Old rules don’t apply to LED lighting design

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Steve Landau from Philips Lumileds Lighting Company looks at the different approach needed by lighting engineers when working with high-power LEDs.

As the lighting and architectural communities continue to embrace high-power LEDs, there remains a challenge to recognise the new engineering paradigm that comes with solid-state lighting. Many lighting designers and engineers continue to consider the LED as the equivalent of a conventional light bulb, perhaps because LED manufacturers have failed to adequately educate the market about the different approach required to specify LED-based lighting products.
Anyone who works with LEDs must understand four key concepts:
• LED performance is not defined by wattage
• Maximising drive current will maximise light output
• Efficiency must be measured by the total system, not by light source
• Total usable light differs from part number to part number and from manufacturer to manufacturer and from application to application
A Question of Wattage?
First, consider the wattage question. While the light output from conventional bulbs is commonly expressed in wattage (as well as lumens), it is misleading to use wattage as a yardstick when selecting an LED because the actual wattage is affected by multiple factors. For example, at 220 volts, a 60-watt light bulb will use 60 watts of power regardless of the socket it’s attached to.
The power or watts that an LED uses is primarily dependent on the current (usually expressed in mA) that is applied to the LED and the voltage. The current will vary from application to application and the voltage will depend in part on the LED and in part on the source, such as a battery or an electronic driver. A single high-power LED, such as a Luxeon K2 that can be driven from 350mA to 1500mA, uses anywhere from one to seven watts of power in a given luminaire, depending on the drive current as well as other factors such as voltage, thermal conditions and the nature of the LED itself. Wattage is not defined by the LED but instead by the system. To describe an LED as a “one watt LED” only describes the LED given a very narrow set of parameters and does not indicate the light output of the LED. Failing to understand this can result in the wrong LEDs being used and lead to luminaire performance that does not meet expectations.
Increasing light output
To date, high power LEDs typically could not be driven at more than 350mA without potentially reducing the effective life of the device. With many LED applications needing far more light output than is possible at this drive current, LED usage was constrained.
Even incremental gains in light output and efficacy at this level are not enough to meet the growing demands of architects and lighting designers for more available lumens for their applications.
The ability to get more light from fewer LEDs enables entirely new types of general lighting luminaires and applications.
It is this demand that has driven LED manufacturers to develop their technologies further to provide a higher drive current, making significant advancements in die and packaging to manage the additional heat. For example, the latest Luxeon K2 LEDs are the first to be tested and binned at 1000mA with specified minimum performance and no sacrifice in lumen maintenance. The ability to drive the LED at higher currents not only delivers more light, a lower cost per lumen, and more thermal flexibility but also provides the ability to incorporate the same LED in a range of applications to minimise manufacturing costs.
The Bigger Picture
It is essential to consider overall system efficiency rather than the efficacy of the light source alone in order to compare solid-state to conventional technologies. Take the example of an under-cabinet luminaire. Today, the best high-power LEDs achieve efficacies of 50 to 60lm per watt, compared to a fluorescent lamp that may be rated at 70lm per watt. When deployed in luminaires, however, both solutions provide the same illuminance on the work surface. This stems both from the nature of LEDs as a directed light source and from the efficiencies of the power, optical, thermal and electrical components. If in fact the desired illuminace is achieved using fewer watts of power with the LED solution than the fluorescent solution, then the LED solution will deliver better system efficiency. When considered as a system, the high-power LED solution can offer superior performance and lifecycle costs.
Real Results
Finally, one must look beyond the raw luminous flux claims in LED datasheets before determining which LED to use. There is no industry standard for the numbers shown on datasheets so minimum, typical and maximum lumen values may not have the same definition or have been derived under similar conditions.
In fact, LEDs from different manufacturers will deliver different light output when placed into identical lighting fixtures.
In order to make an apples-to-apples comparison of LEDs, one must consider the maximum allowable drive current, how hot the LED can get without sacrificing lumen maintenance, the thermal management system, the voltage, LED binning and other system parameters.
Embracing the differences between LEDs and conventional lamps, and recognising the difference between LEDs from different manufacturers, will go a long way toward improving the design of LED lighting products and delivering superior products to consumers as the market continues to grow and develop.
Everyone knows the size, longevity and other benefits of solid-state lighting, but now we have LEDs that open up a new realm of possibilities, far beyond the previously standard 350mA LEDs.
Designers and engineers using effective system designs and knowledgeable LED selection will maximise the usable light from LEDs and the market opportunities for their products.