When the LED first became commercially available, over 30 years ago, no one really paid much attention to how it was made or what it consisted of chemically. This was in part due to the fact that only a few basic types and colors were available (such as GaP - red and green, and GaAsP - yellow). Today, in order to obtain new colors, or wavelengths, and improve performance and reliability, there are many new types of chemical structures being created. Because of this, LEDs are no longer referred to strictly by their color but also by their chemical name, such as InGaAlP or GaAlAs. If the user is not familiar with LED technology or does not have a degree in chemistry and materials, this hodgepodge of letters can be very confusing. The following information is provided to help alleviate some of this confusion.

The first and primary element used in the manufacture of almost all semiconductor LED devices is gallium. Gallium is a metallic material which is found as a trace element in coal, bauxite and other minerals. The symbol for gallium is “Ga” - (atomic number 31). When combined with arsenic “As” (atomic number 33), a highly poisonous gray metallic element, at temperatures of about 4000 degrees Fahrenheit, the compound gallium arsenide “GaAs” is formed. This dark gray crystalline compound is the basis for the original semiconductor LEDs manufactured more than 30 years ago. When current/energy is applied to this material, photons or particles of light are emitted. GaAs by itself emits light in the infrared range which is not visible to the human eye, however, if another element, phosphorus (a highly reactive white or yellow, non-metallic element, occurring naturally in phosphates, with atomic number 15 and symbol “P”) is introduced, a mixed crystal of gallium arsenide phosphide “GaAsP” is formed. Depending on the proportional amount of phosphorus, light in the visible range from red to yellow is achieved.

In addition to GaAsP described above, the material combination gallium phosphide “GaP” was developed. By properly doping this crystal compound, various colors could be obtained. For example, by adding zinc-oxygen to GaP, the color red is obtained. By adding nitrogen, green light is achieved. It is important to note that in almost all semiconductor LED die material, the added elements such as zinc, nitrogen, beryllium etc. are not usually specified in the general material structure acronym. All of the materials mentioned above although developed many years ago, are still widely available and in use today. (Table. 1)

In the late 1970’s, it was discovered that by adding aluminum “Al” (atomic number 13, and the most abundant metallic element in the earth’s crust) to the GaAs compound, a red color could be produced with a brightness and efficiency significantly increased over existing product. Thus, gallium aluminum arsenide “GaAlAs” was formed. Although the combination of gallium, aluminum and arsenic has been around for more than 20 years, the actual format for the elemental configuration varies. Some manufacturers depict the compound as AlGaAs while others call it GaAlAs. Originally, many thought that the material designated first was found in greater quantities than the succeeding elements. If GaAlAs was the designation, then Ga (gallium) was the primary element in the compound. Al (aluminum) would be second and As (arsenide) would be third. This caused many users to believe that if the element order was different, each of the compounds was significantly different. This is an incorrect assumption. The order each element is placed in the compound does not follow standard chemical sequences nor is it required to do so since the exact chemical structure is not specified. GaAlAs are only the “primary” elements used in the compound. All other additional elements or dopants such as zinc or nitrogen and their exact compositions are not listed. Essentially, the only difference between GaAlAs and AlGaAs is in the way the acronym is written.

Recently, this mish-mash of letters and material types has been even further complicated by the introduction of many new compounds such as indium gallium aluminum phosphide “InGaAlP.” With the addition of indium “In” (a soft malleable silvery white metallic compound found primarily in zinc and tin ores with atomic number 49) it was found that not only would the LEDs brightness and efficiency be improved, but the actual lifetime would be significantly increased over current materials such as GaAlAs. Furthermore, with proper doping, a wide variety of colors and wavelengths can be produced. Similar to gallium aluminum arsenide, the acronym for indium gallium aluminum phosphide can be expressed in a number of ways. The two most common are InGaAlP and AlInGaP. Both forms are chemically the same material.

Elements such as (Al, Ga, and In) are called group “III” elements while (P, As, and N) are group “V” elements. Light emitting semiconductor product is typically referred to as “III-V” material derived from the periodic table. Other compounds such as silicon carbide “SiC” which combines silicon (a non-metallic element occurring extensively in the earth’s crust in silica and used for the manufacture of glass, semiconductor devices, pottery etc. with atomic number 14) and carbon (a naturally abundant non-metallic element occurring in all organic or living organisms with atomic number 6) and gallium nitride “GaN” are used in the manufacture of blue LEDs. The acronym for these compounds is generally consistent throughout the industry, although it could be transposed at any time.

Once a chemical compound has been established, the acronym describing that substance can be very subjective to the whims of the manufacturer or developer. It is important to note that one compound should not be misconstrued as being superior or inferior to another compound with the same chemical composition, but a different chemical order.

It’s been over 30 years since the introduction of the first LED (Light Emitting Diode) and we at long last have a white LED that begins to rival incandescent in many architectural and small area illumination applications.

LEDs have enjoyed a tremendous growth over the last several years with new applications ranging from automotive lighting and VMS (Variable Message Signs) to traffic control devices. Much of this is due to the ever-increasing levels of brightness being achieved with new materials and wafer fabrication processes as well as the advances in package and optics design. Several of the most significant areas of expansion however, have resulted from the introduction of the blue LED in the early 1990’s. This allowed for the manufacture of RGB (Full Color) signage as well as pre-empted the development of white LEDs in the late 1990’s.

White light is currently achieved by using two different methods. The first is by combining a blue 450nm – 470nm GaN (gallium nitride) LED with YAG (Yttrium Aluminum Garnet) phosphor. The blue wavelength excites the phosphor causing it to glow white.

The second method is to combine red, green and blue LEDs in the proper proportion to achieve a white color. The former is presently the most dominant and efficient technique used. Recently, new white LEDs have become available by combining a UV (ultra violet) LED, 380nm, with phosphor. Although this method is relatively new, companies such as TOSHIBA are using this technique to obtain white. In addition, by combining different phosphor types with a UV LED, other colors such as purple, orange, pink etc. can be achieved.

The development of a solid state white light has generated a lot of enthusiasm in the industry. This is primarily due to the significant energy savings possible with LED technology as well as the increased lifetimes over standard incandescent lighting. Although white LEDs are currently not yet the most efficient method for illuminating a large area such as an office or room, the white LED has myriad applications in architectural lighting, small area lighting, aerospace and automotive interior lighting, exit signs and emergency lighting, flashlights etc. The goal of this article is to present some basic information that will help the user effectively consider the use of white LEDs in their design.

One of the most common areas of confusion regarding white LEDs is related to efficiency. The standard unit for characterizing the light output of incandescent and fluorescent lamps is the “lumen” which is essentially the total luminous power (luminous flux) emitted from the source. The luminous efficacy is the measure of the light-producing efficiency of the lamp. It is the ratio of the lumens to the total input power of the source. Luminous efficacy is expressed in “lumens/watt”. Many tests have been performed to evaluate the efficiency of LEDs compared to incandescent or compact fluorescent. Unfortunately, oftentimes erroneous conclusions are made from the data obtained. For example, LEDs are typically focused or directional devices unlike standard incandescent or fluorescent lamps. If data is obtained only by measuring the light falling on a small area such as a photo sensor, it may yield abnormally high efficiency results for the LED. There is still a significant amount of light being produced by the incandescent or fluorescent that is not being measured. If a reflector were placed behind the incandescent bulb, essentially simulating the LED emitting angle, the efficiency values obtained would be more accurate.

This is an important concept to understand since there is incorrect information circulating in the industry stating LEDs are significantly more efficient than most other forms of lighting. This is not always the case especially in large area applications such as general office or room lighting where existing technology such as compact fluorescent would still be a far more cost effective and practical source of illumination.

In the example shown in “table 1″, a comparison was made using a standard 7 watt night light along with white LEDs. If we look at the total lumen output of the incandescent lamp, which in this case is approximately 43 lumens, we would need 43 LEDs at 1 lumen each to obtain the equivalent value. Although the LEDs are more efficient than the incandescent bulb, the cost is significantly higher. In actual use, however, all 43 lumens of the 7W bulb are not effectively utilized since only a small area needs to be illuminated, not the entire room. Therefore, 2 – 3 LEDs may be more than adequate to light the desired area. In this case, not only would switching to LEDs be significantly more efficient, the cost may actually be less. A similar comparison using a 100W bulb would show that hundreds of LEDs would be required to sufficiently illuminate a room, which obviously is not a very cost effective solution.

7W Incandescent
White LED Cluster
White LED

Lumen
43
43
1

Power Consumption
7 Watts
4.8 Watts
2.4 Watts

Cost
<$1.29
<$15.00
<$1.05
White LEDs are slightly more efficient than a 100W incandescent bulb and more than three times more efficient than a 7W night light type bulb as shown in table 2. (Smaller incandescent bulbs are far less efficient than larger types and thus are more suited for replacement by LEDs)

As can be seen, white LEDs are a good choice for use in small area lighting such as in desk lamps, task lighting, exit signs and night lights in addition to backlighting for switch panels, dashboards, palm tops etc.

Compact Fluorescent
100W Incandescent
7W Incandescent
White LED

Lumen/Watt 60
17
6
20

Lumen 1380
1680
43
1 - 2 (Single LED)

Lifetime (hrs.) 3,000 - 10,000
750 - 1500 (std.)
3000
> 10,000 (50% output)

Cost/Lumen <$.01
<$.001
<$.03
<$.35

There are also other issues that may warrant the use of LEDs over incandescent.

The typical incandescent bulb emits a considerable amount of energy in the infrared portion of the spectrum, which cannot be seen but is felt as heat. In an environment where many incandescent bulbs are used, such as a room or in a lighted switch panel, the ambient temperature can significantly increase. This may require additional cooling or air conditioning to maintain a specified temperature which raises energy consumption. By eliminating this radiation with LEDs that virtually run cool, these costs can be reduced.

An additional factor that must be considered is the useful life of the light source being utilized. Lamp life for incandescent and compact fluorescent is usually defined as the time at which 50% of the test samples have “burned out” or failed to properly operate. LEDs, however, do not generally fail catastrophically but decrease in output over time. Therefore, the life of an LED is commonly considered to be the time at which 50% of the initial brightness remains. It is generally assumed that LEDs have an expected lifetime of 100,000 hours or more if operated within the specified parameters, however, this is not always the case. The lifetime of an LED is dependent on many factors including drive current, type of material used (InGaN, InGaAlP, GaAlAs), environment, packaging and assembly. Since the white LED is relatively new and process improvements are being made at an extremely rapid pace, there is very little long-term data available. Currently, lifetimes of 10,000 hours or greater can be obtained from well-made white LEDs. This is likely to improve greatly over the next several years, closer to the 100,000 hour level typically expected from LEDs. Manufacturers are working on several areas to improve reliability including better heat sinking methods, improved phosphors and encapsulation epoxy. The proper assembly for white LEDs is important; therefore, care should be taken when choosing a supplier of white LEDs. The manufacturer should have experience in the assembly of GaN type product (the basis for white) as well as an understanding of phosphors. In addition, they should be able to provide even preliminary data as to the estimated lifetime and reliability of the LED.

The future for solid-state white lighting is very promising. New methods for achieving the required luminosity and efficiency are constantly being introduced such as increasing the size and shape of the semiconductor device, improving phosphor performance and clustering a number of individual LED devices together. Although LEDs need to produce approximately 100 lumens/watt to effectively compete in the general illumination market, at the current 20 lumens/watt (50 lumens/watt possible in the next several years), there are many uses for white LEDs that warrant serious consideration as mentioned previously. In these applications, white LEDs are ultimately the most reliable and efficient choice.

If there were ever any question that LEDs would be such a dominating lighting technology just take a look around. They are virtually everywhere. From standard indicators on stereo equipment, laptops and toys to traffic lights, variable message signs and automotive lighting, LEDs have enjoyed an explosive growth over the last several years with no end in sight.Much of this is due to the ever-increasing levels of brightness being achieved with new materials and wafer fabrication processes as well as the advent of blue and white LEDs for RGB (Full Color) and general illumination applications. As the level of sophistication for using LEDs rises, so does the need for accurate measurements of the LEDs optical properties. I have been in the Optoelectronics industry for nearly 20 years now and by far the most common type of question I am always asked is related to the measurement of an LED. (What is a lumen? How do I convert from lumens to candela? How do I accurately measure brightness? Why is my measurement not the same as yours?) I hope to address these and many similar issues in the subsequent article.

The details of optical measurement in this discussion will be broken down into four separate but interrelated topics. These are: Photometric quantities, Radiometric quantities, Wavelength or Chromaticity quantities and finally Angular or Goniometric quantities. Although an entire book can easily be written on the units, standards and test methodologies of the above, I will try to summarize the more common and basic areas of interest.

Photometry is simply the measurement of light in the visible spectrum. (approximately 380nm – 770nm) This is light seen by the naked eye of an average human observer. There are many different types of photometric units such as nits (cd/m2), lux (lumen/m2), footcandles (lumen/ft2), stilb (cd/cm2) etc. All of these are based on two basic photometric standards, the LUMEN and the CANDELA.

The Candela is the unit of luminous intensity, which can be defined as the amount of luminous flux (total luminous power emitted from a source and expressed as lumens) per unit solid angle in a given direction. The Lumen can be defined as the luminous flux emitted per unit solid angle from a uniform point source whose luminous intensity is 1 candela. (1 candela = 1 lumen/steradian) It is also important to understand the definition of steradian, which is the solid angle (cone) at the center of a sphere of radius “r” that subtends an area “r2″ on the surface of the sphere. (See figure 1) The surface area of a sphere is 4p r2; therefore, a sphere has 4p steradians.

FIG 1 - Solid Angle “w” of a Spherical Sector

Most standard LEDs supplied today are measured in candelas, however, due to the increasing demand for LEDs as a replacement to incandescent in the general illumination market, the Lumen is now often used as a unit of measurement for light output. A simple method for converting from Candela to Lumens is shown in figure 2. Although empirical calculations are possible for converting many different types of photometric units, the actual measured value may differ from the calculated value due to variation in the spatial radiation characteristics of the LED. In most cases, the empirical calculation is sufficiently accurate.

FIG 2 - Converting Candelas to Lumens

Step 1.) Obtain the solid angle of the LED

w = p (q )2

w = p (25)2 , assuming the LED half angle is 25º

w = p (.43633)2, convert degrees to radians

w = .598

Step 2.) Calculate Lumens

f = Iv * w

f = 2.00 * .598, assuming the LED brightness is 2000mcd

f = 1.196 Lumens

Additional photometric conversions are shown in figure 3. Although there are myriad other conversions that are possible, for example candela to nit or lambert to candela, there is no easy direct multiplication factor that can be used. Information such as the area of the source and/or detector, or the measurement distance and angular characteristics may be required.

Illuminance quantities lux (lx)
footcandle (fc)
phot (ph)

lux (lx) 1
0.0929
0.0001

footcandle (fc) 10.76
1
0.00108

phot (ph) 10,000
929
1

Luminance quantities nit (nt)
stilb (sb)
footlambert (fl)
lambert (L)
apostilb (asb)

nit (nt) 1
0.0001
0.2919
0.00031416
3.1416

stilb (sb) 10,000
1
2919
3.1416
31,416

footlambert (fl) 3.426
0.0003426
1
0.0010764
10.764

lambert (L) 3183
0.3183
929
1
10,000

apostilb (asb) 0.3183
0.0000318
0.0929
0.0001
1

The photometric measurement of LEDs can be more of an art than an exact science. There are various geometry, electrical and assembly issues that can greatly affect the optical properties of LEDs. Because no two LEDs are exactly alike there are steps that will greatly enhance the accuracy of your measurement. These include but are not limited to:

• Knowing the LEDs light emission optical center vs. mechanical center. – When placing LEDs into a typical test fixture, it is usually placed in such a way as to assume the light is emanating from the device’s mechanical center. This is frequently not the case. (See figure 4) The optical center often deviates 5º or greater from the LED mechanical center. Although this may not present much of a problem when measuring devices with a wide viewing angle such as 40º or greater, for narrow angle devices, the differences in readings can be considerable. (It should be noted that the CIE – Commission Internationale de l’Eclairage – recommendation is to use the mechanical axis of the LED rather than the optical axis as the measurement reference)
• Measuring the output at a specific time interval or when stabilized. – When the LED is first energized, the temperature of the junction increases due to the electrical power consumed. (The junction temperature of the LED is determined by Tj = Ta + (Vf * If) * Rth (j-a)) It may take several seconds or several minutes before the light output has reached thermal equilibrium and a stabilized value. A decrease in output of 5 –20% or more is not uncommon – This is not permanent degradation and will recover upon de-energizing. It is often not practical to wait extended periods of time when many LEDs require testing, therefore, a set time interval such as 5 seconds is often established even though the output may not be stabilized.
• Insuring the ambient temperature is consistent during testing. – LEDs typically change in brightness and color with temperature. As the temperature rises, the output decreases and the color shifts towards the higher end of the spectrum. This will be elaborated upon in the Colorimetry discussion.
• Always use a constant current source. – The forward voltage (Vf) of an LED can fluctuate from device to device, therefore, if a standard power supply or voltage source is used, each LED may not receive the same current.
• Use an easily reproducible test setup. – Elaborate setups may be fine for laboratory type measurements, however, when many LEDs require testing, each with different package styles, viewing angles, colors etc., a system that can be quickly modified while insuring identical alignment of the mechanical axis and guaranteeing the detector always sees the same section of the emission cone is required.
• Insure all equipment is properly maintained and calibrated

FIG 4 – Angle Deviation

Radiometry refers to total radiation or the measurement of all light whether in the visible, infrared, or ultraviolet spectrum. The basic unit of radiometric optical power (Radiant Power) is the watt (W). The watt is an absolute unit because it is independent of wavelength. One watt of infrared light contains as much power as one watt of visible light. Other radiometric terms that are commonly measured are radiant intensity (Watts/Steradian), Irradiance (W/m2) and Radiance (W/m2 sr). The primary method for measuring total radiant power/luminous flux is by using an integrating sphere. (See figure 5)

FIG 5 – Integrating Sphere

he integrating sphere measures light emitted from the LED in all directions. Generally these measurements are independent of viewing angle and not subject to angular measurement inaccuracies seen when testing photometrically, however, errors are still possible. Sphere diameters of approximately 3 and 6 inches are widely used. If accuracy is critical, the larger diameter types are preferred due to the favorable ratio of the sphere area to the size of the LED and ports, however, this also results in a loss of intensity. A major source of measurement error has been where to position the LED inside the integrating sphere. The latest specification adopted by the CIE , Publication 127, states that the entire package of the LED should be inside the sphere which is called a 2p luminous flux measurement.

The same precautions used for measuring LEDs photometrically should also be followed when making radiometric measurements. As with photometric conversions, there are a myriad of radiometric conversions that are possible given the appropriate information.

Radiometric values are normally required for applications used in conjunction with a photo detector such as in fiber optics, scanning or sensing.

The scientific measurement and quantification of LED color is called Colorimetry. Its units are typically given as chromaticity coordinates or in wavelength. Color perception is very complicated because it not only depends on the various physical properties of light but also on things such as surrounding objects, the devices mechanical properties, the viewer’s eye response as well as their psychological state. The CIE has established standards for the measurement of visible light as it relates to the “standard human eye response.” This so called standard observer curve was first established in 1931 (see figure 6a). From this curve, the tristimulus values for accurately defining a color are obtained. The X, Y, Z tristimulus system is based on the assumption that every color is a combination of three primary colors; red, green and blue. The 1931 CIE Chromaticity Diagram (see figure 6b) is derived from the tristimulus values by the following:

x = X/(X+Y+Z) or x = Red/(Red + Green + Blue)

y = Y/(X+Y+Z) or y = Green/(Red + Green + Blue)

Since (x + y + z) =1, the third axis, z = 1 – (x + y)

FIG 6a – 1931 CIE Tristimulus Color Matching Curves

FIG 6b – CIE 1931 Chromaticity Diagram

The chromaticity coordinates are normally specified by the x and y axis only. In general, most specifications provided by LED manufacturers do not list the chromaticity coordinates, but rather the peak and dominant wavelength (unless the LED is white). The dominant wavelength, specified in nanometers, is obtained from the color coordinates discussed above. It is essentially the color that is actually perceived by the human eye.

The peak wavelength is the wavelength at the maximum spectral intensity. The peak value is easy to obtain and is therefore the most common value specified by LED manufacturers, however, it has little practical significance for applications that are viewed with the human eye since two LEDs may have the same peak wavelength but can be perceived as different colors.

Currently, the most accurate method for measuring color is by using a Spectroradiometer. This device performs a complete spectral power distribution of the source being measured from which all photometric, radiometric and colorimetric parameters can be mathematically calculated. The wavelength accuracy of the equipment should be better than .5nm with .1nm preferred. As mentioned previously, there are several factors that can affect the value obtained. One of these is temperature. As the ambient temperature rises, so to does the LED wavelength. This increase will typically be from .1nm/ºC - .2nm/ºC depending on the type of LED used.

The final topic for discussion is goniometric or angular characterization. A Goniometer is a device that measures the spatial distribution or radiation pattern of an LED (see figure 8). This can be accomplished by either moving the detector around the LED or by tilting the LED while the detector remains stationary. In either case, several output measurements are taken for each angle as a rotation from 0º – 180º is performed. Upon completion, a profile of the radiated beam in one plane is obtained. It is often assumed that since most LEDs are round, the radiation pattern is symmetrical. This even seems to be indicated by the graphical representations of viewing angle provided by many LED manufacturers. This is often not the case. As mentioned previously, the geometric and assembly variations that occur during the manufacturing of LEDs can greatly affect its optical properties. It may be necessary to perform an additional scan and record multiple planes of view. In addition, some special shaped LEDs such as the oval or elliptical type, essentially have two radiation patterns (30º x 70º for example), therefore, both a 0º and 90º scan of the device is necessary. If a goniometer is not readily available, it is possible to obtain a crude radiation pattern by using a photo detector and manually rotating the LED or detector, recording the output levels and plotting the data points, however, this can be very tedious and time-consuming.

FIG 8 – Radiation Pattern

It should be clear from what has been discussed that the measurement of light can be highly inaccurate compared to other more specific electrical characteristics such as voltage, current or resistance. There are many factors such as color, device geometry, alignment of the LED into a test fixture, temperature etc. that can induce measurement error. It is often classified as being more of an art than a science. Although a measurement accuracy of ±5% is still considered standard and widely acceptable in the industry, with careful attention, accuracies of better than ±2.5% are possible.

A light emitting diode (LED) is essentially a PN junction semiconductor diode that emits a monochromatic (single color) light when operated in a forward biased direction. The basic structure of an LED consists of the die or light emitting semiconductor material, a lead frame where the die is actually placed, and the encapsulation epoxy which surrounds and protects the die (Figure below shows both standard lamp type and surface mount type).

History

The first commercially usable LEDs were developed in the 1960’s by combining three primary elements: gallium, arsenic and phosphorus (GaAsP) to obtain a 655nm red light source. Although the luminous intensity was very low with brightness levels of approximately 1-10mcd @ 20mA, they still found use in a variety of applications, primarily as indicators. Following GaAsP, GaP, or gallium phosphide, red LEDs were developed. These devices were found to exhibit very high quantum efficiencies, however, they played only a minor role in the growth of new applications for LEDs. This was due to two reasons: First, the 700nm wavelength emission is in a spectral region where the sensitivity level of the human eye is very low (Figure 2) and therefore, it does not “appear” to be very bright even though the efficiency is high (the human eye is most responsive to yellow-green light). Second, this high efficiency is only achieved at low currents. As the current increases, the efficiency decreases. This proves to be a disadvantage to users such as outdoor message sign manufacturers who typically multiplex their LEDs at high currents to achieve brightness levels similar to that of DC continuous operation. As a result, GaP red LEDs are currently used in only a limited number of applications.

As LED technology progressed through the 1970’s, additional colors and wavelengths became available. The most common materials were GaP green and red, GaAsP orange or high efficiency red and GaAsP yellow, all of which are still used today (Table3). The trend towards more practical applications was also beginning to develop. LEDs were found in such products as calculators, digital watches and test equipment. Although the reliability of LEDs has always been superior to that of incandescent, neon etc., the failure rate of early devices was much higher than current technology now achieves. This was due in part to the actual component assembly that was primarily manual in nature. Individual operators performed such tasks as dispensing epoxy, placing the die into position, and mixing epoxy all by hand. This resulted in defects such as “epoxy slop” which caused VF (forward voltage) and VR (reverse voltage) leakage or even shorting of the PN junction. In addition, the growth methods and materials used were not as refined as they are today. High numbers of defects in the crystal, substrate and epitaxial layers resulted in reduced efficiency and shorter device lifetimes.

TABLE 3

It wasn’t until the 1980’s when a new material, GaAlAs (gallium aluminum arsenide) was developed, that a rapid growth in the use of LEDs began to occur. GaAlAs technology provided superior performance over previously available LEDs. The brightness was over 10 times greater than standard LEDs due to increased efficiency and multi-layer, heterojunction type structures. The voltage required for operation was lower resulting in a total power savings. The LEDs could also be easily pulsed or multiplexed. This allowed their use in variable message and outdoor signs. LEDs were also designed into such applications as bar code scanners, fiber optic data transmission systems, and medical equipment. Although this was a major breakthrough in LED technology, there were still significant drawbacks to GaAlAs material. First, it was only available in a red 660nm wavelength. Second, the light output degradation of GaAlAs is greater than that of standard technology. It has long been a misconception with LEDs that light output will decrease by 50% after 100,000 hours of operation. In fact, some GaAlAs LEDs may decrease by 50% after only 50,000 -70,000 hours of operation. This is especially true in high temperature and/or high humidity environments. Also during this time, yellow, green and orange saw only a minor improvement in brightness and efficiency which was primarily due to improvements in crystal growth and optics design. The basic structure of the material remained relatively unchanged.

Current Developments

To overcome these difficult issues new technology was needed. LED designers turned to laser diode technology for solutions. In parallel with the rapid developments in LED technology, laser diode technology had also been making progress. In the late 1980’s laser diodes with output in the visible spectrum began to be commercially produced for applications such as bar bode readers, measurement and alignment systems and next generation storage systems. LED designers looked to using similar techniques to produce high brightness and high reliability LEDs. This led to the development of InGaAlP (Indium Gallium Aluminum Phosphide) visible LEDs. The use of InGaAlP as the luminescent material allowed flexibility in the design of LED output color simply by adjusting the size of the energy band gap. Thus, green, yellow, orange and red LEDs all could be produced using the same basic technology. Additionally, light output degradation of InGaAlP material is significantly improved even at elevated temperature and humidity.

As a result of these developments, much of the growth for LEDs in the 1990’s was concentrated in three main areas: The first was in traffic control devices such as stop lights, pedestrian signals, barricade lights and road hazard signs. The second was in variable message signs such as the one located in Times Square New York which displays commodities, news and other information. The third concentration was in automotive applications.

The visible LED has come a long way since its introduction more than 30 years ago and has yet to show any signs of slowing down. Blue LEDs, which were introduced in the early to mid 1990’s, have become the cornerstone to an entire generation of new applications. Blue LEDs because of their high photon energies (>2.5eV) and relatively low eye sensitivity (465nm typical wavelength) have always been difficult to manufacture. In addition the technology necessary to fabricate these LEDs is very different and far less advanced than standard LED materials. The blue LEDs available today consist of GaN (gallium nitride) and SiC (silicon carbide) construction with brightness levels in excess of 10000mcd @ 20mA. Since blue is one of the primary colors, (the other two being red and green), full color solid state LED signs, TV’s etc. are becoming commercially available. The first decade of the 21st century will see a large growth in RGB (full color) LED applications. Other applications for blue LEDs include medical diagnostic equipment and photolithography.

It is also possible to produce other colors using the same basic GaN technology and growth processes. For example, a high brightness green (approximately 500nm – 530nm) LED has been developed that is currently being used as a replacement to the green bulb in traffic lights. Other colors including purple are also possible. With the introduction of blue LEDs, it became possible to produce white light probably the most exciting new development in LED technology to date. White light is currently made in one of 2 ways. The first is by selectively combining the proper combination of red, green and blue light. This process however, requires sophisticated software and hardware design to implement. In addition, the brightness level is low and the overall light output of each RGB die being used degrades at a different rate resulting in an eventual color unbalance. The 2nd and most dominant method of achieving white light output is to use a phosphor coating (typically - Yttrium Aluminum Garnet or YAG) on the surface of a blue LED. The blue die excites the phosphor causing it to glow white.

In summary, LED’s have gone from infancy to adolescence and are experiencing some of the most rapid market growth of their lifetime. By using InGaAlP material with MOCVD as the growth process, combined with efficient delivery of generated light and efficient use of injected current, some of the brightest, most efficient and most reliable LEDs are now available. This technology together with other novel LED structures will ensure wide application of LEDs. Further developments on white light output will also guarantee the continued increase in applications of these economical light sources and may eventually replace standard incandescent and fluorescent lighting.

 

 

 

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Super bright LEDs are widely used as signals in instruments and meters.With different emitting light colors like white,blue,green and red colour,can in shape of round,oval and rectangular.
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