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| Light Measurement |
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The branch of optics which deals with the quantitative study of light energy is called photometry. |
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The following three quantities are generally measured in practical photometry : Luminous flux, Luminous Intensity and Illumination. |
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Luminous Flux :
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The luminous flux from a light source is the luminous energy emitted per second by the source. It is denoted by φ.
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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.
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Luminous Intensity : |
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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 :
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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.
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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 : |
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Luminous intensity I = φ / ω Lumen / Steradian or lm / sr |
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The unit of luminous intensity is lm/sr or candela (cd).
1 cd = 1 lm/sr |
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Illumination or Illuminance :
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When luminous flux falls on a surface, it is said to be illuminated. |
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The illumination of a surface is measured by the normal luminous flux per unit area received by it. It is denoted by E. |
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If φ lumen is the flux incident normally on a area A m2, then illumination of the surface E is given by, |
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Illumination E = φ / A lumen / m2 or meter candle |
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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. |
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