The goal of this study was to characterize the chromaticity shift that mixed-color and phosphor-converted white LED systems undergo when dimmed. As light-emitting diodes continue to rapidly evolve as a viable light source for lighting applications, their color shift while being dimmed should meet the current requirements of traditional lighting sources. Currently, LED system manufacturers commonly recommend pulse-width-modulation or PWM dimming schemes for operation of LED systems. PWM has the ability to achieve lower intensity levels and more linear control of light intensity compared to continuous current dimming methods. However, little data has been published on the effect dimming has on chromaticity shift of white LED lighting systems.
The primary objective of this study was to quantify chromaticity shifts in mixed-color and phosphor-converted white LED systems due to continuous current dimming and pulse-width-modulation dimming schemes. In this study, the light output of the LED system was reduced from 100% to 3% by means of continuous current reduction or PWM methods using a PC white LED system and a mixed-color RGB LED system. Experimental results from this study showed that the PC white LED system exhibited very little chromaticity shift (less than a 4-step MacAdam ellipse) when the light level was changed from 100% to 3% using both dimming schemes. Compared to PC white LEDs, the mixed-color RGB LED system tested in this study showed very large chromaticity shifts in a similar dimming range using both dimming schemes. If a mixed-color RGB system is required, then some active feedback system control must be incorporated to obtain non-perceivable chromaticity shift. In this regard the chromaticity shift caused by the PWM method is easier to correct than the chromaticity shift caused by the continuous current dimming method.