Are Power LEDs Ready for the General Illumination Market?

By Christos Sarakinos, Engineer, Future Lighting Solutions

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Since power LEDs were first introduced by Philips Lumileds in 1999, entrepreneurs and environmentalists have envisioned a world in which the majority of lighting was solid-state based. Owing to their seemingly infinite lifespan and the revolutionary design possibilities their compact size enables, power LEDs have gained widespread acceptance by makers of traffic, emergency vehicle, portable and niche medical products. Luminaire manufacturers by contrast have been more reserved, patiently awaiting a rival to traditional technologies, one with higher efficiency and proven reliability. In response to this, some power LED manufacturers have regularly issued press releases announcing exciting performance records – possible only in the confines of their laboratories. Each time, luminaire designers have asked in unison: Are we there yet?

 

 

The Market

One indication comes from the increase in worldwide concern for environmental issues. The evergrowing list of countries, states and provinces that have tabled proposals to ban incandescent bulbs within the next decade include: Cuba, Venezuela, Australia, California (USA) and Ontario (Canada). Much of the “ban-the-bulb” legislation focuses on the increased use of compact fluorescents, mainly due to their high availability since the 1970s. Given their slow startup, the inability to dim them, the electronic interference they cause, as well as the mercury contained within, consumers remain reluctant. An alternative is needed.

When considering the viability of a light source, the following factors are critical:

• System efficiency
• Quality of light
• Lifetime and reliability
• Affordability

The following sections examine each of these factors, explain their relevance, and compare the power LED with traditional technologies.

 

System Efficiency

One of the main pushes for alternative lighting is reducing power consumption. The US Department of Energy (DOE) recently estimated that 22% of all the electricity generated in the US goes towards lighting, representing $58 billion in annual spending. The DOE is presently examining ways to decrease this percentage to 11% by 2025.

Given this, it is useful to examine the efficiencies of traditional sources. (Efficiency can be described as the amount of light outputted by a source, relative to the power consumed to create that light.)

According to the DOE, the average efficiency of a regular light bulb (type “A” incandescent) is 15 lumens per watt. (A “lumen” is a measure of the power of light, as perceived by the human eye.) This means that for each watt of power used, 15 units of perceived light are produced. Tungsten halogen bulbs, commonly seen in the MR16 form factor and used in pot lighting, fare slightly better at a source efficiency of 20 lumens per watt. High pressure sodium lighting (the yellow light often seen in parking garages), leads the pack at 100 lumens per watt, while fluorescent tubes have a very respectable efficiency of 65 lumens per watt.

When it comes to the power LED, Philips Lumileds is among two manufacturers who have made a recent jump in efficiency and are able to produce die for white LEDs that attain 80 lumens per watt.

 

The values represented above, while impressive, are not an accurate representation of actual light fixtures. The distinction must now be made between the efficiency of the source, and that of the system; there are three variables that cause them to differ.

Regardless of the technology used, there will always be a loss during the transformation of power. Additionally, it is virtually impossible to get all the generated light from the source to the targeted surface; in this case, power LEDs fare better than their traditional counterparts in that they are directed sources, rather than omni-directional ones. By contrast, the negative effects of heat are far more pronounced with power LED fixtures than in other systems. Table 1 summarizes the system losses.

 

 

When taking into account losses (namely power, optical and thermal), the power LED has a system efficiency of 50 lumens per watt, bringing it on par with high pressure sodium bulbs. Figure 2 illustrates the danger of considering only source efficiency, rather than system efficiency.

 

 

(For more information on factors that affect LED system efficiency, visit: www.FutureLightingSolutions.com)

 

Quality of Light

In lighting, the Color Rendering Index (CRI) can be loosely defined as a rating of a light source’s ability to make illuminated colors appear as they should, out of 100. (Natural outdoor light, for instance, has a CRI of 100.)

Combining the efficient sources into a single table and adding a CRI field, we see that although high pressure sodium has a relatively high system efficiency, its CRI is low, meaning illuminated colors tend to appear grey, regardless of their color.

 

 

When considering the quality of white light, it becomes apparent that high pressure sodium is not a viable source for most lighting applications.

 

Lifetime and Reliability

Lifetime, robustness and reliability have historically been the most publicized benefit of converting to power LEDs. Comparing their lumen maintenance (70% original light at 50K hours) with that of a fluorescent bulb (16K hours), this advantage of solid-state lighting becomes clear.

 

 

There is an issue with power LED lifetimes that bears elaboration, and it is this:

With appropriate luminaire design, power LEDs very rarely fail catastrophically. Rather, their light diminishes gradually over time. As such, power LED manufacturers have generally spoken in terms of lumen maintenance (i.e. what the percentage of light will be output after a given amount of time). As the human eye can detect 20-30% decrease in light output, the lumen maintenance figure most often quoted is 70% light output after 50K hours.

Makers of power LED-based lighting products have latched on to the “50K hours” claim, in many cases without fully appreciating the assumptions, and the conditions for which it is valid. Ultimately, the luminaire manufacturers may be liable if their fixture underperforms. To guarantee the lifetime of a fixture, they must understand what lies beneath the claims, and adapt their system design accordingly.

Lumen maintenance claims do not and can not exist in a vacuum. They depend on:

• The temperature within the power LED (i.e. the junction temperature)
• The current at which the power LED is driven
• Simple probability

 

Each power LED has an internal temperature for which the lumen maintenance claims are valid. The lower the temperature specified, the more difficult the thermal design, and the less likely that the system’s thermal design will be robust. Philips Lumileds’ LUXEON K2 has a lumen maintenance junction temperature of 120ºC, while the LUXEON Rebel has one of 135ºC. This offers significantly more thermal “headroom” than 80ºC– the lumen maintenance junction temperature specified by most other manufacturers.

Additionally, most lumen maintenance claims are valid only at the rated drive current of 350mA. The fact is: greater drive currents means faster degradation of light output.

Another misconception is that lumen maintenance claims are guarantees; in fact, they are simply averages. The claim of 70% at 50K hours means that half the time there will be more light, and half the time there will be less. Challenges arise when trying to specify the lifetime of a luminaire, based solely on an average.

In an effort to guide luminaire designers, Philips Lumileds is the only maker of power LEDs that has released detailed documentation illustrating the effects of relevant factors on their product’s lifetime. This documentation contains “B&L” curves, where “L” represents the percentage of original light output, and “B” represents the percentage of LEDs that will be below the light output specified in “L”. (For instance, B10, L70 curves show the time at which 90% of the parts will have 70% of their original light output.) Figure 3 illustrates the B10, L70 curves for the LUXEON K2 and the LUXEON Rebel, clearly indicating the effect of junction temperature and drive current.

 

 

These graphs allow for the qualified design of luminaires, as well as confident lifetime declarations and guarantees by their makers.

 

For more information on the reliability of power LEDs, click here.

 

Affordability

The trend over the last three decades has been twofold:

• Decreasing cost of light
• Increasing light output per package

Although cost estimates are obviously applicationand design-dependent, increasing efficiencies and decreasing costs have allowed power LEDs to approach the levels of traditional technologies.

 

 

While recent increases in efficiency have thrown power LEDs into the spotlight, their ability to radically reduce maintenance costs (enabled by their lifetime), has historically been the driving force behind their adoption. Although perhaps marginally more expensive at the onset, their long term savings can be significant, as illustrated in Figure 5.

 

 

Conclusion

With proper system design, power LED lighting is efficient, predictably reliable (with the right tools!), affordable, and of a suitable quality. Acknowledging this, ENERGY STAR® (a joint program of the U.S. Environment Protection Agency and the U.S. Department of Energy) has released a draft of its specification entitled Program Requirements for Solid State Lighting Luminaires.

According to the researchers at Strategies Unlimited, the market for white power LEDs in lighting is widely expected to grow to an average annual rate of 47% for the next five years, expanding from $88 million in 2006 to $610 million by 2011. It stands to reason that the market for power LED-based luminaires will grow along similar lines.

Are we there yet? Yes, we definitely are.

 

 

 

 

DOE ENERGY STAR® Program for Solid-State Lighting

The ENERGY STAR label is a highly valued and widely recognized mark of energy efficiency that helps guide the American public to select cost-effective, energy-efficient products. As part of DOE’s national strategy to accelerate market introduction of high efficiency SSL products, the Department is leading ENERGY STAR management, specification development, and partner relations for SSL devices used for general illumination, including:

• Residential, commercial, industrial, and outdoor lighting SSL applications of all types
• Innovative SSL systems applications of all types (includes "free-form" SSL systems, and those incorporated into furniture, buildings, and equipment)

The DOE ENERGY STAR strategy for SSL general illumination products establishes near-term and long-term requirements. The near-term transitional criteria apply to niche application products that are at least as efficient as fluorescent light sources. The long-term criteria are for future products that are far more efficient. The rapid pace of SSL performance improvements will require DOE to periodically review and amend the criteria to parallel technological advances and ensure the criteria remain up-to-date.

For more information on ENERGY STAR® (a joint program of the U.S. Environment Protection Agency and the U.S. Department of Energy) and its released draft of its specification entitled Program Requirements for Solid State Lighting Luminaries, click here.

 

 

 

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