Re: LED streetlamps

[Jan Hollan <jhollan@amper....muni.cz>]

I just returned from a short holiday and went through lots of discussions
on OLF and DSLF.

I don't think that LEDs could replace discharge sources for strong
illumination of large areas within next 30 years. However, are there some
niches where they could win, soon and justifiably?

I see one such niche:

       lighting of narrow paths.

Proportion of light emitted from a standard fixture which misses these
paths will be probably three fourths or more, if pole height is
substantially larger (say, 2.5 times) than the path width (see e.g. new
tables for DW Windsor fixtures within
    http://amper.ped.muni.cz/light/ies2/dwwindsor/narrow_path/ ).
 This may be improved by using grids, as pioneered for outdoor lights by
Friedel and his collaborators in Flanders, but not tremendously with
common, shallow fixtures.

Using LEDs having a 30 degree light cone, the wasted light proportion may
be kept below one fifth probably. So, the inferior luminous efficacy of
LEDs may be as if improved three times in such circumstances. Considering
the usual proportion of light which emanates from the luminaire being
about 0.7 of that emitted from the discharge tube, we get a factor like
    0.8 / 0.25 / 0.7 = 4.6
in favour of LEDs. This is however still not enough.

A further boost may come from the fact that luminance of the path as lit
by discharge sources is often very uneven if the poles are farther than
some three heights from each other. Standards speak mostly about
average luminances or illuminations. This is however a wrong metrics for
most purposes. The really important values are minimum luminances. Avoid
any luminance over 1.4 the minimum one and you probably spare lots of
light (and lots of pollution) again.

With LEDs, almost any luminous intensity distribution of the luminaire is
achievable, and a comfortable 75 degrees cutoff may be obeyed. Low glare
and excellent uniformity of luminance are favourable to using lower
minimum luminances than those contained (often just implicitly) in
standards. This is a third possibility to get an advantage for LEDs.

LED lifetimes are larger when they do not run at their full strength.
Apart from being switched on and off, they can be dimmed continuously to
one per cent of light output.

The only remaining obstacle is the price of a LED luminaire. I hope it may
be competitive in case that small luminous flux is sufficient and not so
many LEDs are needed. Replacing HID with LED is very different from
replacing incandescents by HID: lumens per watt are low for both LED and
incandescent sources. There is no danger of having more light from the
same power. Using LEDs instead of discharge sources should result in
reducing the amounts of light to the environment at least five times (and
perhaps hundred times when lighting during the needed moments only).

On the other side, the same excellent lighting might be surely achieved
using a faint discharge source and a generously dimensioned fixture with
very sophisticated, several-stage optics. I'm looking forward at such a
competition of technologies. Minimum pollution of the night, minimum
electricity consumption and still adequate task lighting.


My further remark concerns colour:

If luminances over 0.03 cd/m2 are used and photopic vision works quite
well already (as for colour images in winter full Moon light, when we
discern the colours), the yellow LEDs should be sufficient mostly (it
applies to 1.0 lx or more of path illuminance). The adverse impacts of
such longwave light are much smaller than those of white or unfiltered
high-pressure sodium light (with a non-negligible shortwave component).
Environmentally, it is not much inferior to LPS light.

White LEDs for outdoor use should be reserved for
      illumination levels <= 0.1 lx,
 those at or below the full Moon ones. Still enough light for cyclists and
pedestrians with night-adapted eyes.


A personal observation:

Broad-spectrum yellow is pleasant and gives surprisingly good colour
rendition. Since the Graz conference in October (Eva Schernhammer was
among the key lecturers), I have converted most of our night indoor
lighting to pure yellow one (glass paint on incandescent bulbs, yellow
foil on CFLs, really yellow linear fluorescent tubes -- they exist, being
twice more expensive than those with standard fluorescent coatings). If
you have no comparison, you almost don't notice the hue. Colour of seas in
maps is perceived as bluish, thanks to the interpretation by the brain,
even if just green remains in fact. The only task which cannot be done in
this light is making proofs of appearance of images in a preprint of a
colour magazine. I believe that up to 5 lx of eye illumination by such
light does not interfere with our body circadian rhythm (in a sense of
diminishing the natural nighttime production of melatonin), being harmless
in a sense.

My wife Yvonna Gailly noticed another advantage of faint yellow lighting:
in spite of open windows in summer, no insects are attracted inside. All
years before, we could have either open windows, or lights on. Using these
two comfort measures together used to result in a discomfort. Not any
more!

wishing you natural levels of melatonin all the night,
and no stronger outdoor lights than polar summer mesospheric clouds
   (see the incredible  http://www.serifone.de/17_7_2005.html)

 jenik

PS.
 How a dream-like LED luminaire could be made? Imagine a sheet covered
with LEDs, which could be cut simply by scissors. Both sides of the sheet
would be conducting (covered by a protecting layer, which could be scraped
away), being a plus and minus pole. The sheet could be deformed
plastically. Luminous intensity in a given direction would be given by a
number of LEDs pointing there (a yellow solar-powered LED I've been using
had just one candela uniformly within a 30 degree cone and with a sharp
cutoff toward larger angles). Shape the piece or pieces of such a sheet as
needed and put it under a sufficiently large opaque cover to achieve a
complete 75 degree cutoff... Apply any current up to the allowed maximum
one (## A/m2 of the sheet) to get the needed lighting.