Celia Sánchez-Pérez

Coherent reflection of light from a turbid suspension of particles in an internal-reflection configuration: Theory versus experiment.

We compare a recently developed coherent-scattering model for the reflectance of light from a turbid colloidal suspension of particles with experimental measurements. The experimental data were obtained in an internal reflection configuration around the critical angle using a glass prism in contact with a monodisperse colloidal suspension of latex particles, and a polydisperse suspension of TiO2 particles. First, we review the coherent scattering model and extend it to the case of polydisperse suspensions in an internal reflection configuration. The experimental data is then compared with results of the coherent scattering model and results obtained assuming that the colloidal system can be treated as a homogeneous medium with an effective index of refraction. We find that the experimental results are not compatible with the effective medium model. On the other hand, good fits to the experimental curves can be obtained with the coherent scattering model.

Measurement of the effective refractive index of a turbid colloidal suspension using light refraction

We propose and analyse a simple method to measure simultaneously the real and imaginary parts of the effective refractive index of a turbid suspension of particles. The method is based on measurements of the angle of refraction and transmittance of a laser beam that traverses a hollow glass prism filled with a colloidal suspension. We provide a comprehensive assessment of the method. It can offer high sensitivity while still being simple to interpret. We present results of experiments using an optically turbid suspension of polystyrene particles and compare them with theoretical predictions. We also report experimental evidence showing that the refractive behaviour of the diffuse component of light coming from a suspension depends on the volume fraction of the colloidal particles.

Technique for referencing of fiber-optic intensity-modulated sensors by use of counterpropagating signals

We present a new all-optical fiber-referencing scheme for intensity-modulated sensors. It consists of a closed loop traversed by sensing and reference optical signals in opposite directions. With the proposed scheme the noise induced by power fluctuations of the optical source and mechanical perturbations can be greatly reduced. We experimentally demonstrate the efficiency of the scheme and discuss its use in a sensor array.

High-resolution optical angle sensors: approaching the diffraction limit to the sensitivity

We carry out a detailed analysis of angle-sensitive devices based on the critical-angle effect. We consider their use in measuring small angular deflections of a laser beam. We establish the diffraction limit to the sensitivity for optical-angle sensors based on reflection and transmission of a laser beam. We find that this limit is identical to that of the triangulation scheme when using a position-sensitive detector or the autocollimation scheme. We analyze the main proposals to date of optical-angle sensors based on the critical-angle effect, focusing on their maximum sensitivity and their polarization dependence in practical conditions. We propose and analyze theoretically a novel and simple angle-sensitive device for sensing optical-beam deflections with very low polarization dependence and a maximum sensitivity close to the diffraction limit when used with typical laser beams. We discuss the basic principles for designing this type of device, provide numerical results, and point out a convenient fabrication procedure.

Determination of refractive index, size, and concentration of nonabsorbing colloidal nanoparticles from measurements of the complex effective refractive index.

We describe a method for obtaining the refractive index (RI), size, and concentration of nonabsorbing nanoparticles in suspension from relatively simple optical measurements. The method requires measuring the complex effective RI of two dilute suspensions of the particles in liquids of different refractive indices. We describe the theoretical basis of the proposed method and provide experimental results validating the procedure.

Sizing colloidal particles from their contribution to the effective refractive index: Experimental results

In this work we assess experimentally a new methodology for sizing non-absorbing colloidal particles in situ. It requires measuring the real and imaginary part of the effective refractive index per unit volume fraction occupied by the particles. The mean size and refractive e index of the particles are determined from a suitable model for the effective refractive index of dilute colloids. We present results of experiments made with polystyrene and silica nano-particles and compare them with dynamic light scattering and electron microscopy measurements.

Fast measurement of the refractive-index dispersion of dilute liquid solutions of absorbing molecules

We describe a fast method for measuring the refractive-index dispersion of transparent and absorbing liquid solutions. The method is based on measuring the reflectance spectra of an optical beam of white light in an internal reflection configuration near the critical angle defined by the refractive indices of the incident medium (a glass prism) and that of the solvent, which is assumed to be transparent. From a few reflectance spectra taken near the critical angle with the pure solvent and the solution, the dispersion of the contribution to the refractive index of the solute (the sample) can be obtained. We present results with solutions of Rodamine 6G dissolved in distilled water and in methanol showing the feasibility of the proposed method.

Spectroscopic refractometer using a double prism scheme for optical characterization of liquid mixtures

In this paper, we develop a method to determine refractive index changes and its dispersion of diluted solutions by analyzing the refraction of light by a hollow prism containing the medium to be characterized. The system uses a double prism scheme to compensate the dispersion of the medium and the output beam is analyzed with a spectrophotometer resulting in a fast measurement with a simple setup. The system was calibrated using solutions of sucrose in water. We present experimental results of the dispersion of the specific refractive index increment (dn/dC) of sucrose. Finally we discuss the limitations of the method to the characterization of absorbing media.

Photometric principle for passive optical range sensors

We propose a simple technique to determine the distance to an object from two passive photometric measurements. The technique offers the possibility of developing fast and inexpensive passive optical range sensors for different applications.

Solar light collection from buildings façades

We present an analysis of the light collection from the buildings fagade for interior lighting using diffuse reflective surfaces. The use of diffuse surfaces may increase the acceptance angle of such systems, in order to collect more sunlight coming from different directions according to the objects surrounding the building fafade for longer periods of the day without the use of sun tracking systems. We discuss a numerical analysis and experimental results comparing light collection with the proposed system with respect to an equivalent system using specular reflecting surfaces. Results show that the collection is around 50 lx with a system of dimensions 10 cm × 15 cm and is of about of 55% higher than the collection achieved by a system of specular surfaces.