HomeMy WebLinkAboutA017 - Letter from Catherine Scott, concerns about lighting C'w 1 0 2004
Low-Pressure Sodium (LPS) 4 _
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Low-pressure sodium lamps are widely used in parts of Europe and elsewh9;W*A11 '-- -^-
some American cities, particularly those near active astronomical research facilities and
those especially concerned about energy issues and municipal electric bills. The light
produced by LPS lamps is nearly monochromatic at a wavelength near 589 nanometers.
Though the eye is very sensitive to this wavelength(leading to the high efficiency of
LPS),the eye cannot distinguish colors when LPS light is the only source available. Low-
pressure sodium lighting is favored where energy consumption and costs are a major
concern and where color discrimination is either not needed or is supplied by other
lighting. Advantages include the highest luminous efficiency and lowest energy use, low
glare associated with the large lamps, good visibility and low scattering, minimal effects
on insects and other wildlife, and lack of hazardous mercury wastes. Disadvantages
include the lack of color rendition, shorter lamp lifetime and higher lamp replacement
costs compared to UPS, and large lamp size in the higher output lamps.
Low-pressure sodium could profitably see more wide-spread use for what is called "Class
2" lighting; that is lighting situations in which the perception of color is not necessary for
the lighting to be effective. Such a recommendation is made in this Handbook with
particular emphasis for areas near astronomical observatories, but it can be extended to
communities independent of the existence of nearby astronomical facilities. LPS lighting
has many advantages: for a given amount of light, energy consumption and costs are low
with LPS systems; when matched for maintained illumination levels, overall operating
costs of LPS systems are lower or comparable to systems using HPS and other lamp
types; LPS light generally has low glare and provides excellent visibility, especially to
the aging eye and under poor atmospheric conditions like fog; the yellow color produced
by LPS is highly visible at lighting levels used in outdoor lighting, but it is less efficient
at producing sky glow because of the lower luminance of the sky and the decreased
atmospheric scatter suffered by the yellow light; the yellow color of LPS light also
interferes less with many living organisms, such as turtles and insects; and finally, though
this characteristic cannot be appreciated by the naked eye, LPS lighting pollutes only a
limited portion of the visible spectrum,allowing other portions of the spectrum to remain
relatively uncontaminated for astronomical research.
Parking lot lighting is specifically provided to allow automobile drivers to see pedestrians
and other hazards and to provide pedestrians visibility to navigate and avoid any hazards
once they exit their automobiles and the abilitv to find their autos when they return.
Under pure LPS lighting, there is almost no color discrimination, occasionally making for
puzzled searching for a car of a particular color. In practice the hindrance is small,
because we use other information to identify our cars such as make and model and
whatever we left on the seat. The energy advantage of LPS in parking lot lighting is
substantial, in general even larger than in roadway lighting, amounting to as much as a
50%reduction when compared to metal halide systems.
It is recognized however that LPS continues to suffer a poor image within the lighting
industry in many areas. Many designers will not willingly consider its use. This lack of
support stems principally from the poor color rendition and the impression among some
that LPS light provides for inferior visibility compared to broad-spectrum sources such as
HPS and metal halide. There has also been considerable misinformation circulated
concerning LPS lighting, and the damage done to its image has been substantial. More
research is needed in some aspects of LPS lighting, such as lamp lifetimes, optimal
operating conditions, and system operating costs. But the advantages of LPS are
sufficiently well documented to justify the more widespread use of LPS lighting,
particularly in these days of increasing energy consumption, increasing energy costs, and
concerns for carbon dioxide production and global warming.
.14: LPS/Other Lamp 'hype Mix for Color Rendition with LPS Energy
Savings
Though service station canopy lighting could be principally LPS (color perception is not
regarded as necessary to pump gasoline), very good color rendition can be obtained with
a siiiall amount of broad-spectrum lighting such as fluorescent. Studies have shown(see
Boynton and Purl, Lighting Research and Technology, 21:23, 1989)that as little as 10%
white light added to LPS light permits nearly normal color perception. This is supported
by experience in the Tucson and Flagstaff areas, where LPS lighting is extensively used,
and where some luminaries combining both LPS and fluorescent lamps have been used,
even at an automobile dealership.
Information from The Outdoor Lighting Code Handbook, International Dark Sky
Association
Submitted by Catherine Scott