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Street light
Today, street lighting commonly uses high-intensity discharge
lamps, often HPS high pressure sodium lamps. Such lamps provide
the greatest amount of Photopic illumination for the least
consumption of electricity. However when Scotopic/Photopic
light calculations are used, it can been seen how inappropriate
HPS lamps are for night lighting. White light sources have
been shown to double driver peripheral vision and increase
driver brake reaction time at least 25%. When S/P light calculations
are used HPS lamp performance needs to reduced by a minimum
value of 75%. This is now a standard design criteria for Australian
roads.
Dangers of street lights
There are two optical phenomena that need to be recognized
in street light installations. The loss of night vision because
of the accommodation reflex of drivers' eyes is the greatest
danger. As drivers emerge from an unlighted area into a pool
of light from a street light their pupils quickly constrict
to adjust to the brighter light, but as they leave the pool
of light the dilation of their pupils to adjust to the dimmer
light is much slower, so they are driving with impaired vision.
As a person gets older the eye's recovery speed gets slower,
so driving time and distance under impaired vision increases.
Oncoming headlights are more visible against a black background
than a grey one. The contrast creates greater awareness of
the oncoming vehicle.
Safety
A misconception is that installing street lights will automatically
make streets safer and reduce crime, so political pressure
can be a major factor in installation of street lights. Untrained
officials often assume that if some is good, more must be
better, and install the brightest lights possible. Misuse
of street lights can cause accidents, and crime lighting is
an entirely different type of lighting than used for automobile
navigation.
The future
In countries such as South Africa, street lamps are burning
an average of 4000 hours per year. Considering that the average
wattage of a lamp is around 150 watts, considering that Johannesburg
contains about 140,000 lamps, Johannesburg spends around 560
mega watt hours. Considering that producing 1 kWh implies the
emission of 340 grams of CO2 (average in Europe), the streetlights
of Johannesburg are responsible for the emission of 190 tons
of CO2 in the atmosphere per year.
Technologies and techniques now exist to:
save electricity without impacting the lighting level perceived
by citizens
directly use renewable energy, through generators integrated
in the street light itself
Amongst the key technologies and techniques :
Power Regulator can be installed in the cabinet that supplies
electricity to the street lights. Power regulators enable
electricity regulation and can usually dim the lamps in the
darkest time of the night. Unfortunately, power regulators
are less efficient than electronic ballast for several reasons.
The length of the supply links, different technologies of
lamps that cannot be dimmed at the same levels. Power Regulators
can provide electricity savings around 20%. There are other
systems to control conventional HPS ballasts that can cut
power use by 40%. Fortunatally dimming control in LED lights
it much simpler then in HPS lights. Dimming is also directly
proportional to energy saving; ie 50% dimming results in a
50% reduction in electricity.
Street lights with integrated, small wind power and/or solar
power generators. This concept may be combined in designs
using emerging technologies such as high output white LEDs
and supercapacitors. Such designs may represent an option
for new installations. Cost savings come not only from savings
in energy, but also from simplified installation due to the
fact that they do not need to be connected to the electricity
grid.
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