Illuminance
Average illuminance is the ratio of the luminous flux incident on a unit surface (perpendicular component) to the area of that surface. Its unit is lm/m² = lux (lx).
Light
It is a special form of energy affecting the eye, accepted as being propagated in the form of waves or photons. If electromagnetic waves are arranged according to their wavelengths, the electromagnetic spectrum is obtained. The part of this spectrum with wavelengths between 380 nm and 780 nm is called visible light.
Luminous Flux (lm)
Luminous flux is the concept related to the total amount of light emitted by a light source per unit time. It is denoted by the letter Φ. The unit is lumen. It is the radiant flux evaluated according to the spectral sensitivity curve of the normal human eye in daylight vision.
Ko: photometric equivalent of radiant flux (683 lm/W) Φ: radiant flux (W) Vλ: spectral sensitivity of the eye or visibility factor of radiation
Luminous Efficacy
Denoted by η, its unit is lm/W. It is the ratio of the total luminous flux emitted by a light source or luminaire to the total power consumed by that source or luminaire. In other words, it is the luminous flux provided per 1 W of electrical power drawn.
Intensity
Luminous intensity refers to the intensity of light emitted in a given direction per unit time. It is defined for point light sources and depends on direction. Its symbol is I, and its unit is candela (cd). The average luminous intensity of a point source in a given direction y, Iy, is the luminous flux emitted into the unit solid angle in that direction.
Luminance
Luminance generally depends on a point on the surface and the direction of view. It is denoted by L. The unit is cd/m². It is the luminous intensity emitted from a unit area of surface in a given direction. The surface may be a primary light source such as a lamp or luminaire surface, or a secondary light source reflecting incident light from another source.
Light Production
The production of light by lamps generally occurs in three main groups: thermal, luminescent, and electroluminescent.
Thermal Light Production
When liquids or solids are heated to high temperatures, they become incandescent and emit light. Incandescent filament lamps produce light based on this principle. The spectrum of this light is continuous.
Luminescent Light Production
When atoms and molecules return from excited states to their ground state, they release the absorbed energy as radiation. This occurs when a normally insulating gas between two electrodes becomes conductive under electric current, and the resulting electron flow excites or ionizes gas atoms. Electrons colliding with atoms cause them to emit radiation when returning to ground state. Unlike thermal light, the spectrum is not continuous. Depending on the type and pressure of gases used, emission occurs at certain wavelengths. Lamps operating on this principle are called discharge lamps (high- or low-pressure). Commonly mercury or sodium gas is used.
Electroluminescent Light Production: LED (Light Emitting Diode)
This process is based on the direct conversion of electrical energy into light energy. LEDs are solid-state light sources that allow unidirectional movement of electrons and emit light when current passes through them. In LED chips formed by the junction of a P-type and an N-type semiconductor, electrons recombine with holes while moving, releasing photons (electroluminescence). Since 1999, LEDs have rapidly advanced in efficiency and have become prominent with high efficacy, good color properties, and long lifetimes. A typical LED chip consists of the following elements, with commonly available types shown below.
White light from LEDs can be obtained in two basic ways:
- By combining three-color LEDs (red, green, blue – RGB) to generate white light.
- By coating short-wavelength (blue or ultraviolet) emitting LEDs with a phosphor layer to generate white light.
In luminaire designs using LED chips, not only optical design but also thermal and electrical design must be considered. High-power LEDs emit intense light from very small areas, which can cause glare if not properly designed. The luminous efficacy (lm/W) depends on chip type and ranges between 10–40%, while the rest of the energy (60–90%) converts to heat. Poor thermal design leads to heat accumulation, raising junction temperature, reducing luminous efficacy, and lowering luminous flux per watt. LEDs usually operate with direct current, requiring drivers to convert mains supply. Drivers also deliver nominal current to each LED (e.g., 350 mA, 500 mA, 700 mA, 1050 mA). Efficiency of drivers decreases as load factor decreases.
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