LED lamp
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A LED lamp is a type of solid state lighting that utilizes light-emitting diodes (LEDs) as a source of illumination rather than electrical filaments or gas.
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[edit] Models
LED lamps come in different shapes, among them the light bulb shape with a large E27 Edison screw. Other models might have a small Edison E14 fitting, GU5.3 (Bipin cap) or GU10 (bayonet socket).
In LED lamps the luminous flux (in lumen), luminous intensity (in millicandela) and the radiation angle (in °) are related.
[edit] Radiation angle
- At a narrower radiation angle the luminous intensity (clarity) becomes larger whereas the luminous flux (quantity of light) doesn't change.
For example: A 1000 mcd 30° led radiates just as much light as a 4000 mcd 15° led. The radiation angle has been halved in both width and height multiplying the luminous intensity four times. See also MR16.
[edit] Luminous flux
luminous flux is a measure for the quantity of energy that is transmitted by a light source in all directions. The SI unit of luminous flux is the lumen (lm). e.g. the luminous flux of a 40 Watt light bulb is 450 lm.
[edit] Luminous intensity
The luminous intensity of a light source is the density of that flux emitted in a given direction. For LED's This is expressed in millicandela (mcd). 1000 millicandelas equals 1 candela. E.g. the luminous intensity is 590.000 mcd.
[edit] Convert Millicandelas to Lumen
To calculate lumen from millicandelas, take the number of candelas, divide it with the number that belongs to the radiation angle of the lamp (see MR16).
- Example: 590,000 mcd = 590 cd, a lamp with a radiation angle of 40°, 590/2,64= approx. 223,48 Lumen.
| Millicandela | radiation angle | Divide by |
|---|---|---|
| 5° | 167,22 | |
| 10° | 41,82 | |
| 15° | 18,50 | |
| 20° | 10,48 | |
| 25° | 6.71 | |
| 30° | 4,67 | |
| 35° | 3,44 | |
| 40° | 2,64 | |
| 45° | 2,09 |
[edit] Color temperature
Color temperature can be indicated in Kelvin or Mired (1 million divided by the colour temperature in Kelvin).
| Color temperature | Kelvin | Mired |
|---|---|---|
| "Warm white" or "Soft white" | < 2700 Kelvin | 370 M |
| "White", "Bright White", or "Medium White" | 2900 - 3000 Kelvin | 333- 345 M |
| "Cool white" | 4000 Kelvin | 250 M |
| "Daylight" | > 5000 Kelvin | 200 M |
[edit] History
For the past 150 years, lighting technology was mainly limited to incandescence and fluorescence. While derivative technologies such as high-intensity discharge lamps (HID) have emerged, none has achieved energy efficacies exceeding 200 lm/W (for monochromatic low pressure sodium lamps), with incandescent lighting usually achieving an efficacy of less than 18 lm/W. With the advent of commercial LEDs in the 1960s, however, a new kind of lighting became available. LEDs can consume less electricity than conventional lighting and can produce less of the parasitic by-product heat. However, at present, commercial LED systems are not as efficient as fluorescent lighting. See luminous efficacy for a comparison.
Initial LEDs were red in color, with yellow and orange variants following soon thereafter. To produce a white SSL device, however, a blue LED was needed, which was later discovered through materials science and extensive research and development. In 1993, Shuji Nakamura of Nichia Chemical Industries came up with a blue LED using gallium nitride (GaN). With this invention, it was now possible to create white light by combining the light of separate LEDs (red, green, and blue), or by creating white LEDs themselves by means of doping.
SSL has been described by the United States Department of Energy as a pivotal emerging technology that promises to alter lighting in the future.[citation needed] It is the first new lighting technology to emerge in over 40 years and, with its energy efficiencies and cost savings, has the potential to replace many existing fixtures.
[edit] Technology overview
A single LED can produce only a limited amount of light, and only a single color at a time. To produce the white light necessary for SSL, light spanning the visible spectrum (red, green, and blue) must be generated in correct proportions. To achieve this effect, three approaches are used for generating white light with LEDs: wavelength conversion, color mixing, and most recently Homoepitaxial ZnSe.
Wavelength conversion involves converting some or all of the LED’s output into visible wavelengths. Methods used to accomplish this feat include:
- Blue LED & yellow phosphor – Considered the least expensive method for producing white light. Blue light from an LED is used to excite a phosphor which then re-emits yellow light. This balanced mixing of yellow and blue lights results in the appearance of white light.
- Blue LED & several phosphors – Similar to the process involved with yellow phosphors, except that each excited phosphor re-emits a different color. Similarly, the resulting light is combined with the originating blue light to create white light. The resulting light, however, has a richer and broader wavelength spectrum and produces a higher color-quality light, albeit at an increased cost.
- Ultraviolet (UV) LED & red, green, & blue phosphors – The UV light is used to excite the different phosphors, which are doped at measured amounts. The colors are mixed resulting in a white light with the richest and broadest wavelength spectrum.
- Blue LED & quantum dots – A process by which a thin layer of nanocrystal particles containing 33 or 34 pairs of atoms, primarily cadmium and selenium, are coated on top of the LED. The blue light excites the quantum dots, resulting in a white light with a wavelength spectrum similar to UV LEDs.
Color mixing involves utilizing multiple LEDs in a lamp and varying the intensity of each LED to produce white light. The lamp contains a minimum of two LEDs (blue and yellow), but can also have three (red, blue, and green) or four (red, blue, green, and yellow). As no phosphors are used, there is no energy lost in the conversion process, thereby exhibiting the potential for higher efficiency.
Homoepitaxial ZnSe is a technology developed by Sumomito Electric where a LED is grown on a ZnSe substrate, which simultaneously produces blue light from the active region and yellow emission from the substrate. The resulting white light has a wavelength spectrum on par with UV LEDs. No phosphors are used, resulting in a higher efficiency white LED.
To be considered SSL, however, a multitude of LEDs must be placed close together in a lamp to amplify their illuminating effects. This is because an individual LED produces an only limited amount of light, thereby limiting its effectiveness as a replacement light source. In the case where white LEDs are utilized in SSL, this is a relatively simple task, as all LEDs are of the same color and can be arranged in any fashion. When using the color-mixing method, however, it is more difficult to generate equivalent brightness when compared to using white LEDs in a similar lamp size. Furthermore, degradation of different LEDs at various times in a color-mixed lamp can lead to an uneven color output. Because of the inherent benefits and greater number of applications for white LED based SSL, most designs focus on utilizing them exclusively.
[edit] Advantages of SSL
[edit] Technological comparison
SSL is intended to be a cost-effective yet high quality replacement for incandescent and fluorescent lamps. To better understand the technical merits of SSL, it is important to understand the technology behind the lamps it intends to replace.
- Incandescent lamps (light bulbs) create light by running electricity through a thin filament, thereby heating the filament to a very high temperature and producing visible light. The incandescing process, however, is considered highly inefficient, as over 98% of its energy is emitted as invisible infrared light (or heat). Incandescent lamps, however, are relatively inexpensive to produce. The typical lifespan of an incandescent lamp is around 1,000 hours.
- Fluorescent lamps (light bulbs) work by passing electricity through mercury vapor, which in turn produces ultraviolet light. The ultraviolet light is then absorbed by a phosphorus coating inside the lamp, causing it to glow, or fluoresce. While the heat generated by fluorescent lamps is much less than its incandescent counterpart, efficiencies are still lost in generating the ultraviolet light and converting this light into visible light. In addition, mercury is detrimental to health, and should the lamp break, exposure to the substance can be hazardous. Fluorescent lamps are typically five to six times the cost of incandescent lamps, but have life spans around 10,000 hours.
- SSL achieves its purpose by grouping smaller LEDs in an orderly fashion, thereby creating a unified beam. The SSL can be comprised of multiple white LEDs, or from ones that are color-mixed—where LEDs of different colors are mixed to produce white light. The inherent advantages and disadvantages of SSL are the same as those of an LED. Advantages include:
- High durability - no filament or tube to break
- Long life span - LEDs last approximately 100,000 hours
- Low power consumption - reduces overall electricity bill
- Flexible application – small size of LEDs can lead to unique lighting devices. For example, with a cluster of LEDs a wide variety of illumination distributions can be generated[1],[2].
- Low heat generation – very little parasitic energy loss
Currently, however, there is no SSL on the market that can be offered as a true replacement for incandescent or fluorescent lamps, even though several manufacturers have gone forward with the introduction of such products. White LEDs produced today are too expensive to be considered affordable, and the lumens produced by the LEDs today are not as bright as traditional lighting. Future developments in LED technologies, however, will address most of these issues. Based on research conducted by the Department of Energy (DOE) and the Optoelectronics Industry Development Association (OIDA), it is expected that by the year 2025, SSL will be the preferred method of illumination in homes and offices.
The following chart, derived from information from Sandia National Laboratories, compares a perfected SSL device (to be released before 2025) with incandescent and fluorescent lights
| Technology | Future solid state lighting | Incandescent | Fluorescent |
|---|---|---|---|
| Luminous efficacy (lm/W) | 200 | 16 | 85 |
| Lifetime (kh) | >100 | 1 | 10 |
| Flux (lm/lamp) | 1,500 | 1,200 | 3,400 |
| Input power (W/lamp) | 7.5 | 75 | 40 |
| Lumen cost ($/klm) | < 2 | 0.4 | 1.5 |
| Lamp cost ($/lamp) | <3 | 0.5 | 5 |
| Color Rendering Index (CRI) | >80 | 95 | 75 |
[edit] Benefits
In 2001, the United States consumed over 7.2 quads (7.2×1015 BTU = 7.6 EJ) of energy on lighting for commercial, residential, and industrial buildings. (US DOE). With America’s steady growth and limited resources, this continued rate of consumption is not sustainable. Recognizing the need for change, the DOE has set a goal to reduce electric lighting consumption 50% by 2025. SSL technologies are uniquely positioned to address this need, and at the same time
- reduce CO2 emissions, thereby positively affecting the greenhouse effect
- decrease by 50% the global amount of electricity used for lighting
- provide higher quality lighting
- decrease by 10% the total global consumption of electricity (projected to be about 1.8 TW·h/yr, or $120 billion per year, by the year 2025)
- reduce projected 2025 global carbon emissions by about 300 million metric tons per year
- create new industries and jobs
The U.S. Government, by way of the DOE and other agencies, has funded millions of dollars in research grants and projects relating to the development of a high quality yet affordable SSL. A major motive of funding such research, in addition to its environmental impact and energy savings potential, is to decrease dependence on foreign fossil fuels.
[edit] Challenges
[edit] Technological hurdles
The current manufacturing process of white LEDs has not matured enough to be produced cost-effectively. Among the manufacturing hurdles to overcome include improving the processes used to deposit the active semiconductor layers of the LED, thereby increasing yields and throughput as well as decreasing costs. Problems with phosphors and their ability to emit a broader wavelength spectrum light have also been an issue. In particular, the untunability of absorption and emission, and inflexibility of form in phosphors have been issues in their spectral capabilities.
More apparent to the end user, however, is the low Color Rendering Index (CRI) of current LEDs. The CRI is widely used to measure how accurately a lighting source renders the color of objects. Sunlight and incandescent lamps have CRI of 100, while fluorescent lamps have CRI >75. The current generation of LEDs, which employs mostly blue LED chip + yellow phosphor, has a CRI around 70, which is much too low for widespread use in lighting particularly indoors. In order for SSL to effectively replace incandescent lamps, more research must be done on developing alternatives to the techniques currently used that address these concerns.
Variations of CCT (color correlated temperature) at different viewing angles present another formidable obstacle against widespread use of white LED. It has been shown, that CCT variations can exceed 500 K, which is clearly noticeable by human observer, who is normally capable of distinguishing CCT differences of 50 to 100 K in range from 2000 K to 6000 K, which is the range of CCT variations of daylight.
[edit] Adaptation hurdles
Potential pitfalls to the widespread adaptation of SSL devices include lighting fixture issues and general consumer resistance. Fixture issues can be overcome either by replacement of the fixture, or a modified SSL device that would fit into the socket. With the ubiquity of SSL, it is believed that any customer resistance will be dissipated over time.
[edit] Research and development
In order to further the development of SSL technology, the DOE has committed more than $50 million on over 45 applied research projects, including short- and long-term projects at large and small businesses, universities and national labs alike. Part of the department's goals include developing a better quality, lower cost, and highly efficient white LED.
Other agencies and universities contributing to SSL development include:
- Sandia National Laboratories
- Rensselaer Polytechnic Institute
- University of California, Santa Barbara
- National Electrical Manufacturers Association
[edit] Future
Every 18 months the luminous intensity of LED lamps doubles. The expectation is that in 2010 the luminous intensity will be high enough to replace the extra long fluorescent lamps.
[edit] See also
[edit] External links
- Solid State Lighting Design
- The Promise and Challenge of Solid-State Lighting
- Variations of light characteristics of white LEDs
- LED professional
| Incandescent: | Conventional - Halogen - Parabolic aluminized reflector (PAR) |  |
| Fluorescent: | Compact fluorescent (CFL) - Linear fluorescent - Induction lamp | |
| Gas discharge: | High-intensity discharge (HID) - Mercury-vapor - Metal-halide - Neon - Sodium vapor | |
| Electric arc: | Arc lamp - HMI - Xenon arc - Yablochkov candle | |
| Combustion: | Acetylene/Carbide - Candle - Gas lighting - Kerosene lamp - Limelight - Oil lamp - Safety lamp - Petromax | |
| Other types: | Sulfur lamp - Light-emitting diode (LED) - LED lamp (SSL) - Fiber optics - Plasma - El wire | |
