What is Flood Lighting? – Definition and Calculations

Flood lighting means flooding of large surfaces with light from powerful projectors. It is employed to serve one or more of the following purposes.

  1. Aesthetic Flood lighting: For enhancing beauty of building at night such as public places, ancient buildings and monuments, religious buildings on important festive occasions etc.
  2. Industrial and Commercial Flood lighting: For illuminating railway yards, sports stadiums, car parks, construction sites, quarries etc.
  3. Advertising: For illuminating advertisement boards and showcases.

For flood lighting it is necessary to concentrate the light from the light source into a relatively narrow beam. The particular type of reflector and its housing used for concentrating the light into narrow beam is known as floodlight projector. Since it is usually installed in remote positions, therefore, it must be robust and weatherproof in construction. The reflecting surface is the most important part in a projector. This may be made of silvered glass or chromium plate or stainless steel ; the efficiency of silvered glass is about 90% while that of polished metal is only about 70%. Metal reflectors being more robust are usually preferred.

The casing and its mounting are arranged so that the inclination of the beam can be varied in both a vertical and a horizontal direction on site. For permanent installations use of cast-metal cases is made to achieve robustness and protection against weather and for temporary installations or those in sheltered situations, use of sheet metal casing is made. When 500 or 1,000 watt lamps are used in projectors, ventilation may have to be provided for adequate cooling. This may be achieved by providing sufficient radiating surface. The front of the projector is usually of clear glass, often bowed outwards to protect it from the heat of the lamp ; use of diffusing glass is made when a diffuse beam is required. As far as possible the projectors should not be visible to the passers-by. In some cases the projectors may be housed in ornamental stand standards.

Projectors are classified according to the beam spread.

  1. Narrow beam projectors with beam spread between 12-25°. These are used for distance beyond 70 metres.
  2. Medium angle projectors—Projectors with beam spread between 25-40°. These are used for distance between 30 to 70 metres.
  3. Wide angle projectors—projectors with beam spread between 40-90°. These are used for distance below 30 metres.

For economic reasons use of wide angle projector with high wattage lamp is encouraged over narrow beam projector with low wattage lamp. High wattage lamp is more efficient than low wattge lamp used in narrow beam projectors. Standard gas filled tungsten filament lamps of 250, 500 or 1,000 watts are used in medium and wide angle projectors, although for narrow angle beams which require accurate location of the light source at the focal point of the reflector, special lamps having bunched filaments and known as projector lamps are generally employed.

Location and Mounting of Projectors: One of the most important factor which affects the choice of projector is the location of the projector. There are two possible locations of projectors in practice. Fig. 7.52 (a) shows symmetric projector kept 20 to 35 metres away from the surface to be floodlighted and providing approximately parallel beam having beam spread of 25° to 30°. Fig. 7.52 (b) shows the case when the projector cannot be located away from the building. In such a case, an unsymmetric reflector mounted in a basement area or on a bracket attached to the building is used which directs more intense light towards the top of the building.

Flood Lighting

Flood lighting Calculations: The problem of flood lighting calculations may be roughly separated into three steps.

First Step : Illumination Level Required – The illumination level (lumens/m2) required depends upon the type of building, the purpose of the floodlighting, the amount of conflicting light in the vicinity etc. Typical figures are given below :

Flood Lighting

Second Step : Type of Projector – Two considerations enter into the choice of a projector, viz., beam size and light output. The former determines the area covered by the beam and the latter the illumination provided. Beam angle of the projector is decided keeping in view the distance of projector from the surface.

Third Step : Number of Projectors –  For any desired intensity over a definite surface the number of projectors required is obtained from the following relation


  • N = Number of projectors
  • A = Area of surface to be illuminated in square metres
  • E = Illumination level required in lumens/m2

The terms used in flood lighting calculations are described below :

  1. Waste Light Factor: Whenever a surface is illuminated by a number of light sources, there is always a certain amount of waste light on account of overlapping and falling of light beyond the edges of the area to be illuminated. This effect is taken into account by multiplying the theoretical value of lumens required by 1.2 for rectangular areas and 1.5 for irregular areas and objects such as statues and monuments etc.
  2. Depreciation Factor: Dirt and dust depositing on the reflectors surface and on the source of light reduces the effectiveness of the projector and 50 to 100 per cent more light must be provided so that the illumination shall be adequate at the end of the interval between the cleaning periods. Depreciation factor is defined as the ratio of illumination under ideal conditions to the illumination under normal conditions.
  3. Coefficient of Utilization: It is also known as beam factor and is defined as the ratio of beam lumens to the lamp lumens. Its value lies between 0.3 and 0.5. This factor is due to the fact that all the light emitted by the projector is not along the direction of the beam but some of it is absorbed by the reflector and by the front glass. When the above losses are considerable the value of utilization is low. The coefficient of utilization factor goes up with deep reflectors having a good reflecting surface, such as silvered glass.