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On color temperature and the mating habits of butterflies

Lucrative returns on investment from light-emitting diode (LED) lighting have spurred rampant replacement of traditional light sources, with some of the fastest growth occurring in outdoor roadway lighting.  The raw efficacy of LEDs, coupled with tight beam control, offers a substantial reduction in energy consumption “per pole” and a fast payback period.  The alluring opportunity encourages the use of the cheapest and most efficient white LEDs, i.e., those using minimal amounts of phosphor (which is used to convert the primary blue/violet LED-chip light to white) and which thus exhibit high color temperature.

What is color temperature?  The somewhat (unnecessarily) maligned term is just a measure of the chromaticity, or color, of the radiation from objects raised to very high temperatures.  Like for flame, the hotter they are, the bluer they become.  Candlelight has a color temperature of about 1800 degrees Kelvin (K), a typical incandescent lamp about 2700K, while noon-day sun is considered to average 5500K[i].

The LEDs described above for roadway lighting leak a significant portion of their primary (blue) emission through the thin phosphor layer, resulting color temperatures of 4000K or more, much higher than that of common incandescent lamps, or traditional roadway lights, i.e., golden-hued high-pressure sodium (HPS) lamps.

In response to ecological and human-health concerns about the rapid adoption of high color temperature light sources in the outdoors, and amidst silence in the general lighting community on the subject, the American Medical Association (AMA) in 2016 issued guidelines recommending the use of lower color temperatures (3000K or less) for outdoor lighting[ii].  That is, it suggested reducing the amount of short-wavelength (i.e., blue/violet) emission from LEDs used in these applications.

Why the concern over short wavelength light at night?  There are quite a few, in fact.  Blue light at night is well-known to disrupt circadian rhythms and has been linked to a wide range of health issues including cancer, diabetes, heart disease, and obesity[iii].  While some argue that the intensity and duration of exposure of drivers to road lighting is unlikely to cause as much disruption as in, for example, night-shift workers, the ecological situation is a different story.  Animals (bird, bats, insects) are far more likely to endure exponentially higher doses of night-time outdoor lighting, especially since many are attracted to it.  Moreover, such lighting can disrupt the feeding cycles of such animals and upset the natural balance of predator-prey populations[iv],[v].

Short wavelength light at night might even disrupt the mating habits of butterflies, which use such light to assess the wing patterns of potential partners[vi], an activity nature designed to occur during the day (the only time short-wavelength light is available naturally).  The long-lasting ecological impacts of artificial light at night are uncertain, but the recently documented rapid population decline of certain insects[vii] is more than enough of a reason for us to strive for a stance of “do no harm”—to quote my friend, lighting designer Jim Benya—when it comes to artificial light at night.

To see how not to do it, refer to the recent photographs below from Mountain View, CA (hometown of Google Inc.), a community one would like to think would be on top of the best possible solutions for its infrastructure.  Clearly, something is wrong here.  (Policy for outdoor lighting, anyone…?)

Left: high color temperature, high-glare LED street light.  Right: creepy moiré patterns in the shadows caused by the (poorly designed) pixelated-LED light fixture — Mountain View, California, USA

The lighting community’s responses to the AMA statement, while respectful on the surface, seemed to mask some resentment.  Indeed, shortly after the announcement, a slew of reports emerged claiming LEDs are “no worse” than traditional light sources when it comes to light pollution and that LEDs should be adopted on their original merit (energy savings).

One study[viii], sponsored by the U.S. Department of Energy (DOE), was a comparative simulation of sky-glow (brightness of the night sky resulting from light pollution).  The study concluded that LEDs with up to 4000K are “no worse” than HPS lamps.  However, a more careful review of the data shows that 4000K spectra in fact are worse under cloudy conditions, even with the assumption LEDs produce no up-light and require only half the light output of HPS.  Moreover, the range of atmospheric conditions considered in the study does not include very humid conditions that can exist, for example, near the North Sea.

I recently experienced this phenomenon first-hand during a nighttime road-trip to Amsterdam. A colleague and I came upon a stretch of highway where the street lights switched from HPS to high color temperature LEDs. The effect was dramatic, and negative.  A cascade of bright balls of bluish light began to hurl over us.  This unambiguous increase in glare, which no doubt contributes to increased sky-glow, suggests to me that a wider range of atmospheric conditions[ix] should be included in the DOE’s simulations. It further raises the likelihood that the AMA’s guideline regarding color temperature limits may in fact not be aggressive enough.

All this suggests more work is necessary to achieve “do no harm” when it comes to selecting LEDs for outdoor lighting applications.  The exciting part is that very interesting options exist.  Amber emitting phosphor-based LEDs, demonstrated almost a decade ago[x], are now commercially available and emit almost no short wavelength emission.  Luckily, the DOE study included this spectrum and the results suggest that, under the same condition mentioned above, amber LEDs produced less than half the sky-glow of HPS lamps. To me, here is the real opportunity, to not only achieve reduced energy consumption, which is critical to our earth’s climate, but to simultaneously and substantially reduce the negative impacts of artificial lighting on our planet’s ecosystem.

Amber LED based outdoor light fixtures are already on the market[xi], and safety concerns regarding such single-color street lighting are potentially overexaggerated[xii].  Currently, amber LED efficacy is not as high as that of white LEDs, and the costs are higher, but with increased deployment and the integration of controls, this approach will provide substantial cost-of-ownership savings over HPS.  Furthermore, the lower efficacy presents an opportunity for LED technologists to invent even better solutions, whether based on new phosphors or on direct, primary-amber LEDs from breakthroughs in materials science.  Finally, completely new spectral engineering, enabled by emerging single-color emitters such as quantum dots[xiii] and line-emitting phosphors[xiv], will bring even more opportunities to engineer LEDs in a holistic fashion that addresses energy savings, ecological concerns, and safety/security simultaneously, resulting in dramatically better products in our future for necessary outdoor lighting.

In memory of Jerri Ann Krames (1972-2017)

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[i] Experts will point out that color temperature is insufficient to describe the details of a light source’s emission spectrum.  This is true.  However, for LEDs, pinpointing a color temperature locks in the range of possible spectra to a reasonable degree, as there is limited amount of “wiggle” room since the color temperature primary tracks with the amount of primary blue light leaked by the LED.  While not sufficient in and of itself, color temperature is a necessary tool to be able to talk practically about a light source’s color properties.

[ii] American Medical Association, CSAPH Report 2-A-16, Human and Environmental Effects of Light Emitting Diode (LED) Community Lighting, June 2016.

[iii] Evans JA, Davidson AJ. “Health consequences of circadian disruption in humans and animal models,” Prog Mol Biol Transl Sci. 119: 283-323 (2013).

[iv] Longcore T, et al., “Tuning the white light spectrum of light emitting diode lamps to reduce attraction of nocturnal arthropods,” Phil. Trans. R. Soc. B 370: 20140125 (2015).

[v] Somers-Yeates R et al., “Shedding light on moths: shorter wavelengths attract noctuids more than geometrids,” Biol Lett 9: 20130376 (2013).

[vi] DJ Kemp, “Female butterflies prefer males bearing bright iridescent ornamentation,” Proc. R. Soc. B 274, 1043-1047 (2007).

[vii] Macgregor CJ et al., “The dark side of street lighting: impacts on moths and evidence for the disruption of nocturnal pollen transport,” Glob Change Biol, 23: 697–707 (2017).

[viii] DOE SSL Program, An Investigation of LED Street Lighting’s Impact on Sky Glow, April 2017.

[ix] International AERONET Federation. D van der Zande, Principal Investigator.  Zeebrugge, Belgium.  Available:  https://aeronet.gsfc.nasa.gov/cgi-bin/webtool_opera_v2_new?stage=3&region=Europe&state=Belgium&site=Zeebrugge-MOW1&place_code=10

[x] Mueller-Mach R, et al., “All-nitride monochromatic amber-emitting phosphor-converted light-emitting diodes,” Phys. Status Solidi RRL, 1–3 (2009).

[xi] See, for example: http://www.ignialight.com/en/projects/detail/id/29/pc-amber-street-lighting

[xii] Steinbach R, et al., “The effect of reduced street lighting on road casualties and crime in England and Wales: controlled interrupted time series analysis,” J Epidemiol Community Health 69, 1118–1124 (2015).

[xiii] Mangum BD, “Exploring the bounds of narrow-band quantum dot downconverted LEDs,” Photonics Research 5.2, A13-A22 (2017).

[xiv] Van de Haar MA et al., “Increasing the effective absorption of Eu3+-doped luminescent materials towards practical light emitting diodes for illumination applications,” Appl. Phys. Lett. 112, 132101 (2018).

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ROME needs red


“…these horrible [LED] lights!  Mama mia!!” laments Nathalie Naim, municipal council member for the city of Rome, in a recent New York Times article[1].  The complaint is a legitimate one.  Recent outdoor luminaire installations there employ bluish-white LEDs whose illumination contrasts sharply with the “golden” hue of the city’s legacy light sources (e.g., sodium-vapor lamps), leading to a garish viewing experience probably best summed up by the analogy:  “candlelit dinner versus the frozen-food aisle of your local grocery store,” as quoted by one resident of the city.

The unfortunate thing about the article, and in my experience a common misperception, is the implicit linkage of LEDs to garish blue-white lighting only, as if there were no alternatives.  Those of us in the field know that almost every conceivable form of common illuminant is achievable with LEDs, from (yes) bluish-white light that is common for applications like car headlamps, to the warm glow of a dimmed incandescent.  So, what gives?  Why is Rome in this situation?

Driven purely by energy savings and the associated return on investment, stakeholders in LED lighting projects are incentivized to use the cheapest materials possible, including the LEDs.  The cheapest LEDs on the market are those that contain the lowest materials expense, that is, blue-emitting LED chips coated with a single, inexpensive and widely available yellow-emitting phosphor (“YAG”[2]).  The resulting mixed white light from these LEDs contain almost no red and are simply not capable of delivering warmer color tones.  “Warm white” LEDs, on the other hand, do exist but require a second component to be added, a red-emitting phosphor.  The most common red phosphor on the market (“CASN”[3]) is several times more expensive than YAG.  Moreover, it is inefficient.  A very broad emitting material, CASN has substantial emission in the infrared wavelength regime, light that is not visible to the human eye.  The summary result is that warm white lighting solutions are expensive for three reasons: 1) the LED bill of materials costs more, 2) a reduction in efficiency (typically 20% or so) caused by the red phosphor means that more of these (higher priced) LEDs are required for a given installation, and 3) energy savings are reduced, substantially increasing the investment paypack period.  None of this is at all enticing for the lighting project stakeholders, who for their own livelihoods are keenly concerned with margin-sharing down the project pipeline.

The situation described above is a good example of why public policy with respect to lighting is so important.  Meager efforts regarding color quality by the Environmental Protection Agency in the U.S. have been overtaken by newer measures, such as California’s (sometimes maligned) Title 24 program, which requires high color rendering for residential lighting installations.  Championed by the likes of Prof. Michael Siminovitch at the University of California at Davis, this is protection of the consumer in action, and is a strategic play to avoid a backlash against LEDs like that experienced in the U.S. in the past against compact fluorescent lamps.  In the near term, the best course of action for Rome would be a similar initiative, which should be straightforward to specify based on chromaticity targets combined with color rendering and illuminance minimums.  No doubt LEDs will be able to meet that specification, and the Eternal City can legislate the return of its golden glow while simultaneously reaping the awards of energy saving LEDs.

However, the challenge regarding warm white LEDs is much more widespread than the example highlighted by the New York Times.  Everywhere there is a push to cash in on LED energy savings at the expense of quality of light.  Once beautiful lighting scenes at restaurants in the evening have been replaced by situations more akin to the atmosphere of a medical examination room—cool white lights with little (or even no) dimming capability.  (Sadly, this has been the fate of several of my favorite haunts in Silicon Valley.)  While one response to this challenge is further efforts to protect end-users, as California has shown, another is a duty of the LED lighting industry.  We must develop more efficient means of producing red light. CASN phosphor has been the incumbent for more than a decade.  Newer, better solutions are now demanded.

Hope is on the horizon, as demonstrated in presentations at the recent Phosphor Global Summit and Quantum Dots Forum conferences in San Diego, CA.  One, from Lumileds, focused on developments in red-emitting semiconductor nanoparticles (aka “quantum dots”) and demonstrated warm white LED efficacy increases of more than 15%[4] above the conventional approach.  Another presentation from Seaborough Research summarized a European effort looking at harnessing the power of trivalent europium, a well-known red-emitting ion, in new phosphors that are practical for more efficient warm white LEDs.  While the former bears a Restriction of Hazardous Substances (RoHS) challenge due to the use of cadmium, the latter is a RoHS compatible path and one that, if successful, should lead to low-cost red-emitting materials capable of providing substantial gains in warm white LED performance.  Once such solutions are in the market place, the current unpleasant trade-off between energy savings and light quality will subside, and we will no longer have the beautification of our world subjugated by the mandates of environmental responsibility.  LED industry, it is time to lead the way here and bring these solutions forward!  Rome deserves better.  The world deserves better.

  1. E. Povoledo, “Streetlight Fight in Rome: Golden Glow vs. Harsh LED,” The New York Times (New York ed.), 28 March 2017, p. A6.
  2. G. Blasse and A. Bril, “A new phosphor for flying-spot cathode ray tubes for color television: yellow-emitting Y3Al5O12-Ce3+,” Applied Physics Letters 11.2 (1967): 53-55.
  3. K. Uheda et al., “Luminescence Properties of a Red Phosphor, CaAlSiN3:Eu2+, for White Light-Emitting Diodes,” Electrochem. Solid-State Lett. 9.4 (2006): H22-H25.
  4. K. Shimizu et al., “Toward commercial realization of quantum dot based white light-emitting diodes for general illumination,” Photonics Research, 5.2 (2017): A1-A6.
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Do we really need better LEDs?

A quarter of a century has transpired since the Nobel Prize winning work that delivered us commercially viable blue- (and thereby, white-) emitting LEDs.  In the decades since, tremendous resource investment in gallium-nitride semiconductor based materials, devices, and packaging technologies has revolutionized backlighting for flat-panel displays (both large and small) and is now aimed at the general lighting industry, an approximate 100 B USD market transformation that promises a simultaneous reduction in worldwide electricity consumption of more than 10% [1].

What more is there to do?  Indeed, maximum power conversion efficiencies of the most sophisticated blue-and violet-emitting LED chip architectures are nearing the theoretical limits [2].  Their light output is conveniently down-converted to longer wavelength (i.e., green/yellow/red) emission by available phosphors to every imaginable quality of white light, from the high-tech brilliance of an automobile headlight to the warm glow of an incandescent bulb.  In addition, any shortcomings of existing phosphors are likely to be addressed by newer ones, or by the the emergence of quantum dots, a new semiconductor-based down-conversion technology that is already penetrating the large-area television market.

However, the down-conversion approaches come with a price:  the energy of a blue or violet photon is significantly (about 20-30%) higher than that of green/yellow/red photons, and the associated conversion generates that much waste heat, even if the processes involved are 100% efficient (which they are not).  Instead, using individual LEDs for each primary (e.g., red/green/blue) would eliminate the fundamental energy waste of the down-conversion approach, reduce rare earth material consumption, and also simplify LED application to color-tunable lighting and to display applications which currently require filters to convert white light into primary colors required for information visualization.

So, why aren’t multiple primary LEDs widely used today?  The answer is efficiency.  Unlike the case for blue- and violet-emitting LEDs, the performance of LEDs at longer emission wavelengths is fairly low and, even more worrying, has largely stagnated [3].  Indeed, the latest, best reported green-emitting LED efficiency is barely higher than it was a decade ago, and remains less than half as efficient as its blue- and violet-emitting counterparts.  The situation for amber- and red- emitting LEDs is similar, and comprises a large part of the reason down-conversion materials like phosphors and quantum dots are developing into robust industries today.

Clearly, we do need better LEDs.  The original inventor of the LED, Nick Holonyak, Jr., prophesied that LEDs of every imaginable wavelength would eventually be realized.  However, today’s best performing gallium-nitride based LEDs largely employ the same technologies introduced 25 years ago.  This has well served blue- and violet-emitting LEDs, but not nearly as much their longer wavelength counterparts.  Evidently, a breakthrough is required and, as becomes more apparent day by day, one that will have to go beyond the Nobel Prize winning work from the 1980s and 1990s.

161003-figs

  1. “Energy Savings Forecast of Solid-State Lighting in General Illumination Applications,” Prepared for the U.S. Department of Energy by Navigant Consulting, Inc. (2014)
  2. C A Hurni, A David, M J Cich, R I Aldaz, B Ellis, K Huang, A Tyagi, R A DeLille, M D Craven, F M Steranka, and M R Krames, “Bulk GaN flip-chip violet light-emitting diodes with optimized efficiency for high-power operation,” Applied Physics Letters 106, 031101 (2015)
  3. M R Krames, 6-1: Invited Paper: Status and Future Prospects for Visible-Spectrum Light-Emitting Diodes, SID Symposium Digest of Technical Papers, 47, doi: 10.1002/sdtp.10594 (2016)