[Darksky] Re: [Ida] White LED
Fri Nov 9 12:19:34 CET 2007
(was Re: [Ida] lighting engineers meeting in Germany)
(sorry, the letter became long at last... I will post it to the
open-access list http://amper.ped.muni.cz/darksky as well to make it
readable without passwords etc. any time)
> > The aspect of the heat and led's is a known problem. I know some
> > researchers working on to solve that. You can have further leds in all
> > color so there is no reason to use not yellow LEDs in stead in my
> > opinion, but the trend is currently more and more white lighting.
This is the trend of the business whose aim is to change night into day.
There are heavy-weight arguments why this aim is a menace to the world, a
menace which is to be averted from it.
Faint yellowish (or even orange) light, akin to oil lamp or candle-flame
colour, is what people really like for evening -- in any case where then
can choose for themselves.
> Is any research being done to construct or use LED's that have similar
> spectra to high pressure sodium lamps? That is, are predominantly
> orange but have blue and green components to appear to illuminate in
> white light.
> That would allow 'white' lights to be constructed that would have a
> limited effect on insects and human melatonin production.
LEDs are usually used in groups, so there is no need to have a single LED
with an optimum spectrum. Pure yellow (or amber) for late night, dimmed
deep below the early evening values (5x or even 10x, regardless of the
current stupid standards), are the solution. When dimmed, the problem of
heating the semiconductor ceases, and the LED lifetime becomes almost
endless. LEDs can be dimmed down to 0.1 % of their nominal output easily.
For an early evening, when perhaps more colour recognition might be wished
for places where people like to appreciate each other's evening robes,
it's easy to switch on white LEDs in addition to the amber ones. Remember
the Chris's LPS pages (as accessible from
Christian B. Luginbuhl's Homepage
URL: http://www.nofs.navy.mil/about_NOFS/staff/cbl/ )
I see no near future of LEDs to achieve 1 cd/m2 road luminances (which are
a stupid recommendation anyway, it would be much more reasonable to have
strict and often verified legal limits to car headlamps luminance and
luminous intensity, as visible by drivers and pedestrians from the
opposite direction; even dimming the red back lights from day values to
night ones (10x perhaps, without fog) would help, and of course, imposing
lower speed limits for non-daytime -- these would reduce CO2 emissions and
However, for 0.6 lx to 2 lx illuminance of narrow paths LEDs should be the
best choice already.
It needs another approach, however, than HID. Relative spacing
(distance/height) of LED luminaires should be perhaps 3 at most, so that
directly visible light from them goes steep down enough (glare is not so
painful then). The tiny luminaires might be attached to a strip hung over
the street (remember cables over most downtown Vienna streets) rather than
to individual poles, then the additional luminaires bring little
additional cost and no additional obstacles to pedestrians and cyclists.
The strip might become a copper sheet around luminaires, to conduct heat
well away from LEDs.
Andrej is true that it's maybe upon us to make first tiny and light
luminaires of such type... The problem is, that rather probably, special
lenses might be needed for that, not just the common Luxeon ones making a
cone with some width. It should be possible to achieve almost any light
distribution with a properly squeezed or otherwise deformed cone lens of a
similar type like the Luxeon one. If the lens would be very sophisticated,
it might even reduce the luminance of the luminaire, so that it would be
not so terribly glaring from the proximity (reduce the luminance: magnify
the space angle from which the needed luminous flux comes).
LEDs with precious optics are a solution to get below 0.1 lx window
illuminance even for the ground floor, close to the lit pavement (it's
really possible to achieve almost no direct light to the windows, even if
perhaps allowing some light to the adjacent facade...) -- Wim is true that
it's an almost impossible task with discharge lights close to the wall.
White light late at night, this is something to be tolerated when the
ground illuminance does not exceed winter full moon levels (0.2 lx).
For road lighting, LPS over centre of the road is still the best solution.
White light is a nonsense: in the worst weather (falling snow) it enhances
eye (and brain) fatigue, as the eye peripheral vision inevitably notices
movement on the sides. This has been found as (the only, as far as I know)
explanation for a well-known preference of yellow light by drivers under
snowfall and fog (I could dig out the reference somehow, if needed). The
other reason concerns old drivers: spectrally pure yellow light disperses
less in their eyes, so that the contrast is much better than with white
light. The whole science behind road lighting is to change, to be made
primarily for drivers over 70 or 80 (or even 90, nothing uncommon within
20 years) rather than for young eyes as up to now. Young eyes need no
stationary road lighting... and tolerate almost any glare.
jenik (with no-more-young eyes, as >50).
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