Electric Illumination Definition and Laws:
Electric Illumination > Luminous flux:
Luminous flux is the rate of energy radiation in the form of light waves. The unit is lumen.
Electric Illumination > Lumen:
Lumen is the unit of luminous flux. It represents the flux emitted in unit solid angle of one steradian by a point source having a uniform intensity of one candela. Thus a uniform point source of one candle power emits 4π lumens.
Lumens = Candle power x Solid angle.
Electric Illumination > Candlepower:
Candlepower is the capacity of a source to radiate light and is equal to the number of lumens emitted in a unit solid angle by a source of light in a direction.
Electric Illumination > Luminous intensity:
Luminous intensity in a given direction is the quotient of the luminous flux emitted by a source (or by an element of a source), in an infinitesimal cone containing the given direction by the solid angle of that cone. Hence it is the luminous flux / unit solid angle.
Electric Illumination > Candela:
Candela is the unit of luminous intensity. The luminous of a full radiator (back body) at the temperature of solidification of platinum is 60 candles per square centimeter.
Illumination:
The luminous flux reaching a surface, per unit area of that surface. One lumen per square meter is termed as one lux. Also
Illumination = Flux / Area = Lumens / Area
= ( Candle power x Solid angle ) / Area
where R is the distance between the surface and a point where the solid angle is formed.
Electric Illumination > Luminance:
Luminance is the luminous intensity in a given direction of an element of a surface, per unit projected area of that surface.
Units for
Illumination:
|
|
Candelas per sq. inch
|
Candela per sq. cm (Stilb)
|
Lamberts
|
Milli-lamberts
|
Foot lambert
|
|
Candela per sq. inch
|
1
|
0.155
|
0.4869
|
486.9
|
452.5
|
|
Candela per sq. cm (Stilb)
|
6.46
|
1
|
3.142
|
3142
|
2920
|
|
Lambert (1 lumen per sq. cm)
|
2.054
|
0.318
|
1
|
1000
|
929
|
|
Milli-lambert
|
0.00205
|
0.000318
|
0.001
|
1
|
0.929
|
|
Lambert (1 lumen per sq. ft)
|
0.00221
|
0.000342
|
0.00108
|
1.076
|
1
|
PROPERTIES OF GOOD ELECTRIC ILLUMINATION:
1. Illuminating source should have sufficient light.
2. It should not strike the eyes.
3. It should have suitable shades and reflectors.
4. It should be installed at such a place so as to give uniform light.
Mean horizontal candle power (MHCP):
It is the mean of the candle powers in all directions in the horizontal plane containing the source of light.
Mean spherical candle power (MSCP):
It is the mean or average of the candle power in all planes.
Mean semi-spherical candle power (MHSCP):
It is the mean or average of the candle powers in all directions below the horizontal.
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