The design of
lighting system depends upon following methods.
- Watts per square meter method
- Lumen or Light flux method
- 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.
- 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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