Friday 16 October 2009

LED street lights

At first, this looks like just yet another of the many cycle paths in Assen. A normal cycle path with normal lighting on it to increase the feeling of social safety after dark.

Nothing special at all ? Take a look at the sign.

There is currently a trial going on with several different types of LED street lighting.

The sign tells how these lights use 30% less energy and reduce light pollution.

In this case, the lights have a greenish tint. They use two colours of LED, green and red, meaning you can make out colour quite well. They do a good job of lighting the path.

Also note that they are more directional than the lights which came before them. This means less light pollution.

It's all part of the strive for energy efficiency locally. Assen is commited to be carbon neutral by 2020. Many Dutch cities have made similar pledges.

This photo shows a different cycle path, just a few hundred metres from home, which has the same greenish LED lights. I took this photo on my way to work.


dr2chase said...

Okay, so speaking as an LED weenie, why green and red? You can get pretty good, and somewhat more efficient, light from a modern so-called "neutral-white" LED. I use some in my kitchen, if there is any problem with the light, it might have a hair of a greenish tinge, but nothing like that in the photo here. The spectrum is quite civilized. (For anyone who is not quite an LED weenie, most "white" LEDs that you see in bicycle lights are "cool white", and tend well to the blue, much like an arc welder. They are trading color rendering quality for lumen quantity.)

David Hembrow said...

dr2chase: I have to admit that I'm not a great fan of the funny colour as such. There are also white LED street lights at other locations in the city and I suspect over time that these will be more popular.

The green and red mix at least gives some colour rendition and as a result it's a lot nicer than orange sodium lamps. Why green and red ? Because these colours can be produced directly by the LEDs with greater efficiency than white light.

White LEDs are less efficient as they actually are blue LEDs with phosphors to provide the other colours. Often just a yellow phosphor is used which leads to quite uneven light production from some LED lights. Also, they use a lot of energy producing blue light, which the human eye is not all that sensitive to.

They are still improving, of course. As well as bike lights, we have LED lighting in our home, too. An older light is rather blue, but newer "cool white" ones produce a very pleasant light quite difficult to distinguish from incandescent lights.

dr2chase said...

Except that it doesn't seem to work the way you say it does. If you consider the latest from Philips/Luxeon, the "Rebel", green and white have equal efficiency, but red has less. So as soon as you mix red in there, you lose. And, for whatever reason, the latest (neutral) white from CREE is about 15% brighter.

Maybe they did it this way to avoid blue, maybe they preferred a European LED supplier, where there is less advantage to neutral white.

Anneke said...

Also, night animals aren't as affected by red and green light as whitre or yellow.

Rob said...

I've been fitting 'warm white' LED downlighters in the house which look just the same colour as halogens to me. I've got older 'cool white' ones in the workshop which look more photo-flash in colour.
But I'm guessing for street lighting that the directional nature of LEDs is helpful - the sodium lighting in these parts seems to be aimed every which way. I'm sure they could halve the power requirements by using 180 degree reflectors and lower power bulbs...

Rob said...

@Anneke: 30% of men have a degree of red-green colourblindness so may also not be effected by the lights either! Could be an interesting experiment to study which lights have the higher accident or near miss rate!

dr2chase said...

(I thought I replied to your reply, maybe I forgot to finish.)

As I read it, despite what the Wikipedia link claims, white LEDS are in fact more efficient than a green+red combo, at least in terms of how well it works for our vision. If you look at the product data sheets for modern power LEDs (e.g., CREE XRE, or Luxeon Rebel), you find that the green LEDs are at best equal in their lumen output, and the red LEDs do worse. The brightest neutral white LED from CREE (93.9 lumens, 1.155W, 81.3 lm/W) are more efficient than any green LED that I know of (best is Luxeon Rebel, 80 Lumens at 1.1025W, 72.3 lm/W). None of the high-end white LEDs that I have seen is playing the blue+yellow game; their "white" looks white.

Note that a "lumen" is a human-weighted metric with its highest weight applied at green, so the metric here already accounts for our ability to use the light, and gives bonus points to green. Green LEDs, as photon emitters, are not especially efficient, but we are very good at seeing them, if we don't mind the hue.

Because white LEDs emit radiation in other colors that we don't see as well as green, yet still obtain the same (or better) lumen rating, they are emitting more light energy, apparently contradicting the conclusions about phosphor inefficiency in the Wikipedia article you cited -- at least in the case of green. I suspect that the article is talking about the most efficient emission of photons by an LED (either red or blue, but not green) and comparing that to phosphor photon-emitting efficiency.

However, further down in the same article, they mention other issues with "white" LEDs that might be relevant, namely "blue pollution" and "blue hazard", and both cool white LED (which they mention) and neutral white LED (which they do not) have the same blue peak (580 nm) of about the same power.

Sorry to go off on this tangent, but I spent a lot of time learning about this, and when Someone is Wrong on the Internet, I need to correct them.

David Hembrow said...

dr2chase: If I could find an email address for you I'd have replied in private. Please, please, no more "LED weenie" stuff on this blog ! It's really not what it's for. If you want to reply, my email address is on the RHS of every page on my blog.

Neither of us are party to the reason why the green colour was chosen, nor when the decision was made, I think there is little point in speculating about exactly what parts were available to the designer, what constraints they faced etc.

It's perhaps worth pointing out that there is much more green than red with these lights. Enough red was added merely to give some colour perception.

There is no doubt a good reason for the design. I don't for one moment believe it is because they "preferred a European LED supplier". For a start, there is no company more Dutch than Philips, who are the source of the Luxeon LEDs which you refer to.

As for me, I've already said that I personally prefer the white colour. However, if there is a benefit to the green...

I agree that Luxeon have a great series of LEDs. I also use them. There are seven of them in my Mango !

Duncan said...

Hi, just to let you know that Peterborough have been using solar powered LED lighting for their cycle paths although these are 'route markers' rather than streetlights. (sorry about long web link - only available from Google's cache it seems:

David Hembrow said...

Duncan: I've seen exactly the same thing in Cambridge. In fact, I took a photo of them. Unfortunately, it's not actually very good.

The problem with these is that they are in the pavement shining upwards, so don't actually light up the surface you ride on. The result is that they tell you nothing about potholes, ice etc. or even whether there is an obstacle on the path.

They also don't actually tell you where the edges of the cycle path are. The photo shows a junction where I fell from my bike as the lights on the path appeared to indicate that it was possible to cycle straight on when this wasn't actually true.

Cyclists need proper lighting which illuminates the way from above - just the same as drivers get. Not this second rate and cheap treatment.

dr2chase said...

It's worse than you describe -- the (relatively) bright lights in the pavement, make it impossible for you see the markings. That's pretty appalling. Where they (bluish) white? That would be worst of all -- amber, at least, does not take out your night vision quite so efficiently.

Rob said...

I love the design of that junction - stopping the cyclist with turning traffic from behind appoaching at, what, 120 degrees?! All 'facility' designers should be forced to go spend a month cycling in the Nederlands to see how it is done properly.