To the ordinary person, a street light is
made up of a lamp in a box with some wiring and a glass bowl to
let the light out.
However, street lights are much more complex
than that, as each part can often be customised to suit the
customer's requirements.
This is a brief guide to the parts of a
luminaire, and will make the
examples list easier to
understand!
Discharge
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Mercury vapour light
An earlier type of high-intensity discharge lamp, mercury lights were popular
because they were bright, efficient and long-lasting. However, their main
drawback is that the bulb becomes gradually dimmer over time but uses requires
the same amount of power. This would cause the light to develop a greenish
tinge.
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Low pressure sodium vapour light (SOX)
This type of lamp produces the well-known yellow glow that is synonymous with
street lighting. Housed within a glass discharge tube, a sodium block is
immersed within a small amount of neon and argon gas. Once the lamp is turned
on, a dim red glow is emitted whilst the sodium warms up and begins to vapourise.
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High pressure sodium vapour light (SON)
Smaller than their SOX counterparts, high pressure lights contain additional
elements (such as xenon) to help create a whiter glow. Once warmed, the lamps
emit a pinkish-white light. Although they are not generally as bright as SOX
lights, the whiter glow makes differentiating between colours much easier.
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Metal Halide light
A modern version of the high pressure sodium light, the metal halide luminaire
emits an intense, virtually pure white light that is becoming increasingly
popular. The only downside is they don't last as long as traditional high
pressure lights.
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LED light
A new type of street lighting that is being trialled in some residential areas,
LED lighting offers an reliable and longer-lasting alternative to sodium
lighting. Even though they last longer, they are very expensive despite the cost
of production falling. However, it is expected that more councils will install
them as they become increasingly cheaper.
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Mounting
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Side entry
The most common type of mounting, where the light fixing is located at
the side of the luminaire. This allows for different lengths of bracket arm to
be used depending on where the light is to be situated.
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Top entry
The fixing point is located at the top of the luminaire. Not very common
with modern lights, except for some varieties of "contemporary" lighting which
are designed to look old-fashioned.
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Post-top
This is a luminaire that is directly attached to the top of the mounting
post, without the need for an additional bracket arm. A spigot cap is required
for this, as it involves the light being directly attached to the pole without
the need for a bracket arm.
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Dual fitting
This type of luminaire can be used for either side entry fitting or for
attachment directly onto the top of a pole. Such lights have one of two types of
connection:
- two separate attachments, a normal side fitting and a
separate spigot cap on the underside of the luminaire; or
- a spigot cap that can be rotated into place, meaning the
light can be attached to the arm or directly onto the pole.
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Power
Many street lights
are available in a variety of power ratings - a light with a
lower wattage is designed for use on residential streets,
whereas more powerful lights (with a higher wattage) are used on
main roads and motorways.
Control gear
The control
gear is the equipment used to power the light. It is comprised
of a ballast, a capacitor and the ignition.
This image shows a test version of a lamp's control gear.
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Integral
The control gear is housed within the lamp. In most units this is in the
casing that surrounds the lamp, however in some lights, the control gear is
housed within the same compartment as the lamp itself.
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Gear-in-shoe
A casing attached to the side of a luminaire that houses the control
gear, rather than it being housed within the lamp compartment. This can
be seen to the left of the light in the adjacent picture.
Only certain models of Philips lighting feature this unusual
arrangement.
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Remote
This means there is no control gear within the luminaire itself.
Instead, the gear is found in a separate compartment, usually in the base of the
pole the light is housed in.
In some places, especially rural areas, the lighting is attached to a
telegraph pole or the side of a building - here, the control gear is housed in a
dedicated box located just below the lamp.
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Photocell
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The photocell is the little device that monitors the level of
light around the luminaire, usually found on top of the unit. It switches the
light on or off when light levels reach a certain point, although there is
usually a time delay in case its merely a passing dark cloud!
In the adjacent picture, the photocell is the small object in the centre of the
light canopy.
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Screening
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Cut-off
Cut-off lights have a design which means that light is concentrated in a
downward direction. This is usually because the light can only escape through a
small flat panel, although in some older lights, the metal canopy at the top of
the light is wider than the glass bowl.
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Semi cut-off
This type of luminaire has a design which allows light to be emitted
from any point below the top canopy. This is generally because the bulb is
housed within a glass bowl. In most cases, this can cause glare to occur, making
the unit look like a oval-shaped "blob".
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Images on
this page are Copyright to Mike Barford, Tim Luckett,
Claire Pendrous and Brian Randall
and used
with permission.
