Cree talks streetlamps after AMA and LRC differ

While broadly supporting led street lighting, the American Medical Association recently raised concerns about the technology, and the US Lighting Research Center then criticised some of the AMA’s conclusions (link).

LED street light LRC

As a reality check, Electronics Weekly spoke to Erik Milz, v-p for marketing outdoor lighting at streetlight maker Cree.

Don’t get the wrong idea here, the AMA report is thoughtful and not sensationalist – there is a link to it in the middle of this article. Broadly, it supports well-designed (and therefore glare-free) lighting, applauds the energy efficiency of led lighting, and cautions against having too much blue in the spectrum.

This last bit it does by advocating the use of ‘warm white’ 3,000K leds after taking into account the typical spectrum of lighting leds.

The LRC has some detailed technical arguments about the way the AMA came to its conclusions, and these too are well and moderately argued – there is a link to it at the bottom of this article.

And the two disagree on how close the link is between led-derived blue light and sleep-inhibition.

“Generally, we applaud the AMA’s pursuit of better lighting,” said Cree’s Milz.

On the blanket use of 3,000K; Cree’s position is that every application deserves an appropriate colour temperature, he said.

It is almost always true of white leds that cooler colour temperatures (~>5,000K) deliver more lumens per watt than warmer colour temperatures (~3,000K).

In the early days of led lighting, only cool whites could deliver enough efficiency to make swapping from an incumbent technology worthwhile.

“You had to get efficiency to justify installation, and return on investment started to make sense first in cooler whites,” said Milz.

Cree RSW streetlampNow 3,000K streetlamps are easily available at >100 lm/W (mains-to-light – so that includes all the electronics). Cree’s RSW series (right), for example,  delivers 3,000K 80CRI white at 115 lm/W.

This means that warm white led lighting, with its inherently lower blue light fraction and inherently better colour rendering compared with cool white, can be had with enough efficiency gain over incumbent technology to pay for itself in a few years – even compared with high-pressure sodium streetlights.

That said, cool white will pay back quicker.

Where is cool white the best answer?

“We don’t have specific applications,” said Milz. However, for example, “cooler colour temperatures have higher visual accuity if properly installed:  people report seeing clearer if lighting is nearer daylight”,” said Milz. This is especially true for applications such as highways.

Sunlight has a 6,500K continuous spectrum and perfect colour rendering (CRI=100), while a 6,500K led is going to have a huge blue peak and will struggle to get CRI=70, so who knows how much of this is psychological?

“Most people have 3,000K or 2,700K light bulbs in their homes, so there is somewhat of a shock factor if your neighbourhood streets are suddenly lit at 5,000K,” said Milz. “But a highway interchange is not a place where people might be so concerned.”

Away from how-much-blue-is-ok arguments, is the question of glare – where led streetlights can be victims of their own technical superiority.

Streetlamps we are used to have a one big source and one big reflector, which together produce a puddle of light immediately beneath the lamp – a puddle which rapidly dims away from the lamp. The street beneath is lit in waves of dim and bright.

Led streetlamps have sources small enough that each can have its own complex light-distribution optics. And different leds in the same streetlamp can have different optics. This new situation allows streetlamps to be designed that can throw light a long way sideways – the street can be lit more evenly, and streetlamp poles can be spaced further apart.

However, it is only a small step from spreading light out to achieve even illumination, and spreading it so far that distant road users are inconvenienced or dazzled.

And even if road users are well served, the windows of dwellings in a converted street could receiving more light than before – which is something the AMA says should be thought through before selecting replacement led streetlights.

With improved beam-shaping comes the opportunity to waste less light so, not only is led technology more efficient than high-pressure sodium or mercury vapour lighting, but it allows extra gain through reduced spill.

“The goal is to reduce energy and maintenance, and increase uniformity, and make it more comfortable,” said Milz. “We have often reduced the number of lumens for a given application.”

And with reduced lumens comes less light at all wavelengths, so there might be less blue that before, he added.

Lastly, leds are dimmable and retro-fit time is a chance to add better controls – to dim streetlight after peak traffic flow, for example.

There were, and still are, bad led streetlamps on sale. And any streetlamp can cause trouble in the wrong place.

The message from the AMA, the LRC and Cree is: buy good led streetlamps, and buy appropriate led streetlamps.


Further reading

According to Milz, the US Department of Energy didn’t want a repeat of what happened with compact fluorescent lamps – a flood of low-quality products, low-quality light, and disappointed consumers – so it got involved early with solid-state lighting.

Part of this has been to commission reports on streetlight retrofits from Pacific Northwest National Laboratory.

PNNL produced this excellent document after the AMA report. Thanks to Cree for pointing this out.

The same PNNL author, Bruce Kinzey, has produced several detailed before-and-after reports on led streetlighting retrofits – such as ‘Restoring Detroit’s street lighting system‘.

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