ILLUMINATION ENGINEERING - PART – 12 - FLOOD LIGHTING - IMPORTANT TERMS AND TWO PROBLEMS

FLOOD LIGHTING 
An artificial light so directed or diffused as to give a comparatively uniform illumination over a rather large given area.
Floodlights are broad-beamed, high-intensity artificial lights.
They are often used to illuminate outdoor playing fields while an outdoor sports event is being held during low-light conditions.
More focused kinds are often used as a stage lighting instrument in live performances such as concerts and plays.
Flood lighting is the use of high-powered light bulbs to illuminate a large, outdoor space.
It is very common to use flood lighting to accentuate the architectural features of a historical or impressive building.
The use of flood lights enhances the details of the building.
The dimensions of the building determine the physical size of the lighting unit.
To illuminate a very tall building, the lighting unit must be large enough to project the level of light required.
In general, the unit should be placed at least 75% of the building height away from the base of the building.
Sign lighting often requires more than one flood light.
These units should be installed equidistant from each other and centered on the sign. A standard rule of thumb is that the lights should be less than half the distance from the base of the sign apart from each other.
General area lighting is used for security and sporting events.
These lights are usually installed on the top of very tall poles to illuminate a large area.

IMPORTANT TERMS IN FLOOD LIGHTING

BEAM LUMENS
The luminous flux contained within the beam as defined in beam angle.
It is usually taken as 25 to 30 percent of the lamp lumens.
No. of flood light required = Average illumination x Area / Beam angle

BEAM ANGLE 
The angle within which the diversity of illumination produced on a surface or right angles to the beam does not exceed 10 to 1.
For symmetric flood lights there will be two beam angles, one in elevation and another in azimuth (The azimuth of a celestial body is the angle between the vertical plane containing it and the plane of the meridian).

BEAM FACTOR (OR) COEFFICIENT OF UTILIZATION
It is defined as the ratio of beam lumens to the lamp lumens.
Its value is 0.3 and 0.5.

DEPRECIATION FACTOR 
Due to dirt and dust on projector surface, the effectiveness of the projector produce from 50% to 100% and hence more light should be provided than is theoretically required so that illumination should be adequate.

WASTE LIGHT FACTOR
When several flood lights are illuminating a surface, there is bound to be some amount of overlap and also some of the light will fall beyond the edge of the area to be illuminated.
Waster light factor has a value of 1.2 for rectangular areas and 1.5 for irregular areas such as monuments or statues.

FLOOD LIGHTING CALCULATIONS

STEP ONE – The illumination level lux required - Depends upon the type of building, the purpose of the flood lighting.

STEP TWO – Selection of type of projector.
CLASSIFICATION OF PROJECTORS
1. Narrow beam projectors with beam spread between 12 and 25 degree
[Used for distance beyond 70 metres]
2. Medium angle Projectors with beam spread between 25 and 40 degree.
[Used for distance between 30 – 70 metres]
3. Wide angle Projectors with beam spread between 40 and 90 degree.
[Used for below 30 metres]

STEP THREE – The number projectors required is obtained from the following relation.
N = (Area of surface to be illuminated  x Illumination x depreciation factor x waste light factor)/( Beam factor x wattage of lamp x luminous efficacy of lamp)

PROBLEM – 01 – The front of a building 50m x 25m is to be illuminated by twenty five thousand 1000W lamps are arranged so that uniform illumination is obtained. Determine the illumination on the surface.
Assume - waste light factor = 1.2, Depreciation factor = 1.3, Utilization factor = 0.4 and luminous efficacy of 1000W lamp = 18 lumen per watt.

PROBLEM – 02 – The front of a building 100m x 15m is to be illuminated 100 lumens per square metre on the surface by flood light projectors of 1000W lamps arranged to get a uniform illumination. Determine the number of projectors required.
Assume - waste light factor = 1.2, Depreciation factor = 1.3, Utilization factor = 0.2 and luminous efficacy of 1000W lamp = 18 lumen per watt.