Spotlighting opportunity: LED lighting market set for rapid growth.

Power consumption and lifetime costs are the key factors when choosing LEDs over traditional incandescent and compact fluorescent light bulbs (CFL). Improvements in the lumens per watt ratio are the major focus of interest. Today's typical cool white LEDs provide around 100lumen, with specialist manufacturers pushing the envelope to values in the region of 139lm. Neutral and warm white LEDs, well suited to indoor lighting applications, typically deliver 80lm and 60 to 70lm respectively. Again, specialist manufacturers offer higher output levels: up to 122lm for neutral white or 100lm for warm white sources. Cree has even developed a lamp with 19 LEDs that produces 969lm at 102lm/W. The output is equivalent to that of a 65W incandescent bulb, yet it consumes only 9.5W. The key advantage of these LEDs is that less power is required in order to light a given area. While switching to CFLs can reduce lighting bills by 80% compared with incandescent lamps, LEDs can more than double that saving. High brightness LEDs extend the savings even more, when the true cost of the system is considered across its lifetime. Lowering cost over the lifetime First and foremost, using brighter LEDs means fewer LEDs are required per lighting module. A 20% improvement in efficiency means four LEDs can be used instead of five, lowering the bill of materials accordingly. Evaluating the potential savings therefore demands the creation of a cost model specific to the application. Questions to be answered should include: overall light budget; lifetime expectations; and potential performance degradations, such as variation in luminous flux over time, thermal design statistics and colour point stability. Customers should also understand their current lighting cost and performance model, in order that a fair lifetime comparison can be made with the new LED based product. For most LEDs, light output does not vary linearly with drive current. Efficacy reduces as the drive current and input power increase, so designers need accurate data in order to determine the available luminous flux, power consumption and cost per lumen. It is also important to scrutinise manufacturers' longevity data in the light of target product lifetimes. With longevity measured in years, rather than months, this requires a good understanding of how performance degrades over time: in particular, the variation in output power and spectral shift. A commonly used definition of lifetime is the time taken for the power output to decrease to 70% of the original value: research has shown that, in most cases, the eye cannot detect a reduction in lighting of less than 30%. These so called lumen maintenance figures are often quoted as L70 and L50, the latter corresponding to the time after which output is halved. Some applications may demand a more stringent measure of lifetime: for example, when lights are used side by side, reduced output is more apparent and designers may have to work to an L80 metric. While L50 can be useful for decorative lighting, it is not suitable for general illumination. Another consideration for white LEDs is a large and permanent shift in the exact colour of white light output – known as the white point or colour point. Colour point shifts may not be detectable in one LED, but become obvious in a lighting installation with many high power LEDs. As with fluorescent light sources, white high power LEDs will experience shifts in white point over their operating lives. Recognising this, manufacturers of quality LEDs have invested major R&D efforts – for example, the design of phosphors and packaging – in order to minimise spectral shifts and to reduce them to levels undetectable by the eye. Temperature and drive current Both types of degradation are exacerbated by high temperatures and higher drive currents. In particular, there is a direct relationship between LED junction temperature and the permanent loss of light output over time. Lifetime is extended by improvements to the LED's operating current, ambient temperature and thermal resistance of the thermal path to the ambient environment. In general, it is essential to keep the junction temperature as low as possible. The goal, therefore, is to find the most efficient way to remove heat from the LED chip and it is important to look carefully at package design. Other features to look for include a substrate with an extremely low thermal resistance, indicating a highly efficient way of channelling heat from the LED junction into the package's thermal path. For example, XLamp XR and XR-E LEDs from Cree boast a thermal resistance of 8°C/W, due to substrate materials. From there, designers can use a range of thermal management techniques to pass heat to the outside. In selecting a heat sink, designers need to walk the line between maximising surface area and providing quick and easy airflow. It is also important to ensure free flowing thermal transfer from the heat source to the fins and to ensure perfectly flat contact between heat sink and the LED or pcb module. Of course, the mounting method also needs to offer good thermal transfer. A number of techniques, including thermometric measurement and CAD analysis, can determine how effective these methods are likely to be. Emerging standards To gain these advantages, it is important to base performance and lifetime calculations on up to date product information, including all relevant test results. For many years, designers have been at the mercy of vendors when it came to using such data reliably. Fortunately, there are now international and de facto standards to help provide a consistent factual basis. In 2008, the American National Standards Institute established key standards to define colour temperature regions (C78.377), luminous flux (LM-79) and lumen maintenance (LM-80). For example, the LM-80 standard describes the measurement of lumen maintenance of LED light sources, requiring manufacturers to provide a range of data – all observable, measurable and supplied with quantified uncertainties. The standard covers LED packages, modules and arrays, but does not allow for prediction of lifetime beyond the testing period. Around a dozen further standards for LEDs have now been published, with 16 or so more in progress, covering issues from nomenclature, photometry and colour to EMC and performance standards. The industry is also making progress towards harmonisation of standard in Europe and elsewhere. This promises benefits from design to development, manufacture and production. All along the product chain, recognised standards make it possible to compare different manufacturers' products fairly. This will enables products to be developed that offer better performance and higher quality output at a lower cost, fuelling the predicted explosive growth in demand for LED lighting. Author profile: Dan Scott is Anglia Component's led lighting business development manager