1 Introduction Over the last decade phosphor converted light emitting diodes, pcLEDs, have impacted most lighting applications, including general illumination, backlighting of displays, automotive and specialty lighting. In many applications, high efficiency at high power density is desired to maximize LED "lumens per dollar", and leverage greater source brightness to enable novel optical system design or reduced cost from smaller optics. So far, the focus on efficiency vs power density has been predominantly on the primary light source, the blue emitting InGaN LED. The word 'droop', uncommon a decade ago, describes the decrease in efficiency with drive which can be traced back to rather fundamental processes [1, 2], of which, Auger recombination involves three charge carriers interacting non-radiatively, subtracting from the radiative recombination, a two-carrier process. A similar kind of higher order recombination process, in this case a process closely quadratic in the concentration of excited activators, as will be shown, appears to decrease quantum efficiency of otherwise highly efficient phosphors, pumped with high brightness blue LEDs. This decrease of efficiency with (optical) excitation, will be referred to as 'droop in phosphors'.
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