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.