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Purpose This study aims to propose and study a refractive index sensor based on measuring variations of the internal diffuse reflectance from a glass interface in a functional design. The device is uncomplicated to assemble with simple optical elements and it can be built as a robust and stable sensor. Design/methodology/approach This study presents a simplified theoretical model of the signal obtained with the proposed device and perform a detailed analysis of its potential resolution and merits. Findings The authors report proof-of-principle experiments with a home-made device to evaluate its performance as a refractometer and index of refraction sensor. Originality/value The main novelty of the device is the use of a diffusing surface to couple light into a glass plate with a wide range of angles of refraction, including angles larger than the critical angle with the external medium, and using the same diffusing surface to couple reflected light out of the glass plate, including light that suffered total internal reflection.

The use of the angular spectrum method (ASM) to simulate the reflection of airborne ultrasound beams from a thin membrane separating air from a mixture of air and another gas is examined. The main advantage of this method is its high computing speed and efficiency for practical design calculations, suitable for sensing applications. The implemented ASM code is validated against custom Rayleigh integral code in a pure propagation simulation. In addition, ultrasound beam reflection calculations using ASM with finite element numerical results and experimental measurements are compared, finding good agreement in both cases. Then, ASM is used to estimate the sensitivity of specular reflection signals to variations in the composition of the incidence medium as a function of the angle of incidence. Conditions for which a reflection signal using inexpensive commercial ultrasound emitter/receiver at 40 kHz, in a simple configuration, offer a high enough sensitivity suitable for monitoring air quality indoors are found.

We propose and test a method for determining a fluorescent medium’s absorption or extinction index while it is fluorescing. The method uses an optical arrangement that records changes in fluorescence intensity at a fixed viewing angle as a function of the angle of incidence of an excitation light beam. We tested the proposed method on polymeric films doped with Rhodamine 6G (R6G). We found a strong anisotropy in the fluorescence emission and, thus, limited the method to TE-polarized excitation light. The method proposed is model dependent, and we provide a simplified model for its use in this work. We report the extinction index of the fluorescing samples at a selected wavelength within the emission band of the fluorophore R6G. We found that the extinction index at the emission wavelengths in our samples is appreciably larger than the extinction index at the excitation wavelength, which is the opposite of what one might expect from measuring the absorption spectrum of the medium with a spectrofluorometer. The proposed method could be applied to fluorescent media with additional absorption other than by the fluorophore.

Light’s internal reflectivity near a critical angle is very sensitive to the angle of incidence and the optical properties of the external medium near the interface. Novel applications in biology and medicine of subcritical internal reflection are being pursued. In many practical situations, the refractive index of the external medium may vary with respect to its bulk value due to different physical phenomena at surfaces. Thus, there is a pressing need to understand the effects of a refractive-index gradient at a surface for near-critical-angle reflection. In this work, we investigate theoretically the reflectivity near the critical angle at an interface with glass assuming the external medium has a continuous depth-dependent refractive index. We present graphs of the internal reflectivity as a function of the angle of incidence, which exhibit the effects of a refractive-index gradient at the interface. We analyze the behavior of the reflectivity curves before total internal reflection is achieved. Our results provide insight into how one can recognize the existence of a refractive-index gradient at the interface and shed light on the viability of characterizing it.

A theoretical and experimental assessment of an optical technique for the rapid quantification of the density of biological cells adsorbed on a flat surface, as well as for monitoring in real time the surface coverage is presented. The method relies on the high sensitivity of the optical reflectivity of a surface near grazing incidence to the presence of particles large compared to the wavelength of light, a fact mostly unknown and not yet utilized in optical sensing. First, we perform a theoretical assessment of the method for sensing surface-coverage by biological-cell type particles. For this, we use a recently developed analytical model for the coherent reflection of light from a flat interface partially covered by a disordered monolayer of particles. Then, we investigate the viability of the technique experimentally with spores of Penicillium digitatum (PD) deposited on microscope slide. These fungus spores, along with those of Penicillium italicum (PI) are the main cause of losses in the citrus industry. We illustrate the possibility of monitoring in real time, the detachment of spores from a surface by an air flow. Experimental measurements are found to follow qualitatively theoretical predictions, showing that the theoretical model can be used for general sensor design purposes. From the experimental tests performed, we conclude that resolutions better than 1% in surface coverage of a flat surface by unicellular organisms are achievable without difficulty.

We report a measurement methodology to sense electrically the presence of nanoparticles in suspensions in droplets and assess their type and density. The nanoparticle suspensions used in this work were solvent-based. A small amount of the suspensions was placed on the sensitive surface of an interdigitated capacitance sensor and left to evaporate at ambient temperature while the capacitance signal was registered. Our sensor has a driving and a sensing interdigitated spiral electrode array and the output signal is proportional to the impedance difference between similar sensors, reference and sensing. They were designed and fabricated over a glass-substrate. A low noise measurement system based on a Lock-in Amplifier monitored continuously in time the evaporation process of nanoparticle suspension droplet by registering variations of the imaginary component of the differential current. The results show that it is possible to infer the presence of nanoparticles in suspension and distinguish, at least between two types of nanoparticles, at very low volume concentrations.

In this work we study theoretically and experimentally, the reflectance of a monolayer of nanoparticles randomly located onto a substrate, immersed in a homogeneous medium. The system is illuminated with a laser of wavelength λ = 543 nm in an internal incidence configuration and both p and s polarizations are investigated. The samples are prepared by means of the direct current magnetron sputtering technique, using short erosion times to allow the growing of a non-percolated single layer of gold nanoparticles over the surface of a glass substrate. The particles’ average radius is 5.8±1.25 nm and the surface coverage is estimated to be 29.6±1%. We compare the reflectance predicted by five theoretical models—equivalent-volume, Van de Hulst, Maxwell Garnett, dipolar and coherent scattering—with the measured reflectance of the samples, obtaining a good agreement between the prediction of the dipolar and Maxwell Garnett models with the measured data. These results motivate further studies to consider an application of this system as an ultrasensitive sensor.

Monitoring lysis in cell’s suspensions can be a good indicator of cell’s integrity and health. The current techniques to measure the number of lysed cells in some process are generally time consuming. Objective: In this paper, we investigate the viability of a simple method for sensing hemolysis in real time through the electrical impedance of a parallel-plate capacitor that holds a biological cell suspension. Materials and methods: The method is based on measuring variations of the effective dielectric function of an erythrocyte suspension during a lysis process induced by osmotic stress. Our sensor is strongly affected by the so-called electrode polarization phenomenon which prevents sensing variations of the real part of the effective dielectric function. However, measurements of the imaginary part, and thus of the electrical resistivity of the suspension, is straight forward. Results: We show that from resistivity measurements, it is possible to sense variations of the lysed erythrocytes’ volume fraction with a resolution better than 0.1%. Conclusions: The proposed sensor can offer a high resolution in monitoring hemolysis and is a viable option for developing new rapid diagnostic medical tests

We analyze the sensitivity of electrical measurements to lysis of biological cells in suspensions. We aim at discerning the electrical parameter and measuring conditions that are more convenient to monitor the kinetics of a lysis process. We consider single-frequency measurements of the capacitance and resistance of an electrical capacitor containing a suspension of biological cells at mid-radiofrequencies. We use well-established models to estimate the relative sensitivities to variations of the volume fraction of cells during a lysis process. Then we estimate values for the resolution in measuring the fraction of cells that have been lysed with commercially available LCR meters. Unexpectedly, our results indicate that, despite the total number of ions in solution remains constant during a lysis process, in general, the electrical resistance provides a more appropriate signal for sensing the evolution of cell lysis than the capacitance. This is contrary to the case when the cell's volume fraction without lysis is to be measured. We predict that, with single-frequency resistance measurements, a resolution better than 0.1% in volume fraction of lysed cells should be attainable without much difficulty. Keywords: Electrical sensing, Biological-cell suspensions, Lysis, Resistance, Capacitance, Effective dielectric function

In this paper, we develop a detailed theoretical model for the optical reflectivity of a bio-tissue film confined between two flat interfaces based on the anomalous-diffraction approximation. We consider bio-tissue films consisting of a few layers of spheroidal cells surrounded by extracellular medium. We explore numerically the predictions of our model and compare them with simple effective medium theories, sometimes used as a first attempt to understand the optical properties of biological media. Then, we fit the model to experimental reflectivity-versus-angle-of-incidence curves of confined whole-blood films measured in an internal reflection configuration. Measurements were performed by confining a drop of fresh blood between a prism and a coverslip. Our experimental results show that it is possible to measure the coherent reflectance with small enough error to infer microstructural parameters with a good precision. The errors in measuring the coherent reflectance depend on the reflectivity magnitude. For instance, for a reflectivity of about 0.3 the error is below 2%, and the refractive indices of cells and surrounding medium can be obtained with a precision better than 1%. These results also indicate that the present model can readily be used to figure out the physical changes experienced at the microscale in bio-tissue films during a physicochemical process.

In this work we investigate an efficient method for calculating the reflected and transmitted acoustic fields from diffracting ultrasound beams incident to a gas/gas interface. The study is motivated by the application to gas sensors of an experimental setup consisting of a 40 kHz acoustic incident beam at oblique angles to an interface separating two gases. The numerical method consists of performing a plane wave expansion of the incident beam using standard Fourier transform methods. Expressions for the reflected and transmitted acoustic fields are obtained in terms of integrals of simple kernels. These can be performed numerically in a straightforward way. The main approximation in this method comes from guessing the acoustic pressure field at a plane just in front of the ultrasonic emitter. We compare calculations of this Plane Wave Expansion Method (PWEM) with finite element method simulations in a 2D geometry, finding good qualitative agreement. Among the advantages of the PWEM we have that there is no need to calculate the whole acoustic fields in a large volume, compared to the wavelength of the frequency used, while still considering rigorously diffraction effects. We present calculations for airborne ultrasound reflection measurements of interest in developing novel acoustic gas sensors.

Currently, there are available a few simple analytical approximations to the complex effective refractive index that may be used for nanofluids. Namely, the Maxwell Garnett mixing formula with scattering corrections, the Maxell Garnett Mie approximation, the Foldy-Lax approximation and the small particle limit of the quasi-crystalline approximation. These approximations are valid either for very small nanoparticles (below a few nanometers in radius) or for very dilute nanofluids (below about 1% in particles’ volume fractions) and therefore, do not cover the whole domain of particle suspensions referred to as nanofluids. Here we propose a new simple analytical approximation based on local field corrections to the Foldy-Lax approximation. The new mixing formula coincides with the mentioned approximations when they are expected to be valid and provides physically sound predictions when the mentioned approximations are no longer valid, within the realm of nanofluids. We compare predictions of the analytical approximations considered in this work with experimental data published earlier for nanofluids of polystyrene in water.

The van de Hulst formula provides an expression for the effective refractive index or effective propagation constant of a suspension of particles of arbitrary shape, size and refractive index in an optically homogeneous medium. However, its validity for biological matter, which often consists of very dense suspensions of cells, is unclear because existing derivations of the formula or similar results rely on far-field scattering and/or on the suspension in question being dilute. We present a derivation of the van de Hulst formula valid for suspensions of large, tenuous scatterers -- the type biological suspensions are typically made of -- which does not rely on these conditions, showing that they are not strictly necessary for the formula to be valid. We apply these results specifically to blood and epithelial tissue. Furthermore, we determine the true condition for the formula to be valid for these types of tissues. We finally provide a simple way to estimate -- and, more importantly, correct -- the error incurred by the van de Hulst formula when this condition is not met., 10 pages, 8 figures

We propose and investigate an alternative optical method to determine the thickness of metallic nanofilms thinner than about 50 nm deposited on glass substrates. The method consists of measuring the internal TM reflectivity as a function of the angle of incidence and fitting the curve with Fresnel formulae for a bilayer system to find the film thickness. Gold nanofilms were fabricated by electron beam evaporation (nominal thicknesses of 20 nm, 30 nm and 50 nm) and characterized by common methods (SEM, AFM, Profilometer). The reflectivity curve of TM (or p) polarized light presents a clear peak around the critical angle with air and then a dip due to the excitation of a surface plasmon-polariton mode at the gold/air interface. Fitting the theoretical model around the reflectivity peak readily gives the film’s thickness, and fitting other sections of the curve away from the critical angle with air without altering the fit around the peak, provides the thickness and effective refractive index of the adhesive layer (Cr, Ti). The present approach to gold nanofilm’s thickness measurement is robust and expeditious. It is less sensitive to surface roughness, contamination and nanofabrication defects such as nanometer sized holes.

The purpose of this article is to calculate the generalized nonlocal conductivity tensor of a spherical particle made of isotropic and linear materials. The generalized conductivity tensor is a crucial element in the formulation of the mean-field theories of the electromagnetic response of random particulate systems. This is equivalent to what is called the T-Matrix in multiple scattering theories. Here, a new method is proposed for finding explicit expressions for this tensor directly from its definition including the magnetic response of the spheres. Its relation with the S-Matrix in the theory of single scattering is stated as a generalization. Several approximations and limit cases of possible interest in specific systems are analyzed and the results of some calculations are presented as a numerical example.

In this work, we solve the problem of modeling the generation of an acoustic pulse produced by the incidence of a pulsed laser light upon an elastic material. Our concern is about the heat transport during the absorption of electromagnetic radiation. We assume that the pulse duration is of the order of nanoseconds, and asses if under these conditions the contribution of the heat transport in the sample is an essential consideration in the description of the phenomena or if we can ignore it in the model. We begin with the energy balance analysis over the initial interaction of radiation with matter in the context of the formulation Meixner-Prigonine which is called the linear irreversible thermodynamics to describe the induced temperature field. Then we carry a momentum balance which yields the macroscopic elasticity equations with a heat source for the induced pressure field. Once established the equations for temperature and displacement fields, we solve them for the one-dimensional case, showing that the induced pressure has two components, one fast component and one slow component which is due to heat transport in the sample, which is one of the main contributions of the paper.

We develop simple models for the optical reflectivity of an interface in optical contact with random media consisting of discrete volumes of arbitrary form and different refractive indices. Examples of interest are surfaces sprinkled with microdroplets or an interface with biological cells adhered to it at random locations. We focus our attention to the case of internal reflectivity, in which the incidence medium has a larger refractive index than the refractive indices at the other side of the interface. Assuming an incident plane wave, we provide simple approximate expressions for the surface’s coherent reflectance and for the surface’s total reflectance. We compare predictions of the surface coherent-reflectance model with numerical simulations. Then we use the surface’s reflectance models to interpret experimental measurements obtained with an optical prism and a thin vegetable tissue adhered to its base. In general, the surface reflectivity can be used to determine fractional contact area between the interface and microdroplets or biological cells and infer their refractive indices with an accuracy of about 0.5%.

We study the optical reflectivity of confined colloidal films as a function of the angle of incidence in an internal reflection configuration. Two effective medium models and an extended coherent-scattering model for thin colloidal films are compared against experimental measurements with gold, latex, and titanium dioxide colloids. A derivation of the coherent scattering model for confined colloidal films used in this work is presented in a comprehensive way. The model lies within the framework of the multiple-scattering theory and is valid for any angle of incidence and for colloids of small or large particles compared to the wavelength of light, however, only for small and moderately small particles' volume fraction. Reflectivity versus angle of incidence curves for an opaque colloidal film in an internal reflection configuration show the effects of two critical angles. Within the two critical angles, there is a high sensitivity to the presence of colloidal particles, while the volume of colloidal samples needed is in the microliter range. Upon comparing theory with experiment, no model fitting was done in any case. The experimental setup and its calibration procedure are discussed. The results provide physical insight into applications involving optical properties of colloidal systems.

Reflectance spectra at normal incidence were recorded from different latex applications during their film formation processes at ambient temperature. Besides oscillations in the reflected spectra due to two-wave interference phenomenon in an effective film, a clear asymmetric Fano-like resonance appeared during long periods of time in some of the latex films studied. We develop an optical model to explain all the observed interference phenomena and elucidate in simple terms the underlying mechanism of the observed Fano resonances. The model reproduces qualitatively very well all the features in the experimental reflectance spectra. We find that the Fano resonances arise due to the interference between the surface reflection and a Bragg reflection from self-assembled photonic crystals within the film.

A parallel plate capacitor can be used as a high resolution sensor of the electrical characteristics [...]

We provide new expressions for the reflection amplitudes of a half space of randomly located identical spherical particles that can be regarded as an extension of Fresnel's formulas when scattering is prominent. We derive them rigorously from Maxwell's equations by solving an integral equation for the electric field within the effective-field approximation. The integral equation is given in terms of the nonlocal conductivity tensor of an isolated sphere. Approximate expressions for the reflection amplitudes are also proposed and their accuracy is analyzed, first for the case of a self-sustained suspension of silver particles, and then for the more realistic situation of silver particles in water. In this latter case the integral equation is modified by introducing the half-space Green's function dyadic instead of the one in free-space, but the method of solution is analogous in both. This extension of Fresnel's formulas, together with the numerical comparison of the different approximations proposed here, is necessary for an accurate interpretation of reflection-spectroscopy measurements in dilute colloidal suspensions of practical interest. The connection between the nonlocal conductivity tensor and the T-matrix operator of scattering theory is also made manifest.

We study an optical method to infer the size of nanoparticles in a thin film of a dilute nonabsorbing nanocolloid. It is based on determining the contribution of the nanoparticles to the complex effective refractive index of a suspension from reflectivity versus the angle of incidence curves in an internal reflection configuration. The method requires knowing only approximately the particles' refractive index and volume fraction. The error margin in the refractive index used to illustrate this technique was 2%. The method is applicable to sizing nanoparticles from a few tens of nanometers to about 200 nm in radius and requires a small volume of the sample, in the range of a few microliters. The method could be used to sense nanoparticle aggregation and is suitable to be integrated into microfluidic devices.

We propose a passive indirect light collector system for interior lighting with the use of diffuse reflectors. This system is based on the idea that diffuse reflectors may increase the acceptance angle of passive systems therefor improving the overall effectiveness and also achieving a better response to light directionality in certain hours of the day. We search the most efficient way to collect optical power by scaling the collection area with a multi-level configuration. We also analyse the light collection on buildings facades through the simulation of the behaviour of the system under cloudy day conditions showing these systems as a feasible and economic option for indirect and direct light collection for interior lighting, allowing a less invasive way to conduct light to interiors with acceptable efficiency and avoiding the use of heliostats or expensive coating materials.

First, we present a brief review of the theory of the complex effective refractive index of a disordered suspension of nanoparticles that take into account the effects of scattering by the particles. We present numerical examples of the dependence of the scattering losses in nanocolloids with the concentration of non-absorbing and weakly absorbing nanoparticles. Then, we explore a way to reduce scattering losses in colloidal suspensions of non-absorbing nanoparticles. Finally, we provide some physical insight into the dependence of scattering losses with the particle concentration.

We derive simple approximate expressions for the reflectivity and transmissivity of light from disordered monolayers of tenuous particles of dimensions larger than the wavelength and supported by a flat interface. The expressions derived can be used for different particle shapes and for moderate angles of incidence. We then investigate the effects of particle shape and orientation on reflectivity and transmissivity spectra of a monolayer of tenuous particles containing an optical chromophore in a solution in their interior. We also simulate the effects of a particle’s shape and orientation on the angle dependence of the optical reflectivity and transmissivity. In our examples, we consider disordered monolayers of particles analogous to some biological cells.

6 págs.; 9 figs.; 3 tabs., The refractive index of nanometric (, This research work was partially supported under Project PAPIIT-UNAM No. IG100614-2. We also acknowledge the funding from the Spanish Ministry for Economy and Competitiveness through TEC 2012-38901- C02-01 and TEC 2015-69916-C2-1-R projects.

Submicron patterning of the spatial arrangement of spherical silver nanoparticles on a substrate is reported. The patterned samples were obtained by irradiating a thin silver film in the quasipercolated state with short laser pulses. Light reached the sample through a slit on a highly reflecting silicon mask placed parallel to the surface of the sample and was reflected between the sample and the inner face of the mask. The incident and reflected light interfered on the sample’s surface creating a complicated pattern of thin lines along which spherical-shaped silver nano-particles are concentrated. We provide a simple model to explain the observed patterns and briefly discuss how could one control the shape and thickness of the patterned lines of nanoparticles.

The reflectivity variations of a dielectric thin film during the deposition of a disordered monolayer of metallic nanoparticles are studied. We present experimental results and provide theoretical physical insight into the behavior of the reflectivity signal and its dependence on the dielectric thin-film thickness and structural features of the monolayer of nanoparticles. A closed-form expression is used to describe the reflectivity of a disordered monolayer of particles on a flat substrate within the frame of a coherent-scattering model approach. It is shown that the model reproduces qualitatively the behavior of the reflectivity signal during the experiment. Finally we study the optical response in the limit of small particles for low surface coverage fractions of the monolayer to evidence the main parameters that dictate the evolution of the reflectivity signal during the growth of a monolayer of nanoparticles. © Springer-Verlag 2012

It has been recently shown that the effective electromagnetic bulk response of a dilute colloidal system, composed by a large collection of identical big spheres, located at random, is spatially dispersive (non-local). Here, we extend this effective-medium approach to the calculation of the reflection and transmission amplitudes of the same system but with a flat interface. We use an integral-equation approach for the calculation of the average electric field. The integral equation is solved within the effective-field approximation, by proposing a plane-wave solution with effective parameters that are calculated by solving a set of consistency equations. We obtain explicit expressions for the transmission and reflection amplitudes as a function of the filling fraction, the radius of the inclusions and the angle of incidence. We show and discuss numerical results for a system of silver particles.

In this work, we discuss the effect of surface roughness on the measurement of the diffuse to near-normal viewing reflectance factor of coatings and evaluate its impact on the corresponding color coordinates. We compare specular component included (SCI) and excluded measurements. We introduce a gloss-factor to account for surface roughness in specular component excluded measurements. We present experimental results on samples with different degrees of surface roughness. Samples in this study were chosen to expose the contribution of the surface in the reflectance factor. For slightly rough surfaces, the influence on the measured reflectance factor depends on whether the specular-component is included or excluded. As the surface roughness increases, the specular-excluded reflectance factor increases approaching its value with the SCI further roughness lead to similar measurement results in both configurations. © 2012 Wiley Periodicals, Inc. Col Res Appl, 38, 177–187, 2013.

We review a formalism developed previously to deal with the non-local (spatially dispersive) effective electromagnetic response of colloidal systems. We use this formalism to find, in the long wavelength limit, a general expression for the local energy conservation theorem in terms of parameters associated to the non-local response of the system. We find an explicit expression for the Poynting vector in terms of these non-local parameters and show that there are several ways to write it in terms of the electric permittivity and the magnetic permeability. The significance of this problem in the treatment of refraction is also discussed.

We analyze the retrieval of the radius and complex refractive index of colloidal particles from the measurement of the effective optical properties of a dilute colloid. We point out some necessary precautions for the measurement of the effective refractive index of colloids and discuss the main sources of uncertainty.

In this paper, we compare three different models that have been used to interpret reflectivity measurements of supported monolayers of nanoparticles. Two of them: (i) isotropic Maxwell Garnett and (ii) anisotropic two-dimensional-dipolar model are based on an effective-medium approach, while the third one (iii) coherent-scattering model, lies within the framework of multiple-scattering theory. First, we briefly review, on physical grounds, the foundations of each model and write down the corresponding formulas for the calculation of the reflectivity. In the two-dimensional-dipolar model, the dilute limit of the pair-correlation function (also called hole-correlation function) is always used in the calculation of the effective optical response. Then we use these formulas to plot and analyze graphs of the reflectivity of a monolayer of gold nanoparticles on a glass substrate, as a function of several relevant parameters, for two different commonly used experimental configurations. Finally, we discuss the importance of our results and how they can be used to infer the limits of validity of each model.

Most colloidal samples have a non-negligible imaginary component of the effective refractive index due to scattering and absorption losses. In this paper we analyze the requirements on a differential refractometer so that it can be used to measure the real part of the effective refractive index increment in colloids for particle size analysis.

We assess the validity and possible use of a simple formula for the complex effective refractive index of a colloidal suspension of very small particles obtained from the quasi-crystalline approximation. This approximation takes into account the so-called dependent scattering effects, which are strongest, in relative terms, in suspensions of non-absorbing nanometer-sized particles. We test experimentally the predictions of the model for the extinction of light in dispersions of sodium dodecyl sulfate micelles in tri-distilled water. We find that the theoretical model can be fitted to the experimental data and provide estimates of the micelles' size and the minimum distance between each other in suspension.

We address the problem of measuring the refractive index of biological materials. We limit our analysis to the case of colloids and consider the use of common automatic critical-angle refractometers to measure their effective refractive index. We provide rough guidelines to when these refractometers will incur in large errors and alternative techniques should be used.

In this work we show that in many practical situations it is possible to obtain the dielectric constant of coatings on plane conductive substrates by means of capacitance measurements with a single electrode without precise knowledge of the thickness.

We compare light reflectivity measurements as a function of the angle of incidence for an interface between an optical glass and a turbid suspension of small particles, with theoretical predictions for the coherent reflectance calculated with different available theoretical models. The comparisons are made only in a small range of angles of incidence around the critical angle of the interface between the glass and the matrix of the colloidal suspensions. The experimental setup and its calibration procedure are discussed. We considered two Fresnel-based approximations and another two based on a multiple-scattering approach, and we present results for monodisperse latex colloidal suspensions of polymeric spherical particles in water with particle diameters of 120 and 520 nm, polydisperse titanium dioxide (rutile) particles suspensions in water with a most probable diameter of 404 nm, and suspensions of copper particles in water with diameters of 500 nm. The comparisons between experiment and theory are made without fitting any parameters.