Temporal Luminescence of Broadband Light-Emitting Diodes and Their Use for Generating Customizable White Light

Broadband visible light-emitting diodes (LEDs) are now being used for applications where high-quality tunable light is needed. These LEDs are based on phosphors with wide emission bands (full width at half maximum, FWHM, of 70 nm or more). We describe the time-dependent characteristics of light emission from such devices. We found time lags in both luminescence buildup and decay as the broadband red, green and white LEDs are pumped by square pump photon flux. The reasons for such delays are photon percolation in phosphor/light scatterer blends and luminescence persistence due to defects in phosphors. The latter part of the paper describes a setup for combining light from phosphor-based red and green broadband LEDs with that of a band-edge emission blue LED to produce white light. It is shown that the chromaticity of white light can be changed stably by changing the frequency/duty cycle of PWM waveforms that are used for controlling red, green and blue LEDs. We also introduce a new LED drive scheme where PWM signals are converted to steady DC levels before driving LEDs. The setup described here enables the generation of full-spectrum white light with electrically controllable chromaticity.