The whats and watts of LED lighting

Lumileds lighting
The capabilities of LEDs for lighting are demonstrated by this installation for Buckingham Palace created by Philips Lighting using Luxeon power LEDs. (Photo: Edmund Sumner)
For the first time in over a hundred years, the look and feel of general lighting solutions are poised to change as LED technology develops. Steve Landau sets the scene. It was 1974, and my younger brother, using his life-savings, purchased a miracle of modern technology — a Commodore electronic calculator featuring the fantastic ability to perform basic mathematical functions that previously required a pencil. Not only did the calculator perform the required functions, it displayed the result in a bright red light-emitting-diode (LED) display. Some 30-odd years later, the calculator is so common it is given away. The tiny LEDs which were once just bright enough to light the display are now poised to become the dominant light source in homes and offices in the foreseeable future. Rapid adoption We can already see the rapid adoption of LED light sources in the architectural, automotive, entertainment, signage and signalling markets. The low energy consumption, long life and ruggedness of LEDs have made them an obvious choice for coloured lighting. That LEDs are of a particular colour is crucial in some applications, such as red LEDs in the stop lamp of a traffic light. The old technology used a white incandescent bulb and a red filter. With LEDs, there is equivalent or better light output with a much lower energy consumption (about 90% less). For general lighting, however, white light is required. Within the past 18 months white LEDs suitable for general lighting applications and with energy efficiency to rival fluorescent tubes have become available. These latest power LEDs, such as the Luxeon range of products from Philips Lumileds, offer efficiency, lifetime and design benefits that are driving new designs and applications for use in office, retail, hospitality, healthcare and residential settings. Architects and those who specify lighting in buildings need to make appropriate comparisons between conventional light sources, such as fluorescent tubes, and power LEDs. One crucial parameter to compare is efficiency. Improving lighting efficiency reduces the direct electrical consumption of the lighting system and cuts the amount of heat it generates. The higher efficacy lightens the load on HVAC systems, further increasing the energy savings. Incandescent and halogen light sources are fairly inefficient, delivering only 12 to 20 lm/W (lumens per watt). Fluorescent lamps can deliver 45 to 100 lm/W. The most advanced power LEDs deliver 40 to 80 lm/W. On a simple lumens-per-watt basis, therefore, the technology decision might seem simple. In fact, it is not as easy as comparing lumens per watt. Delivering light Ultimately, the purpose of a lighting solution is to provide a pre-determined amount of light to a particular place or space. The effectiveness of the fixture is more important than the raw lumens per watt. What actually matters is how much energy it takes to get the right amount of light where it is needed. This means evaluating the complete system or luminaire performance, not just that of the light source. The performance of the system must take into account the driver electronics, reflector or lens optics, and the light source; only then can we make an apples-to-apples comparison.

LEDs are very small devices as demonstrated by this shot of a Luxeon power LED alongside an AAA battery 44.5 mm long and 10.5 mm in diameter. Note, too, how all the light is given off in one direction.

An important factor is that an LED is a directional light source. All the light emitted by the LED falls within a hemisphere. A fluorescent tube, by contrast, distributes its light 360° around the long centre axis of the tube. If we consider the case of an under-cabinet light, it is easy to see that all the light output from LEDs will go in the desired direction, and that half the light of the fluorescent tube is going in the wrong direction. An important function of the housing of a fluorescent tube is simply to capture and redirect light that is going the wrong way. Clearly, the efficiency of the complete system will be less than that of the raw light source. The electronic circuitry of the system also has an impact on the overall system’s efficiency. A fluorescent system uses a ballast, whereas an LED system uses an electronic driver. Neither component is 100% efficient. Optics are a third factor that can significantly affect system efficiency. Any lens, filter or diffuser will ultimately cause light losses and reduce overall system efficiency. In our example of an under-cabinet light, it is possible that the overall efficiency of an LED system is equivalent to a fluorescent system even when the LEDs themselves are less efficient than the fluorescent tube. Many other factors can affect the overall performance of LED and conventional technology systems. For instance, refrigeration lighting (as in a supermarket’s freezer cabinets) is commonly served by fluorescent technology. Fluorescent tubes become less efficient as temperature drops. LEDs behave in the opposite manner, increasing in efficiency in cold environments. Such environmental factors can definitively swing the argument in favour of power LEDs. For the first time in over a hundred years the look and feel of general lighting solutions are changing dramatically. The power LED is a small light source that can, and will, be distributed over a large area. We will begin to see new solid-state lighting applications that use its unique properties — for instance LEDs embedded in furniture or building materials, and controlled digitally to provide different lighting conditions at different times of day or year. New guidelines Regulatory agencies such as the US Department of Energy are already developing new guidelines that will help the market evaluate and compare total lighting solutions, enabling sound decision making. Over the next year we will see a broad range of new solid-state lighting applications such as downlights, spotlights, area lighting and task lighting on the market which will provide facilities managers, architects and others with new tools for their efforts to reduce energy consumption and reduce the life-cycle costs of lighting. In addition, at the end of their useful life, unlike fluorescent tubes, LEDs contain no mercury, clearly a green solution.

Power LEDs are small light sources that can, and will, be distributed over a large area.

Ultimately, for the building or facility manager the technology and solution decisions should be fairly straightforward and easy to make. Luminaire or system providers, either directly or through an architect or lighting specifier or designer, must provide efficiency and photometric data for the solution, along with appropriate maintenance projections. Although the light source is an important factor, it should not be the criteria for decision making. With this approach, an informed life-cycle analysis, including energy savings, can be well understood. Steve Landau is with Philips Lumileds Lighting Company.
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