Friday, November 25, 2005

Roads to Hell- the Sodium Menace

It's time for a quick refresher as to the basics of why we need to become a bit more conscious of the harm being done by the present trend towards ever-higher levels of street lighting. So pay attention and I'll thank you to not shuffle your feet. Basically, it's a re-write of Scared of the Dark, only a bit techier and with the addition of an impressive graph. If you've read that instructive piece already and feel tempted to skip any of the following, don't. For here be revelations...

Perpetual Dawn

Now you may feel the term "Light Pollution" smacks of pee-ceeness, but it describes an effect which deprives all of us from experiencing the blue-black tapestry of stars that is the night-sky in it's true colours. The sense of vastness, of the universal. The subtle frosting of silver moonlight across the landscape; natural order, a blue horizon. The wonder of the night sky as experienced by all generations before us. Our ability to appreciate natural beauty overwhelmed now by the dirty orange stain cast into the sky from every town and city in the land, from every out-of-town retail development and from every Industrial Estate. We are told it's all for our own good. We are told brighter lighting cuts crime. We are told it makes the roads safer. We are encouraged to feel vulnerable without it.

The majority of UK streetlighting utilises two types of sodium-discharge technology. Low Pressure Sodium, which produces a monochromatic deep orange colour and has been around since the 60's [but really proliferating in the 70's and 80's] and then the newer High Pressure Sodium type, characterised by a glaring peach-coloured light, usually much brighter than the LPS fitments they replace.

Roads to Hell

Sodium streetlighting produces it's characteristic orange glow as a result of electricity exciting the sodium gas contained in the tube-fitment. This reaction produces the light and is responsible for the garish colour. In terms of lumens produced, Low Pressure Sodium is extremely efficient at converting electricity into light. This is the beginning and end of the justification for their proliferation over the last thirty years. However; credible, peer-reviewed researches have now found that, because the human eye is extremely unresponsive to orange light at low ambient lighting levels, sodium streetlighting is actually APPALLINGLY inefficient when it comes to the production of USEFUL illumination. As the light level reduces from daylight to the twilight levels typical of those achieved by streetlighting, our eyes' colour-sensitivity shifts to favour blues and greens; colours not rendered at all by Low Pressure Sodium and rendered poorly by most types of High Pressure Sodium. Most of the orange light is wasted, because it's in the wrong part of the spectrum for the human eye. Researchers such as Dr. I. Lewin of Lighting Sciences Inc., Pennsylvania, have now shown that, in streetlighting tasks, orange light must be between 3 AND 15 TIMES the brightness of white light, in order to achieve the same effect on drivers' reaction-times. The difference really is that dramatic.

See Pat Mullins' excellent article, for a more detailed analysis of these researches.

This graph uses data from practical vision experiments; under four types of artificial light, subjects' reaction-times to hazard-stimulii were measured at various lighting-levels, indicative of those found in street-lighting applications. To make sense of the Luminance figures on the x-axis, 0.1 cd/sq.m is about ten times the level of starlight, whilst luminances above 3 cd/sq.m relate to daylight conditions. Streetlighting will be designed to give average luminances in the range 0.3-2.0 cd/sq.m. Lighting on major commercial roads in the U.S. is designed to give an average luminance of 1.2 cd/sq.m. [click on graph to enlarge]


Assuming this high 1.2 cd/sq.m luminance is provided by High-Pressure Sodium, the graph shows drivers' average reaction-time to be about 800 milliseconds. Under cool white Mercury-Vapour, only 0.4 cd/sq.m would be required to produce the same result. If Metal-Halide is used the results are even better, as this light can be tuned to produce it's peak output at the optimal colour-temperature for visual response. Despite this, MH [which is a development of MV] is rarely used for streetlighting due to a slight cost/maintenance disadvantage against HPS. These cost increases could be more than offset by electricity savings achieved through running lower wattages, but for the way in which Lighting Authorities pay for their electricity as a flat rate, regardless of how much they consume. So although, say, an 80 watt Metal-Halide would easily outperform a 150 watt HPS fixture in terms of effective lumination [and at about a third of the apparent brightness], there is, at present, no cost advantage to Local Authorities in reducing the amount of energy their streetlights are wasting.

Electric Starlight

Before the 1980's turned our night sky orange, public lighting tended to be of the Mercury-Vapour type: another gas-discharge lamp but, with mercury as the reactant gas, the light-output is a cool silver-white. Not entirely dissimilar to the colour of starlight, which made for a more natural and peaceful night-time aesthetic. There's still a few of these lights left in the villages, but, as they fail with age, replacements tend to be in the form of putrid peach-coloured HPS. Check that graph again and compare the performance of MV against HPS. The uncomfortable reality is that ugly high-intensity dusky-peach HPS revoltingness needs to be at City-Centre type levels of illumination in order to provide the same effect on drivers' reaction-times as MV can achieve with fifty watts of electric starlight.

In this picture, a modern semi-cutoff fixture houses a mercury-vapour lamp shining it's gentle and benificent light on a quiet corner in the beautiful Oxfordshire village of Childrey. Notable here is the appearance of the road surface. What can you tell from this? The picture was taken on a cold November evening, with ice starting to form on the wet tarmac. Tyre-tracks are clearly visible in this shot. Under the tangerine glare of HPS, there would be no such information available, as the road-surface would be reflecting so much of the wrong type of light that conditions would just look uniformly damp.

There are other serious safety issues concerning the proliferation of over-bright sodium lighting; the transition between lit and unlit areas causing the retinas of our eyes to expand and contract, for example. As we age, our eyes take longer to achieve this transition, so elderly drivers may experience moments of near-blindness when moving from a bright to a dark area, or vice-versa. Very bright HPS in full-cutoff lumieres can be uncomfortable to travel under, as there's a strobing effect with the sudden bright glare every few seconds. [It is advisable to lower one's sun-visor to mitigate against this.] And the driver's peripheral vision is affected, too. Because the roadway's lit up so bright, the boundaries of the carriageway assume less importance. And it's the boundaries, or what might leap out from them, that we address with our peripheral vision. Our peripheral vision is, you've guessed it, extremely unresponsive to orange light! So there's a cumulative effect akin to tunnel vision. The road's lit up bright as far as we can see, and it looks clear, so we drive fast. It is a dangerous optical-illusion.

Come to think of it, could there be, possibly perhaps, a Very Good Reason as to why we respond so well at night to the cool white light which replicates moonlight? Could it be that we have, in fact, evolved to suit our natural environment and that it is by design that our faculties work best in naturalistic conditions?

Heroes and Villains

So what about the influence street-lighting may have on criminality? Are the Home Office accurate in their assertion that "brightly-lit streets cut crime"? There appears to be a contradiction in their approach: they are rightly critical of the widespread misuse of highly-powered domestic security lights, making the case that these are counter-productive because, behind the glare, they create shadow and this can provide cover for miscreants. And yet, their own guidance to street-lighting agencies still runs along the lines of "more is better", regardless of how these high lumen-levels are obtained and without adequate investigation into the alternatives.

Their opinion is informed primarily by the Farrington and Welsh Report: Home Office Research Study 251 pub.2002: Effects of improved street lighting on crime: a systematic review. This is a piece of flawed meta-analysis: the researchers reinterpreted data from previous studies, some up to forty years old and badly-designed, in order to arrive at their [predetermined?] conclusion. Farrington and Welsh made exaggerated claims not backed by the authors of the original studies. In any case, these did not compare areas on a true like-for-like basis; low-crime control areas being compared to re-lit crime hotspots. The studies were not designed to take into account that statistical-effect known as "regression to the mean": that extreme results would be the result of chance variation. This is particularly relevant here, as incidences of criminal-behaviour are not isolated events: one individual can be responsible for a crime-wave. Read Paul Marchant's critique of HORS251 and wonder at how anyone with a grounding in statistical-modelling could have ever taken it seriously.

Also critical of HORS251 is Dr. B. Clarke. In this 2002 paper written for the Astronomical Society of Victoria, he debunks the "brighter is better" view, arguing in favour of cutting low-level criminality by reducing artificial lighting levels. Consider when you last saw unruly yoof largeing it up in an unlit area... or a graffiti-artist working without light. Bright artificial lighting is an urbanising influence. Noisy environments will attract noisy behaviour.

From UK Parliament Select Committee on Science and Technology Seventh Report:

"... the Committee notes that in the August 2003 electricity blackout in parts of North America, the feared crime wave did not materialise. Similarly, in 1998, Auckland was victim to a black out lasting several weeks. A police inspector was reported as saying "It's almost a crime-free zone. The normal levels of muggings, violence, fights, burglary and robbery have just not happened.""

Now, what good would that be to the politician whose livelihood is dependent upon instilling a sense of fear amongst the voters before being seen to address said fear with a "solution"?

"When all our roads are lighted
By concrete monsters sited
Like gallows overhead,
Bathed in the yellow vomit
Each monster belches from it,
We'll know that we are dead."

From "Inexpensive Progress" [1966] by John Betjeman

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2 Comments:

Anonymous coolight said...

I'm not sure if you already have this link to Sam Berman's new paper on Human vision and color perception, so here it is. That paper came from Yahoo Outdoor Lighting Group

Also there is a blog for Outdoor Solid State Lighting

5:11 am  
Blogger Basil Brown said...

Thanks for the info. More evidence supporting the case for blue-white light.

6:35 pm  

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