PORTLAND, Ore. " What are claimed to be the first white light-emitting diodes (WLEDs) to achieve a high color-rendering index were recently demonstrated by an international collaborative team of researchers. This could answer the last remaining stumbling block to universal adoption of white LEDs to replace incandescent bulbs and fluorescent tubes.
White LEDs use a fraction of the power of the venerable light bulb. Unfortunately, traditional LEDs by themselves emit only a single wavelength. Consequently, the only way to get white light from LEDs is to house red, green and blue LEDs in the same package, add yellow phosphors to the inside of the lens of blue LEDs, or turn to new materials, such as zinc selenide, which can simultaneously emit blue light from its active region and yellow light from its substrate. Any of these LED construction methods emit light that appears to be whitish, which makes them suitable for general-purpose lighting, but color rendering index (CRI) tests verify that none of these approaches are suitable for professions who must have lighting that is able to reproduce all the colors in the visible spectrum.
Now, Professor Elisabeth Holder and her colleagues at the University of Wuppertal (Germany) have collaborated with professor Hilmi Volkan Demir of Bilkent University, in Ankara (Turkey), on a new approach to crafting white LEDs that they claim yields a very high color-rendering index, making them suitable for professional users from graphic artists to horticulturists.
The key to the new technique is to embed various types of inorganic luminescent nanoparticles into a highly fluorescent organic polymer base, then pumping the mixture with a near-ultraviolet LED. The result, according to the researchers, is hybrid organic/inorganic LEDs that produce white light that contains almost every wavelength in the spectrum, and, thus, can render most any color accurately.
Key to the collaborative effort was marrying the organic and inorganic materials. By taking a layer-by-layer approach, the researchers were able to embed fluorescent yellow, green and red nanoparticles (using cadmium-selenium cores coated with a zinc silicon shells) into a fluorescent blue polymer matrix (polyfluorene), and subsequently tune the mixture to attain a color-rendering index in excess of 80 (compared with 100 for sunlight, over 90 for incandescent, 70 for phosphor-based LEDs, and 65 for fluorescent tubes). With further optimization, the researchers hope to improve CRIs upward toward 100.