Chapter: Electric Energy Generation and Utilisation and Conservation - Illumination

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Properties of good lighting scheme

1. Incandescence 2. Luminescence: 3. Fluorescence: 4. Phosphorescence:

Properties of good lighting scheme:

1. Incandescence


Thermo luminescence is by definition radiation at high temperature. The sources employing this process are Incandescent Lamp, Gas Lamp, (flames and in oil Lamps and wax candles). They lead to a continuous spectrum of radiation.


2. Luminescence:


Luminescence Electro luminescence by definition Chemical or Electrical Action on gases or vapour radiation. Here color of radiation depends on the material employed. Usually this process leads to Line or Band Spectrum.


3. Fluorescence:

Fluorescence is a process in which radiation is absorbed at one wavelength and radiated at another wavelength eg: UV impinging on Uranium – Fluorescent oils. This re radiation makes the light radiated visible.


4. Phosphorescence:


Phosphorescence is a process when energy is absorbed at some time and radiated later as glow. Examples of this process are luminous paints that contain calcium sulfide that lead to Phosphorescence. They produce light Radiation after exposure to light.In practice good efficient lighting is obtained by combining Luminescence and Fluorescence. Fluorescent lamp is Luminescent source of low luminous value activating Fluorescent surfaces which lead to visible radiation. Here intensity depends on gas or vapor involved and phosphor material. However, the temperatures of  the material play a role in radiation.




The lumen (symbolized lm) is the International Unit of luminous flux. It is defined in terms of candela steradians (cd multiplied by sr). One lumen is the amount of light emitted in a solid angle of 1 sr, from a source that radiates to an equal extent in all directions, and whose intensity is 1 cd.


Luminous intensity


Luminous intensity is an expression of the amount of light power emanating from a point source within a solid angle of one steradian.

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