By electric lighting we understand the production of light by means of electricity and its application to provide for efficient, comfortable and safe vision.
The most familiar light to the householder is the common incandescent bulb lamp. In this a very fine spiral of tungsten wire is subjected to the current. Its resistance is extremely high and is heated to a brilliant whiteness. Its melting point is also very high so it is able to withstand this degree of heat without damage. The resistance of the filament is great because it is sealed up in an airtight glass bulb from which all air was first extracted and a small amount of an inert gas called argon put in.
Other forms of lighting are much more efficient than the gas-filled incandescent lamp. Thus, the arc lamp, for instance, is a source of light used where great intensity of illumination is necessary, such as in searchlights and cinematograph projectors. Arc lamps work at low voltages - 40 is enough - but they consume heavy currents: a large one needs almost 120 amperes.
years ago it was discovered that an electric discharge put into
a tube filled with neon made the gas glow with a flaming red colour.
Further experiments with other gases showed that mercury vapour
glowed with a cold blue; nitrogen, an apricot colour, carbon dioxide,
white; and helium, a pale heliotrope. This discovery is put into
practice as follows: the gases are sealed into tubes, they are at
a very low pressure and, of course, all air is previously drawn
out. They require little current and transformers supply the high
voltage easily. These lamps are relatively cheap to make and keep
running but they do not last indefinitely and the gas in the tube
must be replaced from time to time. These discharge tubes give a
close approach to the idea of cold light and they are more efficient
users of electricity than the filament lamps.
One of the latest forms of electric lighting is the electric discharge lamp also called fluorescent lamp. It does not utilize a filament but relies on the property of certain gases and metallic vapours of emitting light when excited by the passage of an electric current. This is made to flow between two points inside the tube containing a conducting gas, the colour of the light so produced, depending on the gas or vapour. The light given out by the lamp is further modified by employing fluorescent powders in the tube or incorporating such substances in the glass. These lamps are only highly efficient when they act with A.C.
Fluorescent lamps produce light by fluorescence. They are electricity-discharging lamps with a fluorescing coating (phosphor) on the inside of the arc tube. This coating turns some of the ultraviolet energy generated by the discharge into visible light.
The fact that some minerals are capable of producing light when being excited by ultraviolet radiation has long been known. Indeed, as far as 1859, Becquerel published a paper describing fluorescent lamps that he had produced. We see that there is nothing basically new in producing light in this way.
High-voltage fluorescent lamps for advertising purposes were introduced in Europe approximately in the middle of the third decade of XX century, though they were not yet applied to interior lighting. Small discharge lamps known as Beehive lamps from the shape of the electrode inside, are often used to provide a faint glow that just kills complete darkness or as a switch indicator in dark places. The bulb has a small quantity of neon gas in it, and a voltage of only 200 volts is sufficient to start the glow which plays in the form of a rosy light around the electrodes. They are very economical using no more than 5 watts and when employed as switch indicators they can be wired up so that their light is extinguished as soon as the main light is switched on.
produced by fires and torches has been used by man since the most
remote times. Later, is was obtained from oil lamps. Excavations
provided examples of the latter, made of gold or carved from stone.
Six thousand years later, despite the great progress of science,
the oil lamp was still the best artificial source for general use,
though gas illumination from central stations became popular at
the beginning of XIX century. None of the early sources produced
much light and consequently, with such low luminous output, there
was no possibility of lighting rooms to the level usually obtained
during the day. Suitable interior lighting had to wait for the development
of improved lamps that had higher light output for the energy consumed.
Edison's first lamp, which gave about the same amount of light as
sixteen candles, developed 1.4 lumens per watt, compared with 10.4
and 20.1 lumens per watt for a gas-filled tungsten bulb rated at
25 and 200 watts respectively. In 1960, the development in electric
illumination resulted in 77 lumens per watt attained with fluorescent
lighting. Thus, with this improvement, it became feasible to light
interiors conveniently and the struggle for quantity of light was
replaced by that for higher quality of illumination.
General Principles in Electric Lighting
The factors we must bear in mind when applying light to provide efficient and comfortable vision are: amount of light, quality including colour and difussion of light, and elimination of direct and reflected glare, aesthetics and economics.
Visual rendering improves as the amount of light or level of illumination increases. The values of quantity of light required for certain visual jobs are based upon available research data, economics and feasibility. The quality of light is as important as its amount, mainly if hard visual tasks must be accomplished. The quality of illumination has to do with the distribution of brightness in lighting installation. In order to produce lighting of high quality, the chief consideration is keeping low brightness ratios, that is to say, both the brightness of the task and that of its surroundings must be almost the same.
The principles we have just mentioned are applied to the lighting of an interior location, but in places such as theatres and stores the consideration of lighting quality is less important to an in operating rooms, offices, shops and classrooms where good visual conditions are essential.
Most offices, schools and public buildings employ either fluorescent or incandescent sources whilst many industrial plants, specially where ceilings are high, use mercury sources as well as incandescent and fluorescent. However, the sources should be protected from view for reducing direct glare and, wherever it is practicable, to reduce reflected glare. A type of lighting unit producing a minimum of direct and reflected glare sends the greater part of its light output towards the ceiling, the latter acting as a secondary source of large area and low brightness.
The use of high-reflectance finishes for ceilings, walls, floors and furniture is another important practice in interior lighting.
As to exterior lighting, though the general principles are the same, the quantity of light is usually inferior to that used in interior areas as the visual tasks are less important. As regards its quality, in spite of not being a problem, direct glare should be avoided, or at least, reduced to a minimum.
When designing roadway lighting the factors to consider are: traffic density, accident records, types of vehicles, speed, parking, construction characteristics such as dimensions and materials, intersections, bridges and underpasses.
The main light sources for roadways are incandescent, mercury and fluorescent lamps.
Flood lighting, that may be produced through incandescent, mercury and fluorescent lamps as well as reflectors, is used in lighting the outside of buildings and other outdoor areas. It was first employed on a large scale at the Panama-Pacific International Exposition in San Francisco in 1915, where the total lighting load was about 8,000 kilowatts. Since then this type of lighting has been used extensively to emphasize the architectural beauty of outstanding buildings and as an effective method of advertising. It is also used in many outdoor areas such as those intended for the practice of sports.
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