Illumination Interview Question Answer - 2

Describe the advantages of providing lighting voltage transformer for lighting distribution system.

The following are the advantages of providing lighting voltage transformer for lighting distribution

  • It reduces voltage fluctuation.
  • Increase the life of lamps due to better voltage regulation
  • Increase the efficiency of lighting system

Describe the advantages of Sodium vapour lamp as compare to Mercury vapour lamp for street lighting.

Advantages of Sodium vapour lamp over Mercury vapour lamp

  • A 150-watt sodium vapour lamp produces approx. 15000 lumens whereas 250-watt mercury lamp produces approx. 12700 lumens therefore sodium vapour lamp consumes less energy and produces more lumen output. 
  • The life of the 150-watt sodium vapour lamp is 12000 hours whereas the life of 250-watt mercury lamp is only 5000 hours. 
  • The CRI (colour rendering index) in the sodium vapour lamp is not as good as that of mercury vapour lamp(disadvantage)

Write the relation between angstrom and Milli - meter

One angstrom = 10-10 mm

Explain the term: Radiant efficiency

Radiant efficiency: When a current passes through conductor, the radiant energy in the form of heat at low temperature. The radiant energy in the form of heat as well as light at high temperature. The radiant efficiency is the ratio of energy radiated in the form of light to the total energy radiated from the body.

Radiant efficiency Energy radiated in form of light / Total energy radiated from body

Describe the difference between plane angle and solid angle

Plane angle: It is defined as the ratio of length of arc to the radius. 

Plane angle = Length of arc / arc radius

Solid angle: It is defined as the ratio of area of the spherical surface to the square of its radius.       Solid angle = Area of sphere / (radius)2

How the luminous flux emitted by the light source is determined?

Polar curves

What is meaning of the Polar curves?

Polar curves : It is drawn by taking luminous intensity by the light source in various directions at an equal angular displacement in the sphere. It is plotted between candle power and angular position.

Describe the various types of polar curves.

Types of polar curves

  • Vertical polar curve
  • Horizontal polar curve

Describe the function of the Photometry.

Function of photometry: The function of the photometry is to measure the candle power or luminous intensity of a given light source.

Describe the operating principle of the arc lamps.

Principle of arc lamps : The arc lamp consists of two electrodes which are separated by some distance apart.  When a DC supply is given to these electrodes, current flow through them.  The discharge between two electrodes occurs in the atmosphere. The arc between two electrodes is a source of light.

Describe the different types of arc lamps.

Types of arc lamps

  • Carbon arc lamp
  • Magnetic arc lamp
  • Flame arc lamp

Whether the size of the positive electrode and negative electrode should be same in the carbon arc lamp? Why?

Size of electrodes : The diameter of the positive electrode is approximately twice to that of negative electrode when it is operated on DC supply because the carbon particles transfer from positive electrode to negative electrode therefore the positive electrode consumed earlier than the negative electrode. The size of positive and negative electrodes is same when the carbon arc lamp operated on alternating supply.

Whether the positive and negative electrode emits same light in the carbon arc lamp when it is operated on DC supply?

The positive electrode emits about 85% of light, negative electrode emits 10% of light and 5% light is given out by the air.

Describe the operating principle of the Incandescent lamp.

Principle of incandescent lamp : The incandescent lamp consists of fine metallic wire. When a current passes through it, it produces heat energy at low temperature. The incandescent lamp produces light energy and heat energy at high temperature.

Describe the properties of the filament material for incandescent lamp.

Properties of filament material

  • High melting point
  • Low temperature co – efficient
  • High resistivity
  • High mechanical strength to withstand against vibration
  • Ductile 

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