Diseño y desarrollo de componentes ópticos delgados para aplicaciones anidólicas/design and development of thin optical components for nonimaging applications

Resumen

Classical optics has focused grounded in the design of imaging systems in which it has reached a high level of development. However, classical optics solutions to the problems of light energy transfer are only appropriate when the light rays are paraxial. The paraxial condition is not met in most applications for the concentration and illumination. Nonimaging optics eliminates the constraints of image formation and solves problems of efficient light transfer. Moreover, in general, these new optical systems can be made with fewer components and thus with more tolerance to manufacturing errors. This makes nonimaging optics an essential tool in the optic designs for illumination systems, concentrating photovoltaic, among others. Illumination systems are designed to achieve a specific illuminance distribution. An example is backlight system used for the LCD illumination (screens of mobile phones, computer monitors or TV). A backlight behaves as a square light source that emits light uniformly across its exit surface toward the LCD screen. In this thesis, the backlight is designed using the flow line design method. The light coming from LEDs enters the backlight by one of its side being confined within it first, and then extracted toward the LCD by a micro-structured (sawtooth type) surface properly calculated and distributed along the guide. Another field of application of nonimaging optics is in concentrating photovoltaic. Concentration is a technique that seeks to minimize the cost of the photovoltaic systems using high-efficient solar cells. Due to high cost of the cells, these systems are interesting only if the required solar cell area is reduced by using an optical concentrator. Nonimaging concentrators are well suited for the collection of solar energy, because the goal is not the reproduction of an exact image of the sun, but instead the collection of its energy. Most of the concentrators comprise two optical elements, the primary reflector (X) and the secondary lens (R). The use of reflectors for concentration is highly desirable for several reasons: They result in very compact systems (such as XR concentrators), has no chromatic aberration and use very little material. The main disadvantage of the high-reflective mirrored reflectors is their high cost. In order to reduce the cost of the system, the mirrored reflectors can be replaced by metal-less TIR grooved reflectors. A design procedure for these grooved reflectors is presented in this thesis. Most of the optical systems designed for nonimaging aplications are free-form. These systems are formed by specific surfaces without symmetry. The absence of the restriction of symmetry allows us to design systems with less surfaces and more efficient. Free.form optical systems become especially relevant lately with the development of molding machines, for example 5-axis diamond turning machine. Although some components are very complicated for shaping, the manufacturing costs are relatively insensitive to the complexity of the mold especially in the case of mass production (such as plastic injection), as the cost of the mold is spread in many parts. The volume of the used material for each component is very important for mass manufacturing. This is for two reasons: the price of the material itself and the loss of efficiency that is usually associated with the components that use a lot of material. So, thin optical systems (small volume but large surface area) have big potential to replace the conventional systems. In this thesis, several design methods, such as the Simultaneous Multiple Surface method in two and three dimensions (SMS2D and SMS3D) and the flow line design method, are explained. These methods are based on very efficient control of the light flow, providing excellent results in the nonimaging optic designs. The thesis comprises seven chapters. In the first chapter, an introduction to nonimaging optics with some basic concepts is given. In Chapters 2-7 there are presented few thin nonimaging components made of plastic. All these devices are designed as thin plastic pieces, so that, there are suitable for mass production by injection molding process. Due to possibility of mass production, their price is low. In Chapter 2 it is presented a design procedure for geodesic lenses, included all necessary mathematical background theory. In order to solve the problem of the abrupt lens transition toward the horizontal plane (as happens in Rhinehart lens), few non-full aperture geodesic lenses are designed. The geodesic lenses have been used in the integrated optoelectronic components for year, but herein they are implemented in illumination systems, as well. It is shown that a Rhinehart lens performs rotation of the phase space, which is used in illumination optic devices, such as geodesic integrator CHMSL (mostly used in vehicular lighting) and geodesic kaleidoscope (a colour mixing device). The ray-tracing of these devices demonstrates their good optics performances: high uniformity in illuminance and intensity patterns. Chapters 3 and 4 presents a backlight design procedure based on the flow-line method, and characterization of the first linear prototype. Using the flow-line method the light was controlled efficiently, so the backlights with high efficiency (up to 80%, included all losses and absorption) were designed. All presented designs are thin and compact dielectric pieces with possibility of inclusion of the polarization recycling system, which make them ideal for LED illumination systems. Considering efficiency, the best models are the conical backlight design and modified constant thickness design (more than 79%, included all losses and absorption). The third most efficient design, the linear backlight (71.3% of efficiency) was chosen for the prototype due to its simpler form. The linear model is made of PMMA by direct cutting process. The measurements have shown worse results than it was expected according to the Light Tools simulations. The measured efficiency of the prototype is lower, 51.7%. Also, the measured irradiance pattern is less uniform than the simulated one. The maximum variation of the irradiance respect to the mean value is 12.3% that is higher than in the designed model (about 6.8%), but still good enough for illumination purposes. These undesired losses are caused by surface errors at the ejector's edges and the surface roughness. The efficiency drop can be compensated by introducing the polarization recycling system. Although its function is to recycle the unwanted polarization, it is shown that this system also recovers the lost power. The simulation of the prototype model gives an efficiency of 67.85 %. However, the efficiency recovery increases the irradiance non-uniformity, which can be corrected by proper redesign of the backlight ejectors. Chapter 5 and 6 describes a novel design procedure for V-groove reflectors. The general design problem of perfect coupling of two wavefronts after two reflections at the groove is defined by a system of functional differential equations. The problem has been solved using the SMS method starting at a polynomial approximation of the surface near the groove peak. The presented canonical examples in two and three dimensions show perfect coupling of two wavefronts. Also, using Van Brunt theorem we have proved the existence and uniqueness of the analytic solution, once the initial conditions are prescribed. Chapter 6 explains a design procedure for thin TIR dielectric grooved reflector. The measurements of a parabolic TIR prototype show good reflectivity, up to 98%. The concept of the grooved reflectors is implemented in the last chapter. It is designed a metal-less RXI collimator. The measurements of the first prototype show good efficiency (about 94% of the theoretical value). Also, the measured far field pattern coincides with the simulated one. In this chapter it is shown that unlike to the conventional designs the V-groove RXI provides good colour mixing. Good experimental results for the V-groove optical components encourage their use in optical systems. These devices will be a potent alternative to conventional metalized ones, especially because of the potential for mass production, by injection molding. However, they are more sensitive to the surface roughness and surface errors, since there are two reflections instead of the one of conventional metalized surfaces. This thesis demonstrates that nonimaging optic designs can achieve highly effective illumination systems, such as presented backlights and RI3R collimator. The absence of mirrored surface in presented devices makes them cheap especially when mass production via injection molding is applied.