Monthly Archives: January 2014

What is LED?

LED Module

LED Module

LED – The replacement for every light source we possess? Thanks to the tightened regulations and more energy efficient approach, LED has started to replace all the traditional lighting. But what do those three letters hold inside? What actually IS LED?

Literally, it is a light-emitting diode. Technically it is PN-junction. Functionally it is a light source.

What actually happens in a functional sense for lighting? By driving the diode in forward biased mode light can be produced if materials are selected in a proper way. Basically, this means that materials have to be selected so, that the energy driven from power supply is transferred mostly to light emission, not to heat. More and more efficient structures have been developed to produce light more efficiently by using optically different material layers, which keep the light inside those layers.

In electrical sense, PN-diode consists of positively doped (p-type) and negatively doped (n-typed) semiconductor layers. When power supply’s positive pole is on p-type and negative on n-type, the supply voltage is forward biased. At certain voltage the diode starts to give remarkable output current, this voltage is called threshold voltage. In another words, the PN-diode conducts current if it is connected forward biased to power supply. If the power supply’s positive pole is on n-type and negative on p-type, the diode does not conduct current. Then it is connected to reverse biased power supply. This means that it is in OFF state. If the reverse bias voltage grows too large, the diode will damage permanently.

Generation of white light

There are basically two ways to generate white light. One is to use individual LEDs consisting of three colors (red, green, blue) and then mix those colors to form white light. Another way is to use a phosphor material to convert monochromatic light from a blue LED to broad spectrum white light.

Let’s take a look at the generation of white light with the help of phosphor in more detail. In these kinds of LEDs, blue emitting LEDs are coated with phosphors of different compositions to form white light. Color temperature, which measures how white colors experienced, depends on the dominant wavelength of the blue LED and the composition of the phosphor.

 

Citizen LEDs

Citizen SMD and COB LEDs

The measures for LED

Besides color temperature (CCT), other important measure for white LED is color rendering index (CRI). It is sometimes also called Ra index. CRI is a measure of how well the white (LED) light source can reproduce the colors of various objects in comparison with an “ideal” or natural light source. Maximum value of CRI is 100. Current LED generations reach already CRI>90, even CRI 97. There is also new discussion on color rendering challenging this old way of thinking. Is CRI the best way to measure the “quality” of light? Is it accurate enough and does it really tell anything? But that’s a topic for another post.

 

In addition to Color temperature and CRI, other important measures for general lighting LEDs are:

  • Lumen output

Lumen output tells the amount of lumens the LED produces (luminous flux).

  • Luminous efficacy.

Luminous efficacy tells the amount of lumens you get per watt fed to LED.

  • Threshold voltage

Tells you what voltage must be applied over PN-junction to get the LED conducting current.

  • Forward current

Tells you what current you need to feed the LED with.

To get the picture, here are the same measures of a traditional incandescent lamp:

  • CCT: ~ 2700K-3000K
  • CRI: 100
  • Lm Output: ~ 600-700lm
  • Efficacy: ~ 9lm/W -16lm/W

One important characteristic for white lighting LED is Tj-temperature. The letter ‘j’ refers to junction. So basically Tj-temperature is the temperature of the PN-junction. Absolute maximum temperature for Tj-temperature is around 150 Celcius degrees. Already at that temperature LED lifetime drastically reduces. Tj-temperature can be calculated from Tc-temperature (temperature of the cathode). Tj-temperature depends on how well the heat is conducted away from the diode. All lifetime estimations are based on the junction temperature. Tj is all that matters when LED diode operation is in question.

So, that concludes the main details of how LED works. If you have any comments or questions, don’t hesitate to leave your comment below.

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