Moreover, the Toshiba LED is brighter than its equivalent among conventional white LEDs. The output of the Toshiba part is 160 mcd, which the company expects to raise to 200 mcd this year. But the technique for generating white light is interesting for another reason. Varying the mix of RGB phosphors creates new colors, such as pink and violet (Fig. 2).
Other vendors also are developing UV LED chips for applications beyond just white LEDs. In its X-Bright family, Cree has developed a 290- by 290-µm GaN-based LED chip that generates 18 to 20 mW of UV light. (That value reflects packaged performance.) With a drive current of around 20 mA, this chip can be implemented in standard LED packaging.
Another company, Uniroyal, produces a 400-nm InGaN chip that generates 2 to 3 mW of UV output from a die measuring 12 to 13 mil (250 to 300 µm) per side. (This power specification represents the unpackaged die.) One use for this chip is in currency authentification where a fluorescent tag is embedded within the bill. When exposed to the UV light created by the LED, the tag glows visibly. UV LEDs also are finding applications in biomedical equipment with uses in photodynamic therapy and bacteria detection.
Meanwhile, development of InGaNbased blue LEDs continues as a means of making brighter white LEDs. In terms of chip development, Cree's X-Bright family includes a blue device that generates 15 mW. This device is intended for use in standard surface-mount and 5-mm LED packaging.
Moving from the chip to the package level, InGaN blue LEDs also have reached new heights in performance. Consider Nichia's NSCW215/335 ser-ies of 0.8- and 1-mm-high side-view LEDs. With output rated at 600 mcd, these devices have achieved 50% greater light output than the company's existing SMT series.
Side-emitting LEDs are especially suited to the LCD backlights because they improve the optical efficiency of the light guide by eliminating the energy-wasting bends required by conventional top-firing LEDs. With the improved performance of side-view white LEDs, backlight developers now have a greater ability to reduce the number of LEDs in backlight designs
Unlike backlighting small LCD screens, using HB LEDs to illuminate larger displays like those in notebook and desktop monitors isn't common. That's changing, though, as vendors look to increase battery runtime.
LumiLeds has re-ported that display maker FIMI Philips is using its Luxeon series of emitters to backlight a 15-in. medical-grade LCD monitor. LEDs offer several benefits over the CCFLs that they replace, including tunable white points, without reducing grayscale levels. Plus, because the backlight deploys a combination of red, green, and blue LEDs, it provides a wider range of colors while letting users adjust the color in real time. Greater brightness and longer life expectancy (50,000 hours) are other advantages of the LED backlight.
Now, the quest is on to improve the efficiency of HB LEDs to apply them in consumer-grade applications. In a typical 15-in. monitor today, the CCFL draws about 18 W. In the same application, the current generation of Luxeon LEDs would draw about 25 W. But LumiLeds' roadmap says that value should drop to 11 W by next year. The change to LEDs is expected to double notebook runtime. In addition to cutting lamp power, the move to LEDs eliminates the high-voltage inverter, plus the RF protection circuitry and UL approvals accompanying it.
References:
- "The Solid-State Lighting Industry Initiative: An Industry/DOE Collaborative Effort," by Stephen Johnson, Architectural Lighting Magazine, November/December 2001, http://eetd.lbl.gov/btp/papers/47589.pdf.
- "The Promise and Challenge of Solid-State Lighting," by Arpad Bergh, George Craford, et al., Physics Today, Dec. 2001, see www.aip.org/pt/vol-54/iss-12/p42.html.