High and Low pressure sodium lamps

Hi,

I was writing an article on LP earlier and started to get confused about whether High Pressure Sodium (HPS) was better or worse for light pollution etc. I sent the following to the IDA list and the replies are included in the followups.



Hi folks,

I was wondering if any of you could point me at information regarding the
use of low pressure versus high pressure sodium lamps? Dublin City Council,
Ireland, is currently replacing all its low pressure streetlights with high
pressure ones as they need replacing, and I want to build up information on
what this means for us.

Furthermore the high pressure lamps that are being used are semi-cut-off,
using a bowl of glass under the light rather than flat glass under the
light. From phone conversations with some of the people involved in this
I've been told that the use of FCO with flat glass would require more
lights and a lesser distance between posts. The distance isnt much of an
issue when you are replacing existing fittings, but this argument is being
used for some new developments around the country and I want to have my
facts straight before talking to people about it!

I'd be interested to hear any opinions you folks have on this, in
particular any studies, preferably in Europe, that can put figures on the
use of FCO -v- semi cut off in terms of cost, pole separation etc.

Cheers,
~Al
--
International Dark-Sky Association - Ireland Section
www.irishastronomy.org/ilpac

LPS are big lamps and mostly very expensive to develop a good mirror that
increase the performance. The result is that there are so good as no good
fixtures for LPS on the Europe market. HPS are a lot smaller and so there
can be developed better mirrors and when you take the complete application
of performance of the fixture together with the performance of the lamp, in
a good fixture the efficiency and performance of the HPS is in most cases
better then the LPS. Because there are better fixtures for HPS it is mostly
better for light pollution. The light of HPS is more difficult to filter
out, but when doing observations of variable stars you are not able of using
filters, CCD pictures you can't use a lot filters... And people that want to
do cheap astronomy has better a good sky then a polluted sky they can
filtered out.

Pole distance is mostly lower with flat glasses. Flat glasses has in a lot
of occasions also bright light on one point and less light between, and a
higher reflection when you want to have a specific minimum luminance at the
site of the regions to lighting.
Bowls glass under the fixtures you can increase the distance between poles,
but lose again a lot of light by reflection in the glass it self. Refractor
glasses are a disaster and can better never be used.
Sagged glasses can be also used to increase distance between the poles in
the same way as the bowls. The reflection in the glass with upwards lighting
is there but quite nothing and there is a better equal spread over the
lighted zone with result in less reflection. The part still losing upwards
by the sagged glass can be covered by a HDG inside the fixture or louvers
outside the fixture. The HDG give also a better equal spread of the light on
the length of the road and less upwards lighting then without. Especially
there is less lose of lighting in beside the road in the wide of the road.

When using the right wattage for the right luminance of the road there (not
overpowering) then the sagged glass under a fixture with the right mirror
and the bulb in the right position mounted together with a HDG or paralumes
or louvers can be in some occasions the solution with the less of light
pollution and a maximum of distance between the poles.

The sagged glass may be also not to deep sagged. Only slightly. When more
deeper, you not win in pole distance but increase the light pollution a lot.

More concrete on fixtures available on the Belgium market. Choose for the
Safier of Schreder with the HDG, or the Iridium of Phillips with louvers and
both with the sagged glass. The Iridium sagged glasses is the best sagged
glass available, but the HDG of the safier is better then the louvers. So
both are excellent choisses where the one is better for some specific
applications and the other for some other.
There is also the Aurora distributed in Belgium by Techniligth, but a German
fixture of I believe Industria. The HDG is a development of our group and
bring in production by Schreder in the Safier, but we make also some that
are implemented in this aurora.

I send you pictures, measurements and so on of a pilot project and the
results it gives on your private e-mail.

This means not that sagged glasses are always better; they can be better in
some occasions.

Regards,

Friedel

The frequently heard claims that closer spacing is always required with full
cutoff fixtures is sometimes true under extremely limited conditions but it is
not usually true for most cases when the lighting system is designed
properly. It depends on the lighting standards that must be met and the configuration
of the area that needs to be lit.

The semi-cutoff cobrahead installed at standard mounting heights, spacing,
and setbacks typically used for most municipal and residential lighting will
usually exceed the maximum glare allowance (veiling luminance ratio), whereas
only the cutoff and full cutoff fixtures can meet that vital criterion under
these conditions. See the two studies in the Files / Research folder of DSLF that
compare similar luminaires in the four IES cutoff classifications under
otherwise identical conditions to see the results based on the American National
Standard Practice for Roadway Lighting IESNA/ANSI RP-8-00. Visual quality is
significantly better with the full cutoff.

I do not know what lighting standard is applied in Ireland currently, but it
is probably either BS5489 or CEN prEN 13201-1 opposed to AASHTO or IESNA
RP-8-00. If the CEN standard presides there will be no problem meeting the
lighting standards with well designed full cutoff fixtures like the Vectra with flat
glass lens from WRTL Exterior Lighting, Ltd., or the GE M250R2 and 150 watt
HPS lamps at 10 or 12 meter mounting heights. They did it on the M65 in
Lancashire without encountering any problems. If more vertical illuminance is needed
the Vectra is also available with a shallow clear curved bowl lens that does
not have refracting prisms and it is an efficient luminaire marketed in the UK
that meets the IES specification for Cutoff (16% maximum lamp lumens above
the horizontal).


>> I'd be interested to hear any opinions you folks have on this, in
>> particular any studies, preferably in Europe, that can put figures on the
>> use of FCO -v- semi cut off in terms of cost, pole separation etc.


Uniformity is not improved by any appreciable degree when using semi-cutoff
fixtures since they put LESS light on the ground where it is needed and more
light where it is not needed lumen for lumen when the wattage of the fixtures
remains the same between semi-cutoff and full cutoff cobraheads. The downward
efficiency of light sent *to the task area* offered by a well designed full
cutoff fixture like the GE M250R2 is far superior to its semi-cutoff counterpart
in the same luminaire family. Spill will still occur beyond the road limits,
but not nearly as much as with a less stringent cutoff classification.

One characteristic with the way luminaire efficiency is currently rated often
results in confusion causing designers and engineers to stumble in their
assumptions when reviewing the utilization curve and photometric performance as it
relates to total light emitting from the luminaire at beneficial angles. One
really cannot tell how the fixture will perform until it is modeled in a
lighting design program that has 3D evaluation capability.

Light has no trajectory, therefore what light goes up never comes down.
Light sent above the horizontal (31% total lamp lumens is allowed with a
semi-cutoff) is pure waste that serves no useful purpose other than to create more
skyglow and cause more significant light trespass impacts. It is beneficial to
the utility company because more watts are needed than with efficiently shielded
fixtures that put more of their light where it is needed. The common
argument claiming full cutoff only sends light down rather than sideways is absurd
because all luminance emitting from the luminaire at angles greater than 0
degrees above nadir is vertical in nature until it strikes the ground plane, at
which time it then becomes horizontal luminance.

Although LPS (i.e., SOX) lamps are more energy efficient from a photopic
adaptation lumen per watt standpoint, their monochromatic characteristics do not
render any colors at all and the driver's peripheral vision when dark adapted
to mesopic levels are not as sensitive to the wavelength offered by LPS. The
eye responds more favorably to HPS under those conditions, although the lamp
efficacy is about 40% lower than that for LPS (SOX) lamps. CEN lighting
standards recommend not using lamps that offer a color rendering index (CRI) below 20
because safety marking colors cannot be identified properly. The CRI of HPS
lamps is 22 and they do support that important criterion. Although filtering
the spectral qualities from visual telescope fields might be a bit more
challenging with HPS opposed to how it is with LPS, it is not nearly as damaging as
the spectral qualities offered by metal halide with regard to skyglow and
increased glare sensitivity. Additionally the HPS equipment is usually not as
expensive nor is it as bulky and heavy as most LPS fixtures typically are. Hope
it helps!

Clear skies and good seeing,
Keep looking up!

Cliff Haas
Author Light Pollution Awareness Website (LiPAW)
members.aol.com/ctstarwchr
www.crlaction.org

Member: IESNA, CRL, NELPAG, AARP

I recently spoke to a representative of the ESB who informed me that when designing lighting on roads, FCO luminaries require approx 20-25% more poles than semi-cutoff.
BS 5489 is, from my understanding, the standard used in Ireland. There is also another design standard specifically used for lighting design on roads - TD 30/87. However, all this document basically does is refer to the BS (British Standard).
One interesing thing I came across in the BS is the reference to astronomical observatories. There is a reference in this document which recommends the use of cutoff lighting in the vicinity of suchstructures (I can get the exact wording if anyone's interesting). This may be useful for those who have an observatory whose floor area is less than that required for a structure to warrant planning permission by your local authority - typically 25 sq metres i.e. 16.4ft x 16.4ft. If you are such a person, it may be worth having a chat with your local authority area/town engineer stating that you have such a structure and request/ask if they would be willing to replace all old lamps with cutoff versions whenever they need replacing in the vicinity of your house. Should be intersting to find out what reaction you get.
Michael

Interesting stuff....

Now, I wonder what they use for lighting near airports? The lighting I've seen seems to work really well at keeping light from going skywards and I presume distracting pilots coming in to land. I'm sure it's not cheap either and this is the crux of the matter... mmmm.

Anyone know what's so special about this type of lighting?

[quote="John O'Brien"
Now, I wonder what they use for lighting near airports? The lighting I've seen seems to work really well at keeping light from going skywards and I presume distracting pilots coming in to land. I'm sure it's not cheap either and this is the crux of the matter... mmmm.

Anyone know what's so special about this type of lighting?[/quote]

I went on a fact finding mission to Dublin Airport last night on the way home from my girlfriends.

The lighting within the airport looks mainly to be semi cutoff HPS, though there are some FCO lights around. Eiter would be enough to cut out glare to pilots.

Outside the airport we have a complete mix most roads where there is lighting use orange LPS lamps. While the motorway in parts at least uses FCO lamps on very high posts. There are some semi cut offs where the council (I presume) have replaced the LPS lamps.

In conclusion the lighting around the airport is not as good as you would expect, though better than average environments. It is not in my opinion as good as I have seen on some Motorway intersections which exclusivly use FCO.

Cheers,
~Al
--
Deputy Chair,
ILPAC