From SSL Postings: All Fired Up About Down-Conversion

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“April 19, 2017
All Fired Up About Down-Conversion
As you probably know, the process of down-conversion by phosphors has been central to the development of white-emitting LEDs. That’s because, for most white-LED systems, the LED emits blue photons, most of which are converted to green-yellow and red by a layer of phosphors that typically rests on top of the LED, with the resulting color mix perceived as white by the human eye.

This process, by which photons are converted from more-energetic colors to less-energetic colors, is known as down-conversion. And while down-conversion phosphors have progressed considerably over the past decade, they still need significant improvements in order to meet the performance targets described in the DOE SSL R&D Plan. That’s especially true for red phosphors, which could benefit from being less thermally sensitive and having a narrower spectral width to better match the eye response. With DOE R&D support, GE has commercialized a promising narrow red phosphor, PFS, that’s making its way into GE lighting products with excellent color quality and efficacy. Another narrow red phosphor that has made it into LED products with similar benefits is the SLA phosphor developed by Lumileds.

However, phosphors aren’t the only materials that can be used as down-converters. For quite some time now, scientists — some of them funded by DOE — have been working on developing alternative down-conversion materials called quantum dots (QDs), which are more tunable than phosphors in terms of emitted wavelength, and can be spectrally purer (that is, they have a narrower spectral width). In addition, QDs have demonstrated very high conversion efficiencies that match or exceed those of existing phosphors.

QDs are engineered materials that convert light through a different process than do phosphors, which are limited by their own material emission and absorption properties, whereas QDs re-emit light as a function of their size. What’s more, the ability to develop specific sizes of QDs gives them the potential for greater spectral control. For the past few years, QDs have been used in TV displays, because their narrow emission expands the color gamut, resulting in a more vivid image. However, the rapid degradation of QDs under on-chip LED operating conditions — notably, high temperature, humidity, and blue-flux intensities — has been the biggest barrier to their use as a narrow-band red solution for white LEDs in general illumination applications.

That barrier appears to have been overcome — or, at least, to be toppling. Scientists at Lumileds, reporting in the journal Photonics Research, say they’ve used red QDs with tunable peak emission and narrow spectral width to demonstrate the first commercial production-ready white QD LEDs for the general illumination market. This has resulted, they say, in LED efficacy improvements of 5-15% over commercial phosphor-based LEDs, at CCTs that range from 2700K to 5000K — and without the degradation that plagued previous attempts.

In other words, up until now, no one had been able to successfully deploy QDs on chips, because of the degradation from the flux, temperature, and moisture. Lumileds has succeeded by using QDs that are developed by Pacific Light Technologies and are able to withstand the extreme environment of a conventional LED package, while showing similar stability as the phosphors that they’re displacing.

This is an important breakthrough, because it puts a whole new technology in the LED lighting toolbox, enabling higher luminous efficacy and even more-refined control of the emission spectrum for down-converted LEDs. DOE is currently funding an R&D project at Columbia University that aims to further improve the manufacturability of the QDs that were used to get this result, and Lumileds is planning to commercialize the breakthrough.

The development of quantum dots illustrates the importance not only of taking an integrated approach that considers all elements and requirements of the system, but also of casting a wide net for R&D that takes into account alternative approaches that can help improve SSL performance.

Best regards,
Jim Brodrick

As always, if you have questions or comments, you can reach us at postings@akoyaonline.com.

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— Jim Brodrick

New CALiPER Snapshot on TLEDs

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“Linear fluorescent lamps — energy-efficient, long-lived, and relatively inexpensive — have been a staple of ambient lighting in offices, classrooms, and other commercial spaces, where they’re usually housed in troffers. Linear LED lamps, commonly known as TLEDs, typically draw about 60% of the power of linear fluorescents and have become a viable alternative, used mainly in retrofit situations. DOE’s CALiPER program has released a new Snapshot report on TLEDs that’s based on LED Lighting Facts® data.

Among the key findings:
• TLEDs now comprise more than 50% of all lamps listed with LED Lighting Facts, and more than 10% of all listed products.
• TLEDs offer the highest mean efficacy of any lamp type, and also include the listed product with the highest efficacy (190 lm/W).
• In aggregate, TLED efficacy decreases by 3 lm/W for every 1000K decrease in CCT.
• While the raw efficacy of TLEDs exceeds that of dedicated LED troffers, the reverse is true if TLED efficacy is adjusted to account for luminaire efficiency. In other words, dedicated LED troffers tend to exceed the efficacy of troffers fitted with TLEDs.
• Almost all (98%) of the listed TLEDs have a CRI in the 80s, with most between 80 and 85.
• A vast majority (97%) of TLEDs that are currently listed by LED Lighting Facts (and that report this optional metric) have a power factor of 0.90 or greater.
• Nearly 90% of the currently listed TLEDs (which include 2’- and 4’-long products) emit between 1,000 and 3,000 lumens. This is generally less than the emission of a typical 4’ linear fluorescent lamp. Of the more than two-thirds of TLED products that are identified as having a 4’ length, the mean output is 2,094 lumens.
As the numbers from LED Lighting Facts attest, TLEDs seem to be everywhere, and their numbers are growing rapidly. But while their rise to prominence is indisputable, they’re not necessarily a clear favorite when evaluating performance.

LED Lighting Facts data show that TLEDs consistently draw less power and emit fewer lumens than the linear fluorescent lamps they’re intended to replace. On balance, they have somewhat higher efficacies, but the energy savings achieved are in large part due to the lower power draw. Importantly, TLEDs offer more of a directional emission than linear fluorescent lamps, meaning they can make troffers or other luminaires more efficient, delivering equal illuminance to the work plane, with fewer lamp lumens. However, sometimes the increased luminaire efficiency can’t compensate for the reduced lamp lumens. In such cases, energy savings are derived from reducing the light levels, which may or may not be acceptable. The change in distribution, something that’s not obvious from the LED Lighting Facts data, presents yet another issue, as it can change both the appearance of the luminaire and the distribution of light within a space.

TLEDs are often compared to other LED options for replacing a fluorescent lighting system — such as using retrofit kits or dedicated LED fixtures. At first glance, TLEDs may appear to be superior, with higher efficacy and likely lower product and installation costs. But accounting for factors such as luminaire efficiency may tip the balance against TLEDs in some scenarios, and their long-term costs may be increased by factoring in the remaining life of existing fluorescent ballasts, if they’re to be reused.

Nevertheless, viable TLED options are increasingly available, which was not the case a few years ago. And as they push the efficacy limits for LED products, TLEDs can be compelling replacements for fluorescent tubes, as long as other tradeoffs are appropriately accounted for. But there are thousands of choices when specifying TLEDs. And as the new Snapshot report shows, there’s considerable diversity in performance, even when examining only basic attributes. The Snapshot doesn’t address the electrical and safety considerations when changing from fluorescent to LED lamps, nor does it examine features such as distribution of light or lifetime. It also doesn’t distinguish between the different types of TLEDs (UL Type A, those that can operate directly on a fluorescent ballast; UL Type B, those with an integrated driver; UL Type C, those with an external driver; and hybrids), because they don’t differ appreciably in photometric performance. Their distinguishing features, however, are very important considerations during specification and installation.

When evaluating TLEDs, it’s critical to examine the expected performance of the complete lamp and luminaire system, understand the complexities of installation, and be cautious in considering long-term performance.

For a closer look at the findings, download the full report. For additional guidance, see the DOE Fact Sheet Upgrading Troffer Luminaires to LED.

Best regards,
Jim Brodrick

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— SSL Postings, US Department of Energy