The Light Emitting Diode Principle and Behavior

1. Characterizing a Light Source

A light source used for general lighting is characterized by:
→its color temperature or its position on the chromaticity diagram (see point 1.4)
→its color rendering
→its luminous flux

1.1 Color temperature, hot or cold light

The light source color temperature is determined by comparing its spectrum with that of a black body (1) heated between 2000°K and 10000°K.

For multi-chromatic fluorescent or electroluminescent (LED) sources, which have an irregular spectrum and thus resemble less than the spectrum of a black body, a Correlated Color Temperature (CCT) is calculated. In practice, we speak of color temperature and not of correlated temperature for all multi-chromatic light sources.

A light of low color temperature such as the standard incandescent lamp is called hot, its spectrum contains lots of yellow and red which gives the impression of a flame; it is perceived as soft and comfortable. From 5000°K onwards, a light source is called cold; it becomes blinding and is not used for general lighting.

The technical sheets of lighting products give the color temperatures of sources. For example, for office lighting, we recommend using light having temperature ranging from 3000 to 4000°K.

1.2 For LED

The white light of an LED generated from a blue source exciting a yellow phosphor gives a cold color ranging from 5000 to 10000°K (see the different methods for producing white light from LEDs in point 6.1). When the color temperature is not specified by the manufacturer, it is generally more than 7000°K.

Hot white LEDs have appeared recently with the development of phosphors that convert blue into a large spectrum or by adding a yellow phosphor and a red phosphor to the blue LED that reduces the color temperature between 2500 and 5000°K. The reduction of the color temperature also results in the reduction of the LED yield associated with the use of thicker phosphors or with the addition of an additional phosphor to give the red color.

1.3 Color rendering

The color rendering index gives the capacity of a source to restore the colors of an illuminated object with regards to an ideal source. The color rendering depends on the spectrum of light emitted. The more this spectrum is conditions and thereby closes to that of sunlight, the better shall be its rendering. A light source may be used for general lighting as soon as its color rendering exceeds 80%.

1.4 For an incandescent light source

→An incandescent light source has a color rendering index close to 100%.

1.5 For a discharge luminescent source or LED

→A TL tube or a compact fluorescent lamp may have color rendering indices from 80 to more than 90%
→A white LED produced from a blue LED covered with yellow phosphor generally had bad color rendering (<80), as the emitted light lacked red and did not restore this color correctly. Today, we find white LEDs with a color rendering index greater than 80.
→When the color rendering index exceeds 80 it is specified in the technical characteristics of the LED.
→A low-pressure sodium lamp (used for lighting roads) has a rendering of 22, nearly monochromatic emission of yellow.

2. Luminous Efficiency

The efficiency of a light source is determined by the luminous flux [lm] emitted per unit of power consumed by the light source [W]: lumen/Watt [lm/W]. The luminous efficiency also takes into account the perception of light by the human eye. This perception varies with the wavelength.

2.1 Transition from radiometry to photometry

→Radiometry studies the electromagnetic waves in general
→Photometry pertains to electromagnetic waves visible to the human eye, that is, light.

2.2 Luminous flux perceived by the human eye for 1W of radiated energy

In day vision, a light source that radiates 1W of power will be perceived differently based on the emitted wavelength (color). In day vision, a wavelength of 555nm (yellow-green) is best perceived by the eye.

The International Commission for Lighting has defined the luminous flux perceived by 1W radiated at a wavelength of 555nm as being equivalent to a flux of 683 lumens. Figure 2: Luminous flux perceived by the human eye for a radiation of 1W at different wavelengths under day vision and night vision (International Commission for Lighting). In night vision (2) the 507nm blue-green wavelength is best perceived by the human eye (1700lm/W).

3. The Light Emitting Diode Principle and Behavior

3.1 Principle

LED is a particular diode which generates photons (light) when a stream of electrons passes through it. To build a diode we use a crystal (electric insulator) which is doped by atoms which have one more electron on their valence band (N doping) or missing one electron on their valence band (P doping).

3.2 N doping

We use an electron donor an atom which has 5 electrons on its valence band. Four electrons will participate to the crystal structure the fifth will stay free capable of moving in the crystal as a negative charge.

3.3 P doping

We use an electron acceptor element which has 3 electrons on its valence band. These will participate in the crystal structure but is fails one electron which creates a fixed hole like a positive charge. Examples of P-doping elements: boron (B), aluminium (Al), gallium (Ga), indium (ln).

3.4 Working principle

The LED is a diode that restricts the direction of movement of charge carriers. The current can flow from the P-type side (the anode) to the N-type side (the cathode), but not in the opposite direction. In a diode a n-type semiconductor is brought into contact with a p-type semiconductor creating a 3.5 p-n junction.

When a p-n junction is first created, mobile electrons from the N-doped region diffuse into the P-doped region where there is a large population of holes (places for electrons in which no electron is present) with which the electrons "recombine". When a mobile electron recombines with a hole, the hole vanishes and the electron is no longer mobile. Thus, two charge carries have vanished. The region around the p-n junction becomes depleted of charge carries and thus behaves as an insulator. However, the depletion width cannot grow without limit. For each electron-hole pair that recombines, a positively-charged dopant ion is left behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone which acts to slow and then finally stop recombination. At this point, there is a 'built-in' potential across the depletion zone.

If an external voltage is placed across the diode with the same polarity as the bult-in potential, the depletion zone continues to act as an insulator preventing a significant electric current. This is the reverse bias phenomenon. However, if the polarity of the external voltage opposes the built-in potential, recombination can once again proceed resulting in substantial electric current through the p-n junction.

4. Examples of LED Application in Signaling

Details about only two LED applications from technical and economical angles for the region's traffic lights or for signage.

The Brussels-Capital Region is replete with companies that design, manufacture, Install or market products based on LEDs for emergency vehicles, road signals, Signage, lighting and even Christmas decorations.

4.1 The LED traffic lights of the Brussels Capital Region

Most of the world's large cities are slowly replacing their incandescent traffic lights with LED traffic lights. Brussels- Capital, the first region in the country to replace the incandescent bulb based traffic lights by LED traffic lights. Brussels Capital was the first Belgian region to switch to LED traffic lights. 
The Region manages around 600 crossroads with traffic lights, which means, 24000 light units. During renovations and the installation of new lights, the Service des Techniques Speciales de I'Administration de I' Equipement et des Deplacements replaces the incandescent halogen lamps with LED units. As of date 3000 LED units have been installed.

4.2 With LEDs: drastic reduction in energy consumption

The 10W LED units (42V supply voltage) use 6 times less energy than the 60 w incandescent sources (220v supply voltage).

4.3 With LEDs: drastic reduction in maintenance costs

→For safety reasons, the incandescent lamps are replaced 3 times per year (after approximately 1200 hours of operation for a life span estimated at 3000 hours),which is a cost of EUR 1.4/unit including the replacement of the light source and cleaning the unit.
→The LED light sources are guaranteed for 5 years by their manufacturer. It is presumed that there shall be o replacement during the first 10 years. The region therefore only incurs the cleaning expenses, that is, EUR 0.6/unit, 3 times per year.

The first LED lights were installed by the Region 2 years ago and so far no unit has had to be replaced.

4.4 With LED units, there is increased safety on crossroads

In addition to saving on energy and maintenance costs by using LEDs instead of incandescent light sources, safety at crossroads also increases by installing the LED units:

→Greater reliability of LEDs and therefore the reduction in the risk of light source failure contributes to the safety of crossroads. Moreover, the LED units have several LEDs, when one source malfunctions the others continue to light, preventing any interruption in the operation of traffic lights.
→Almost instantaneous lighting of the LED source allows the automobile driver to have a faster reaction to the alternating changes.
→The luminous flux is greater with LEDs than with incandescent sources. The LED directly emits a monochromatic light of the desired color without needing filters.
→In addition, to fight against the phantom effect (or infra) the lens of the incandescent lights is designed to absorb sunrays that fall on the light along with the absorption of luminous flux emitted by the incandescent light source. Since the LED units are less prone to phantom effect, the transmittance of the lens can be better. In the sources studied by the Katholieke Hogeschool Sint-Lieven (KAHO), the light intensity of the LED traffic light units is 4 times greater(13) than that of incandescent traffic lights making them more perceptible to the user.

The phantom effect is reduced. The phantom effect results from the reflection of incident sunlight by reflectors encircling the light module, mainly when the sun is facing the module. In case of phantom effect, the sunrays reflect within the unit and are reflected back through being able to make out the phase of light.

The phantom effect is higher in the incandescent units that have large reflector surrounding the lamp than in the LED units where the source reflectors are very small. Moreover, the LED units generate a higher flux (see above) which simply identifying the light phase.

5. Some Other Developments

→LED applications in general illumination
→Fundamental research on the crystalline structures to be done for generating light
→Improvement in phosphor conversion outputs
→Improvement in production technique of semi-conductors having fewer defects and therefore generating more light
→Production of bigger size semi-conductors. The semi-conductors used in high power LEDs have an area of mm2
→Improvement in heat dissipation to increase the current density in the semi-conductor and thereby increase the generation of light, etc.

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