14-02-2013, 04:20 PM
LIIGHTIING
LIIGHTIING.pdf (Size: 787.27 KB / Downloads: 112)
INTRODUCTION
“Light is the first element of design; without it there is no color, form, or texture.”
Background
From the dawn of civilization until recent times, human beings created light solely from fire,
though it is more a source of heat than light. We are still using the same principle even in
the 21st century to produce some light and more heat through incandescent lamps. Only in
the past few decades have lighting products become much more sophisticated and varied.
For example, considerable chemistry and physics are required to create an electric arc
within a fluorescent lamp, and then to convert the energy from that arc into useful light.
Lighting energy consumption contribute to 20 to 45% in commercial buildings and about 3
to 10% in industrial plants. Most industrial and commercial energy users are aware of
energy savings in lighting systems. Manufacturers are aggressively marketing their
products these days and help the users to take a decision. Often times significant energy
savings can be realized with a minimal investment of capital and common sense. Replacing
mercury vapor or incandescent sources with metal halide or high pressure sodium will
generally result in reduced energy costs and increased visibility. Installing and maintaining
photo-controls, time clocks, and energy management systems can also achieve
extraordinary savings.
LIGHTING FUNDAMENTALS
Basic Theory
Light is just one portion of the various electromagnetic waves flying through space. These
waves have both a frequency and a length, the values of which distinguish light from other
forms of energy on the electromagnetic spectrum.
Light is emitted from a body due to any of the following phenomenon.
Incandescence Solids and liquids emit visible radiation when they are heated to
temperatures about 1000K. The intensity increases and the appearance become whiter as
the temperature increases.
Electric Discharge:
When an electric current is passed through a gas the atoms and
molecules emit radiation whose spectrum is characteristic of the elements present.
Electro luminescence: Light is generated when electric current is passed through certain
solids such as semiconductor or phosphor materials.
Photoluminescence: Radiation at one wavelength is absorbed, usually by a solid, and reemitted
at a different wavelength. When the re-emitted radiation is visible the phenomenon
may be termed either fluorescence or phosphorescence.
Luminous Intensity and Flux:
The unit of luminous intensity I is the candela (Cd) also known as the international candle.
One lumen is equal to the luminous flux, which falls on each square meter (m2) of a sphere
one meter (1m) in radius when a 1-candela isotropic light source (one that radiates equally
in all directions) is at the center of the sphere. Since the area of a sphere of radius r is 4pr2,
a sphere whose radius is 1m has 4pm2 of area, and the total luminous flux emitted by a 1-
cd source is therefore 4p1m.
The Inverse Square Law
The inverse square law defines the relationship between the illuminance from a point
source and distance. It states that the intensity of light per unit area is inversely
proportional to the square of the distance from the source (essentially the radius).
Colour Temperature
Color temperature, expressed on the Kelvin scale (K), is the color appearance of the lamp
itself and the light it produces.
Imagine a block of steel that is steadily heated until it glows first orange, then yellow and so
on until it becomes “white hot.” At any time during the heating, we could measure the
temperature of the metal in Kelvin (Celsius + 273) and assign that value to the color being
produced. This is the theoretical foundation behind color temperature.
For incandescent lamps, the color temperature is a "true" value; for fluorescent and highintensity
discharge (HID) lamps, the value is approximate and is therefore called correlated
color temperature. In the industry, “color temperature” and “correlated color temperature”
are often used interchangeably. The color temperature of lamps makes them visually
"warm," "neutral" or "cool" light sources. Generally speaking, the lower the temperature is,
the warmer the source, and vice versa.
Colour Rendering
The ability of a light source to render colour of surfaces accurately can be conveniently
quantified by the colour-rendering index. This index is based on the accuracy with which a
set of test colours is reproduced by the lamp of interest relative to a test lamp, perfect
agreement being given a score of 100. The CIE index has some limitations, but is the most
widely accepted measure of the colour rendering properties of light sources.
Incandescent (GLS) Lamps
An incandescent lamp acts as a ‘grey body’, selectively emitting radiation, with most of it
occurring in the visible region. The bulb contains a vacuum or gas filling. Although this
stops oxidation of the tungsten filament, it will not stop evaporation. The darkening of bulbs
is due to evaporated tungsten condensing on the relatively cool bulb surface. With an inert
gas filling, the evaporation will be suppressed, and the heavier the molecular weight, the
more successful it will be. For normal lamps an argon: nitrogen mixture of ratio 9/1 is used
because of its low cost. Krypton or Xenon is only used in specialized applications such as
cycle lamps where the small bulb size helps to offset the increased cost, and where
performance is critical.