Smooth light extraction in lighting optical fibre A. A. Fernandez-Balbuena*a, D. Vazquez-Molinia, A. Garcia-Botellab, J. C. Martínez-Antona, E. Bernabeuc a
Dept. of Optics UCM, School of Optics, Arcos de Jalón 118, 28037 Madrid, SPAIN Dpto. de Física Aplicada a los Recursos Naturales, UPM, E.T.S.I. de Montes. Madrid, SPAIN c Dept. of Optics, University Complutense of Madrid, Faculty of Physics, Madrid, SPAIN
ABSTRACT Recent advances in LED technology have relegated the use of optical fibre for general lighting, but there are several applications where it can be used as scanners lighting systems, daylight, cultural heritage lighting, sensors, explosion risky spaces, etc. Nowadays the use of high intensity LED to inject light in optical fibre increases the possibility of conjugate fibre + LED for lighting applications. New optical fibres of plastic materials, high core diameter up to 12.6 mm transmit light with little attenuation in the visible spectrum but there is no an efficient and controlled way to extract the light during the fibre path. Side extracting fibres extracts all the light on 2π angle so is not well suited for controlled lighting. In this paper we present an extraction system for mono-filament optical fibre which provides efficient and controlled light distribution. These lighting parameters can be controlled with an algorithm that set the position, depth and shape of the optical extraction system. The extraction system works by total internal reflection in the core of the fibre with high efficiency and low cost. A 10 m length prototype is made with 45º sectional cuts in the fibre core as extraction system. The system is tested with a 1W white LED illuminator in one side. Keywords: compound parabolic concentrator, nonimaging optics, natural lighting, prismatic film, daylight, hollow light guide.
1. INTRODUCTION Nowadays optical fibre is used for telecommunication all over the world; the use of optical fibre for lighting is used wide spread for signaling but not for lighting because it requires solving two problems: first one is to introduce light inside the fibre bundle in guiding conditions and the other one is extracting light over the optical fibre in an efficient and controlled way [1, 2]. Optical fibre lighting is getting more important in environments with some hazard risk like chemical, explosives, etc. where the use of electricity is dangerous. Lighting fibre systems cannot be applied in places where it is needed high illuminance level but it can be used in places where low levels are enough. With fibre it is possible to guide the light close to the working plane with not heat transfer. For example, in cultural heritage lighting is easy to put out the IR and the UV radiation and light with low levels following the conservation criteria. Optical fibre has several advantages compared with conventional lighting systems. For example, optical fibre has many advantages in big publicity panels, signalization and places with difficult access. Also the concentration of the radiation source on a box may involve some advantages in energetics and economics maintenance. Lighting with optical fibre is powerful  against conventional systems if the light extraction is made over the entire optical fibre surface with a controlled light beam. * [email protected]
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Illumination Optics II, edited by Tina E. Kidger, Stuart David, Proc. of SPIE Vol. 8170, 81700S © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.896777
In order to extract light of a fibre it is necessary to break the guiding conditions. It can be obtained by several ways: introduce diffuser materials as drop inside the fibre or as a skin, using curvature segments. These systems have not control on the light output flux and therefore are low efficient. Other extraction system is to use air bandgap, or layer with lower refraction index, which produce a TIR reflection and drives the light out of the fibre. In this case the output flux will have a controlled photometry profile. The fibre optic smooth extraction system consists in a several cuts perpendicular to the axis of the fibre. These cuts are defined by two parameters that are tilt angle α and cut depth h, figure 1. Light that is guided in the optical fibre is extracted by means of total internal reflection (TIR) on the surface of each extractor. 2.1 Individual extraction system characterization In order to characterize the extraction system a 2000 mm length fibre is designed with 8.5 mm core and a clad of 0.5 mm over it. The core has a refraction index of 1.6 and the clad has a refraction index of 1.4. Also an extraction system  that is tilted 45º is evaluated for different depths, h [0 7.5 mm].
Figure 1: Raytracing on detector plane and extraction system geometry.
Figure 2: Evaluation of individual extraction system. Flux extracted (blue) and flux not extracted (green) for different extraction system depth and a tilt of α of 45º.
2.2 Numerical calculation of extraction system depth The objective of the extraction system is to obtain a uniform illuminance distribution over the detector plane that is a plane of 2000 mm length in the longitudinal section and has 45º detection angle in the transversal section. Previous optical evaluation of individual units is useful for designing an algorithm to perform a smooth extraction over the selected fibre. The attenuation of the fiber is not taken into account, all calculation is evaluated for 555 nm. We are going to design a 2000 mm length guide with 10 extrators over it, figure 1. The first extractor is set to have a depth h of 0.5 mm. So,
φ1 = φsourceυ extrator1 ,
where ф1 is the flux over the detector plane due to the extractor 1 and ν is the efficiency of extraction at detector plane due to extractor 1. This efficiency depends on depth and is easily obtained with figure 2 data.
φ2 = φ pass1υ extrator 2 ,
where ф2 is the flux over the detector plane due to the extractor 2. We want extractor 2 to extract the same flux than extractor 1 so ф2= ф1, then the necessary efficiency for extractor 2 is:
υ extrator 2 =
As we know the necessary efficiency, equation 2, for extractor 2 operating with figure 2 data we can obtain the necessary depth h and design the second extractor. New passing flux for extractor 2 is фpass2=фpass1 νpass2. This process is done recursively for all the extractors and the best depth design is obtained. Extractor
Calculated depth (mm)
Estimated flux on detector (lm)
Passing flux (lm)
Table 1: Numerical calculated depth for best uniformity, depth * is fixed as a start parameter. Also initial flux for the source is set to 2500 lm.
In table 1 result of this algorithm is presented. The extractor 1 is the closest to the lighting source, the depth is fixed at 0.5 mm. Is we choose a higher depth for extractor 1 all other extractors can be calculated as well, in this case the total amount of flux over the detector plane will be increased.
2.3 Experimental prototype: Mock-up design The Optics Department has built a mock-up of a lighting optical fibre for verifying the concept. This mock-up is done with a 10 m length optical fibre of 9.5 mm diameter. The experimental prototype is measured with a 3M 1W LED as source injector, figure 3
Figure 3: LED light used for experimental mock-up.
Figure 4: 10 m length prototype at 50 cm distance over floor.
In figure 4 we show a 10 m length prototype of the optical fibre extraction system, the black line corresponds to 0.8 m distance from the fibre. Figure 5 show the measured illuminance for a 0.6 m section of the fibre.
Figure 5: Experimental measurement of illuminance (lux) for z (longitudinal) 2.8 m to 3.4 m and x (transversal) 0 to 80 cm.
3. RESULTS In this section we analyze the extraction system by raytracing software . The objective is to determine the irradiance distribution on the detector plane for two different extraction configurations. Configuration “R1”, figure 6, is conformed of ten extractors with the same depth of 1 mm tilted each one 45º. Configuration “R3”, irradiance plane in figure 7, consists on the analysis of the optimized numerical calculation that has different depth for each extractor as can be seen on table 1.
Figure 6: Irradiance distribution over the detector plane for extraction system with depth 1mm in all extractors
Figure 7: Irradiance distribution over the detector plane for extraction system with optimized depth for extractors.
For better understanding of results we show in figure 8 three profiles for configurations “R1”, “R2” and “R3”. The profiles are the mean longitudinal (along optical fibre axis) illuminance distribution in the detector plane. R3 configuration corresponds to the numerical optimized calculation and shows good longitudinal illumination uniformity that was the desired result.
Figure 8: Illuminance profile for different extractors and optimized extractor (blue) for better uniformity.
4. CONCLUSIONS The extraction system design is a useful way to obtain smooth and regular uniformity distribution from a lighting optical fibre. With this extraction system it is possible to control the light output flux in order to light controlled areas. This system makes possible to light cultural heritage without IR or UV radiation, it is a secure system in order to avoid electrical risk. The proposed numerical design method is able to optimize not only uniformity but also the desired distribution function over a detector plane. Here is presented only the depth change in each extractor for obtaining good uniformity but also change in tilted angle, curvature and distance are suitable for study.
ACKNOWLEDGEMENTS This project has been supported by Spanish Department of Science and innovation with its Research Program project number HAR2009-12862.
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