White LEDs can also be fabricated using organic dye molecules as wavelength converter materials. The dyes can be incorporated in the epoxy encapsulant (Schlotter et al., 1997). Dyes can also be incorporated in optically transparent polymers. A drawback of organic dyes is their finite lifetime. Dye molecules “bleach out”, i. e. become optically inactive, after […]

PRS-LEDs have been demonstrated using a GaInN/GaN LED emitting in the blue and an electrically passive AlGaInP photon-recycling semiconductor emitting in the red part of the spectrum (Guo et al., 1999). The emission spectrum of the device, depicted in Fig. 21.17, shows the emission line of the primary LED at 470 nm and a second […]

Next, we calculate the light-power ratio between two sources required for the emission of white light and the luminous efficiency of the photon-recycling semiconductor LED. We refer to X1 and X2 as the primary (short) and secondary (long) wavelength, respectively. For white light, X1 and X2 are pairs of complementary wavelengths. We define the color […]

Light-emitting diodes using semiconductor wavelength converters have been demonstrated by Guo et al. (1999). The schematic structure of the photon-recycling semiconductor LED (PRS- LED) is shown in Fig. 21.13. The figure indicates that a fraction of the light emitted by the blue GaInN LED is absorbed by a AlGaInP secondary active region and re-emitted (or […]

White LEDs can also be fabricated with optical excitation of phosphor in the ultraviolet (UV) wavelength range (Karlicek, 1999). Semiconductor sources emitting in the near-UV (320­390 nm) and in the violet, close the edge of the visible spectrum (390-410 nm) are frequently used for such white sources. Semiconductor diodes emitting near 400 nm with remarkably […]

Phosphors consist of an inorganic host material doped with an optically active element. Common hosts are garnets, which have the chemical formula A3B5Oi2 where A and B are chemical elements and O is oxygen. Among the large group of garnets, yttrium aluminum garnet (YAG), Y3Al5Oi2, is a particularly common host material. Phosphors having YAG as […]

The conversion efficiency of short-wavelength light to long-wavelength light by a wavelength — converter (X-converter) material is determined by two distinct factors, namely (i) the external quantum efficiency of the X-converter and (ii) the inherent quantum-mechanical-energy loss incurred in wavelength conversion. The external quantum efficiency of the converter material, next, is given by П _ […]

The generation of white light by a semiconductor LED whose light is partially or fully used to optically excite one or several phosphors, is a viable and common method to generate white light for general illumination applications. There are several different approaches to generate white light based on phosphors excited by semiconductor LEDs, which are […]

Light is perceived as white light if the three types of cones located on the retina of the human eye are excited in a certain ratio, namely with similar intensity. For the case of white light, the tristimulus values are such that the location of the chromaticity point is near the center of the chromaticity […]

As the trend of higher efficiencies in LEDs continues, the number of possible applications increases as well. A highly interesting application with a very large potential market size is general daylight illumination in homes and offices. The field of solid-state lighting (SSL) is concerned with the development of solid-state sources for illumination applications. LEDs are […]