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A cerium-doped gadolinium-yttrium garnet yellow emitting phosphor was used to optimize the phosphor layer for down-conversion of blue light from InGaN inorganic light-emitting diode (iLED) to white. Optical, morphological and elemental characterizations of phosphor were carried out. Various amounts of the phosphor were dispersed in polymethyl methacrylate polymer (PMMA)-chlorobenzene solution. The phosphor layer thickness was optimized by varying the volume of the phosphor-PMMA mixture. Using a blue emitting (458 nm) iLED, a two times larger luminance efficacy (lm/W) was obtained from down-converted white light using a 37.5 μm thick 25 mm × 25 mm film prepared from 30 mg of phosphor in 500 μl of the PMMA-chlorobenzene solution.