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What’s a LED?

Light Emitting Diode (LED) is a semiconductor that transforms directly the electrical energy in visible light by means of electroluminescence.

The electroluminescence occurs when the LED is polarized by a voltage differential; the negative charge (electron) and positive charge (proton) are attracted to the union zone, some electron can pass to the driving band to the Valencia layer, going down from a high energy level to a more stable lower energy level, this loss of energy can generate a spontaneous emission of photons. For this spontaneous emission to be usable, the LEDs are designed with the purpose that these photons aren’t reabsorbed.

The use of different types in different layer of semiconductor material, it will delimit the colour spectre emit: ultraviolet, visible or infrared light. The LEDs used to illuminate must avoid emitting UV or infrared radiation during their life, to be a safe and efficient LED. This not only depends on the LED quality, it also the technic with this LED has been implemented in the lamp or luminaire, mainly because the union temperature of semiconductor will shorten his life, being able to substantially deform the structure of the encapsulation. A bad use, adaptation or design thermic could cause that the phosphor layer not absorb homogeneously the UV or blue light and it affects directly the surface or living organism.

As usual a LED that emits white light, convert the blue or ultraviolet light that generates he diode to a white spectrum by photoluminescence. This spectrum conversion is obtained adding a phosphors series in the package. These phosphors absorb ultraviolet radiation and emit white light on visible frequency, like the magnetic induction luminaire or fluorescent tubes. In the more used method the LED is compound of Indio-Galio-Nitride (InGaN) and phosphor layer.

Summarizing the LED is considered a light emitter in solid state (SSL), that produces visible light with less heat generation and parasitic energetic, due to the collisions of electrons, energy is also released in the form of heat, greater when the current is greater. This heat should be dissipated, this being the main enemy of its correct operation, causing the permanent depreciation of maximum luminous flux and emitted. From certain temperature values, the LED degrades rapidly, causing its collapse and destruction. To avoid this, the luminaires must be tested in an integrating sphere with controlled temperature , in our laboratory we can perform spectral surveys by subjecting the luminaire up to 80ºC at room temperature.

  • An LED is one or more diode formed by several layers of semiconductor and an electrical circuit encapsulated in a resin or ceramic housing according to its technology.
  • An LED only allows the passage of current in only one direction.
  • An LED according to its encapsulation, and / or electrical circuit can be classified in different technologies: DIP, SMD, COB, MCOB.
  • A cooler white LED has greater efficiency, but lower colour rendering index (CRI), since it requires fewer phosphors in its encapsulation.
  • A warmer white LED has lower efficiency, but higher colour rendering index (CRI), since it requires more phosphors in its encapsulation.
  • An LED is characterized by a very long life that can be defined by the LM80-08 standard of the IESNA.
  • An LED varies its life depending on the connection temperature reached and its current in circulation.
  • An LED InGaP (indium phosphide gallium and aluminium) emits a red and amber light.
  • An LED InGaN (indium nitride and gallium) emits a light close to UV, green and blue light.
  • An LED to be regulated, the colour of the light emitted remains constant.
  • An LED needs an auxiliary power supply (DRIVER) to obtain the appropriate service currently.
  • An LED allows to direct the light with great accuracy.
  • An LED must have a good dissipation, its radiation power, useful life decreases with increasing its temperature.

The colour temperature is measured in Kelvin (K), therefore when we say that an LED emits a colour temperature of 5000K, we refer to the colour that would produce a piece of metal "black body" if it were heated to 5000 degrees Kelvin (4726 ºC). Pure sunlight has a colour temperature of 5300K.

The chromatic reproduction of the LED (CRI) is an important value in the quality of the light. An ideal light source like the sun, contains the full range of colours and has a colour rendering index of 100 (Ra = 100). This makes the illuminated surfaces well defined and look natural. The smaller the value of the Ra, the less definition and less natural the visual perception of those illuminated surfaces will be.
Therefore, in order to achieve optimal light comfort values, we must take into account:
1) A suitable colour temperature depending on the use and need.
2) A chromatic performance (CRI - Ra) that allows us to define visually and correctly and safely the tasks to be performed.

Types of LEDs:

We can classify LEDs into two main types:

1) Low Power LED:
They produce a small amount of light, between 5 and 7 lumens, are of the DIP or SMD type, are usually used in television screens, LED strips, orientation and signalling systems. They require a low current, approximately 20mA.

2) High Power LED:
They produce a lot of light, up to 160 lumens per watt, they are of the SMD, COB, MCOB type, used in road lighting, tertiary and industrial sectors. They require a much higher operating current, between 350mA and 1000mA, it's very important to correctly implement this LED in the thermal design of the luminaires, the heat dissipation will be a decisive factor.

Useful life and reliability:

In order to evaluate the useful life and reliability of the LED the manufacturers of the diode led provide us the tests, certificates and tables in which are reflected the maintenance factor of the lumen per watt provided by the LED different hours of operation (5000h., 50000h., 60000h., etc.) depending on the bonding temperatures to which it is exposed ( See certificate and test LM-80 led Nichia implemented in Venalsol lighting ).
From the designer of the luminaire depends that a good LED is correctly implemented, contribute the useful life and efficiency expected from it. There is no use for a large LED, in a badly designed luminaire or in a poor adaptation in luminaires designed for other technologies.

Considering that:

- The driver implemented is of quality in its electronic components, does not generate electromagnetic interference (EMI), support and does not generate transients.
- An LED of a recognized manufacturer has been implemented, with quality guarantees in the manufacturing processes, tests and certificates.
- The design of the luminaire has been carried out correctly, causing a low thermal resistance between the luminaire heatsink and the led diode.
- The junction temperature of the LED diode remains within the temperature range required by the manufacturer for proper operation.
- The test has been carried out at a minimum at the maximum ambient temperature for which the luminaire is offered.

We will obtain, based on the LM80-08 standard of the IESNA, the maintenance classification of lumen at 50000h. of use of the luminaire.
So a luminaire that is offered with an L70 will maintain 70% of the light flow at 50000h. of use as long as the ambient operating temperature in the installation does not exceed that specified by the manufacturer of the luminaire.