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Light is a special form of energy affecting the eye and it's considered to spread in forms of waves or photons. If electromagnetic waves are ordered according to their wavelength, the electromagnetic spectrum is obtained. The section of this spectrum which lies between 380 nm and 780 nms of wavelength is the visible part which is referred to as light.
Luminous flux is a term related to the total amount of light emitted by a light source per unit time. It is denoted by . Its unit is lumens. Luminous flux describes the energy flux emitted by a light source which is assessed according to the spectral sensitivity curve of a healthy eye's day vision.
Ko::photometric equivalent of energy flux (683 lm/W) F: energy flux (W) Vλ : spectral sensitivity of the eye or visibility factor of radiations.
Denoted by e, unit's lm/W. Luminous efficacy is the ratio of the total luminous flux emitted from a light source or luminaire to the total power of the source or luminaire. In other words, it's the amount of luminous flux emitted by a light source or luminaire corresponding to 1W of power consumed from the grid.
Luminous intensity is related to the intensity of light emitted in a certain direction per unit time. It is defined for point light sources and it's a direction dependent quantity, its symbol is I and its unit is candela. The average luminous intensity of a point source in any direction , I is the amount of luminous flux coming out of the unit solid angle in the same direction.
Average illuminance level is the ratio of the vertical component of incident luminous flux to the area of unit surface. Its unit is lm/m2 = lux and is denoted by lx.
Luminance, in general , is related to a particular point on a surface and the direction of view. Luminance is denoted by the letter L. Its unit is cd/m2. It is a term related to the luminous intensity travelling in a specific direction from a unit area of a surface. The surface that emits light can be a primary light source such as a lamp that produces light itself or a translucent luminaire surface as well as a secondary light source that reflects the light coming from another source. The luminance of a point on a surface in the direction of is the luminous intensity emitted from the apparent unit surface in that direction.
Light generation in lamps generally occurs in three different groups as thermal, luminescent and electroluminescent.
Thermal Light Generation
When liquids or solids reach a superheated state at high temperatures, they become incandescent and emit light. Incandescent lamps generate light according to this principle. Incandescent light has a continuous spectrum.
Generation of Luminescent Light
Atoms and molecules release the energy they gain as they pass from the excited state to the ground state, as radiation. This phenomenon takes place by the gas between two solid electrodes which is normally non-conductive, becoming conductive through an electric current and the created electron flow exciting or inoizing the gas atoms. The electrons flowing from one electrode to the other collide with gas atoms on their way. If the speed the of electrons is high enough to excite the atoms, the electrons excite the atoms and radiation occurs while the atoms move to their ground state. Unlike the generation of thermal light, the spectrum of light in generation of luminescent light is not continuous. Light is emitted in specific wavelengths according to the type and the partial pressure of the used gases. Lamps operating on this principle are referred to as high pressure or low pressure according to the pressure of the gas within. In discharge lamps used today, mercury or sodium gases are generally used.
Electroluminescent Light Generation: LED (Light Emitting Diode)
This process is based on the principle of conversion of electrical energy directly to light energy. LEDs are solid-state light sources which allow unidirectional movement of electrons and emit light when an electric current is passed through them. In LED chips which consist of a junction of a P-type semiconductor with an N-type semiconductor, electrons emit photons through combining with holes while passing from the negative side to the positive side (electro-luminescence). LEDs with their efficacy increasing rapidly especially after 1999, stand out for their high efficacy, good color characteristics and long lifetime today. A typical LED chip consists of the following elements.
White light can be obtained from LEDs basically in two ways:
1)Obtaining white light using three colored red, green, blue (RGB) LED combinations.
2)Obtaining white light through covering short wavelength (blue or ultraviolet) LEDs, with a layer of phosphor.
In designing luminaires using LED chips, optical design being the most notable, thermal and electrical designs of the system should also be taken into consideration. Especially in high-power LEDs, an intense amount of light is released from a very small area. Therefore possible glare problems should be prevented through a good optical design. The luminous efficiency, expressing the ratio of conversion of electrical energy into light energy by LEDs, varies between 10-40 % depending on the type of LED chips. The remaining energy (60-90%) is converted to heat. Heat will accumulate in an LED luminaire without a good thermal design and there will be a temperature rise in the junction point where the light is produced. Increases in the LED junction temperature, will reduce the luminous efficiency of the LED and the luminous efficacy (lumens / watt) value, which indicates the luminous flux amount created per unit power, will be reduced. As the LED chips usually work with direct current, they use a driver in order to draw energy from the existing grid. A secondary task of the drivers is to provide the necessary nominal current to each LED. LED drivers are generally set to give constant current and as a standard, they are produced in current ranges such as 350 mA, 500 mA, 700 mA, 1050 mA etc. The efficiency of LED drivers scale down as the load rate decreases.