This is a reply to the comments by Hu et al. directed to a previous paper “Toward scatter-free phosphors in white phosphor-converted light-emitting diodes” by Park et al., [Opt. Express 20(9), 10218 (2012)].
© 2013 OSA
This letter is intended as a Reply to the Comments by Hu et al. directed to one previous paper  proposing the use of large micro-size cube phosphors (~100 μm) as an outstanding potential candidate for scatter-free phosphors in white phosphor-converted light-emitting diodes (pc-LEDs).
First of all, we would like to thank the authors for taking such interest in the subject. Large micro-size cube phosphors satisfying the boundary conditions required for scatter-free phosphors based on Mie’s scattering theory [2–4] were exploited in  in an effort to enhance the conversion efficiency (CE) and packaging efficiency (PE) of white pc-LEDs during implementation, to minimize scattering loss, and to provide efficient phosphor capabilities beyond those of conventional micro-size powder-based white pc-LEDs. Micro-size cube phosphors can lead to higher luminous efficiency (LE), higher PE, and reduced influences of ambient temperature and applied current compared with those characteristics of conventional powder phosphor-based white LEDs. This is because micro-cube phosphors show reduced scattering loss due to their large-size cubes with small grain boundaries.
In their Comment, Hu et al. pointed out that besides the LE and PE, angular color uniformity (ACU) is also a significant property of high white light quality [5, 6]. The effect of phosphor particle size on ACU was discussed, and especially the ACU characteristics of the large micro-size phosphors were inspected by simulation method with a consideration of the effects of phosphor concentration [7, 8]. The increase of phosphor particle size from 30 μm to 100 μm contributes to the decrease of ACU, which implies that the large phosphor particles deteriorate the ACU of pc-LEDs. The increase of phosphor concentration could enhance the ACU. However, the effect of phosphor concentration on ACU for large particles is weakened compared with that for small particles. These effects may be attributed to the strong anisotropy of light scattering intensity of large particle size. Hence, for those applications for which ACU is an important property, the large micro-size phosphors may not be used.
However, the shape of the large micro-size cube phosphors prepared as scatter-free phosphors by our process is a little different from the ideal shape of spherical particles used in the Mie calculation [2–4]. This means that the fabricated large micro-size cube phosphors still have a slight scattering effect, although they satisfy two of the morphological conditions required for scatter-free phosphors based on Mie theory. Therefore, we anticipate that the effect of the phosphor concentration of the fabricated large micro-size cube phosphors on ACU will be increased compared to that of the ideal scatter-free phosphors due to the remaining scattering effect. We measured the correlated color temperature (CCT) of the conventional micro-size powder-based and large micro-size cube-based pc-LEDs in the normal mode in order to select and compare the angular dependence of the two pc-LEDs that have similar CCTs. We then selected three pairs of two different types of pc-LEDs (powder pc-LEDs with phosphor concentrations of 10, 20, and 25 wt%, cube pc-LEDs with phosphor concentrations of 30, 40, and 50 wt%), which have similar CCTs of 11500 K, 4500 K, and 4300 K, respectively.
As can be seen in Fig. 1 , we measured the angular dependence of the two different type pc-LEDs as a function of the phosphor concentration in order to analyze the change of the CCTs in an angle range of 0° to 70° and evaluated the concentration effect on ACU. The ACU is defined as the ratio of the minimum CCT (angle of 70°) to the maximum CCT (angle of 0°). The two different types of pc-LEDs with CCT of 11500 K, which have relatively low phosphor concentrations (powder pc-LED: 10 wt%; cube pc-LED: 30 wt%), exhibited low ACUs (powder pc-LED: 0.64; cube pc-LED: 0.56) because the directional emission of unscattered blue LED plays a more important role in the white emission at low concentration of both samples. However, as the phosphor concentration of both samples (powder pc-LED: 20, 25 wt%; cube pc-LED: 40, 50 wt%) increased, the ACU (powder pc-LED: 0.92, 0.96; cube pc-LED: 0.93, 0.93) significantly increased because the large cube phosphors still have a non-negligible scattering effect (Fig. 1(b, b’, and c, c’)). Figure 1 also indicates that when the concentration of conventional powder phosphor changes from 10 to 25 wt%, the ACU increases from 0.64 to 0.96, an increase of 50.0%; the ACU for the cube phosphor increased from 0.56 to 0.93, an increase of 66.1%, when the concentration of the cube phosphor changes from 30 to 50 wt%. This clearly indicates that the concentration effect of the fabricated large cube phosphors on the ACU could be sufficiently large for these materials to be used in white LED applications; this finding is in contrast to the comment proposed by the authors, even though the ACU of the large cube phosphors is slightly lower than that of the conventional powder phosphors. Therefore, because the large micro-size cube phosphors prepared by our process have a non-negligible scattering effect and more advantages in terms of optical properties such as improved LE and CE, these materials can be used in applications in which ACU is less important. In addition, we expect that secondary optics equipment, such as lenses, reflectors, and diffusers can compensate for the slightly lower ACU problem of scatter-free large micro-size cube-based white pc-LEDs. This implies that the scatter-free micro-size cube-based white pc-LEDs can be effectively applied in lighting and displays.
In summary, we appreciate the authors for inspiring our research interests and provoking discussions that have advanced our understanding of the use of large micro-size phosphors for high white pc-LEDs. The ACU described by the authors is a very important property for high white light quality. As for the consideration of ACU in fabricated scatter-free large micro-size spherical particles, we agree with the comment of the authors. However, large micro-size cube phosphors can increase the ACU with an increase of the phosphor concentration because these materials are not ideally spherical particles. In addition, the ACU can be increased in large micro-size cube-based white pc-LEDs using secondary optics equipment. Therefore, the large micro-size cube phosphors can be used for various applications due to the enhanced optical properties of white LEDs.
This work was supported by the National Research Foundation (NRF) grant (no. 2011-0017449) funded by the Ministry of Education, Science and Technology (MEST) of Korea.
References and links
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