A Guide to LED Luminaire Design Considerations

By Armando Emanuel Roggio, Sr.

Contributed By Hearst Electronic Products


LED luminaire design begins with understanding the specific application the design must serve, its requirements and what incumbent lighting solutions may be vying for the design win. Once these specifications are in place, the designer may begin the process of electronic component selection, thermal design, and lens choice.

Despite the many advantages to designing with LEDs, not the least of which is an expected lower total cost of ownership thanks to much longer life and far superior electrical efficiency than other lighting technologies, LED fixtures, nonetheless, still face stiff competition from compact fluorescent lights, halogens, or even high intensity tungsten lamps depending on the specific application.

To help, this article describes a step-by-step luminaire design process that should aid lighting and fixture designers. The steps involved are based on suggestions from LED manufacturers such as Cree and Osram.

Establishing basic lighting requirements

It might go without saying that the first step in LED luminaire design should be to identify critical (must-have) application characteristics and understand how those characteristics will be measured and compared. For example, some applications may be subject to an industry standard, so that any design that does not meet the standard's specific requirements would be effectively unusable. Others — such as outdoor lighting or some down lighting applications — will base performance requirements on specifications from other, perhaps entrenched, technologies. Finally, some newer applications may have completely unique requirements.

Consider actually listing these critical requirements in a table or a design document. It is also important to understand what unit of measure the application's industry or the end user will employ for the critical requirements and under what circumstances those measurements should be taken.

The next step, according to suggestions from LED makers, should be to identify other important application characteristics. While these luminaire characteristics are not critical (deal stoppers if you will), they may ultimately represent competitive advantages that will make one lighting solution more marketable than its competition. For example, imagine an application that required a specified luminous flux and electrical power consumption. Given that this was a requirement, competitive solutions will also try to meet these specifications. If, however, a luminaire designer was also able to deliver superior performance over, say, a wider breadth of operating temperatures, this operating temperature range characteristic, while not critical, could become the point of difference in the marketplace.

When thinking about these important characteristics and the potential competitive advantages they represent, a designer may also want to consider what the luminaire will be lighting and how humans will interact with the light. For example, Osram, in its lighting design suggestions, encourages designers to consider "the three dimensions of light.” Without exploring the intricacies of what Osram calls "the theory of optimum vision with the aid of artificial light sources," the key takeaway is to consider — however briefly — how humans will respond to a lighting design on a biological level. Although this approach can seem somewhat esoteric, it may result in a better overall design.

At a more concrete level, it is at this design consideration stage that designers may wish to consider whether the project should be electrically isolated, and whether the luminaire will need to be dimmable.

With a set of critical and important requirements identified, the designer next should set up design goals and thereby measure the project's completeness and success. It is also a good idea at this stage to study and test competitive solutions and technologies, even taking into consideration published road maps, to ensure that luminaire design goals will still be relevant and competitive at the time of production.

Estimating system efficiencies and component selection

With the design goals set and an overview of the competitive landscape complete, luminaire designers should turn to considering the proposed system's optical, thermal, and electrical efficiencies in order to begin the process of selecting required components.

"An LED’s luminous flux depends on a variety of factors, including drive current and junction temperature," Cree writes in its Luminaire Design Guide. "To accurately calculate the necessary number of LEDs, the inefficiencies of the optical, thermal, and electrical systems must be estimated first. Personal experience with previous prototypes, or the example numbers provided in this document, can serve as a guide to estimate these losses."

To estimate optical system efficacy, Cree suggests considering the affect of secondary optics on light loss and the amount of light lost within the proposed fixture.

"Secondary optics are any optical system that is not part of the LED itself, such as a lens or diffuser placed over the LED," according to Cree. "The losses associated with secondary optics vary depending on the particular element used. Typical optical efficiency through each secondary optical element is between 85 [percent] and 90 [percent]."

Similarly, rays from the LED that are cast into the fixture housing where they are absorbed, result in an overall reduction in luminaire efficiency, so that designers will want to take care with LED selection and fixture design.

As a specific example of components that may be chosen to avoid secondary optical and fixture loss, Cree's XLamp XR-E Natural light LEDs are a significant contributor to improve fixture efficacy. Specifically, a comparison of a compact fluorescent light and a Cree XLamp XR-E found that while the fluorescent had a 65 lm/W efficacy compared to about 58 lm/W for the XLamp XR-E, the XR-E's superior light output pattern resulted in a total fixture efficacy of 44 lm/W compared to just 35 lm/W for the compact fluorescent.

Cree XLamp XR-E

Figure 1: Cree XLamp XR-E LEDs contributes to overall fixture efficacy.

Next, LEDs lose relative flux output as junction temperatures rise, so the luminaire designer may wish to estimate efficiency lost due to operating temperature. This can be determined using data from LED documentation obtained when selecting LEDs or when determining how many LEDs will be required to meet or exceed the design goals. Since many LED datasheets estimate luminous flux at a junction temperature of just 25°C, it may be necessary to make adjustments based on the application.

LED drivers are also important in understanding system efficiency, since electrical losses in the driver may decrease the overall system performance.

Finally, with these various system efficiencies estimated, the designer is now well prepared to calculate the number of LEDs required and the type of LEDs needed.

Summary

With an estimated 20 billion or more light fixtures using incandescent, halogen, or fluorescent lamps worldwide, there are plenty of opportunities for LED lighting design wins. However, to assure success, a logical design process must be put in place. Following the steps that this article has outlined should aid LED luminaire designers in establishing the basic requirements, identifying the key application characteristics, estimating the system efficiency and in the selection components.

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