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Why? Lots of reasons,
none more important to you than the economics of watching machinery
that cost you tens or even hundreds of dollars an hour sitting around
idle half the time rather than doing what they’re designed
to do. Then, too, Departments of Transportation increasingly are
restricting work along highways to off-peak hours where construction
activities will necessitate lane closures, and as you’re well
aware, work proceeds around the clock for most heavy construction
projects. So what does this mean to you? Chances are you’ll
have your crews working under lights almost as often as your favorite
ball team, so you’d do well to study up.
What
Kinds of Lights Are Available?
The majority of portable
lighting systems on the market make use of high-pressure sodium
(HPS), metal halide (MH), or tungsten halogen (TH) bulbs for illuminance
(see Tables 1 and 2 for characteristics and capacities).
Table 1. Characteristics
of Available Light Sources
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Light
Source
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High-Pressure
Sodium
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Metal
Halide
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Tungsten
Halogen
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Watts
per Floodlight
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1,000
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1,000
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1,000
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Initial
Lumens
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140,000
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110,000
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21,500
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Mean
Lumens
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126,000
(92%)
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88,000
(80%)
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20,850
(97%)
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Average
Life (hours)
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24,000
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12,000
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2,000
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Color
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Soft
Orange
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Bright
White
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Natural
Daylight
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Features
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High
lumen output; low lumen depreciation glare; low total system
cost; fast restrike
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High
lumen output; good color rendition and overall lighting
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Lowest
initial cost; instant on; no ballasts; natural daylight color;
good light aiming
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Table 2. Lighting
Capacity: Approximate Area Illuminated by 1 Foot-Candle
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Avg.
Foot-Candles
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ft.2
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ac.
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m2
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ha
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HPS
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0.5
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105,000
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2.41
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9,755
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0.98
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MH
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0.5
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82,000
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1.88
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7,618
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0.76
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TH
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0.5
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26,500
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0.61
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2,462
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0.25
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Illuminance is the measurement
of how bright a point source of light appears to the eye. It is
measured in foot-candles or lux (see box). The foot-candle is defined
as the illuminance on a uniform surface 1 ft. away from the light
of one candle and is equal to 1 lumen/ft.2 This means
a light source’s output of 1 lumen flowing through a sphere
with a surface area equal to 1 ft.2 would produce an
illuminance of 1 foot-candle on the surface of the sphere.
1 foot-candle equals
1 lumen/ft._, while 1 lux is equal to 1 lumen/m2
To convert from foot-candles
to lux, multiply foot-candles by 10.76391
To convert from lux to
foot-candles, multiply lux by 0.092903044
To determine what light-source
output (L0 in lumens) would be required to produce an
illuminance of 1 foot-candle at a distance of 1 ft., the formula
is Lo/A, where Lo is the
light-source output in lumens and A is the surface
area in square feet of a sphere centered on the light source (see
Figure 1). Thus, at a distance of 1 ft., A = 4 p ft2;
so when I = 1 foot-candle, Lo = IA = 4 p lumens,
which is approximately 12.57 lumens. In other words, for a light
meter to read 1 foot-candle from a uniform point source of light
that is 1 ft. away, a light source of about 12.57 lumens is required.
In general terms: I =
Lo/(4 p d2) assuming the light can be
considered a uniform point source (no reflector). From this, one
can determine a light bulb’s minimum peak illuminance when
one knows its specified initial output rating) and the distance
to the observer. This is illustrated in Figure 2, showing the illuminance
curves for lights of three different initial output ratings in lumens.
The nationally recognized
standard of 0.1 foot-candle as a maximum level of light spillage
off a property is indicated by the dotted line. If codes were designed
to reflect this standard (or others), the formula for determining
light placement could be as follows:
For a light bulb with
an output rating of Lo (lumens) and a code limiting illuminance
to less than I (foot-candles), the minimum allowable distance from
property line to the bulb would be d, where: d = Ö (Lo
/ [4 p I]).
Reflectors
In the case of a light
source within a reflector, the illuminance is increased because
the light is being directed into a cone rather than in a sphere,
as is the case with no reflector. The surface area of the spherical
section of the cone (the bowl shape) is the area illuminated by
the light source. This area is given by A = 2 p d2 (1
- cos [q /2]) so that the general illuminance formula for illuminance:
I = Lo/A becomes I = Lo/2p d2 (1
- cos [q /2]).
Note that when q
= 360º (no reflector), I = Lo/(4p d2).
From this, one can determine
a reflected light bulb’s minimum peak illuminance (foot-candles)
when one knows its specified initial output rating (lumens), the
angle (degrees) of the light cone and the distance (feet) to the
observer. For example, a 60-W (840-lumen) floodlight creating a
90º light cone would have to be placed almost 50 ft. farther
away to achieve an illuminance level of 0.1 foot-candle.
This shows the energy
savings that can be obtained by using properly directed lighting.
By not spraying light indiscriminately as is done with unshielded
fixtures, the desired illuminance level can be maintained by focusing
the light to the proper location and reducing the light bulb’s
power consumption.
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In
the construction industry, so much is said about job-site
safety that it’s easy to think you have heard it all.
The reality is that new safety issues frequently arise that
contractors must address.
Job-site
lighting is a perfect example. Today’s jobs have tighter
deadlines and stricter regulations that are causing contractors
to work during nontraditional hours. These factors force contractors
to work in the early morning and late evening and into the
night when nature simply cannot provide a safe amount of light.
In
response to such conditions, contractors have turned to an
alternate source of light: portable light towers. But all
portable light towers are not the same, and many factors need
to be considered when selecting and setting up light towers
to ensure that the most efficient and safe job-site lighting
is provided.
What
to Look For
Because
light towers are often a necessary obstruction on a job site,
contractors need the most light from the fewest towers. To
determine how much light a given tower can deliver, look at
the source of the light: the lamp and the fixture.
Most
light-tower manufacturers recommend and utilize 1,000-W lamps.
The 1,000-W units are cost-effective and durable and deliver
an excellent amount of light–up to 110,000 lumens in
a perfect, nonreflective environment.
When
placed in a coaxial (round) fixture, most of a lamp’s
light output is reflected off the fixture surface, dropping
light output to far below the lamp’s capability. Independent
testing has shown that fixtures offering the most direct,
nonreflective light output, such as parallel (oval) lamp fixtures,
provide the most usable lumens directly to the job site.
Additionally,
parallel lamp fixtures provide a more even distribution of
light over the entire job site. Coaxial fixtures will typically
produce spots of light, referred to as "hot spots,"
that are extremely brighter than the rest of the illuminated
area.
On
job sites, space is at a premium, and the addition of more
equipment can lead to congestion and unsafe conditions. To
combat this problem, contractors should look for light towers
with a compact design. Compact units can save valuable space
on a job site and might allow a contractor to close only a
single lane of traffic; for example, to relieve congestion
problems often associated with road construction and improvement
projects. Compact units are also lighter, leading to better
maneuverability, and are easier to ship, transport, and store
for contractors, distributors, and rental houses.
How
Many Towers Do You Need?
The
Illuminating Engineering Society, a group of experts from
the lighting industry, established the guidelines listed below
to help contractors determine how many foot-candles of light
particular job sites require. A foot-candle is the intensity
of light per square foot of a surface area and can be measured
with a light meter.
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Job Site
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Foot-Candles
|
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Airport
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0.5—2.0
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Explosives
Handling
|
30.0
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General
Construction
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10.0
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Excavation
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2.0
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Haul
Roads and Industrial Roadways
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0.5—1.0
|
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Industrial
Yard/Material Handling
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5.0
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Loading
and Unloading
|
20.0
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Parking
Areas
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1.0—5.0
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Piers
|
20
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Railroad
Yards, Switching
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2.0
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Quarries
and Open-Pit Mining
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5.0
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Sports
Fields/Recreational
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10.0—50.0
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How
many foot-candles a light tower can provide depends on many
factors. Are you working on black dirt, concrete, or asphalt?
Obviously a contractor might need more light towers to reach
the recommended foot-candle measurement if he’s working
on black dirt than if working on concrete. Is the sky clear
or is it cloudy? A clear, moonlit night, for example, might
require fewer light towers than an evening with overcast skies.
Every
application must be evaluated with these considerations in
order to determine how many light towers are necessary to
safely light each site.
Setting
Up
When
setting up a light tower, always level the unit completely
before extending the mast. Position the fixtures at the highest
location possible to minimize glare on or at the work surface.
On typical job sites, the fixtures should be tilted 20º
toward the work area.
Contractors
can further reduce glare by adding a visor to each fixture.
A visor is, essentially, a piece of aluminum bent to the shape
or curve of the fixture to capture excess reflected light
and direct it both toward the job site and away from traffic,
residential areas, and so on. Visors can provide as much as
a 13% increase to a fixture’s usable light. Most manufacturers
offer visors as optional equipment.
Other
Safety Considerations
Just
as job-site lighting is a key safety consideration, so is
noise contamination, and adding another piece of equipment
to an already busy job site can bring noise to a dangerous
level. Many light-tower manufacturers have minimized vibration
noise with heavy-duty enclosures and well-mounted engines.
Some light towers also offer sound attenuation packages to
further reduce noise levels. A light tower’s noise level
should be no more than 65 dba.
Many
light towers feature a mechanism that locks down the tower
and prevents it from extending inadvertently when towing.
Other safety features include self-locking stabilizers and
captive latches on the tower support. These latches have eliminated
the need for troublesome pins, resulting in a safer and simpler
unit. Some light towers also feature what’s referred
to as a "ground rod," which grounds the units to
protect operators from electrical shock.
A
Wrap-Up
To
stay competitive, contractors are forced to do their work
at all hours of the day regardless of the natural light available.
To keep their job sites safe and productive, contractors must
be aware of what manufacturers are doing to improve light
towers and what their specific lighting needs are.
Jerry
Greenquist, vice president of sales and marketing at Allmand
Bros. Inc.
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