Although it was established long back that solid state diodes can emit light it had to wait several years before it could be molded and perfected to be used as a new generation light source. Over the last decade LED as a greener light source has made significant progress. Application to general lighting, in particular, has gained momentum with the development of AlGaInP high power LED. But the dominance of LED in much sought after street lighting segment is yet to be established. There are three major barriers, which researchers all over the world are trying to solve in order to make LED streetlight a viable alternative to the traditional streetlights.

  1. Like its conventional counterpart LED light source is not a stand-alonetype. Hence, if there is failure or problem in a particular streetlight luminaire, servicing or replacement becomes a remote possibility.
    2. Optical design considerations, which has to meet complex photometric distribution for different street lighting installation.
    3. Thermal management of high power LEDs used Street lights.

Figure 1:  Illustrations show reflector-refractor optic for a typical LED package and concept of FOCAL SMEAR which makes the chip an extended source of light

Development towards LED in General Lighting

In the first place in order to overcome the drawbacks of LED in street lighting application many major LED manufacturers like Philips, Osram, Cree, and Bridgelux have developed LED modules. An LED module is a single unit in which light source, heat sink, optic and driver electronics are integrated. Although such modular approach is the future of LED Lighting, a breakthrough will happen only when the portfolio of traditional HID lamp and luminaire is broadened by addition of LED module and luminaire. For street lighting application the next generation VLED optical module has created interest because of optical precision of VLED which is highly desirable in street lighting design. These modules are available in square or round shape and include 64 emitters (75 system watt), 80 emitters (94 system watt), and 120 emitters (141 system watt). Despite such remarkable developments there are still no standards for LED light engine and power supplies. This compels Luminaire manufacturers to develop customised products around a given light engine. This makes replacement a difficult task as LED systems are typically hardwired into a luminaire.

Optic

Optic Design consideration is another complex and critical entity in LED street lighting. This is because a LED chip is very small in size and is virtually a point source emitting light in one direction only. So it needs integration of reflector-refractor optic to make use of the physical size of a chip. It is treated as an extended source while designing the reflector cup and epoxy dome for refraction. This makes the light come from different directions. This primary optic is included in the LED package. To modify the output beam of LED so as to meet the desired photometric application there is need for designing and modeling of secondary optic. Secondary optics is used to spread the incoming light (diverging optics) or provide collimated beam (collimating optics). Normally Pillow Lens is used for diverging optics, whereas collimating lens come in two main varieties – reflecting or refracting. Reflecting elements are typically metalised cavities with a straight or parabolic profile. Refracting elements are plano convex, dual convex and Fresnel (collapsed plano convex) lenses. Apart from these conventional spherical lenses, other more efficient lens designs such as hyperbolic planar, spheroelliptic and free form lenses are also available. Another class of lens exist whichcombine both refraction and total internal reflection (TIR) and is referred as reflective-refractive or catadioptric lens. These lenses are powerful and most efficient. Like refractor designs, there are many efficient reflector designs available. They are parabolic reflectors, linear parabolic reflector and on-imaging compound parabolic reflectors (CPC). The reflector material used widely for LED is normally vacuum metalised ABS plastic. Optical design is critical in case of street lighting luminaire as it calls for high precision design and development of optimum optic combination to meet the desired photometric characteristic.

Thermal Management

Thermal Management plays a major role in sustaining the efficiency of a street light luminaire and the declared rated life of the LED module. Although LED has become popular as a cold radiator, on the contrary like any other semiconductors a large portion of electric energy (70%-80%) is converted into heat. That is why unlike the traditional light sources which are basically thermal radiators, cooling or thermal management becomes absolutely imperative in case of LED to maintain the efficiency which is radiated luminous flux to applied electrical energy. Hence, if thermal technical boundary conditions are adhered to a white radiating high efficiency high power LEDs can work trouble free and last its rated average life. Be it SMT design in PLCC housing, hexagonal or octagonal designs, COB (Cup on Board), MCOB (multiple Chip on Board) optimal thermal management is necessary to achieve highest possible luminous flux for high performance LEDs for lighting purpose. The ambient temperature and the chip temperature directly influence the efficiency and life span of an LED. It has been observed that increased electrical power for achieving higher luminous flux causes big temperature difference and shortens life span considerably. This also adversely affects the synthetic materials used for the enclosures and lenses (epoxy, resin, silicon etc) resulting in cloudiness on the lenses. Since the optimal heat engineering interpretation for definite cooling is extremely complex, there are several methods adopted to dissipate the heat away from the junction. The possibilities available are artificial surface magnification of the LED assembly contact zone; PCB (conductor paths, metal clad PCB); heat sinks either glued or soldered on to the PCB or mounted separately. The cooling path follows two partial path – 1) Junction to contact pins to the ambient air. But for high power high performance LEDs used in street lights these methods are not that reliable particularly in India where outside air temperature varies considerably. Here design of appropriate Heat Sinks become necessary after taking into account the thermal criteria for calculating the thermal resistance of the system and consideration of the mounting situation. In one of the recent installations of 80W LED streetlight in India the side plates of the luminaire were made from extruded aluminium and designed to serve as a heat sink for dissipating heat to the air. The module, whether it is 60W, 80W or 120W can be directly connected to the side plates. Another milestone in heat sink connection to LED has been achieved through solder mounting connection by means of reflow or IR soldering.

Conclusions

Since thermal management is most critical to LED performance LED system manufacturers all over the world are trying to address this challenge by seeking out improved heat sink designs, high efficiency circuit boards, high thermal conductivity enclosures and other thermal design techniques. The most recent development is CAD-embedded thermal and fluid simulation software which enables design engineers to diagnose thermal problems, evaluate alternative designs, and iterate rapidly to an optimal solution.

The rapid pace of development in LED lighting will hopefully overcome the barriers soon in order to meet performance, lifetime and cost requirements for creating a demand for greener street lights.

References

Secondary Optics Design Considerations for Super Flux LEDs – application brief AB20-5 replacing AN1149-5

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