User:Vigneshwaran Aiyappan

                ELECTRICAL ILLUMINATION

 Contents                                   Page no.

1). Electrical Illumination [3] 2). Definitions [4] 3). Laws of Illumination [8] 4). Types of lamps [9] 5). Illumination for different purposes [12]

Electrical Illumination:-

                The degree of visibility of environment by producing natural or artificial radiance of a luminous source is known as Illumination. Application of electricity over the principle of illumination, for the production of lighting phenomenon is known as the electrical illumination. ( lighting phenomenon,  i.e., lamps). 

Radiations from a hot Body:- The usual method of producing artificial light consists in raising a solid body or vapour to incandescence by applying heat to it. It is found that as the body gradually heated above room temperature, it begins to radiate energy in the surrounding medium in the form of electromagnetic waves of various wavelengths. The nature of this radiane energy depends on the temperature of the hot body. Thus, when the temperature is low, radiated energy is in the form of heat waves only but when a certain temperature is reached, light waves are also radiated out in addition to the heat waves and the body becomes luminous. Further increase in the temperature produces an increase in the amount of both kinds of radiations but the color of light or visible radiations changes from bright red to orange, to yellow, and then finally if the temperature is high enough, to white. As temperature is increased, the wavelength of the visible radiation goes on becoming shorter. It should be noted that heat waves are identical to light waves except that they are of longer wavelength and hence produce no impression on the retina. Obviously, from the point of view of light emission, heat energy represents so much wasted energy.

Definitions:- 1). Solid angle:- Consider an area A which is part of a sphere of radius r. let us find the solid angle ω subtended by this area at the centre C of the sphere. For this purpose let point C be joined to every point on the edges of the area A. Then, the angle enclosed by the cone at point C gives the solid angle. Its value is

                   ω = A/ r2    (steradian)

The unit of solid angle is steradian (sr). if, in the above equation, A=r2, then ω=1 steradian. Hence, steradian is defined as the angle subtended at the centre of a sphere by a part of its surface having an area equal to (radius)2 . Obviously, the solid angle subtended at the centre by whole of the spherical surface

                  ω = (4π r2) / r2 =   4π steradian (sr).

2).Candela:- It is the unit of luminous intensity of a source. It is defined as the 1/60th of the luminous intensity per cm2 of a black body radiator at the temperature of solidification of platinum (2045 K). A source of one candela (cd) emits one lumen per steradian. Hence, total flux emitted by it all around is 4π *1=4π lumen.

3).Luminous Flux (F or Ф):- It is the light energy radiated out per second from the body in the form of luminous light waves. Since, it is a rate of flow of energy, it is a sort of power unit. Unit of luminous flux is lumen (lm). It is defined as the flux contained per unit solid angle of a source of one candela or standard candle. Approximate relation between lumen and electric unit of power i.e., watt is given as 1lumen = 0.0016 watt (approx.)

4).Lumen-hour:- It is the quantity of light delivered in one hour by a flux of one lumen, which is similar to watt-hour (Wh).

5).Luminous Intensity (I) or Candle-power:-

        Candle power of a point source in any particular direction is given by the luminous flux radiated out per unit solid angle in that direction. In other words, it is solid angular flux density of a source in a specified direction.
        If dФ is the luminous flux radiated out by a source within a solid angle of dω steradian in particular direction, then   I=(dФ)/(dω).

5).Reduction Factor:-

        Reduction factor of a source is given by the ratio,
        F=(mean spherical candle power)/(mean horizontal candle power).
        It is also referred to as spherical reduction factor.

6).Illuminance or illumination(E):-

       When the luminous flux falls on a surface, it is said to be illuminated. The illumination of a surface is measured by the normal luminous flux per unit area received by it.
      If dФ is the luminous flux incident normally on an area dA, then E=dФ/dA or E=Ф/A.
      Unit. Since flux Ф is measured in lumens and area in m2 , unit of E is lm/m2 or lux. The alternative name is metre-candle. 

7).Reflection Ratio or Coefficient of Reflection or Reflectance(ρ):-

      It is given by the luminous flux reflected from a small area of the surface to the total flux incident upon it.
       ρ = M/E i.e., ratio of luminous existence and illuminance. It is always less than unity. Its value is ‘zero’ for an ideal black body and unity for a perfect reflector.

8).Specific Output or Efficiency:-

      Efficiency of a lamp is the ratio of luminous flux to the power intake. Its unit is lumen/watt (lm/W). 

9).Specific Consumption:-

     It is the ratio of the power input to the average candle- power.

    The summary of the above quantities along with their units and symbols is given in table:-

Sl. No. Quantity Unit Symbol 1). Luminous flux Lumen F or Ф 2). Luminous intensity

(candle- power)	Candela	l

3). Illumination or illuminance Lm/m2 or lux E 4). Luminance or brightness Cd/m2 l or B 5). Luminous existence Lm/m2 M

Laws of illumination:-

       The illumination (E) of a surface depends upon the following factors. The source is assumed to be a point source or is otherwise sufficiently away from the surface to be regarded as such.

(1). E is directly proportional to the luminous intensity (I) of the source or E ∞ I. (2). Inverse Square Law:-

         The illumination of a surface is inversely proportional to the square of the distance of the surface from the source.
   In other words, E ∞ 1/r2. 

(3). Lambert’s Cosine law:-

      According to this law, E is directly proportional to the cosine of the angle made by the normal to the illuminated surface with direction of the incident flux.

Types of lamps or lighting schemes:-

    Different types of lighting schemes can be classified as 
 
              [1] Direct lighting,

[2] Indirect lighting, [3] Semi-direct lighting, [4] Semi-indirect lighting & [5] General diffusing system.

[1]. Direct lighting:-

          As the name indicates, in the form of lighting, the light from the source falls directly on the object or the surface to be illuminated. With the help of shades and globes and reflectors of various types most of the light is directed In the lower hemisphere and also the brilliant source of light is kept out of the direct line of vision. Direct illumination by lamps in suitable reflectors can be supplemented by the standard bracket lamps on the desks or by additional pendent fittings over counters. The fundamental point worth remembering is planning any lighting installation is that sufficient and sufficiently uniform lighting is to be provided at the working or reading plane. For this purpose, lamps of suitable size have to be so located and furnished with such fittings as to give correct degree and distribution of illumination at the required plani. Moreover, it is important to keep the lamps and fittings clean otherwise the decrease in effective illumination due to dirty bulbs or reflectors may amount 15 to 25 % in offices and domestic lighting and more in industrial areas, as a result of a few weeks neglect.
     Direct lighting, through most efficient, is liable to cause glare and hard shadows.

[2]. Indirect Lighting:-

      In this form of lighting, light does not reach the surface directly from the source but indirectly by diffuse reflection. The lamps are either placed behind a cornice or in suspended opaque bowls. In both cases, a silvered reflector which is corrugated for eliminating striations is placed beneath the lamp.
    In this way, maximum light is thrown upwards on the ceiling from which it is distributed all over the room by diffuse reflection. Even gradation of light in the ceiling is secured by careful adjustment of the position and the number of lamps. In the cornice and bowl system of lighting, cornice lighting, a frieze of curved profile aids in throwing the light on the working plane is received by diffuse reflection, it is important to keep the fittings clean.
   One of the main characteristics of indirect lighting is that it provides shadowless illumination which is very useful for drawing offices, composing rooms and in workshops especially where large machines and other obstructions would cast troublesome shadows if direct lighting were used.
    However, many people find purely indirect lighting flat and monotonous and even depressive. Most of the users demand 50 to 100% more light at their working plane by indirect lighting than with direct lighting. However, for approaching relief, a certain proportion of direct lighting is essential.

[3]. Semi-direct System:-

      This system utilizes luminaries which send most of the light downwards directly on the working plane but a considerable amount reaches the ceilings and walls also.
     The division is usually 30% upwards and 45% downwards. Such a system is best suited to rooms with high ceilings where a high level of uniformly- distributed illumination is desirable. Glare is such units is avoided by using diffusing globes which not only improve the brightness towards the eye level but improve the efficiency of the system with reference to the working plane.

[4]. Semi- indirect lighting:-

    In this way system which is, in fact, a compromise between the first two systems, the light is partly received by diffused reflection and partly directly from the source. Such a system, therefore, eliminates the objections of indirect lighting mentioned above. Instead of using opaque bowls with reflectors, translucent bowls without reflectors are used. Most if the light is, as before, directed upwards to the ceilings for diffuse reflection and the rest reaches the working plane directly except for some absorption by the bowl.

[5].General Diffusing system:-

     In this system, luminaries are employed which have almost equal light distribution downwards and upwards.

Illumination Required for Different Purposes:-

     There has been steady movement towards higher intensities for artificial illumination during the last few decades. The movement is likely to continue because the highest intensities in average installations are much less than those of the diffused daylight. The human eye posses a tremendous power of accommodation and it can work comfortably within an enormous range of illuminations.
    For example, at full noon , sun provides about 120,000 lm/m2 ,diffuse day-light near a window is of the order of 600 lm/m2 and full moon-light gives 0.1 to 0.3 lm/m2 . For reading, usually 20 to 30 lm/m2 is generally considered sufficient, though daylight illumination is much higher.
   Some persons can read without much strain even when illumination is as low as 3 lm/m2 , because of this, it is difficult to lay down definite values of illumination for various purposes but the following summary will be found useful:

Sl. no Purpose and places lm/m2 1). Precision work, displays Above 500 2). Extra fine machines, fine engraving 200-500 3). Drawing, bench work, sustained reading 100-200 4). Usual reading, art exhibition 60-100 5). Places of simple nature not important 40-60 6). Bed rooms, auditoriums, factories 20-40 7). Hospital yards, railway platforms 5-10

05:41, 2 May 2010 (UTC)05:41, 2 May 2010 (UTC)05:41, 2 May 2010 (UTC)05:41, 2 May 2010 (UTC)05:41, 2 May 2010 (UTC)05:41, 2 May 2010 (UTC)05:41, 2 May 2010 (UTC)05:41, 2 May 2010 (UTC)05:41, 2 May 2010 (UTC)05:41, 2 May 2010 (UTC)