The LED lightbulb: are we there yet? progress and challenges for solid state illumination

Rapid improvement in light emitting diode (LED) performance and an annual multibillion dollar lighting market is motivating worldwide development of LED based illumination systems. In addition to the traditional figures of merit of efficiency and high flux density, the white LED lamps must also have appropriate color characteristics. Current commercial white LED lamps have undesirable color temperatures and uniformity, and inferior color rendering. As discussed below, color uniformity in single phosphor lamps has been improved by deposition of thin uniform phosphor layers conformal to the LED die. Also, 2-phosphor LED lamps with white color characteristics typical of conventional incandescent and halogen sources are described. Furthermore, LED illumination offers additional features not practically achievable with conventional sources.