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Light Measurement
The branch of optics which deals with the quantitative study of light energy is called photometry.
The following three quantities are generally measured in practical photometry : Luminous flux, Luminous Intensity and Illumination.
Luminous Flux :
The luminous flux from a light source is the luminous energy emitted per second by the source. It is denoted by φ.
The unit of luminous flux is lumen, symbol lm. A lumen is a unit of energy per second or power, so it must be related to watt.
Luminous Intensity :
The luminous intensity of a light source is the light radiating capacity of the source in a given direction. It may be defined as under :
The luminous intensity of a light source in any direction is the luminous flux emitted by the source per unit solid angle in that direction. It is denoted by I.
If a uniform light source emits luminous flux of φ lumens within a solid angle of ω steradian, then luminous intensity I of the light source is given by :
Luminous intensity I = φ / ω    Lumen / Steradian  or   lm / sr
The unit of luminous intensity is lm/sr or candela (cd).
1 cd  =  1 lm/sr
Illumination or Illuminance :
When luminous flux falls on a surface, it is said to be illuminated.
The illumination of a surface is measured by the normal luminous flux per unit area received by it. It is denoted by E.
If φ lumen is the flux incident normally on a area A m2, then illumination of the surface E is given by,
Illumination E  =  φ / A    lumen / m2  or   meter candle
One lux is equal to the illumination produced at the inner surface of a sphere of radius 1 m when a source of 1 cd is placed at the centre of the sphere.