air density determinations is within 1 ? 10?5 kg m?3. A discrepancy of the order of 10?4 kg m?3 is observed between the CIPM-81/91 formula and artefact methods. This difference needs to be clarified by future measurements. The relative combined standard uncertainty obtained on the air density determination when using the air buoyancy artefacts is about 7 ? 10?6.

The phase of light propagating through a bent optical fibre is shown to depend on the refractive index of the medium surrounding the fibre cladding when there is resonance coupling between the guided core mode and cladding modes. This shifts the spectral maxima in the bent fibre-optic Fabry — Perot interferometer. The highest phase and spectral sensitivities achieved with this interferometer configuration are 0.71 and 0.077, respectively, and enable changes in the refractive index of the ambient medium down to 5×10-6 to be detected. This makes the proposed approach potentially attractive for producing highly stable, precision refractive index sensors capable of solving a wide range of liquid refractometry problems.

We investigate the changes in the transmission spectrum of long period fibre gratings and tilted short-period fibre Bragg gratings versus the refractive index of the surrounding medium. The metrological characteristics of tilted short-period fibre Bragg gratings and an analytical method enabling their potential use in accurate refractometry are discussed.

We have prepared 28Si, 29Si and 30Si single crystals with enrichments above 99.9 at % and a silicon single crystal of natural isotopic composition. The oxygen and carbon concentrations in all the crystals are within 5×1015 cm-3, and the content of metal impurities is 0.01 — 0.1 ppma. The refractive index of the crystals has been determined in the range 1.05–25.5 μm using interference refractometry, and its dispersion and material dispersion have been determined.

This paper explores the application to mammography of phase contrast produced by variations in x-ray refractive index. As a spatially coherent x-ray beam propagates through an x-ray transparent medium, the phase of the incident wavefront becomes modified in a manner related to the electron density of the medium. The resulting phase gradient across the wavefront is equivalent to a small change in direction of the propagation of the wave. For a general object, the change in propagation direction will vary from point to point depending on the structures within the object. The net effect can be recorded in a radiographic image using an appropriate geometry to produce the visual appearance of edge enhancement at interfaces between materials with differing x-ray refractive indices. Normally these materials will also have differences in attenuation coefficient, so the overall effect is to increase the visibility of interfaces between materials. It is proposed that mammographic images can be subtly enhanced by the use of phase contrast information to overcome some of the known limitations of the imaging process whilst leaving the gross radiological appearance of the images substantially unchanged. The design trade-offs required to utilize phase contrast information were investigated using a conventional mammographic x-ray generator and film-screen system. The Leeds TORMAM mammographic image quality test object was then used to demonstrate a considerable improvement in image quality for the phase contrast enhanced images over those produced in the conventional geometry with no increase in radiation dose to the patient. The results are discussed in terms of their possible practical application.

Clarification is provided for certain expressions quoted in Hedlund and Pendrill (2006 Meas. Sci. Technol. 17 2767–72).

High-precision critical-angle refractometers, with automatic refractive index computation and readout, are now available from several manufacturers. In such instruments light is incident from within a transparent reference medium, typically an optical prism, onto a sample material in good optical contact with the prism. The critical angle of reflection is measured at the interface between the prism of known refractive index and the sample material of unknown refractive index. Critical-angle refractometers are calibrated for transparent samples but are commonly used for measurements of refractive index in optically absorbing or optically heterogeneous materials. The instrument will then read an apparent refractive index which differs from the true refractive index of the sample. The optics of the critical-angle refractometer are investigated to quantify errors which arise from the neglect of absorption and heterogeneity in the transparent sample interpretation of the critical angle. Refractive index reading errors will be important when they become larger than the refractive index precision of modern instruments, about . In critical-angle refractometry it is shown that this occurs for samples of surprisingly weak absorption. The critical-angle and Brewster-angle methods are compared for optically absorbing and heterogeneous samples. Errors arising from sample absorption in the Brewster-angle method are shown to be much less than in the critical-angle method.

This paper derives an analytical model for investigating the effect of the distribution of absorbers upon light attenuation in a scattering medium. Results from this are found to agree with those of Monte Carlo simulations. The implications of this model are then examined for their likely effect upon the measurement of cerebral blood volume (CBV) using near-infrared (NIR) spectroscopy. We conclude that, given the small diameter of the majority of cerebral blood vessels, the distribution of the blood will have little effect upon the measurement of CBV. Where changes to the blood volume occur in the larger (> 0.2 mm diameter) vessels on the surface of the brain, NIR spectroscopy is likely to underestimate the change.

This paper describes a comparative study of in situ cure monitoring by three methods: (i) evanescent wave spectroscopy; (ii) refractive index change; and (iii) near-infrared spectroscopy. The cure characteristics of an epoxy/amine reaction were followed in real-time during the crosslinking reaction via the above-mentioned techniques. The evanescent wave spectroscopy technique was based on monitoring the characteristic infrared absorption bands of the resin system to compute the concentration of the amine hardener as a function of cure time. Good correlation was obtained between the evanescent wave spectroscopy data and a conventional method of studying cure reactions, i.e. infrared spectroscopy. During the cure reaction, the refractive index of the resin system increases as a function of the crosslink density. This increase in the refractive index was monitored using two optical fibre techniques. In the first case, a declad region of the optical fibre was immersed in the resin system and in the second method an optical fibre reflectometer was used to track the changes in the refractive index. Once again, good correlation was obtained between the optical fibre techniques and infrared spectroscopy cure data. The results obtained from the optical fibre sensor experiments were used to model the cure kinetics of the resin system. The cure kinetic models were found to predict the cure reaction up to approximately 60% of the reaction.

The potential of several physical methods for investigation of the botanical origin of honey has been discussed. Samples from the three most prevalent types of honey in Bulgaria (acacia, linden and honeydew) have been used. They have been examined by laser refractometry, UV, VIS and FTIR spectroscopy, electric conductivity measurement and differential scanning calorimetry. The purpose of this study was to reveal the physical characterizations of honeys from different flora produced in Bulgaria and to identify honeys with a high apitherapy potential for future studies.

A plasma channel created by a high intensity infrared laser beam was observed in a long scale-length plasma (L ~ 240 μm) with the angular filter refractometry technique, which indicated a stable channel formation up to the critical density. We analyzed the observed plasma channel using a rigorous ray-tracing technique, which provides a deep understanding of the evolution of the channel formation.