Illumination Engineering Interview Questions and Answers:

1. What are the advantages of artificial lighting produced electrically ?

Ans. Artificial lighting produced electrically have the advantages of cleanliness, ease of control, reliability, steady output as well as low cost.

2. What is Angstrom unit (AU) ?

Ans. Angstrom unit (AU) is unit of wavelength of light and equals to 10-10 m.

3. What is difference between illumination and light ?

Ans. Light is the cause and illumination is the result of that light on surface on which it falls.

4. Explain candle power and mean horizontal candle power.

Ans. Candle power. Candle power is the light radiating capacity of a source in a given direction and is defined as the number of lumens given out by the source in a unit solid angle in a given direction. It is denoted by symbol CP.

candle power

Mean horizontal candle power. It is defined as the mean of candle powers in all directions in the horizontal plane containing the source of light.

5. Define and explain “illumination” and “luminous intensity”.

Ans. Illumination. When the light falls upon any surface, the phenomenon is called the illumination. It is defined as the number of lumens, falling on the surface, per unit area. It is denoted by symbol E and is measured in lumens per square metre or lux or metre-candle.

If a flux of F lumens falls on a surface of area A, then the illumination of that surface is

E = F/A lumens/m2 or lux (lx)

Luminous Intensity. Luminous intensity in any given direction is the luminous flux emitted by the source per unit solid angle, measured in the direction in which the intensity is required. It is denoted by symbol I and is measured in candela (cd) or lumens per steradian.

If F is the luminous flux radiated out by source within a solid angle of ω steradians in any particular direction then I = F/ω lumens/steradian or candela (cd).

6. Differentiate between candela and lux.

Ans. Candela or lumens per steradian is the unit of luminous intensity while lux is the unit of illumination.

7. Define illumination. Mention properties of good illumina­tion.

Ans. When the light falls upon any surface, the phenomenon is called the illumination. It is defined as the number of lu­mens, falling on the surface, per unit area. It is denoted by symbol E and is measured in lumens per square metre or lux or metre-candle.

If a flux of F lumens falls on a surface of area A, then the illumination of that surface is

E = F/A lumens/m2 or lux (Ix)

Good illumination means adequate illumination level, uniform distribution of light all over the working plane, suitable light colour and absence of glare and hard shadows.

8. What do you understand by foot candle and luminance ?

Ans. Foot-Candle. It is also the unit of illumination and is de­fined as the luminous flux falling per square foot on the sur­face which is every where perpendicular to the rays of light from a source of one candle power and one foot away from it.


Luminance. When the eye receives a great deal of light from an object we say it is bright, and ‘brightness’ is an important quantity in illumination. It is all the same whether the light is produced by the object or merely reflected from it. The object sends out light as though each small piece of its surface were of a certain luminous intensity. Generally, the brightness of an object is not the same from all points of view. When brightness is considered in terms of mea­surable quantity, the term luminance is employed.


Brightness or luminance is defined as the luminous intensity per unit projected area of either a surface source of light or a reflecting surface and is denoted by L.

If a surface of area A has an effective luminous intensity of I candelas in a direction θ to the normal, then the lumi­nance (brightness) of that surface is


i.e., nit is defined as candela per square metre. Bigger unit of luminance is stilb which is defined as candelas per square cm. Lambert is also the unit of brightness which is lumens/cm2. Foot-lambert is lumens/ft2.

9. What is relation between candela (cd) and lumens per stera­dian ?

Ans. Candela (cd) and lumens per steradian, both, are units of luminous intensity. 1 Candela = 1 lumen per steradian.

10. What is relation between lux and lumens/m2?

Ans. Lux and lumens/m2, both, are units of illumination. 1 lux = 1 lumen/m2.

11. Differentiate between lamp efficiency and specific consump­tion.

Ans. Lamp efficiency is the ratio of the luminous flux in lu­mens to the power input in watts while the specific con­sumption is the ratio of the power input in watts to the average candle power in candela.

12. What is difference between maintenance factor and depre­ciation factor ?

Ans. Maintenance factor is defined as the ratio of illumination under normal working conditions to the illumination when every thing is perfectly clean. Depreciation factor is merely inverse of the maintenance factor.

13. What are the two laws of illumination ?

Ans. (i) Law of inverse squares (ii) Lambert’s cosine law.

14. What are laws of illumination ?

Ans. There are two laws of illumination (i) Law of inverse squares and (ii) Lambert’s cosine law.

According to law of inverse square the illumination of a surface is inversely proportional to the square of the dis­tance between the surface and the light source provided that the distance between the surface and the source is sufficiently large so that the source can be regarded as a point source.

According to Lambert’s cosine law, the illumination at any point on a surface is proportional to the cosine of the angle between the normal at that point and the direction of lumi­nous flux.

15. Why none of the practical type of lamp provides uniformly distributed light in all directions ?

Ans. Because of unsymmetrical shape of the practical types of lamps.

16. What do you understand by polar curves ?

Ans. None of the practical type of lamp provides light uniformly distributed in all directions because of its un-symmetrical
shape. It is often necessary to know the distribution of light in various directions to ascertain how the candle power of a light source varies in different directions. The luminous intensity in all the directions can be represented by polar curves. If the luminous intensity in a horizontal plane passing through the lamp is plotted against angular position, a curve known as horizontal polar curve is obtained. If the luminous intensity in a vertical plane is plotted against the angular position, a curve known as vertical polar curve is obtained.

17. How is polar curve useful to an illumination engineer ?

Ans. The polar curve are used to determine the mhcp and mscp of a lamp. They are also used to determine the actual illumination of a surface by using the cp in that particular direction as read from the vertical polar curve in illumina­tion calculations.

18. What for an integrating sphere is used ?

Ans. Integrating sphere is a piece of apparatus which is com­monly used for measurement of mscp.

19. What are various methods of producing light ?

Ans. Artificial light can be produced by any one of the follow­ing methods :

  1. By passing electric current through a wire raising its temperature to incandescence.
  2. By establishing an arc between two electrodes.
  3. By maintaining an electric discharge through a vapour or gas.

20. Why the glass globe of a filament lamp is completely evacu­ated ?

Ans. The glass globe of a filament of incandescent lamp is com­pletely evacuated to prevent the oxidization and convec­tion currents of the filament and also to prevent the tem­perature being lowered by radiation.

21. Why is tungsten selected as the filament material ?

Ans. Nowadays tungsten is the most commonly used metal for filament due to its high melting point (3,400°C), high resistivity, low temperature coefficient (0.0051), low vapour pressure, being ductile and mechanical strong to withstand vibration during use.

22. Why does the efficiency of a filament lamp increase with the increase in operating voltage ?

Ans. The efficiency (lumens/watt) of a lamp increases with the increase in operation voltage owing to increase in tem­perature and is proportional to the square of the operating voltage.

23. Why sodium vapour discharge lamps are not used for gen­eral lighting ?

Ans. Sodium vapour discharge lamps are not used for general lighting because they have got drawback of colour dis­crimination.

24. Why thermal type starter for use in fluorescent tubes have become obsolete nowadays ?

Ans. Because of more complicated construction, greater cost and greater power loss.

25. What is the role of condenser and choke used in fluores­cent lamp ?

Ans. When the supply is switched on to a fluorescent lamp, the starter provides a path through the electrodes and choke for providing preheating. The starter switch then opens automati­cally thereby interrupting the current. Since this heating current reduces to zero instantaneously in the choke circuit also, the choke field collapses releasing stored energy and so developing a high voltage between the electrodes and thus causes the preionized tube to strike.

Thus choke performs two functions : (i) it provides inductive voltage surge to start the discharge of electrons and (ii) it limits the current in the lamp circuit during normal working. Power factor of fluorescent lamp is about 0.5 (lagging) due to presence of choke, so condenser is connected across the supply to improve the power factor. Overall pf of the fluorescent tube circuit is about 0.95 (lagging).

26. What is meant by stroboscopic effect ? How this effect is eliminated in fluorescent tube lightings ?

Ans. At the usual alternating current supply frequency of 50 Hz, a discharge lamp will he extinguished 100 times per sec­ond. Although this effect is seldom noticeable in normal conditions. It is possible that this may, in some instances, give rise to a stroboscopic effect, that is moving objects such as rotating parts of machinery, illuminated by this light, may appear to be moving in disunity or jerks, or rotating more slowly than their actual speed.

In actual applications, however, where this effect might cause annoyance, it can be practically eliminated in a three lamp unit by connecting each lamp on a separate phase of a 3-phase system and it can be greatly reduced in a two lamp unit by the use of a two lamp control unit, which employs a condenser in the ballast of one of the lamps. The current through the lamps is almost 90° out of phase and under these conditions light output of one of the lamps is at a maximum. This method has an additional advantage of giving an overall power factor of nearly unity for the unity of two lamps. In this arrangement one circuit re­mains at a low power factor at about 0.5 lagging, while the other circuit incorporating a series capacitor C remains at a power factor of about 0.5 leading.

27. What are the drawbacks of direct lighting system ?

Ans. The drawbacks of direct lighting scheme are that hard shadows and glare are caused.

28. What system of lighting should be recommended for draw­ing office ?

Ans. Direct lighting scheme may be adopted on the working table in drawing office. Light should be concentrated by reflector and shadow and glare should be avoided.

29. What is floodlighting ?

Ans. Floodlighting means flooding of large surfaces with light from powerful projector(s).

30. How projectors are classified according to the beam spread ?

Ans. Projectors, according to beam spread, are classified as nar­row beam angle projectors (beam spread 12-25°); medium beam angle projectors (beam spread 25-40°) and wide beam angle projectors (beam spread 40-90°).

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