Handbook of Electrical Installation Practice - BeKnowledge

490 Handbook of Electrical Installation Practice
direct light from the arc tube dazzling spectators on the opposite side of the pitch.
Control gear is housed separately from the floodlight.
FLOODLIGHTING CALCULATIONS
Floodlights mounted on poles are extensively used for lighting areas such as loading
docks, parking lots and so forth. The calculation of the illuminance is not difficult,
but it varies from the method used in the interior of buildings in certain respects
and other terms are introduced.
Beam angle
This is the width or spread of the beam and is defined as the total angle over which
the intensity drops to a fixed percentage, 50%, 10% and 1% of its peak value. A
single figure is given for a floodlight with a symmetrical distribution, two for a unit
with an asymmetric distribution.
Beam flux and beam factor
Beam flux is the total flux contained within the beam angle to 10% of peak intensity
and is usually expressed as the proportion of lamp lumens contained within the
beam (beam factor). It is usually presented in tabular form by the manufacturers.
Instead of the usual ‘utilisation factor’ a ‘waste-light factor’ multiplied by the
‘beam factor’ is used. The waste light factor is usually taken as about 0.9, but in
awkward shaped areas may be as high as 0.5. It is influenced by the width of the
floodlight beam.
Illuminance diagrams
A number of manufacturers publish illuminance diagrams which provide an easy
method of calculating the illuminance over an area from a specific floodlight at a
known mounting height and aiming angle. These can be superimposed to calculate
the illuminance from an array of floodlights. A typical diagram is shown in Fig. 18.4.
Conversion factors are used to adapt the figure for different mounting heights.
Zonal flux diagrams
Another method, which is very adaptable, is the use of ‘zonal flux diagrams’, a typical
example of which is shown in Fig. 18.5. It is divided through the plane of symmetry
with isocandela lines plotted on the angular grid and figures denoting the flux per
1000 lamp lumens in each angular zone on the right. It can either be used to calculate
point values of illuminance or the flux intercepted by the area to be lit, which
can be drawn as an overlay using the same angular scale. Elevation of the floodlight
can be shown by moving the diagram up and down the vertical grid line, but a fresh
diagram must be drawn for adjustments in azimuth. The beam factor of the floodlight
is expressed by the dotted line on the diagram.The beam lumens per 1000 lamp
lumens is twice the value of the flux values enclosed within it.