LED is the acronym for “Light Emitting Diode”. LEDs are semiconductor devices that produce light. These were initially used as indicator lights but are now used extensively for indoor, outdoor and decorative lighting. Here is a picture of an LED:
White LED picture showing the electrodes housed in a plastic case. (Picture credit http://commons.wikimedia.org/wiki/File:2007-07-24_Light_emiting_diode_(LED).jpg)
Below is the electronic symbol of the LED:
Symbol of the light emitting diode showing the cathode, anode and the direction of light emitted.
The LED has a long history dating back to 1907 when a British physicist discovered that silicon carbide crystals could produce light when subjected to electric currents. Rubin Braunstein of the Radio Corporation of America and Robert Biard and Gary Pittman of Texas Instruments contributed to the development of the infrared LED. In 1962, a GE scientist, Nick Holonyak developed the first visible light LED. This LED produced red light. Later George Craford developed the yellow light LED. The humble LED then cost close to $ 200 for a single LED! Within a few years the cost had fallen to 5 cents. Since then the cost of the LED has been falling while light output has been going up.
How does an LED work?
The P-N junction is the basis of the functioning of the LED. The LED has an anode and a cathode separated by a crystal of semiconductor material. Addition of impurities to the semiconductor material produces P-N junctions within the chip. The entire assembly is housed within a plastic cover that can also double up as a lens to guide the light emitted by the LED.
A schematic diagram of a Light Emitting Diode. (Picture credit http://commons.wikimedia.org/wiki/File:LED_Device.jpg)
When a voltage is applied across the electrodes the current flows from the anode (P side) to the cathode (N side). When an electron meets a hole at the P-N junction it falls in to a lower energy state. The difference in energy of the two states is called the ‘Band gap’which is a characteristic of the material comprising the P-N junction.
The excess energy of the electron is emitted as a Photon. More is the ‘Band Gap’ higher is the energy difference and shorter is the wavelength of the light emitted.
The picture above shows the spectrum of electromagnetic radiation. Of the different constituents of visible light red has the longest wavelength of 700 nm (least energetic) while violet has the shortest wavelength 400 nm (most energetic).
Since an LED produces light in a narrow band of wavelengths phosphors are often used to improve the spectrum of white light produced by an LED. It is also possible to combine several LEDs each producing a different wavelength to produce full spectrum light.
The following are important for increasing the lighting efficiency of LEDs
- Advances in material sciences to create materials with better band gaps
- Better fabrication techniques for reducing the cost and increasing the efficiency
- Improvement in heat dissipation
- Light extraction from the material comprising the diode. New materials allow more light to be extracted, thus, improving the lumen per watt characteristics of LEDs.
- Improvements in phosphor technology to increase the efficiency of conversion of light from one wavelength to a wider band of wavelengths.
A single LED is very small and produces a defined amount of light according to its design type. There are low-, mid- and high-power LEDs. Several LEDs need to be combined to produce the desired amount of light.
The picture below showing 9 LED menorah resting on a fingertip indicates the relative size of an LED:
This small size makes it possible to combine LEDs in any combination and imparts versatility to LED powered lighting devices. According to the design of the printed circuit board each LED will fail independently or a group of LEDs will fail together. It means the chances of failure of all LEDs simultaneously are very remote.
Now that you know what an LED is, it is time to understand what are LED bulbs made of. LEDs need to be combined to increase the light output per bulb to suitable values. You could try to do it on your own by soldering several LEDs on a metal core printed circuit board but it is better to choose packaged bulbs. Packaged bulbs are better because all LEDs have the same voltage and current requirements, the integration of the sub parts of the bulb is better and the aesthetic appeal of a manufactured LED bulb is difficult to imitate.
The image below is a graphic representation of a section of an LED light:
A LED product the following parts –
LED cluster – Produces the required amount of light.
Driver Electronics – Change the household Alternating Current to Direct Current and maintains the right current to power the LED. The electronic ballast converts input voltage to 12V or 24V which is the often occuring input voltage for LED circuit boards. Other Voltages are also possible, even AC driven LEDs. The driver is the brain of the LED bulb. The two main components of an LED driver are
a) The driver integrated circuit and
b) The driver circuit the driver circuit is also called the electrical control gear.
Instead of using ICs and driver circuits, the required voltage drop can be achieved by simply using a resistor. But resistors can lead to unacceptable voltage drops and far higher currents than the LED is designed to handle optimally.
Heat sink – The heat generated by the electronic parts and the LED need to be dissipated. If heat sinks are not designed properly or excess drive currents result in overheating it can result in elevated junction temperatures that in turn will compromise both - the life and the light produced by the LED.
For good thermal management three things are important
a) Substrate material – Often a metal core PCB is used to mount LEDs. Besides providing a substrate for mechanical mounting of LEDs, the metal core spreads the heat over a larger area and helps to transfer it to the heat sink.
b) Interface materials – Usually a film or grease is employed as an interface material. These help in removing heat while simultaneously electrically separating the passive heat sink from the energized components.
c) Heat sinks - Heat sinks are of two types. Active heat sinks often employ fans to circulate air. Passive heat sinks use metal fins to dissipate heat. Additional structural features can also be used to improve air flow around the metal fins and ensure better heat dissipation. Active heat sinks dissipate more heat but given the advances in passive heat sink design these are not needed in most applications. Only when several LEDs are being used in a confined space an active sink may be needed to control temperatures.
Optics – LED light is directional light. The standard light distribution angle of a LED is 180 degrees, the light is emitted into the upper half-space. For some LEDs the distribution angle is adjustable, there are narrow-, wide-beam till batwing optics available. The viewing angle can be altered by lenses. Lenses can be built into the structure of the LED (first optic) or a secondary lens can be used to further control the viewing angle. Either there can be one secondary lens for several LEDs in a bulb or each LED may be given a separate secondary lens for tighter control of light output. Polycarbonate lenses are preferred as they have very little light loss and are relatively easy to manufacture. The surface quality and accuracy of shape are vital to ensure even spread of light and for restricting losses in light output.
LED devices at myledlightingguide.com have high quality LEDs with excellent light output, high quality driver electronics, aluminium heat sink and patented optical diffuser for optimum performance.
Advantages of LED Technology
Light output - In 2002, light output from LEDs was in the region of 20 lumens per watt. Today commercial LED lighting devices routinely produce 100 Lumens per watt. Experimental LEDs are reported to produce as much as 208 Lumens per watt. This is far in excess of the light produced by incandescent bulbs (15 lumens per watt) or fluorescent tubes (80-95 Lumens per watt). Lower power consumption means that a typical LED light at myledlightingguide.com can pays for itself in a little under 2 years.
Life span - LED lights last anywhere between 30,000 to 100,000 hours. Most commercially available LED lights are rated for a 30,000 to 50,000 hour life span. This means that once installed an LED will last anywhere between 10 to 30 years, depending on the running hours per day. The long life span reduces maintenance expenses and makes these bulbs particularly suitable for difficult to reach locations and for streetlights where maintenance costs can be significant.
Operating characteristics – LED operate at lower temperatures, are not sensitive to low temperature and unaffected by on off cycling. This makes them safer, more efficient in cold environments (outdoor lights, refrigerator lights and cold room lights) and better for applications requiring frequent switching of and off lights. These bulbs are unaffected by vibrations making them the best choice for places like bridges.
Shock resistant – The energized components of the LED are well separated from the outer surface with high quality insulation. The electrodes are embedded in the bulb matrix and the driver electronics are encased in its shell. A layer of interface material between the LED and the heat sink ensures that no current can leak to the heat sink.
Vibration resistant – If you look that at the picture of the LED at the beginning of the page you will notice that the electrodes are encased in transparent acrylic. There are no suspended filaments. Consequently, LEDs are resistant to vibrations. Many avid off road driving fans use LED automotive lights in their SUVs because of this feature alone.
No bulb replacements for decades – what will lighting companies do?
Some people wonder that if LEDs will not need replacements for decades lighting companies will be devoid of customers. The reality is quite the opposite. The world is witnessing a drive to bury the incandescent bulb. The versatility of LEDs is ensuring that for a decade or more lighting companies will be busy supplying the need of customers who are replacing their burnt out bulbs and tubes. Driven by an understanding of the eyes’ response to light and the importance of pupil lumens high-pressure sodium lamps and mercury vapor lamps will witness replacement in favor of LEDs.
A new wave of intelligent lighting systems shall probably be the biggest business opportunity for lighting companies. It will also translate into one of the biggest savers of carbon dioxide emissions. San Jose is already experimenting with intelligent LED street lights and so are several retail stores.
Given its technical superiority LEDs are rapidly being adopted in a range of applications including automotive lighting, billboard lighting, indoor lighting in art galleries and museums to protect rare paintings and artifacts, flood lights, grow lights, wall washers, troffer lights, high bay lights among others.