Planar Lighting System Using Array of Blue LEDs to Excite Yellow Remote Phosphor Film

This investigation demonstrates a novel light-emitting system with an array of blue light-emitting diodes (LEDs). Unlike a conventional lighting system that is constructed from white LEDs, the proposed planar lighting system uses blue LEDs to excite a YAG : Ce³⁺ yellow phosphor film remotely, yielding a high lumen efficiency with uniform and planar light-emission. The phosphor film herein acts as a wavelength converter and a light diffuser simultaneously. The optical properties of the proposed lighting system, angular color deviation and light-emitting uniformity, are studied. Eventually, a high lumen efficiency (73.0 lm/W) and thin lighting module (7.5 mm-thick) was demonstrated without the use of any conventional diffuser plate or light guiding plate (LGP). A low color deviation (Δu'v' = 0.025) and a high light-emitting uniformity (U_niformity = 82%) were achieved. Hence, the proposed lighting system exhibited superior optical performance using a compact module.

I. Introduction

In recent years, white light-emitting diodes (wLEDs) have become more important sources of illumination in TFT-LCD backlighting (BL) and general lighting applications than conventional incandescent, fluorescent (hot cathode or cold cathode) or halogen lamps, because wLEDs are compact, mercury-free, and energy-efficient [1], [2]. However, in conventional wLEDs, phosphor is dispersed in an epoxy resin that surrounds the LED die [see Fig. 1(a)]. Since the phosphor is close to the LED die in the wLEDs package, a significant proportion of the blue light is backscattered by the phosphor and lost by absorption of the LED chips [3]–[5]. Additionally, the high temperature of an operating wLEDs causes thermal quenching, which reduces the light radiation efficiency of the YAG phosphor and the InGaN blue LEDs [6], [7]. Hence, the placing phosphor away from the die, as in a remote phosphor converter (RPC), has been developed to increase lumen efficiency [Fig. 1(b)]. The RPC method enables the backscattered photons to be extracted and the effect of thermal quenching to be reduced, increasing the ultimate overall light output and lumen efficiency [8]. Fig. 1.

Schematically depicts the packing method of LED. (a) Package of dispensing phosphor. (b) Package of remote phosphor.