7 April 2019

Design of Lighting System

The design of lighting system depends upon following methods.
  1. Watts per square meter method
  2. Lumen or Light flux method
  3. Point to point or Inverse square law method

Lumen or light flux method

  • This method is useful for uniform illumination in the working plane. 
  • The illumination in the working plane is calculated by following formula.
Illumination in working plane 
= ( number of lamps ) * ( rating of lamps in watt ) * ( lamp efficiency in lumen per watts ) * ( co – efficient of utilization ) / Depreciation factor
                            OR
Illumination in working plane 
= ( number of lamps ) * ( rating of lamps in watt ) * ( lamp efficiency in lumen per watts ) * ( co – efficient of utilization ) * Maintenance factor

Co – efficient of utilization or Utilization factor

  • A light source radiates light in the working plane but working plane does not get 100% light from light source. 
  • The utilization factor is defined as the ratio of lumen on working area to the output lumen of lamp.
Utilization factor 
= lumen on working plane / output lumen of lamp
  • As the utilization factor increases, the working plane gets more output from lamp lumen. 
The utilization factor depends upon following factors.
  • Height of lamp mounting
  • Illumination area
  • Ceiling and watt colour
  • Lighting system : direct, in direct, semi direct
The value of utilization factor always lies less than unity.

Maintenance factor

  • The illumination level of light decreases due to dust on lamp and reflector after some times. This matter is considered in the maintenance factor.
Maintenance factor 
= meter candle on working plane / meter candle at starting
= 0.8 when lamp is clean
= 0.6 when there is dust on lamp and reflector

Depreciation factor

  • It is defined as the reciprocal of maintenance factor. It’s value always greater than unity. 
  • Depreciation factor = 1 / maintenance factor

Calculation of illumination

N = ( E )( A ) / ( O )( CU )( MF )
Where
  • N = number of light fitting
  • E = Illumination ( in Lux )
  • A = working area ( in meter square )
  • O = Illumination of lamp output ( Lumen )
  • CU = Co – efficient of Utilization
  • MF = Maintenance factor
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