Design Process for a Led Based Luminaire in Exterior Lighting Application

Design Process for a Led Based Luminaire in Exterior Lighting Application Giovanni CIAMPI1, Antonio ROSATO1, Michelangelo SCORPIO1, Sergio SIBILIO1, Carla LANGELLA2, Antonia AULETTA3

(1)Department of Architecture and Industrial Design “Luigi Vanvitelli”, Second University of Naples, via San Lorenzo, 81031, Aversa, Italy - Built Environment Control Lab (2)Department DICDEA (Ingegneria Civile, Design, Edilizia ed Ambiente), Second University of Naples, via Roma 29, 81031, Aversa, Italy - Hybrid Design Lab (3)Designer [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]. Abstract Light Emitting Diode (LED) or Solid State Lighting (SSL) represents a promising technology for future lighting application; compared with other light sources, the LED lights allow efficiency comparable with those sources actually best performing, longer life and increased flexibility as it can be regarded as an effective "point source". On the other hand, the design of a LED based luminaire has to take into account many specific aspects of this technology such as photometric characteristics variation with the junction temperature, current vs. input voltage forward variation, environmental conditions (ambient temperature, humidity, and wind), and other aspects related to the heat dissipation (heat sink design and integration), control (current driving and power supply) and lighting fixture design for which is of a crucial importance the integration of different parts of a LED luminaire considering existing standard requirements. In order to achieve the final target of developing a LED based luminaire in exterior lighting application, it has been firstly analysed and revised a set of up-to-date and trendy LED-based luminaires today commercially available and their principal characteristics have been listed; the behaviour of the main components of a LED system has been analyzed and their relationships have been taken into account. The first results of design process have been reported and the proposal of some prototypes fully described.

Keywords: LED, Hybrid Design Methodology, LED based luminaires design, LED system components design, Energy saving. 1. Design case studies, market analysis and trend In the first phase of the design project some case studies were chosen to examine the different categories of LED outdoor lighting systems existing on the market. The case studied were classified in different categories of application:

1) Lighting for residential areas, parks, gardens and urban streets; 2) Lighting for parking lots and city large streets; 3) Lighting for extra-urban streets.

To this study were added: a specific study of some LED headlamps available on the market with different powers, a repertoire of retrofit devices and an analysis of some devices based on traditional light sources competing with LEDs because still widely used. A comparison with these devices was

carried out in order to define design strategies to overcome the LEDs weakness and to enhance their benefits. The analysis was conducted on two different levels. A first level of systematization was carried out cataloguing and indexing the devices in relation to some identifying features, some of those useful to compare their potentiality in terms of market competition. The features were:

• name; • producer; • site of production; • type of source; • degree of protection; • orientation; • electrical power; • light efficiency; • technical specifications; • photometric simulation files; • catalogues.

A further and more detailed analysis, was carried out on a limited selection of products, considered more interesting in relation to the specific business context and market related to the company that funded the research project. In this second step a detailed report of data and graphs was carried out. The analysed features were defined in relation to the needs of the business market, in order to be able to build a strategy database, covering several sectors. The analysis could be continuously updated in order to drive the company's innovation strategy and the new products development, both from the technical and design points of view. The repertory obtained is easily implementable and linked to instructions for updating to obtain a durable strategic instrument for the future of the company beyond the end of the project. As result of the survey performed, all the gathered informations have been organized, for each luminaire, as reported in figure 1.

Fig. 1: Example of luminaire record. After the activities of selection, cataloguing and characterization, a trend scenario was carried out. Besides the products on the market, some design concepts developed by designers or by design students were considered at an international scale. This study allowed for foreseeing the advanced functional and aesthetic trends useful to define the evolution paths of technological opportunities, users needs and design ideas. The result of the analysis on a specific target of devices with a power of 250 W shows a prevailing aesthetic uniformity in terms of design and of the LEDs geometrical organization in the projector. In contrast, the analysis of the more advanced concepts associated to lower power devices, smaller, lighter and more free from technical limitations, has provided some clues to the evolutionary trends that could be transferred and implemented on devices with high power. Inspired by these design trends, though the necessary technical and technological considerations, our research aimed for giving new design solutions to develop new products oriented to emergence on the mainly uniform market. 2. Design evolutionary framework and corporate identity strategy The innovative formal and technical solutions emerged from the evolutionary framework were:

• modularity of the projectors starting on a basic module composed with various geometries in order to be flexible and adaptable to different contexts with different lighting requirements;

• use of new LED assembly geometries, alternative to the conventional rectangle. There are various examples of compositions. One of the most interesting is a continuous bend organization in which LEDs are mounted on a continuous bended profile (see Mira 380 by Hei Solarlight, Pilled by KXdesigners, SoleCity by Philips, PALO ALTO by Vibia). This kind of "bendy" sign could be interpreted as a characteristic identity of LED technology, which specifically allows this type of geometric arrangement. An opportunity to propose a recognizable character to associate to the LED devices connecting this new language with the values of innovation and sustainability, which should strengthen the image of these products on the market;

• in parallel with the previous bendy tendency oriented to harmonic continuous morphologies without interruptions, in other examples, emerges also the exaltation of a polilinear segmented geometry, with the contrast between the net vertical and horizontal lines (see Plan by Schréder group , Archilede and UFO by Guzzini);

• optimization of the structural supports and reduction of matter, oriented to reducing both the economical and environmental impact. The essential and minimal shape, resulting from structural optimization, seems to facilitate the integration of devices in different context, new or ancient. Geometries are clean and coherent with the contemporary in contrast with the approach of many companies of imitating morphologies and details of old lampposts for the historic contests. At the international level this approach is now considered as "historical fake";

• use of systems of reflection, refraction and diffusion to increase the LEDs efficiency and to overcome the spot effect that characterizes them, providing more homogeneous and continuous lighting conditions;

• tendency to integrate LED lighting systems with renewable energy sources, like PV systems and microwind, connecting the LEDs energy saving potential with other solutions for environmental sustainability;

• tendency to transfer morphologies from biology to propose new morphologies different from the conventional ones as well more compatible with nature and able to communicate the value of sustainability included in LED technology (such as the Philips's concept evoking a flower that opens and closes).

From this evolutionary framework the requirements of the various project proposals were developed, reaching to the definition of 4/5 devices corresponding to different target of market and fields of application e.g. parks and green areas, main streets and urban minor streets. The analysis developed shows the need to impress the design of new devices on real users needs, deepening the differences, in terms of visibility and quality of light between traditional light sources and LED sources, in order to design systems that enhance the potential of new technologies exceeding their limits: many of the LEDs barriers to diffusion could be overtook by a good design. Some fundamental restrains, that constitute a barriers to market, can be recognized in the high initial costs and the reduction of the useful life due to design errors in terms of dissipation (higher junction temperature), assembly and power. The benefits to highlight are, however, a better visibility of objects and walking people, durability, low maintenance costs, reduced energy consumption, possibility of dimming, colour rendering and greater visibility of road signs. Among the several proposals developed, the research project has been closely related to various types of roads with specific lighting requirements (ie. road lighting class). From the survey on the state of the art, it can derived that the LED lighting market, particularly for outdoor use, suffers from a general morphological and linguistic uniformity that interprets, in an automatic way, a source that is often perceived as cold and artificial: as if the appearance was sacrificed by the virtue of energy saving and technological innovation. The study of design emerging trends leads, moreover, to identify two parallel and often conflicting approaches: a first one that continues the path of minimalism and "smoothness" of the forms that become "universal" because seem abstracted from all contexts, and a second one inclined to "naturally complex" forms that find in the "parametric design trend" the ultimate expression of affinity between mathematics and nature under a new digital aesthetics. In the design phase of the research it was decided to combine these two approaches through a "bio- inspired" approach which refers to the "intelligence of biological systems" to define a new line of products more efficient in terms of optimization, adaptability and flexibility: an identity strategy, based on the integration between the concept of sustainable innovation with a nature-inspired approach, was defined. A "new generation" of devices was conceived, by intersecting high LED technology with innovative complex morphologies, bio-inspired textures and organizations as well as structures taken by nature, with the aim to approximate the artificial dimension to the biological one.

The ultimate goal was to shift the perception of these systems from a "cold" and "technological" image toward a more natural one, more compatible and similar to man and to his biological nature. The design concepts developed for the research project described below outline a new scenario of possible development paths characterized by a perspective of design for sustainability that intersects technological innovation and scientific evolution. In these "design concepts" nature was interpreted as a conceptual reference for designing systems that are more compatible with nature, but also as a communicative potential to sensitize the stakeholder on the environmental issues facilitating the assimilation of ethical messages. The nature is conceived, so, even as conceptual and communication tool to demonstrate an interest in issues of sustainability. In this project, a bio-inspired design approach was applied though the adoption of a specific bio-inspired methodology particular called Hybrid Design Methodology [1] developed within the Hybrid Design Lab [2], a design lab dedicated to bio-mimetic design born in the Second University of Naples in 2006. In the Hybrid Design Lab the complex qualities drawn from biological world are transferred to the design of innovative products and services as a kind of "new genetic code". A code that can be extended from a single product to composite systems, defining new forms of relationships between production systems and user system. 3. Methodology and outcomes The bio-inspired design methodology used in the project is an evolution of the "hybrid design methodology" geared to the specific characteristics of the LED lighting sector. The design process steps were:

1. analysis of the evolutionary scenario and trends; 2. definition the project brief as a list of "design problems" to compare with biological references.

The issues identified were: modularity, light efficiency, reduction of environmental impact throughout the life cycle, energy efficiency, structural efficiency, lightness and reduction of materials, economic sustainability, adherence to the user requirements, adaptability to different contexts, aesthetic and formal quality;

3. check the solutions currently available, using the present scenario, the information coming from the state of the art but identifying the related limitations;

4. preparation of a list of biological systems in which it is possible to find answer to "life problems" similar to the "design problems";

5. interpretation of the nature model from a “design” point of view; 6. translation of the nature model into design concepts; 7. selection of the "hybrid concept “ more suitable to be developed; 8. development of the design and technical details.

The application of methodology allows to obtain a selection of strategies inspired by nature applicable in the field of LED streetlights. The strategies were:

• structural optimization; • aggregation; • modular; • organization of space; • organization of light; • mimicry.

Fig. 2: References of the "Hybrid Design" approach specifically applied to the research.

Inside these strategies two families of design concepts were developed: 1. One's Parametric: this concept was based on the principle of modular organization in some biological structures like: the cobweb, the cells of a honeycomb structure of a leaf, the eye of an insect, the ribs that strengthen the wings of the dragonfly. These all are the result of shapes repeated several times that creates a rhythmic composition and a modular organization that seems to be born to perform specific mechanical, physiological or optical needs. The texture present in many species of diatoms, for example, in the form of hierarchical holes on the shells is an interesting nature model as the structure of Amazonian Victoria leaf that seem to respond to the need of differentiation in terms of strength, flexibility and shape. This strategy was interpreted as an evolutionary response to the flow of specific forces on a surface. 2. One's Mehndi (Diatom top, Diatom01, Diatom0): these concepts are different versions of the same “type” inspired by the complex patterns of diatom shells. Diatoms are unicellular algae that have always been a great object of biological studies and research. Many species of diatoms exist in nature and the variety of shapes and ornamentations is very wide. These perforated patterns have in nature many motivations like filtering, structural optimization and lightness. In the design concepts this nature model was transferred to obtain heat dispersion, structural optimization and material reduction. The founding principle of the project consists in a proposal of a minimal base model adapted to be inserted in both historical and contemporary contexts and in places with different "degrees of artificiality". The new product collection includes some possible solutions inspired by textures of nature, which lighten the structure, in order to optimize the structure but also to dissipate the heat with a great advantage in terms of increasing the efficiency of the LEDs, thus of saving energy. Textures can also be customized, according to the architectural character of the place of installation, to the trends changing and to the different aesthetic languages characterizing different cultures. For example, the propensity of Arab culture decorations to choose very complex patterns or Scandinavian preference for minimal morphologies. The choice of materials and surface effects allow to choose different colours promoting the opportunities of integration in different contexts, according to a predetermined colour palette and the opportunity of customizing finishes and colour effects.

Fig. 3: Hybrid Design: One's Parametric luminaire concept developed for street light application. 4. Light source and optic From the technological point of view, the growing efficiency of the Light Emitting Diode (LED) technology involves the possibility to use LED based luminaires in wider application fields. If from one side LEDs allow a greater efficiency, flexibility and longer life than traditional light source, on the other side their application presents new problems to take into account to guarantee their correct operation. Comparing a traditional source luminaire vs. LED based luminaire, it can be noted the presence of some parts that specifically characterize LED luminaire type. In particular, in a LED based luminaire, there are components such as a heat sink, secondary lens, Printed Circuit Board (PCB), driver, etc… that are not usually considered in the design process of a present luminaire or they are present with different form or function: just considering the light source itself, the traditional luminaire was often assembled with a single light source of considerable size whereas LED based luminaire is characterized by an array of light source that can be approximated as "point sources". Within this perspective it raises the necessity to a suitable approach for defining the behaviour of every single component as well as for the whole assembly, to analyse not only reciprocal relationship, but also the influence of environmental condition or the operating condition of the application field. The main parts of a LED luminaire system that is hence necessary to consider can be summarized as:

• LED, as the light point source;

• secondary optic, that provides for optimizing and redirecting the luminous flux emitted by the source in precise directions;

• heat sink, that has the function to dissipate the heat power produced by the LED source; • power supply and control system, that provide for electricity supply, flux control and system

regulation under several operation conditions. The Light Emitting Diode represents the core of the system and at the same time the most fragile element of a LED system as it acts as "basic point" for the choice and the design of all other components. A LED is a chip of semiconducting material impregnated, or doped, with impurities to form a junction positive-negative (PN); the main feature of these devices is to allow current flow in one direction, as a normal diode, to which is added the ability to emit light. The main characteristics of a LED can be summarized as reduced size (usually 5 x 5 mm), longer life time (greater than 50000 hours), higher efficiency (up to 160 lm/W or more [3]) and a high Colour Rendering Index (CRI) (greater than 90 for white light LED [4] [5]); these features allow to use the LED not only in retrofitting operation but, above all, to develop new and innovative products. Considering a scenario where can be detected a market situation made of an impressive and massive LED sources availability, it is fundamental to correctly select the LED that fits the design targets and requirements necessary for each lighting application such as defined during the "design process" previously considered. Above all, the main aspects to consider for the choice of a LED light source can be summarized as: • type and its field of application; • efficiency; • commercial availability; • life time; • luminous flux emitted; • unit cost. In the following table, the LED considered during all the phases of the research project have been reported with their main parameters.

Manufacturer Model Color CCT (K) Flux (lm)

@ 25 °C Thermal resist. (°C/W)

Viewing angle (°)

LED junction

temp. (°C)

Drive current

typ. @25°C (mA)

Drive voltage

typ. @25°C

(V) Min Typ. Max

Cree XLamp XB-D

Warm white 2600 6200 114 6.5 115 150 350 3.2

Cree XLamp XP-E

Warm white 2600 3700 87.4 9 115 150 350 3.05

Edixeon KLC8 series

Warm white 2670 3800 45 10 140 125 350 3.2

Edixeon K series Cool white 5000 10000 140 8 120 125 700 3.5

Edixeon ARC series Warm white 2670 3800 60 13 130 125 350 3.5

Osram Golden Dragon

Plus White 71÷150 11 170 125 350 3.2

Seoul

Semicon. Acriche White 3000 180 (@ 110V) 7 130 125 20 220

Getian High Power LED

Warm White 2700 3000 3300 1250 115 1400 16


Warm White 2700 3000 3300 3250 115 3200 16


Cold White 6000 6500 7000 6500 115 3200 33

Tab. 1: Main parameters characterizing selected LED.

Another aspect that has been evaluated during the development of the LED luminaire for the specific road lighting application, is the application of existing regulations and standards; in this field many technical standards claim for the enhancement of the vision quality and the reduction of light pollution. Thus, it rose up the need to integrate a lens system able to provide for the necessary photometry to optimize the luminaire and adapt it to the specific requirements. This "secondary optic" is mainly divided into two types:

• Total Internal Reflection (TIR) lens; they are constituted by a full body, realized using polymer with a high transmission factor of the light, with a characteristic shape of a revolution paraboloid with central emptying. The area without material along the central axis has the function of directing the high intensity radiation of the central portion of the photometric solid. The area with material works by exploiting the phenomenon of total internal reflection to redirect the light; each reflection involves minimal absorption that must be adequately considered to reach a high luminous efficiency;

• Reflector lens; they are built using polycarbonate with a high thermal resistance and with an internal surface treated with burnished aluminium oxide in order to achieve the higher reflectance. At any reflection, a light absorption greater than the TIR lens occurs but, between two reflections the light runs through an empty space with negligible absorption; therefore only a little difference in efficiency is observed comparing TIR and Reflector lens. Nevertheless, the reflector lenses offer a lesser control of the radiation; a part of the light rays are emitted directly (without modification), causing an expansion of the beam.

For application where a carefull control of the luminous beam is required, such as road light application, a new type of secondary optics have been developed, called “free-form”. The free-form lens can be classified as a TIR lens, and its importance is related to the possibility to accurately design its internal and external shape for precisely controlling of the light beam of LED source. In our design application, the use of free-form lens has been considered within the design of lighting fixtures to achieve the desired photometry as well to improve the optical efficiency; ie. it is important not only to consider the efficiency of the apparatus in terms of how much light is emitted from but also how much light effectively illuminate the task area. Presently, on the market are available many lenses type for different application fields and LED source; each manufacturer makes available all necessary information for a correct selection of a lens for a specific application. For our process, the design of the correct lens type for a specific application, has been made by a suitable selection among those commercially available; whereas the lens adopted has not been defined by a photometric point of view, a laboratory measurement where carried out by a simplified system set up at the Built Environment Control Lab of Department of Architecture and Industrial Design of Second University of Naples, for the photometric characterization of LED based sources. The facility is provided with photometric bench within a dark room, equipped with both traditional luminance meter and videoluminancemeter (LMK) that acquires luminance map of a suitable target upon which falls the luminous flux emitted by the LED and LED & lens system under consideration.

(a) (b)

Fig. 4: Facility layout (a) and candlepower distribution of LED Cree XP-E equipped with a free-form lens for street lighting (b). 5. Thermal management Considering the working principles of a LED, it can be seen that about the 70% of the power supplied is converted into heat and that its life time and efficiency strongly depends from the operation temperature of the p-n junction; the major cause of failures in LED devices is due to improper thermal management. For this reason, the thermal management of a LED system assumes a critical aspect in the LED system design process and represents a topic ("heat sink" design) for many researches.

Light source

Targets surface #1

Support structureLuminance meter

For the optimization of the system LED & Heat sink, a preliminary simulation tool able to identify the maximum thermal resistance value for the heat sink, with a predefined junction temperature, has been developed. The value of the maximum thermal resistance, so calculated, was used as input value into the catalogues of different manufacturers for the choice of the optimal heat exchanger, in terms of the total size, shape and characteristics of the fins. To analyze the thermal behaviour of the LED & Heat sink considered, laboratory measurements were carried out using some T-type thermocouples, for different point acquisition, and an infrared thermal imager [6], for the characterization of the temperature distribution on the surface of the LED system. Various combinations (optimized and not optimized) of light sources and heat sinks have been considered and tested; for each combination the experimental junction temperature values have been compared with those simulated. Before starting the thermal acquisition it has been checked that the system achieved electrical and thermal stabilization. Figure 5a shows an example of the LED system investigated and figure 5b shows a typical thermal acquisition.

(a) (b)

Fig. 5: Coupling of LED & Heat sink investigated (a) and its thermography (b).

For the present design project, the cheapest solution for a correct thermal management of a LED system for road application was considered: a fin aluminium heat sink that exploits natural convection heat transfer. The choice of the heat exchanger type was made based on two considerations. The first consideration originates from a market research, the second one from the study of the datasheets and the catalogues of heat sinks manufacturers. The market analysis was aimed to define some manufacturers of heat sinks and to analyse characteristics used in competitor devices for road applications. The survey gave indication about the shape, material, fin type and surface finishing; the research results indicate, as the most used for road luminaires, the aluminium heat sink with straight fins and surface of anodised natural colour. The study of the datasheets and the catalogues of heat sinks manufacturers gave information about the size, the shape, the thermal power dissipated and the efficiency of the products available on the market. In the table 2 are listed some of the combinations investigated with indication of the type, thermal resistance and characteristics of the heat sink and the LED light source coupled; for each combination, the junction temperature has been evaluated and reported. 6. Field test The last step of the design process is the photometrical characterization of the luminaire to know how the light is emitted: it represents a crucial aspect of the development of the luminaire. The luminous intensity distribution measurement is very important also to verify the data obtained through simulation during different design steps or to evaluate the effectiveness of a design choice. The presence of a protection glass, the assembly process or other parts essential for mechanical or protection reasons can modify the original photometry of light source. Therefore, it is fundamental to measure the luminous intensity distribution of the whole luminaire; to this aim, a "test room" was prepared as showed in figure 6, where has been reported a picture of the experimental apparatus realised for the evaluation of the luminous distribution of luminaire. Marks were placed on the room floor defining a "reference system" to use for measurements. The light source was placed at 3.5 m from the floor to respect the hypothesis of considering the light luminaire as a "point source" from all marks; a modular structure was used for supporting the luminaire and for aligning it to the reference system.

The luminous intensity distribution is simply calculated applying the "inverse square law" starting from the illuminance measurements in correspondence of the marks; the illuminance values have been gathered using a multi-heads luxmeter (Konica Minolta, Illuminance Meter T10).

LED system Heat sink R (°C/W) WxLxh (mm)

Fin thickness (mm) Pitch (mm) LED source

TJ (°C)

System 1

1.4 150x95x40 B = 3 b = 2 variable

79.9

System 2

1.5 150x110x40 1.5 4

52.9

System 3

5.9 34x54(diam.) a1 = 1 a2 = 5 3

55.6

System 4

2.0 170x40x35 2 7

58.4

Tab. 2: List of some combination LED & Heat sink analyzed.

During photometric measurements, also temperature values and electrical parameters have been acquired and stored to have a full knowledge of operation condition. A luminaire commercially available was used to calibrate the experimental apparatus, comparing experimental data with luminous intensity distribution provided by manufacturer.

Fig. 6: Facility layout for photometrical characterization of luminaire. Knowing the geometrical position of each mark respect to the light source and the illuminance value on them, the luminous intensity distribution and the luminous flux of the luminaire have been obtained; from experimental data, a EULUMDAT file can be created by a suitable software [8]. EULUMDAT file is a standardized file format utilized to provide information about photometric distribution and power supply of a light source and to exchange those information among simulation software. The EULUMDAT files, obtained for the luminaires considered, has been then used in a lighting simulation software [9] to evaluate the respect of the performance requirements and to compare various lighting solution and different luminaires. 7. Conclusion In the present paper, the issues related to the development of a LED based luminaire in exterior lighting application have been faced. In order to understand the behaviour of a LED system, the specific aspects of this technology have been analyzed and their relationships have been taken into account, while to the aim to increase the overall efficiency, the "design approach" has treated the LED system as an "unique" device with different parts and contributions interacting each other.

The analysis of the LED-based luminaires available on the market and some case studies has been developed showing a prevailing aesthetic uniformity in terms of design and LEDs geometrical organization. In this research a "new generation" of devices was proposed, by intersecting high LED technology with innovative complex morphologies, bio-inspired textures and organizations and structures taken by nature, with the aim to approximate the artificial dimension to the biological one. From technological point of view, the main components of a LED system have been identified and analyzed. The LEDs as light source have been analyzed and their main characteristics, compared with traditional light sources, have been summarized as well as the main aspects to consider for the correct selection of a LED source for a specific application field, have been suggested and listed. In order to achieve the desired photometry as well to improve the optical efficiency, the different type of secondary optics have been examined and a simplified system has been set up and used for the photometric characterization of LED based sources. To understand the thermal behaviour of a LED system, a preliminary simulation tool has been developed and several experimental acquisitions have been performed to know the punctual temperature values and its distribution on the surface of various combinations LED & Heat sink. For the present work, a fin aluminium heat sink that exploits natural convection heat transfer as cheapest solution for a correct thermal management of a LED system for road application, has been considered. Finally, an experimental facility has been used in order to measure the luminous intensity distribution of the luminaires developed. From the experimental data, the EULUMDAT files have been derived and a lighting simulation program has been used to evaluate the respect of the performance requirements and to compare various lighting solution and different luminaires. Acknowledgement The authors wish to thanks the company TELENIA Srl for the strong support provided during all the phases of research project and the availability of luminaires and hardware for performing in-situ measurements. References

[1] C. Langella, Hybrid design. Progettare tra tecnologia e natura, Milano: Francoangeli, 2007.

[2] [Online]. Available: www.hybriddesignlab.unina2.it.

[3] [Online]. Available: http://www.cree.com/LED-‐Components-‐and-‐Modules/Products/XLamp/Discrete-‐Directional/XLamp-‐XPG2.

[4] [Online]. Available: http://www.cree.com/LED-‐Components-‐and-‐Modules/Products/XLamp/Discrete-‐Directional/XLamp-‐XPG.

[5] [Online]. Available: http://www.osram-‐os.com/osram_os/en/products/product-‐catalog/leds-‐for-‐general-‐lighting/oslon-‐square/lcw-‐cqar.cc/index.jsp.

[6] NEC -‐ Thermo Tracer TH7102WL.

[7] Konica Minolta, Illuminance Meter T10.

[8] Acuity brands: Visual photometric tool.

[9] DIALux 4.12.

Design Process for a Led Based Luminaire in Exterior Lighting Application

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