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White light‐emitting diode (LED) luminous efficacy must improve significantly if LEDs are to become useful for general lighting. Phosphors commonly used in white LEDs backscatter more than half of the down‐converted light, of which a significant portion is eventually lost within the package, thus reducing the overall efficacy. This letter describes an experi‐mental study that shows by placing the phosphor away from the die, the backscattered photons can be extracted and the ef‐ficacy can be significantly increased. At low currents, the lu‐minous efficacy exceeded 80 lm/W. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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A study to assess the detection and acceptability of flicker and stroboscopic effects under different lighting conditions was conducted. Frequencies from 50 Hz to 300 Hz were assessed, as well as different modulation amounts, duty cycles, waveform shapes and correlated colour temperatures. For the range of conditions evaluated, direct perception of flicker was negligible at 100 Hz or higher, but stroboscopic effects could be perceived at 300 Hz. Reducing flicker modulation from 100% to 33% decreased perception of stroboscopic effects. A higher duty cycle was associated with slightly lower discomfort. The implications for solid-state lighting systems producing high-modulation flicker are discussed.

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A laboratory experiment was conducted to investigate the performance characteristics of the currently available highpower LEDs under various drive conditions and ambient temperatures. Light output degradation and color shift properties as a function of time were measured for five types of commercial high-flux LEDs, namely, red, green, blue, and white from one manufacturer, and a different high-flux white LED package from a second manufacturer. The major difference between the two manufacturers' products is that the first uses a single LED die per package, and the second uses multiple dies within its package. LED arrays were tested under normal drive current and ambient temperature, normal drive current and higher ambient temperature, and higher drive current and normal ambient temperature. Because each LED type has to operate at a particular ambient temperature, all were tested in specially designed individual life-test chambers. These test chambers had two functions: one, to keep the ambient temperature constant, and two, to act as light-integrating boxes for measuring light output parameters. Overall, the single-die green and white LED arrays showed very little light loss after 2,000 hours, even though the current and the ambient temperature were increased. However, the red LED array seemed to have a high degradation rate. The white LEDs had a significant color variation (of the order of a 12-step MacAdam ellipse) between them. However, the color shift over time was very small during the initial 2,000-hour period. For white LEDs to be accepted broadly for general illumination applications, the color variation between similar products must become much smaller, of the order of a 2-step MacAdam ellipse.