History of LED

  In 1907, 27 year old British scientist Henry J. Round of Marconi Labs discovered the physical effect of electroluminescence, an optical and electrical phenomenon in which a material emits light in response to an electric current passed through it or to a strong electric field. The light produced was very dim and not bright enough to stimulate further research.

  20 years later in 1927 Oleg Lossew, a Russian physicist, informed the world of the creation of the first LED. He had managed to create a device where when electrons fell to a lower energy level, light could be generated through the previously discovered electroluminescence. His work was published in various scientific journals in Russia, Germany and Britain, however no practical uses were found until decades later.

  In 1951 a step forward for the world of science and semiconductor physics as a transistor is developed, making it possible to give an explanation of light emission.

  In 1961 American experimenting duo Gary Pittman and James R. Biard obtain the first US patent for an LED – despite the infrared that was emitted.

  In 1962 the first visible spectrum LED light was produced by Nick Holonyak Jr. and was red in colour. This coined his nickname, 'Father of the Light Emitting Diode'. The red LED's were not bright enough to be seen in daylight so the first LED applications were mainly used as indicator lights for military use.

  M. George Craford, a student of Holonyak, invented the first yellow LED in 1972 and then went on to produce a much brighter red LED.

  In 1994 Nichia Corporation’s Shuji Nakamura presents the first blue LED, which was based on InGaN - a semiconductor material consisting of indium nitride and gallium nitride.

  1995: The discovery of highly efficient blue LEDs swiftly led to the creation of the infamous white LED.

  2012: Lighting manufacturers, establish high power InGaN LEDs. The use of silicon substrates could potentially reduce the cost of production by 90 percent. And till date further research is going on.

 Introduction

  LED is a semiconductor device which work on the property of Electroluminescence in LEDs which 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). For N doping 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. For P doping 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), aluminum (Al), gallium (Ga), indium (ln).

 LED Construction And Working Principle

  LED diode convert current to light when a forward bias voltage is applied across a p-n junction, electron and hole flow across the space-charge region and become excess minority carriers. The excess minority carriers diffuse into the neutral semiconductors regions, where they recombine with majority carriers. If the semiconductor is a direct bandgap material such as GaAs the electron and hole can combine with no change in momentum, and a photon or light wave can be emitted. Conversely, in an indirect bandgap material, such as silicon, whe an electro and hole recombine, both energy and momentum must be conserved. Therefore LED is fabricated from GaAs or other compound semiconductor material. In an LED, the diode current is directly proportional to the recombination rate, which means that the output light intensity is also proportional to the diode current i-e, forward current. Thus the higher the diode current higher the light output. The schematic symbol and basic structure of LED is shown figure.

  Here an N-type layer is grown on P-type substrate by diffusion process. The metal connection to both layers make anode and cathode terminal as shown in figure. The light energy is released at the junction, when the recombination of electrons with holes takes place.

LED Construction

 CRI (Color Rendering Index)

  Color rendering describes how a light source makes the color of an object appear to human eyes and how well subtle variations in color shades are revealed. The Color Rendering Index (CRI) is a scale from 0 to 100 percent indicating how accurate a "given" light source is at rendering color when compared to a "reference" light source. CRI is rated on a scale from 0 to 100 and only Sunlight is classified as having a CRI of 100. Colors look exactly like they should underneath a light scoring CRI of 100. So the simple concept: the higher the CRI, the better colors will look and if the lower the CRI, the worse colors will look.

SourceAchievable CRI
Incandescent/Halogen>95
T8 Linear fluorescent75-85
Compact fluorescent>82
Standard Metal Halide>65
Cool White Linear fluorescent62
Standard HPS22
LED80-98

 CCT (correlated color temperature)

  The correlated color temperature (CCT) is a specification of the color appearance of the light emitted by a lamp, relating its color to the color of light from a reference source when heated to a particular temperature, measured in degrees Kelvin (K). The CCT rating for a lamp is a general "warmth" or "coolness" measure of its appearance. However, opposite to the temperature scale, lamps with a CCT rating below 3200 K are usually considered "warm" sources, while those with a CCT above 4000 K are usually considered "cool" in appearance.

  The correlated color temperature (CCT) designation for a light source gives a good indication of the lamp's general appearance, but does not give information on its specific spectral power density. Therefore, two lamps may appear to be the same color, but their effects on object colors can be quite different. Examples of the CCT of some common light sources are:

Types of lightingCCT (Kelvin)
High-pressure sodium2200
Incandescent (soft white)2800
Halogen3000
Fluorescent (cool white)4000
Daylight>5000
LED (cool white)>8000

 Efficiency

  Efficacy of radiation measures the fraction of electromagnetic power which is useful for lighting. It is obtained by dividing the luminous flux by the radiant flux. Light with wavelengths outside the visible spectrum reduces luminous efficacy, because it contributes to the radiant flux while the luminous flux of such light is zero. Wavelengths near the peak of the eye's response contribute more strongly than those near the edges. In SI, luminous efficacy has units of lumens per watts (lm/W). Photopic luminous efficacy of radiation has a maximum possible value of 683 lm/W, for the case of monochromatic light at a wavelength of 555 nm (green). Scotopic luminous efficacy of radiation reaches a maximum of 1700 lm/W for narrowband light of wavelength 507 nm.

 Lux

  The lux (symbol: lx) is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square meter. Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (measured in lumens) as a measure of the total "amount" of visible light present, and the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance (lux) is inversely proportional to area when the luminous flux (lumens) is held constant. One lux is equal to 1lumen per square meter.

1lx = 1lm/m2 = 1cd.sr/m2

 Lumen

  Light measurements can either be radiometric or photometric. Radiometric measurements measure all the wavelengths of a light source, both visible and invisible. Photometric measurements measure only the visible wavelengths of light. The total electromagnetic energy that a light source emits across all wavelengths is known as radiant flux, and is measured in watts. The total energy that a light source emits across the visible wavelengths of light is known as luminous flux, and is measured in lumens. One lumen is the amount of light emitted in a solid angle of 1 sr, from a source that radiates to an equal extent in all directions, and whose intensity is 1 cd. One lumen is the equivalent of 1.46 mill watt (1.46 x 10-3 W) of radiant electromagnetic (EM) power at a frequency of 540 terahertz (540 THz or 5.40 x 1014 Hz). Reduced to SI base units, one lumen is equal to 0.00146 kilogram meter squared per second cubed (1.46 x 10-3kg multiplied by m2 / s3).

 Candela

  The candela (abbreviation, cd) is the standard unit of luminous intensity in the International System of Units (SI). It is formally defined as the magnitude of an electromagnetic field, in a specified direction, that has a power level of 1/683 watt (1.46 x 10-3 W) per steradian at a frequency of 540 terahertz (540 THz or 5.40 x 1014 Hz).

 Candela

  LEDs are often described in marketing materials as "cool" lighting, and in fact LEDs are cool to the touch because they generally don't produce heat in the form of infrared (IR) radiation. On the other hand, LEDs generate heat in the diode semiconductor structure (in addition to photons) and this heat must exit the system through conduction and convection. Consequently, luminaries’ designers must be conscious of potential heat dissipation challenges and how those challenges may affect LED performance, longevity, and even lamp safety.

  Elevated junction temperatures have been shown to cause an LED to produce less light (lumen output) and less forward voltage. Over time, higher junction temperatures may also significantly accelerate chip degeneration, perhaps by as much as 75 percent with an increase from about 100°C to 135°C during regular use.

  Engineers and material scientists have been and are developing new LED-related thermal management solutions including improved drivers, diaphragm-driven forced convection methods, better heat sinks, and even the introduction of graphite foam as a cooling medium. This article will first describe three junction temperature considerations--basic thermal resistance, power dissipation, and junction temperature measurement--then briefly look at advances in each of the aforementioned approaches to improved LED thermal management.

  The following are the various important parameters in selecting Thermal Management:

  1. Thermal resistance
  2. Airflow
  3. Volumetric resistance
  4. Fin density
  5. Fin spacing
  6. Width
  7. Length

  Calculate the required LED heat sink for thermal management. The basics to do that is to understand the scheme at the right Each part of the design adds up some heat due to individual thermal resistances of each material – the adding up can be calculated as T = Pd x Rth. In this case we have the thermal resistance of the LED module (Rj-c), the thermal resistance of a gap filler (thermal pad or grease) we want to place between the led module and the heat sink (Rb), and the thermal resistance from our heat sink (Rh) which has to make that the total design stays below the maximum required junc_on temperature Tj

Thermal Management

  If our led light is in a recessed environment I want to calculate with an ambient temperature Ta of 45°C Means the maximum temperature added in the total design is Tj – Ta = 135°C – 45°C = 90°C

  The total power to dissipate is of course lower than the total power the LED consumes. Some part of the power becomes light - the more efficient your LED module, the bigger part of the total power will be transferred in to light, easy to verify if you compare the luminous flux to the power. As a fist rule we use 80% of the total power to be dissipated (Pd.

Pd = 14.7W x 80% = 11.76W (example).

  Now we just define mathemacally what would be the maximum thermal resistance our heat sink should have, or define the maximum raise in temperature our heat sink will create when dissipating Pd 11.76W.

  Suppose we will use a phase change gap filler thickness 0.18mm (thermal pad which becomes fluid on first heating cycle) with a thermal resistance of 0.4°C/W.

  Let's see what we know already and what is missing.

Thermal Management

  Only thing missing now is the needed thermal resistance of the heat sink Rh.

Thermal Management

  Choose a heat sink for thermal management with an Rth value of < 4.65°C

 Long Life

  LED Lights have the benefit of a super long life span of up to 50,000 hours which means you can cut maintenance costs as the lamps last up to 8-10 times longer than standard halogen lamps making them an ideal replacement.

 Energy Efficient

  A standard 50W halogen lamp turns 90% of electricity used into heat with only 10% into light. The benefit of LED Lights are that they use only 15% of the energy a standard halogen uses, provide up to 85% of the light output and create less heat making them so cool to touch. This makes LED Lights not only energy efficient but extremely cost effective as air conditioning use can be lowered. Some LED Lights can be operated by mains power, but when used with a Low Voltage LED Driver, LED Lights will produce more light output per watt.

 Ecologically Friendly

  LED lights are free of toxic chemicals. Most conventional fluorescent lighting bulbs contain a multitude of materials like e.g. mercury that are dangerous for the environment. LED lights contain no toxic materials and are 100% recyclable, and will help you to reduce your carbon footprint by up to a third. The long operational life time span mentioned above means also that one LED light bulb can save material and production of 25 incandescent light bulbs. A big step towards a greener future!

 Improved Durability

  LED's have no filaments so can withstand a greater intensity of vibration and shock than standard lights making them durable with less risk of breaking and need to replace.

 Zero UV Emissions

  LED illumination produces little infrared light and close to no UV emissions. Because of this, LED lighting is highly suitable not only for goods and materials that are sensitive to heat due to the benefit of little radiated heat emission, but also for illumination of UV sensitive objects or materials such a in museums, art galleries, archeological sites etc.

 Fast Switching

  LED Lights will start at full brightness, instantly, every time; therefore there is no need for backup lighting. LED Lights are a benefit because they switch on and off instantly making them ideal for flashing signs, traffic signaling and automotive lights, compared to standard compact fluorescent lights which fade in and out or flicker.

 Operational in Extremely Cold or Hot Temperatures

  LED are ideal for operation under cold and low outdoor temperature settings. For fluorescent lamps, low temperatures may affect operation and present a challenge, but LED illumination operates well also in cold settings, such as for outdoor winter settings, freezer rooms etc.

 Low-Voltage

  A low-voltage power supply is sufficient for LED illumination. This makes it easy to use LED lighting also in outdoor settings, by connecting an external solar-energy source and is a big advantage when it comes to using LED technology in remote or rural areas.