## Illumination Interview Questions & Answers

1. Question 1. What Do You Mean By Stroboscopic Effect? How Is This Effect Eliminated In Fluorescent Tube Lighting?

All gas discharge lamps operated on a 50 Hz alternating current supply are really flickering off and on 100 times per second. In normal conditions, the human eye does not notice the fact. But when a moving object like rotating parts of machinery is followed, at the certain speed it may appear to be stationary or moving in series of jerks or rotating slowly than its actual speed. This effect of producing an illusion of reality is known as the stroboscopic effect and this is obviously a possible source of danger in factories, machine shops etc.

In fluorescent tube lighting the stroboscopic effect can be eliminated either by using a three lamp unit in which each lamp is connected to a separate phase of a 3 phase, 4 wire supply or by using a two-lamp unit in single phase supply each having its own choke but there is only one condenser connected in series with one of the lamps.

2. Question 2. Why The Power Factor Of Tube Circuit Low And What Is Is The Value Of The Power Factor?

Due to the presence of inductive choke the power factor of the tube lamp is about 0.50.

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4. Question 3. What Is The Starting And Running Current Of The Fluorescent Tube?

0.55 A and 0.37 A for 20 W, 0.65 A and 0.43 A for 40W, 1.00 A and 0.67 A for 65 W & 1.3 A and 0.87 A for 80 W respectively.

5. Question 4. What Are The Available Sizes Of Fluorescent Tubes?

• 38 cm (15”) for 14 watts
• 46 cm (18”) for 15 W
• 61 cm (2”) for 20 W
• 100 cm (3’- 37/8 “) for 25 W
• 122 cm (4’) for 40 W
• 125 cm (5’)for 65 W and
• 125 cm (5’) for 80 W
6. Question 5. What Is A Fluorescent Tube?

When the fluorescent tube is switched on almost full supply voltage is applied to the starter. The potential across bimetallic electrodes of the starter causes a small glow discharge at a small current not enough to heat up the tube filaments. This discharge is enough to heat the bimetallic strips of the starter itself causing them to bend and make contact. As a result, the large current flows through the electrodes their temperature being raised to incandescence and the gas in the immediate neighborhood inside the tube gets ionized. At the same time when the contacts of the starter are closed potential difference across it is reduced to zero. After two or three seconds glow discharge inside the starter stops and the bimetallic strips cool down which breaks the contacts apart. This sudden break induces a very high voltage in the coke connected in series due to its inertia effect and it is sufficient to initiate the discharge in the main tube light. When the fluorescent tube is a light, current passes through the tube which offers a very low resistance. The voltage across the tube is about 110 volts and rest of the supply voltage of about 110 volts is dropped in the choke. Due to the low voltage, the starter ceases to glow. Starters are so designed that this potential difference across the lamp in running condition is insufficient to restart the glow discharge in the starter. So the contacts remain open, thus the tube is put in operation.

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8. Question 6. Why Is A Condenser Connected In Parallel With The Starter Of A Fluorescent Tube?

To suppress the radio interference a condenser of about 0.05 μF capacity is connected in parallel with the starter.

9. Question 7. Why Is A Resistance Connected In Series With The Radio Suppressor Condenser?

A resistance of about 100 ohms is connected in series with the radio suppressor condenser to check the condenser surges so as to prevent the starter contacts welding together.

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11. Question 8. Why Is A Condenser Connected Across The Supply Line In Fluorescent Tube Circuit?

To improve the power factor a condenser is connected across the supply line. The size of the condenser should be 3.25μF for 40 watt and 7.5 μF for 80 watt lamps.

12. Question 9. Is The Choke Necessary For Tube Light Working In D.c. Circuit?

Yes, the choke is necessary to create the voltage impulse at the time of starting.

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14. Question 10. Why Is An Extra Variable Resistance Connected In Series With The Choke On D.c. Supply?

As there is no effect of inductance on D.C. supply an extra variable resistance is connected in series with the choke to increase the effective resistance to absorb the difference between the mains voltage and the correct lamp voltage during running.

15. Question 11. Why Is A Reversing Switch Used For Fluorescent Tube Light In D.c. Circuit?

A reversing switch is used to reverse the current at intervals to prevent the blackening of the tube at positive end due to migration of the mercury from the positive end to the negative end of the tube.

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17. Question 12. Which Lamp Will Give More Of Incandescent Lamp And Fluorescent Tube Light For The Same Wattage?

Fluorescent tube light.

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19. Question 13. Why Are Fluorescent Lamps Most Commonly Used?

Fluorescent lamps are most commonly used due to its following advantages over other light sources :-

• Increased efficiency
• Longer life of lamp
• Gives illumination in a variety of colours
• Gives glare or glitter free light
• High lighting intensities are achieved without excessive temperature rise
• Gives diffused and shadow light
• Gives more comfortable illumination and
• Through initial cost is more it is economical.
20. Question 14. Why Is Light Efficiency High In Fluorescent Tube Lights?

The light efficiency in fluorescent tube light is high because the invisible radiation is converted into visible radiation with the action of phosphor coating inside the tube which reduces the loss light energy.

21. Question 15. What Do You Know About The Working Of A Gas Discharge Lamp?

A gas discharge lamp consists of a transparent gas discharge enclosure containing a small quantity of gas or vapour at a low pressure and two electrodes provided for connecting the lamp to the supply mains. When the electrodes establish a potential difference, the gas gets ionized. Consequently, an electrical current flows and a luminous discharge is obtained from the excited atom of gas.

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23. Question 16. What Is The Basic Principle Of Gas Discharge Lamp?

Ionisation of gas.

24. Question 17. Why Are Gas Discharge Lamps Preferred To Incandescent Lamps?

Gas discharge lamps are preferred to incandescent lamps due to higher efficiency, longer life, the better color of lighting and uniform intensity of light.

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26. Question 18. What Are The Demerits Of A Gas Discharge Lamp?

• High initial cost
• Low power factor
• Complicated starting requires choke to give voltage surge and to limit the current
• Condenser is required to improve power factor
• Full brilliancy comes after a considerable time
• These cannot be used in any position
• Light output fluctuates producing stroboscopic effect
• Multiple images are formed on the moving objects.
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28. Question 19. What Do You Mean By Hot Cathodes And Cold Cathodes?

In discharge lamps cathodes emitting electrons by heat are termed as hot cathodes and where no heating is employed they are called as cold cathodes.

29. Question 20. How Does An Arc Lamp Work?

In an arc lamp a current flows between two electrodes which are drawn apart resulting an arc being struck in between them. The arc maintains the current and becomes very efficient source of light.

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31. Question 21. What Are The Forms Of Arc Lamps?

Carbon arc, flame arc and magnetite arc the forms of arc lamps of which the carbon arc lamp is most usual form.

32. Question 22. What Do You Know About Carbon Arc Lamp?

Carbon arc lamps have electrodes of hard carbon placed and to end and connected to a d.c. source. When the current flows through them the ends of carbon rods become incandescent due to high resistance. If they are pulled slightly apart about 2-3 cm distance an arc will be formed between two carbon rods producing white light. The arc consists of carbon vapour surrounded by an orange red zero of burning carbon and pale green flames. The arc is maintained by the transfer of carbon particles from positive carbon rod to negative one. It is necessary to maintain the carbons at a constant distance apart otherwise there will be decrease in illumination due to burning of positive carbon. The luminous efficiency of carbon arc lamp is about 9 lumens per watt.

33. Question 23. What Are The Main Applications Of Carbon Arc Lamps?

Cinema projectors and search lights.

34. Question 24. What Is The Working Voltage Across D.c. Arc Lamp?

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36. Question 25. Can We Use Arc Lamp On A.c Supply And What Is Its Working Voltage?

Arc lamp can be used on a.c. supply at its working voltage of about 55 volts.

37. Question 26. What Is Carbon Filament Lamp?

In this type of lamp the filament is made of carbon. The melting point of carbon is 3500℃ but its working temperature should not exceed 1800℃ because at high temperature it starts disintegrating and blackens the inside of the bulb. Its temperature Co-efficient is negative. So the resistance decreases at high temperature and taken more current resulting high power consumption. The efficiency of this lamp is low of about 4 lumens per watt. It gives yellowish light. It is generally used for heating purposes and to create a voltage drop for example in battery charging but not for lighting purpose. The approximate life of this lamp is about 800-900 hours.

38. Question 27. What Is A Halogen Lamp?

Halogen lamp is the latest member in the family of incandescent lamp possessing numerous advantages over the ordinary incandescent lamp. Halogens area group consisting of the elements fluorine, chlorine, bromine and iodine. The life and efficiency of an ordinary incandescent lamp are affected by the gradual evaporation of tungsten and also its operating temperature. The addition of a small amount of any halogen vapour to the argon gas filing of a lamp has the effect that under certain conditions the metal evaporated from the tungsten filament on reaching the relatively low temperature near the wall of the envelope forms a compound of tungsten halogen and being very voltage tungsten halogen suffers from thermal diffusion in the direction of filament and on reaching the filament at high temperature it decomposes into the tungsten and halogen. In this way the evaporated tungsten is returned back and restored to the filament by means of a chemical reaction. Halogen therefore works as a transport gas. This regenerative cycle maintains the interior of the bulb in a clean condition without depositing any metal vapor on the wall. The action requires a higher bulb temperature of about 250℃ that can be withstood by the glass. Therefore a relatively expensive material quality is chosen as the envelope material and its size is kept very small in which high gas filling pressure in made possible. This gives long life of about 2000 hours to the lamp with higher light output of about 22-23 lumens/watt.

Halogen lamps are made in the standard wattage of 300 W, 500 W, 600 W, 1000 W, 1500 W & 2000 W but sometimes these are made upto 5 kW. These lamps are suitable for outdoor illumination of buildings, sports grounds, parks, air ports, fountains etc. and also used in public halls, sports halls, factories, photo film, T.V. studios, overhead projectors, car lighting, signaling etc.

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40. Question 28. What Is An Electric Lamp?

An electric lamp is a glass bulb, often filled with nitrogen or some other chemically inactive gas containing a wire or filament usually made of tungsten in which the passage of an electric current through the filament heats it to a white heat and then to the visible radiation i.e. light.

41. Question 29. What Are The Types Of Lamps?

There are types of lamps for producing light by electricity.

• Incandescent lamp
• Electric arc lamp and
• Gas discharge lamp.
42. Question 30. What Do You Mean By Incandescent Lamp?

The incandescent or filament type consists of a file metallic wire of high resistance known as filament enclosed in an evacuated glass globe. When the electric current passes through the filament, heat is produced and the temperature of the filament increases. At high temperature, it radiates heat as well as light energy thereby producing electric lights by incandescence of a heated filament.

43. Question 31. What Are The Types Of Incandescent Lamps?

The two types of incandescent lamps are:-

• Metal filament lamps &
• Carbon filament lamps.
44. Question 32. What Should Be The Properties Of Metal As A Filament In The Incandescent Lamp?

It should have high melting point, low vapour pressure, high resistivity and low-temperature coefficient. It should be ductile and mechanically very strong to withstand vibrations during normal use.

45. Question 33. Which Materials Are Used For The Filament Of The Incandescent Lamps?

Tungsten, Tantalum and Carbon.

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47. Question 34. Which Material Is Mostly Used For The Filament Of Incandescent Lamps And Why?

Tungsten is widely used for the filament of the incandescent lamp due to its high-temperature co-efficient, high melting point, low vapour pressure, ductility and good mechanical strength.

48. Question 35. What Are The Melting Point And Working Temperature Of The Tungsten Filament?

Melting point is 3400°C and working temperature is about 2000°C.

49. Question 36. What Are The Types Of Filaments Normally Used?

• Straight wire type
• Coiled type and
• Coiled-coil type.
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51. Question 37. Why Are The Modern Incandescent Lamps Made Of Coiled Filament?

The causes are as follows:-

1. Coiled coil filament is mechanically stronger.
2. It can operate at high temperature.
3. It gives greater output.
4. It gives greater efficiency.
52. Question 38. On Which Principle Does The Incandescent Lamp Work?

Incandescent lamp works by the heating effect of electricity.

53. Question 39. What Is Specular Reflection?

Specular reflection means the reflection on some in the form of a beam of light but not scattered. In this reflection unless the eye is placed in the path of the reflected beam the viewer is unaware of the existence of light.

Diffuse reflection means the reflection of light energy in the scattered form in all direction. In this reflection the viewer can see the illuminated surface but not the light source.

54. Question 40. What Is Glare?

Glare means the brightness within the field of vision of such a character as to cause annoyance, discomfort, interference with vision or eye fatigue.

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56. Question 41. What Is Polar Curve Of A Lamp?

A polar curve is a convenient way of showing how the candle power of a lamp varies in different directions. Polar curves are used to determine the M.H.C.P.,M.S.C.P. and the actual illumination of a surface by employing the candle power in the particular direction.

57. Question 42. Why Does Uniformly Distributed Light Not Come From Any Practical Type Of Lamp?

Due to its unsymmetrical shape.

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59. Question 43. How Are Hard And Long Shadows Avoided?

Hard and long shadows are avoided by

• Using large numbers of small luminaires mounted over a minimum height of 2.5 metres.
• Using wide surface sources of light by using diffusing globe over filament lamp or by using indirect lighting system.
60. Question 44. What Do You Mean By Brightness And Its Unit?

The flux emitted per unit area of the source in a direction at right angles to the surface is known as brightness. Its unit is candles / m2 or candles / 〖cm〗2 or candles / 〖ft〗2.

61. Question 45. What Is Depreciation Factor?

It is the ratio of illumination under normal condition of old installation to the illumination under ideal condition of new installation.

62. Question 46. What Are The Laws Of Illumination?

• Illumination is directly proportional to the luminous intensity of the source.
• Inverse square law – The illumination of a surface receiving its flux from a point source is inversely proportional to the square of the distance between the surface and the source.
• Lambert’s cosine law – The illumination of a surface at any point is proportional to the cosine of the angle between the normal at the point and the direction of the luminous flux.
63. Question 47. What Are The Characteristics Of Good Illumination?

• The light should not strike directly the eyes.
• The type and size of the lamp should be correct.
• Proper location should be made.
• Reflecting equipment should be suitable for purpose.
• Hard and long shadows should be avoided.
64. Question 48. What Are The Factors Which Affect The Correct Illumination?

• Nature of the work
• Architectural design
• Surroundings
• Nature of light and
• Maintenance.
65. Question 49. What Are The Factors To Be Considered In The Design Of A Lighting Scheme?

The factors are as follows:

•  Illumination level
• Glare
• Space height ratio
• Mounting height of the lamp
• Area to be illuminated
• Colour of surrounding walls
• Movement of the object
• Utilization factor and
• Depreciation factor.
66. Question 50. What Are The Advantages Of Correct Illumination?

• Production factor increases
• Accident decreases
• Wastage of products decreases.
67. Question 51. What Do You Mean By Direct Lighting?

In this lighting scheme the light does not reach the surface directly from the source, maximum light is thrown upwards to the ceiling from which it is distributed all over the room by diffuse reflectance. The glare being reduced the resulting illumination becomes softer. It is used for decoration purposes in cinemas, theaters, and hotels etc. and in workshops where large machines and other obstructions would cause troublesome shadows if direct lighting is employed.

68. Question 52. What Do You Mean By Semi-direct Lighting?

In this lighting scheme the total light flux is made to fall downwards directly with the help of semi-direct reflector on the working surface and also to illuminate the ceilings and walls. It is best suited to rooms with high ceilings where a high level of uniformly distributed illumination is desirable.

69. Question 53. What Do You Mean Semi-indirect Lighting?

In this lighting scheme the light comes partly from the ceiling by diffused reflection and party direct from the source on the working surface. As it is glare free with soft shadows it is mainly used for indoor light decoration purposes.

70. Question 54. What Do You Mean By General Lighting?

In this lighting scheme lamps made of diffusing glass are employed which give almost equal distribution of light in all direction.

71. Question 55. What Do You Mean By Local Lighting?

Local lighting means an intense illumination on some particular points by means of adjustable fittings. In this lighting scheme lamps are mounted in deep reflectors to avoid glare.

72. Question 56. What Do You Mean By Specular Reflection And Diffuse Reflection?

Specular reflection means the reflection on some in the form of a beam of light but not scattered. In this reflection, unless the eye is placed in the path of the reflected beam the viewer is unaware of the existence of light.

Diffuse reflection means the reflection of light energy in the scattered form in all direction. In this reflection, the viewer can see the illuminated surface but not the light source.

73. Question 57. What Is Light?

Light is a form of energy, which is radiated by heated bodies. The light is the part of the radiant energy which produces a sensation (of light) on the human eye.

74. Question 58. What Is Luminous Flux?

Luminous flux is the light energy radiated per second from a luminous body in the form of light waves. It is measured in lumen.

75. Question 59. What Is Lumen?

Lumen is the unit of luminous flux which is equal to the flux emitted per unit solid angle from a uniform source of one candle power.

1 lumen = 0.0016 watt (approx).

76. Question 60. What Is Luminous Intensity?

Luminous flux emitted by the sources per unit solid angle in any particular direction is known as the luminous intensity.

77. Question 61. What Is Illumination?

The luminous flux falling per unit area on a surface is known as illumination and is expressed in Lumen/m2.

78. Question 62. What Is A Standard Candle?

A standard candle is that weighs 1/6 pound of pure spermaceti wax and burning at the rate of 120 grains per hour i.e. 7.776 grams per hour.

79. Question 63. What Is A Standard Candle Power?

The light coming out from a standard candle used as the unit of illuminating power of a lighting source is called standard candle power.

80. Question 64. What Are Foot-candle And Metre-candle?

Foot candle or Lumen per square foot is defined as the illumination produced on the inner surface of a hollow sphere of a hollow sphere of radius one foot by a point surface at the center of intensity of one candela.

Metre-candle or Lux (Lumen/m^2) is defined as the illumination produced on the inner surface of a hollow sphere of radius one meter by a point surface at the center of uniform intensity of one candela.

81. Question 65. What Will Be The Total Flux Emitted By A Source Of 60 Candle Power?