1) Define luminous flux.

It is defined as the total quantity of light energy emitted per second from a luminous body. It is represented by symbol F and is measured in lumens. The conception of luminous flux helps us to specify the output and efficiency of a given light source.

2) What is meant by candle power?

It is defined as the number of lumens given out by the source in a unit solid angle in a given direction n. It is denoted by CP.

CP=lumens /ω

3) Define MHCP.

The mean of candle power in all directions in the horizontal plane containing the source of light is termed as Mean Horizontal Candle Power.

4) Define utilization factor.

It is defined as the ratio of total lumens reaching the working plane to total lumens given out by the lamp.

Utilization factor= Total lumens reaching the working plane/Total lumens given out by the lamp

5) What are the laws of illumination?

Law of Inverse Squares:

Illumination at appoint is inversely proportional to square of its distance from the point source and directly proportional to the luminous intensity (CP) of the source of light in that direction. If a source of light emits light equally in all directions be placed at the centre of a hollow sphere, the light will fall uniformly on the inner surface of the sphere. If the sphere be replaced by one of the larger radius, the same total amount of light is spread over a larger area proportional to the square of the radius.

Lambert’s cosine law:

The illumination at a point on a surface is proportional to cosine of the angle which ray makes with the normal to the surface at that point.

6) What is meant by luminance?

It 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.

7) Define space-height ratio.

It is defined as the ratio of horizontal distance between adjacent lamps and height of their mountings.

Space-height ratio= Horizontal distance between two adjacent lamps/ Mounting height of lamps above working plane

8) What is polar curve?

In most lamps or sources of light the luminous intensity is not the same in all directions. If the luminous intensity, i.e. the candle power is measured in a horizontal plane about a vertical axis and a curve is plotted between candle power and the angular position, a curve is obtained is called as horizontal polar curve.

The luminous intensity in all the directions can be represented by polar curves. If the luminous intensity in a vertical plane is plotted against the angular position, a curve known as vertical polar curve is obtained.

9) Name the various photometer heads.

  1. Bunsen Head (or) Grease spot photometer
  2. Lummer-Brodhun photometer head

There are two types of Lummer Brodhun heads a) Equality of Brightness type photometer head b) Contrast type photometer head

10) What are all the sources of light?

According to principle of operation the light sources may be grouped as follows.

  • Arc lamps
  • High temperature lamps
  • Gaseous discharge lamps
  • Fluorescent type lamps

11) What is stroboscopic effect of fluorescent tubes?

With  a.c.  supply  frequency  of  50  cycles  per  second,  discharge  through  the  lamp becomes zero, 100 times in a second. Due to the persistence of vision, our eyes do not notice this. If this light falls on moving parts, they may appear to be either running slow or in the reverse direction or even may appear stationary. This effect is called stroboscopic effect.

12) Define beam factor.

The ratio of lumens in the beam of a projector to the lumens given out by lamps is called the beam factor. This factor takes into account the absorption of light by reflector and front glass of the projector lamp. Its values vary from 0.3 to 0.6.

13) Mention the types of lighting schemes.

The distribution of the light emitted by lamps is usually controlled to some extent by means of reflectors and translucent diffusing screens or even lenses. The interior lighting schemes may be classified as

  • Direct lighting
  • Semi-direct lighting
  • Indirect lighting
  • Semi-indirect lighting
  • General lighting

14) What are the drawbacks of discharge lamps?

Drawbacks of discharge lamps:

  • Take time to attain full brightness.
  • High initial cost and poor power factor.
  • Starting requires trigger-starter.
  • Light output fluctuates at twice the supply frequency.
  • The flicker causes stroboscopic effect.
  • These lamps can be used only in particular position.

15) What are the requirements of lighting system

The following factors are required to be considered while designing the lighting scheme.

Illumination level                                        Uniformity of illumination

Colour of light                                             Shadows

Glare                                                           Mounting height

Spacing of luminaries                              Colour of surrounding walls.


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