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.