Halogen PAR and MR integral lamps have been a mainstay for the retail and residential lighting markets for decades. These lamp sources have excellent color rendering and narrow light distribution capabilities but also exhibit relatively short service lifetimes and poor energy efficiency. The latter two weaknesses have been exploited effectively by many successful solid-state lighting (SSL) retrofit lamp products, but until recently the LED technology had not yet advanced enough to simultaneously achieve the high efficacy, long lifetime, focused light distribution, and high color rendering index (CRI) required to truly match the performance of these traditional light sources. The latest LEDs, however, promise to supplant halogen in new SSL designs as we will demonstrate in comparing some reference designs and existing LED and halogen products.
The MR16/GU10 lamp poses perhaps one of the biggest challenges to LED retrofit design. In addition to the stringent requirements mentioned previously, the MR16 lamp is an extremely small size and has relatively little available surface area to dissipate the high power densities required in the application. This article will discuss recent breakthroughs in LED lamp architecture that enable all of these problems to be solved simultaneously.
CRI: The last problem to solve
The most common halogen MR16/GU10 lamps on the market are the 20W, 35W, and 50W varieties. The figure at right shows examples of some of the styles of LED retrofit lamps that have successfully met or exceeded the luminous flux, Center Beam Candle Power (CBCP), efficiency, and lifetime requirements of these common lamp types.
Different numbers, configurations, and models of packaged LEDs were used to achieve these results, but each of them — like nearly 100% of the commercially available LED retrofit lamps currently on the market — was only able to achieve a CRI of around 80. While 80 CRI is normally acceptable for many residential applications, retail applications often demand CRI in excess of 90 to help accentuate the color and quality of fabrics, skin tone, and food items, and this is where many of these early LED retrofit lamps have so far fallen short of the mark.
The technology to produce LED lamps with 90 CRI or greater has existed for several years but the efficacy penalty — generally around 20% — of going from 80 to 90 CRI or beyond has been too much to pay in terms of cost, but just as important is thermal load in this thermally constrained MR16/GU10 application.
To demonstrate that the color/CRI challenges are now achievable, Cree application engineers used 90+ CRI versions of a new Generation 2 series of high-efficacy Cree XLamp LEDs to build working MR16 reference designs. The XM-L2, MK-R2 and MT-G2 depicted at left were used to create 20W-, 35W- and 50W-halogen-equivalent MR16 lamps. The photo below shows one of the reference lamps. The three designs use LED components assembled onto off-the-shelf heat sinks along with off-the-shelf optics and drivers. These new reference designs of high-CRI MR16s are all around 50–55 lm/W as assembled, and all three designs hit or exceed their 20W, 35W, and 50W equivalent targets. With further optimization of the drivers, optics, and heat sink, even higher equivalencies are possible from these LEDs.
The three XLamp reference designs discussed here are not the first or only LEDs on the market to take on the difficult MR16 design problem. Simultaneously attaining high CRI and all the other aforementioned performance constraints in an MR16 format has also recently been achieved by using so-called gallium-nitride-on-gallium-nitride (GaN-on-GaN) LED technology. GaN-on-GaN LED technology, because of the homogenous substrate or wafer, allows for relatively higher drive currents and therefore higher current densities with much reduced efficacy droop. A comparison of the major performance parameters of the two solutions is summarized in the table below.
Heat, color stability, lifetime
All LED lamps — regardless of chip architecture or substrate technology — represent a complex and highly engineered system. These systems consist of the LED chip, the phosphor down-converter, and the encapsulant and packaging materials. As mentioned, GaN-on-GaN LED chip technology affords the opportunity for higher drive currents and higher current densities, with improved efficacy droop capabilities. Even though the GaN-on-GaN chips may perform well under these stressful conditions, the phosphor system and the LED package still have to reliably survive these conditions or the MR16 lamp cannot maintain the performance of halogen lamps over time.
The phosphor conversion efficiency of any LED lamp system can degrade significantly vs. increased operating temperature. This means that as the LED lamp heats up, the color output changes until it reaches a steady thermal state. The phosphor response of the Cree and GaN-on-GaN LED MR16 solutions over temperature is shown in the figure at left.
Both the magnitude and the direction of the color shift are important since a horizontal shift in color is likely to be perceived by the observer as a change in CCT (about 100K in this case), whereas a vertical shift in color runs more or less along iso-CCT lines (about 30K here).
At 3000K, the ability of the human visual system to perceive variation in CCT is about ±50K, according to recognized color science experts Wyszecki and Stiles. Therefore, the difference in color from initial startup until steady state would likely be noticeable for the system that shifts horizontally.
Temperature can also impact CRI. The figure at left shows two spectral power distribution plots, each normalized to their peak intensity. A change in shape of these curves as a function of temperature can indicate a change in color rendering between the initial startup conditions — normally the data sheet conditions — and the thermally stable steady state of the installed MR16 lamp.
High operating temperatures and high drive current densities can also affect the encapsulant and packaging systems of LED lamps. Driving LED lamps at higher power density can lead to package depreciation, even if the chip itself may be stable. There is a range of silicone encapsulants used in the LED industry, and those details are beyond our scope here, but we can discuss the general impact of temperature and materials.
These silicone encapsulant materials typically are polyorganosiloxanes, also called siloxanes. Cree has published a chemical compatibility application note that goes into greater detail on the topic (www.cree.com/products/pdf/XLamp_Chemical_Comp.pdf). At elevated temperatures, volatile hydrocarbons can diffuse through the silicone. With the high temperatures and high amount of radiant energy present, these impurities can darken over time and cause light degradation. Thus, the higher the drive current, the more light being emitted from the chip and the faster the lumen depreciation of the lamp.
The three XLamp LED components used in this work have gone through rigorous, long-term lifetime testing, as defined by the Illuminating Engineering Society (IES) LM-80-2008 long-term test configurations. Based on available LM-80 data, and using the IES TM-21-2011 method to project, all the XLamp LEDs used in the High-CRI MR16 reference design show an L70 value of greater than 36,000 hours at the drive current and operating conditions of the respective three MR16 reference designs. Remember these lifetime figures are projections and not warrantied specifications.
Halogen MR16 B50 lifetimes range between 2,000 and 10,000 hours, and most commercially available halogens typically have rated lifetime of 3,000 hours. The LM-80 data for the GaN-on-GaN LED described here has not been published, but Cree has tested some of these MR16 lamps in real-world scenarios (open air, Ta = 22°C; closed fixtures, Ta = 55°C) and found that the L70 for at least a small sample of these MR16 lamps can be as short as about 4,000 hours — barely exceeding typical halogen rated lifetimes.
The figure at right shows a graphical comparison of a Cree XLamp LED component with regard to two commercially available GaN-on-GaN LED MR16 lamps under two different ambient operating conditions, and also the halogen MR16 B50 rated lifetime. From this we can see the degradation effects the very high operating temperatures and current densities have on the phosphor and packaging elements of the LED lamp.
UV light content
One final point of note with LED MR16 lamps involves applications that are sensitive to ultraviolet (UV) light. Some lighting applications involving fabrics, paints, and sensitive artwork have had problems over the years with the UV content of halogen lamps. To compensate for this, halogen MR16 lamps are sometimes fitted with UV (also sometimes infrared, or IR) filters. One potential drawback of this GaN-on-GaN technology is the use of shorter-wavelength UV light (less than 400 nm) which, like the UV content of halogen MR16 lamps, has the potential to cause photo-degradation and may require UV filtering in some sensitive applications. Cree XLamp LEDs are based on a blue (nominally 460 nm) LED chip, which is generally thought to have much less potential for such photo-degradation than the sub-400-nm UV light content in the other two light sources.
Current LED technology has progressed to the point where all the performance metrics of halogen lamps — including rated lifetime, LPW efficacy, color temperature, color stability, CBCP, and now CRI — can be met or exceeded. This work looked at the most challenging halogen application — the MR16/GU10 lamp — but the findings can be generalized to larger halogen lamp styles like PAR20, PAR30, PAR38, and AR111 as well. These performance metrics were met both with a new generation of traditional blue LEDs as well as GaN-on-GaN LEDs. But care must be taken in the end application to minimize operating temperatures, which will ensure the rated lifetime and color stability. Moreover, designers must guard against UV-induced damage in sensitive applications for the GaN-on-GaN-based LED products evaluated here.