21-09-2008, 11:12 PM
There are two ways of producing high intensity white-light LED. One is to first produce individual LEDs that emit three primary colors ? red, green, and blue, and then mix all the colors to produce white light. Hence the product is called multi-colored white LEDs (sometimes referred to as RGB LEDs). Because its mechanism is involved with sophisticated electro-optical design to control the blend and diffusion of different colors, this approach has rarely been used to mass produce white LEDs in the industry. Nevertheless this method is particularly interesting to many researchers and scientists because of the flexibility of mixing different colors. In principle, this mechanism also has higher quantum efficiency in producing white light. On the other hand, the second method of producing white LED is involved with coating a LED of one color (mostly blue LED made of InGaN) with phosphor coating of a different color to produce white light. Depending on the color of the original LED, phosphors of different colors can also be employed. By applying several phosphor layers of distinct colors, we can effectively increase the color rendering index (CRI) value of a given LED. The term CRI will be defined more elegantly in the following section. Because this method of producing white LEDs heavily employs the usage of phosphor, the resultant LEDs are called phosphor based white LEDs. Although easier to be manufactured than multi-colored LEDs, phosphor based LEDs have a lower quantum efficiency and other phosphor-related degradation issues. However, it is still the most popular technique of manufacturing high intensity white LEDs as well as high intensity LEDs of other colors because it requires much easier material processing and therefore suits today?s applications. Much effort has been spent on optimizing the operating environment, namely temperature and current, for this type of LED.
There are several types of multi-colored white LEDs: di-, tri-, and tetrachromatic white LEDs. Several key factors that play among these different approaches include color stability, color rendering capability, and luminous efficiency. Luminous efficiency is a term expressing the luminous flux per unit electrical input power. It is a key factor in discussing energy efficiency. In principle, if perfect solid-state lighting devices can be fabricated, the same level of luminance can be achieved by using merely 1/20 of the energy that incandescent lighting source requires. Color stability is a self-explanatory term which means the stability of color. Color rendering capability is hard to grasp without being traced back to its origin. In 1777, George Palmer first found that an object?s perceived color strongly depends on the illumination source.[20] He discovered that by varying the illumination sources, an object?s color appeared differently. Because of their conflicting nature, there is always a trade off between the luminous efficiency and color rendering. For example, the dichromatic white LEDs have the best luminous efficiency (120 lm/W), but the lowest color rendering capability. Oppositely although tetrachromatic white LEDs have excellent color rendering capability, they often have poor luminous efficiency. Trichromatic white LEDs are in between, having both good luminous efficiency (>70 lm/W) and fair color rendering capability.
Phosphor based white LEDs encapsulate InGaN blue LEDs inside of a phosphor coated epoxy. A common yellow phosphor material is cerium-doped yttrium aluminum garnet (Ce3+:YAG). Although the phosphor based white LEDs have a relatively easier mechanism, they reach the fundamental limitation due to the unavoidable Stokes energy loss6, a loss that occurs when short wavelength photons are converted to long wavelength photons. Regardless this technique of manufacturing is adopted by most of the LED industry because of its low cost and high output. All the high intensity white LEDs now on the market are manufactured by this method.
What multi-color LEDs offer is not merely another solution of producing white light, but is a whole new technique of producing light of different colors. In principle, all colors in the visible spectrum can be produced by mixing different amount of three primary colors, and this makes it possible to produce precise dynamic color control as well. As more effort is devoted to investigating this technique, multi-color LEDs should have profound influence on the fundamental method which we use to produce and control light color. However, before this type of LEDs can truly play a role on the market, several technical problems need to be solved. These certainly include that this type of LEDs? emission power decays exponentially] with increasing temperature, resulting in a substantial change in color stability. Such problem is not acceptable for industrial usage. Therefore, many new package designs aiming to solve this problem have been proposed, and their results are being reproduced by researchers and scientists.
On the other hand, phosphor based white LEDs are the optimal solution to produce high intensity white light. Since its simplified mechanism, this type of LEDs has attracted much interest from the lighting industry. Because of their more stable performance over a range of temperatures, prototypes as well as products based on this phosphor based mechanism have already appeared on the market. And more high intensity white LEDs are expected to be produced in the near future. However, the biggest challenge these phosphor based white LEDs face is solving the seemingly unavoidable Stokes energy loss. Again this can be done by adapting a better package design or by replacing a more suitable type of phosphor. Philips Lumileds patented conformal coating process addresses the issue of varying phosphor thickness, giving the white LEDs a more consistent spectrum of white light.
White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium-based red and blue emitting phosphors plus green emitting copper and aluminum doped zinc sulfide (ZnS:Cu, Al). This is a method analogous to the way fluorescent lamps work. However, the ultraviolet light causes photodegradation to the epoxy resin and many other materials used in LED packaging, causing manufacturing challenges and shorter lifetimes. This method is less efficient than the blue LED with YAG:Ce phosphor, as the Stokes shift is larger and more energy is therefore converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both approaches offer comparable brightness.
The newest method used to produce white light LEDs uses no phosphors at all and is based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate which simultaneously emits blue light from its active region and yellow light from the substrate
A new technique developed by Michael Bowers, a graduate student at Vanderbilt University in Nashville, involves coating a blue LED with quantum dots that glow white in response to the blue light from the LED. This technique produces a warm, yellowish-white light similar to that produced by incandescent bulbs.
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