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 accidents. The science of illumination engineering is, therefore, becoming of major importance.
Nature of Light:
Light is a form of radiant energy. Various forms of incandescent bodies are the sources of light and the light emitted by such bodies depend upon the temperature of bodies. Heat energy is radiated into the medium by a body which is hotter than the medium surrounding it. The heat of the body, as seen, can be classified as red-hot or white-hot. While the body is red-hot the wavelength of radiated energy will be sufficiently large and the energy available is in the form of heat.
When the temperature increases the body changes from red-hot to white-hot state, the wavelength of the energy radiated becomes smaller and enters into the range of the wavelength of light. Actually the hot bodies emit heat as well as light energy, the velocity of these being equal to 3 x 108 m/s. The light waves have wavelengths varying from 0.00075 mm to 0.0004 mm. The wavelength of light is usually expressed in Angstrom; 1 Angstrom being equal to 10-10 metre.
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The ratio of the energy emitted by the body in the form of light to the total energy emitted by the body is known as the ‘radiant efficiency’ of the body, which depends upon the temperature. Higher the temperature of the body; lower the wavelength of the radiant energy and higher the radiant efficiency. The radiant efficiency will be maximum when the temperature of the body will be such that the wavelength of the shortest wave radiated by the body is 0.0004 mm (4,000 Å).
It is found that maximum radiant efficiency would occur at about 6,250°C and even then it would be 20%. Since this temperature is far above the highest that has yet been obtained in practice, it is obvious that the actual efficiency of all artificial light sources, i.e., those depending upon the temperature of incandescence is quite low.
Colour:
The sensation of colour is due to the difference in the wavelengths of the light radiations. Visible light can have a wavelength between 4,000 Å and 7,500 Å and the colour varies in the way, as shown in Figs. 7.1 and 7.2.
Relative Sensitivity:
The sensitivity of the eye to the lights of different wavelengths varies from person to person and according to the age. The average relative sensitivity is shown in Fig. 7.2. The eye has greatest sensitivity for wavelengths of about 5,500 Å; that is, yellow-green can be seen under poor conditions of illumination when blue or red cannot be seen under dim illumination, the sensitive curve tends to shift, as shown by the shaded region in Fig. 7.2. Therefore, violet disappears first and red remains visible.
Yellow disappears last as the illumination becomes very dim. As each colour disappears, it becomes a grey shade, and finally black. The sensitivity of the eye to yellow-green radiation is taken as unity or 100% and the sensitivity to other wavelengths is expressed as a fraction or percentage of it. The relative sensitivity at any particular wavelength λ is written Kλ, and is known as relative luminosity factor.
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 colour and it is this brightness and colour which the eye sees and interprets as something useful, or pleasant or otherwise.
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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 vapour.
Some bodies reflect light in some measure, and when illuminated from an original source they become secondary sources of light. A good example is the Moon, which illuminates the earth by means of the reflected light originating in the Sun.
Sources of Light:
According to principle of operation the light sources may be grouped as follows:
1. Arc Lamps:
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Electric discharge through air provides intense light. This principle is utilised in arc lamps.
2. High Temperature Lamps:
Oil and gas lamps and incandescent filament type lamps, which emit light when heated to high temperature.
3. Gaseous Discharge Lamps:
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Under certain conditions, it is possible to pass electric current through a gas or metal vapour, which is accompanied by visible radiations. Sodium and mercury vapour lamps operate on this principle.
4. Fluorescent Type Lamps:
Certain materials, when exposed to ultraviolet rays, transform the absorbed energy into radiations of longer wave length lying within the visible range. This principle is employed in fluorescent lamps.