Angel Garcia-Botella

Algorithm for fringe pattern normalization

In this work we present a new algorithm for fringe pattern normalization, that is, background suppression and modulation normalization. Normalization is necessary for several fringe pattern processing techniques. For example, this is the case of the regularization and phase sampling methods. In general, background suppression can be accomplished by high-pass filtering, however if modulation is not constant or almost constant over the field of view, normalization is a difficult task. The solution proposed is based in the use of two orthogonal bandpass filters, from which a normalized irradiance distribution is obtained. We have applied the method to simulated as well as experimental data with good results.

Improved method for isochromatic demodulation by RGB calibration

The red-blue-green (RGB) calibration technique consists in constructing an a priori calibration table of the isochromatic retardation versus the triplet of RGB values obtained with a RGB CCD camera. In this way a lookup table (LUT) is built in which the entry is the corresponding RGB triplet and the output is the given retardation. This calibration (a radiometric quantity) depends on the geometric and chromatic parameters of the setup. Once the calibration is performed, the isochromatic retardation at a given point of the sample is computed as the one that minimizes the Euclidean distance between the measured RGB triplet and the triplets stored in the LUT. We present an enhanced RGB calibration algorithm for isochromatic fringe pattern demodulation. We have improved the standard demodulation algorithm used in RGB calibration by changing the Euclidean cost function to a regularized one in which the fidelity term corresponds to the Euclidean distance between RGB triplets; the regularizing term forces piecewise continuity for the isochromatic retardation. Additionally we have implemented a selective search in the RGB calibration LUT. We have tested the algorithm with simulated as well as real photoelastic data with good results.

New concentrator multifocal Fresnel lens for improved uniformity: design and characterization

The emergence of high efficiency photovoltaic cells is leading the industry into using solar concentrators in order to reduce costs by decreasing the number of cells used. In this paper Optics department of Universidad Complutense de Madrid has designed a multifocal Fresnel lens of PMMA and has studied the main parameters that have influence on its final function. This has been done by taking into account its manufacturing tolerances. The lens is square shaped with sides measuring 270 mm and it is composed of three different zones based on three different criteria: The central zone has been designed by using paraxial formulation, the intermediate one has been designed based on Fresnel classical formula while the marginal zones purpose is to deflect the light by total internal reflection on prism faces. All three zones have different focal areas and different optical axis so the energy distribution will be more uniform whilst avoiding cell damage caused by hot spots. The design stage is feedback through simulations using a ray tracer software. In order to characterize the lens operation a measure of optical concentration was first taken on different lens areas using an integrating sphere. Finally, the lens performance in terms of concentration and in terms of uniformity at the focal spot was studied by processing the images taken with a CCD camera on a screen placed at the focal plane of the lens.

Thermal influences on optical properties of light-emitting diodes: a semiempirical model

The application of LED technology to fields such as alphanumerical displays and traffic control is continuously increasing. Because the technology is used outdoors, it must be able to operate under various environmental conditions. Like all semiconductor devices, LEDs have properties that change with temperature. We propose a semiempirical model, based on semiconductor solid-state theory, that predicts the changes in the emission spectrum including the effect of temperature changes on the optical properties of the LED, within a range appropriate for outdoor applications (0-40 degrees C). This model permits us to evaluate the changes in the output flux and the chromaticity coordinates of the LED. We checked this model with seven different LEDs.

Field method for dielectric concentrator design

Field method, also called geometrical vector flux field method, is a well established technique to design concentrators and in general nonimaging optical systems. The method is based on building reflective concentrators with the geometry of field lines, these concentrators do not disturb the flux field and become ideal. In this paper we study the properties of surfaces orthogonal to the field vector J. For rotational symmetric systems J is orthogonal to its curl, then exist a family of surfaces orthogonal to the lines of J, surfaces of constant pseudopotential. From the definition of J, pseudopotential surfaces can be interpreted as surfaces of maximum flux density and can play an important role in the design of nonimaging systems. We study refractive concentrators with the geometry of pseudopotential surfaces. Dielectric material modifies the field lines and then the geometry of the system, including compactness and reflective parts, it also introduce total internal reflection which must be considered in the design. We apply this concept to study hyperbolic concentrator modified with pseudopotential refractive optics and we shown that it achieves the theoretical limit of concentration.

Ideal flux field dielectric concentrators

The concept of the vector flux field was first introduced as a photometrical theory and later developed in the field of nonimaging optics; it has provided new perspectives in the design of concentrators, overcoming standard ray tracing techniques. The flux field method has shown that reflective concentrators with the geometry of the field lines achieve the theoretical limit of concentration. In this paper we study the role of surfaces orthogonal to the field vector J. For rotationally symmetric systems J is orthogonal to its curl, and then a family of surfaces orthogonal to the lines of J exists, which can be called the family of surfaces of constant pseudopotential. Using the concept of the flux tube, it is possible to demonstrate that refractive concentrators with the shape of these pseudopotential surfaces achieve the theoretical limit of concentration.

Geometric and thermal design for a new concentrator-collimator lighting system based on LED technology

A new concentrator-collimator lighting system based on light-emitting diode (LED) technology is presented. Using an array of LEDs as source, by means of a three-dimensional concentrator and a parabolic collimator, this device makes it possible to obtain an integrated light beam with a high intensity level and the desired divergence, in a compact geometry with small output area. It offers low consumption and a long lifetime. An optical model allowing control of the beam parameters is presented together with a design for thermal stabilization of the device. These characteristics make it applicable in a variety of fields where high luminous intensity, quasi-monochromatic light and narrow angular distributions are needed, among them road signalization. To improve the colour rendering it would be necessary to use a white LED or three different LEDs in an array.

Modulation transfer function of translucent rough sheets

Rough surfaces in translucent protective sheets are used in imaging systems, such as displays, to decrease specular reflections of external sources. However, they modify the quality of the images formed by transmission. Using a geometric approximation, we have modeled the behavior of rough surfaces in imaging systems. This model provides an analytical expression for the modulation transfer function of rough surfaces.

Optics detailed analysis of an improved collimation system for LED light sources

Throughout present study will be discusses the influence of the manufacturing margins, adjustment precision and ray model accuracy of a collimating LED unit in the overall system performance. It will be also analyzed the angular performance and the collimated in relation to the relative position of the LED and the collimators dimensions. Finally the results will be compared with existing publications in this field.

Skylight: a Hollow Prismatic CPC

Many applications involve the use of a compound parabolic concentrator (CPC) like, natural lighting, thermal applications, optics for illuminators, optical fibre coupling and solar energy. The use of a CPC in reverse mode for natural lighting gives the chance to use it as a lighting skylight in ceilings because light output is controlled inside the design angle, on the contrary having a low flux transfer ratio because of the reduced area of the entrance pupil regarding exit pupil. The authors propose an innovative 3D hollow prismatic CPC (HPCPC) made of a dielectric material, which has a high efficiency comparing it with aluminium CPC. The basic idea is to use a hollow prismatic light guide with CPC shape. This paper reports 2D, 3D design and numerical analysis by raytracing software, also experimental results are shown. The system works almost like a true CPC when light enters through standard entrance pupil and also collect light that enters outside entrance pupil. Performance and efficiency of the prismatic CPC is in average 300% higher than standard aluminium CPC for collimated light in a range from 0º to 85º. A prototype has been developed and tested.