Illumination And Light

Illumination differs from light very much, though generally these terms are used more or less synonymously. Strictly speaking light is the cause and illumination is the result of that light on surfaces on which it falls. Thus the illumination makes the surface look more or less bright with a certain color and it is this brightness and color which the eye sees and interprets as something useful, or pleasant or otherwise.

 

Light may be produced by passing electric current through filaments as in the incandescent lamps, through arcs between carbon or metal rods, or through suitable gases as in neon and other gas tubes. In some forms of lamps the light is due to fluorescence excited by radiation arising from the passage of electric current through mercury vapor

 

Design of Lighting Schemes

The lighting scheme should be such that it may provide adequate illumination, light of suitable color and light distribution all over the working plane as uniform as possible and avoid glare and hard shadows as far as possible. The factors to be considered while designing a lighting scheme are:

 

    • Illumination Level

    • Uniformity of Illumination

    • Color of Light

    • Shadows

    • Glare

    • Mounting Height

    • Spacing of Luminaries

    • Color of Surrounding Walls

 

Gas-filled Lamps

A metal filament can work in an evacuated bulb up to 2000°C without oxidation and if it is worked beyond this temperature it vaporizes quickly and blackens the lamp. For higher efficiency it is necessary to use operating temperature above 2000° keeping down the evaporation, which is possible by filling the bulb with an inert gas-argon with a small percentage of nitrogen. Nitrogen is added to reduce the possibility of arcing. Krypton is the best gas for this purpose but it is expensive that it is used only in special purpose lamps. For low wattage lamps, however, the heat loss due to introduction of gas is more than that in medium wattage lamps, so for low wattage lamps (up to 40 W) vacuum type lamps are used. Efficiency of coiled-coil gas filled lamp is about 12 lumens per watt.

 

Halogen lamp is the latest member in the family of incandescent lamps and has luminous efficiency varying from 22 lumens/watt to 33 lumens/watt. It has long life (2000 hours) and better color rendition. Halogen lamps are manufactured in ratings up to 5 KW.

 

Gaseous Discharge Lamps

The incandescent lamps suffer from two drawbacks-low efficiency and color light. The gaseous discharge lamps have been developed to overcome these drawbacks. Discharge lamps are of two types;

 

    • The lamps which give the light of the same color as produced by the discharge through the gas or vapor such as sodium vapor, mercury vapor

       and gas lamps.

    • The lamps which use the phenomenon of fluorescence and are known as fluorescent lamps. In these lamps, the discharge through the vapor

       produces ultra-violet waves which cause fluorescence in certain materials called the phosphors. The inside of the fluorescent lamp is coated

       with a phosphor which absorbs ultra-violet rays and radiates visible rays. Example is fluorescent mercury-vapor tube.

 

Laws of Illumination

There are two laws of illumination. These laws are:

 

    • 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.

    • Lambert’s Cosine Law. According to this 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.

 

High Pressure Mercury Vapor Lamp

The mercury vapor lamp in construction is similar to sodium vapor lamp. It gives greenish blue color light, which causes color distortion. The efficiency is about 30-40 lumens per watt. These lamps (MA type) are manufactured in 250 and 400 W ratings for use on 200-250 V ac supply. Lamps of this type are used for general industrial lighting, railway yards, ports, work areas; shopping centers etc where greenish-blue color light is not objectionable. Another type, which is manufactured in 300 and 500 W ratings for use on ac as well as dc supply mains is MAT type. This is similar to MA type except that it does not use choke as ballast. Lower wattage lamps, such as 80 and 125 W, are manufactured in a different design and using high vapor pressure of about 5-10 atmospheres. These are known as MB type lamps.

 

Fluorescent Tubes

Fluorescent lighting has a great advantage over other light sources in many applications. The tubes can be obtained in a variety of lengths (38 cm, 46 cm, 61 cm, 100 cm, 122 cm, 152 cm), with illumination in a variety of colors. Their efficiency is about 40 lumens per watt. The tube contains small quantity of argon gas at a pressure of 2.5 mm of mercury and one or two drops of mercury. The inside surface of the tube is coated with a thin layer of fluorescent material in the form of a powder. The coating material used depend upon the color effect desired and may consist of zinc silicate, cadmium silicate or calcium tungsten.

 

The tube is provided with two electrodes coated with electron emissive material. A starting switch is provided in the circuit to put electrodes directly across the supply mains at the time of starting so that electrodes may get heated and emit sufficient electrons. A stabilizing choke is connected in series with it, which acts as ballast in running condition and provides a voltage impulse for starting. A capacitor is connected across the circuit to improve the power factor. The effect of voltage variation in case of fluorescent lamps is less marked as compared to the incandescent lamps but their life and performance are adversely affected both by low and high voltage. The normal life of fluorescent tubes is 7,500 hours. The average life is for three burning hours per switching operation. The actual life may vary from 5,000 to 10,000 hours depending upon the operating conditions.

 

Various Types of Electric lamps

The various types of electric lamps in common use are:

 

    • Arc Lamps. Electric discharge through air gives intense light. This principle is used in arc lamps. In an arc lamp electric current is made to

       flow through two electrodes in contact with each other which are drawn apart. The result is an arc being struck. The arc maintains the

       current, and is very efficient source of light. There are various forms of arc lamps such as carbon arc, flame arc or magnetic arc lamps.

    • Carbon arc lamp is the earliest type of lamps and is still used in cinema projectors and search lights. The luminous efficiency of such a lamp

       is 12 lumens per watt.

    • Flame arc lamp operates on the same principle on which carbon arc lamp operates. Though the arc is very efficient but owing to objection

       to its colors it has now been superseded by the electric discharge lamps. Its luminous efficiency is 8 lumens per watt.

    • Magnetic arc lamp makes use of copper electrode as a positive electrode and iron magnetic oxide electrode.

 

Terms Used in Illumination

The devising of modern lighting schemes and the selection of fittings and types of lamps requires knowledge of the terms and quantities in general use for such purposes. Therefore, the following definitions are given in simple form to facilitate easy identification and reference.

 

    • Light is defined as the energy from a hot body which produces visual sensation upon the human eye. It is expressed in lumen-hours.

    • Luminous flux is defined as the total quantity of light energy emitted per second from a luminous body and is measured in lumens (or cd-sr).

    • Luminous intensity in any given direction is the luminous flux emitted by the source per unit angle, measured in the direction in which the

       intensity is required. It is measured in candela (cd) or lumens per steadied.

    • Lumen is the unit of luminous flux and is defined as the amount of luminous flux given out in space represented by one unit of solid angle by

       a source having an intensity of one candle power in all directions. The total lumens given out by a source of one candela is 4 lumens.

    • 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.

    • Illumination is defined as the luminous flux (number of lumens), failing on the surface, per unit area. It is measured in lumensim2 or lux or

      meter candle.

    • Candela is the unit of luminous intensity and is defined as 1/60 th of the luminous intensity per cm2 of a black body radiator at the temperature

       of solidification of platinum (2,043°K).

    • Mean Spherical Candle Power (MSCP) is defined as the mean of candle powers in all directions and in all planes from the source of light.

    • Lamp efficiency is defined as the ratio of luminous flux to the power input and is expressed in lumens per watt.

    • Specific consumption is defined as the ratio of the power input to the average candle power and is expressed in watts per candela.

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

       It is measured in nit (candelas/m2). Bigger unit of luminance is still (canadelas/cm2).

    • Glare may be defined as the brightness within the field of vision of such a character as to cause annoyance, discomfort, interference with vision

       or eye fatigue.

    • Space-height ratio is defined as the ratio of horizontal distance between adjacent lamps and their mounting height.

    • Utilization factor or coefficient of utilization is defined as the ratio of total lumens reaching the working plane to the total lumens given out by

       the lamp.

    • Maintenance factor is defined as the ratio of illumination under normal working conditions to the illumination when the things are perfectly

       clean. It is always less than unity.

    • Depreciation factor is merely the inverse of the maintenance factor and is defined as the ratio of initial meter-candles to the ultimate maintained

       metre-candles on the working plane. It is always more than unity.

 

Sodium Vapor Lamp

Principally sodium vapor lamp consists of a bulb containing a small amount of metallic sodium, neon gas and two sets of electrodes connected to a pin type base. The lamp operates at a temperature of about 300°C and in order to conserve the heat generated and assure the lamp operating at normal air temperatures the discharge envelope is enclosed in special vacuum envelope designed for this purpose. The efficiency of a sodium vapor lamp under practical conditions is about 40-50 lumens/watt. Such lamps are manufactured in 45,60,85 and 140 W ratings. The average life is about 3000 hours and is not affected by voltage variations. The major application of this type of lamp is for highway and general outdoor lighting where color discrimination is not required, such as street lighting, parks, rail yards, storage yards etc.

 

Illumination And Light

Illumination differs from light very much, though generally these terms are used more or less synonymously. Strictly speaking light is the cause and illumination is the result of that light on surfaces on which it falls. Thus the illumination makes the surface look more or less bright with a certain color and it is this brightness and color which the eye sees and interprets as something useful, or pleasant or otherwise.

 

Light may be produced by passing electric current through filaments as in the incandescent lamps, through arcs between carbon or metal rods, or through suitable gases as in neon and other gas tubes. In some forms of lamps the light is due to fluorescence excited by radiation arising from the passage of electric current through mercury vapor.

 

Neon Lamp

It is a cold cathode lamp and consists of a glass bulb filled with neon gas with a small percentage of helium. These lamps give pink colored light. Their efficiency lies between 15-40 lumens/watt. Such a lamp is of the size of an ordinary incandescent lamp. The power consumption is of the order of 5W. These lamps are used as indicator lamps, night lamps, for determination of polarity of dc mains and in large sizes as neon tubes for the purpose of advertising.

 

Illumination

Light is the prime factor in the human life as all activities of human beings ultimately depend upon the light. Where there is no natural light, use of artificial light is made. Artificial lighting produced electrically, on account of its cleanliness, ease of control, reliability, steady output, as well as its low cost, is playing an increasingly important part in modern everyday life. Apart from its aesthetic and decorative aspects, good lighting has a strictly utilitarian value in increasing production, reducing workers fatigue, protecting their health, eyes and nervous system, and reducing accident. The science of illumination engineering is, therefore, becoming of major importance.

 

Various Types of Electric lamps II

Incandescent lamps:

The incandescent or filament type lamp consists of a glass globe completely evacuated and a fine wire known as filament within it. The glass globe is evacuated to prevent the oxidization and convection currents of the filament and also to prevent the temperature being lowered by radiation. The material, which can be used for the filaments of incandescent lamps, must possess the properties of high melting point, low vapor pressure, high resistivity, low temperature coefficient, ductility and sufficient mechanical strength to withstand vibrations during use. The materials which can be used for filaments in incandescent lamps are carbon, osmium, tantalum and tungsten. Because of low operating temperature (1800°C) of carbon filament lamp, its efficiency is quite low (of the order of 3.5 lumens/watt).

 

Osmium is very rare and expensive metal. The melting point of osmium is 2600° and average efficiency of osmium filament lamp is of the order of 5 lumens per watt. The melting point of tantalum is only 2800° and the efficiency of tantalum filament lamp is about 5 lumens/watt. Now-a-days tungsten is the most commonly used metal for filament due to its high melting point (3400°C), high resistivity, low temperature coefficient (0.0051), low vapor pressure, being ductile and mechanically strong to withstand vibration during use. The average efficiency of tungsten filament lamp is about 10 lumens per watt.

 

Types of Lighting Schemes

The distribution of the light emitted by lamps is usually controlled to some extent by means of reflectors and translucent diffusion screens, or even lenses. The interior lighting schemes may be classified as direct, semi-direct, semi-indirect, indirect and general lighting schemes.
Direct lighting scheme is the most commonly used lighting scheme. In this lighting scheme more than 90% of total light flux is made to fall directly on the working plane with the help of deep reflectors.

 

Semi-direct lighting scheme is best suited to rooms with high ceilings where a high level of uniformly distributed illumination is desirable. In this lighting scheme, 60 to 90 per cent of the total luminous flux is made to fall downwards directly with the help of semi-direct reflectors, remaining light is used to illuminate the ceiling and walls.
Semi-indirect lighting scheme is mainly used for indoor light decoration purposes. In this lighting scheme 60 to 90 per cent of total luminous flux thrown upwards to the ceiling for diffuse reflection and the rest reaches the working plane directly except for some absorption by the bowls.

 

Indirect lighting scheme is mainly used for decoration purposes in cinemas, theaters and hotels etc and also in workshops where large machines and other obstructions would cause troublesome shadows if direct lighting is used. In this lighting scheme more than 90% of total luminous flux is thrown upwards to the ceiling for diffuse reflection by using inverted or bowl.

 

Methods of Lighting Calculations

The most common methods used for lighting calculations are:

 

    • Watts Per Square Meter Method. This is principally a ‘rule of thumb’ method very handy for rough calculations or checking. It consists of making

       an allowance of watts/m2 of area to be illuminated according to the illumination desired on the assumption of an average figure of overall

      efficiency of the system.

    • Lumen or Light Flux Method. This method is applicable to those cases where the sources of light are such as to produce an approximate uniform

       illumination over the working plane or where an average value is required. Lumens received on the working plane may be determined from the

       relation.

    • Lumens received on the working plane = Number of lamps X wattage of each lamp X lamp efficiency (lumens/watt) X coefficient of

       utilization/depreciation factor.

    • Point-To-Point or Inverse Square Law Method. This method is applicable where the illumination at a point due to one or more sources of light

       is required, the candle power of sources in the particular direction under consideration being known.

 

 

 

Illumination And Light

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