Illumination differs from light very much. Light is the cause and illumination is the result of that light on surfaces on which it falls. Thus, the illumination makes the surface look bright with a certain color and it is this brightness and color which the eye sees and interprets as something useful, or pleasant or otherwise.
It is defined as the luminous flux (number of lumens), falling on the surface per unit area. It is measured in lumens/m2 or lux or meter candle.
Light may be produced by passing electric current through
filaments as in the incandescent lamps
arcs between carbon or metal rods
suitable gases as in neon and other gas tubes.
In some forms of lamps, the light is due to fluorescence excited by radiation arising from the passage of electric current through mercury vapour.
Light is defined as the radiant energy from a hot body which produces visual sensation upon the human eye. It is measured in lumen-hours.
1.2. Luminous flux
It is defined as the total quantity of light energy emitted per second from a luminous body. It is measured in lumens (or cd-sr).
1.3. Luminous intensity
In any given direction is the luminous flux emitted by the source per unit angle, measured in the direction in which the intensity is required. It is measured in candela (cd) or lumens per steradian.
It is the unit of luminous flux and is defined as the amount of luminous flux given out in space represented by one unit of solid angle by a source having an intensity of one candle power in all directions. The total lumens given out by a source of one candela is 4π lumens.
1.5. Candle power
It is the light radiating capacity of a source in each direction and is defined as the number of lumens given out by the source in a unit solid angle in each direction.
It is the unit of luminous intensity and is defined as th of the luminous intensity per cm2 of a black body radiator at the temperature of solidification of platinum (2,043° K).
1.7. Lamp efficiency
It is defined as the ratio of luminous to the power input. It is measured in lumens per watt.
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