Paper

Journal of the Ceramic Society of Japan 118 [2] 128-131 2010

Influence of light scattering on luminous efficacy in Ce:YAGglass-ceramic phosphor
Shunsuke FUJITA,*,**,† Yoshio UMAYAHARA* and Setsuhisa TANABE*****

Advanced Materials and Technology Division, Nippon Electric Glass, Seiran 2-Chome, Otsu, Shiga 520-8639
Graduate School of Human and Environmental Studies, Kyoto University, Yoshida Nihonmatsu-cho Sakyo-ku, Kyoto 606-8501

The luminous efficacy of Ce:Y3Al5O12 (YAG) glass-ceramic (GC) phosphor increased with the increasing crystal size of precipitated Ce:YAG, while the light scattering coefficient decreased. This increase in luminous efficacy can be attributed to thedecrease in light scattering loss owing to the decrease in light scattering coefficient with increasing Ce:YAG crystal size.
According to Mie scattering theory, the reason for the decrease in the light scattering coefficient is the decrease in light scattering cross-section per unit volume with increasing Ce:YAG crystal size.
©2010 The Ceramic Society of Japan. All rights reserved.

Key-words : Ce:YAG, Glass-ceramic, Phosphor, Light scatterin[Received September 29, 2009; Accepted November 19, 2009]

1. Introduction
The white light emitting diode (LED) is seen as the nextgeneration solid state lighting and is rapidly becoming common
in daily life. To date, we have developed Ce:Y3Al5O12 (YAG)
glass-ceramic (GC) phosphor for white LEDs using a glassceramic process.1)–2) Since Ce:YAG micro crystals are precipitated in the glass-matrix phase, good stability at high temperatureand high humidity can be expected compared with conventional
white LEDs, in which phosphors are embedded in a resinmatrix.3)In Ce:YAG–GC, Ce3+ ions, incorporated in the Y sites of theprecipitated Ce:YAG micro crystals, emit yellow fluorescentlight when they are optically excited by a blue LED. White lightemission can be obtained by mixing the exciting blue light andthe fluorescent yellow light. In the emission process of Ce:YAG–GC, light scattering generally occurs at the phase boundary
between the precipitated Ce:YAG micro crystals and the glassmatrix phase due to the difference in refractive index. Since thelight scattering changes the direction of both the exciting blue
light and the fluorescent yellow light, this directly affects theluminous efficacy of Ce:YAG–GC phosphor.
In this study, the interdependence between Ce:YAG particle
size, luminous efficacy, and the light scattering coefficient wereinvestigated in order to improve the luminous efficacy ofCe:YAG–GC phosphor. Since the luminous efficacy should beaffected by the light scattering, as mentioned above, an understanding of light scattering phenomena is critical for the development of Ce:YAG–GC phosphor with higher luminous efficacy.

2.

ExperimentalThe glass composition of Ce:YAG–GC is 42.5 mol% SiO2, 20mol% Y2O3, 34 mol% Al2O3, 3 mol% Li2O, and 0.5 mol%Ce2O3. The mixed raw materials were melted in a platinum crucible at 1650°C for 2 h. Then, the molten melt was cast into acarbon mold followed by annealing. The glass obtained wascrystallized using four kinds of heat treatment: 1380°C 9 h,†Corresponding author: S. Fujita; E-mail: snfujita@neg.co.jp

1281440°C 9 h, 1440°C 36 h, and 1440°C 72 h. Plate-shaped
samples with a thickness of 0.4 mm were prepared for all of thefollowing measurements. The samples were polished with a diamond slurry containing 3 μ m diamond particles, to obtain a mirror surface.
An electron probe micro analyzer, EPMA (JEOL Ltd., JXA–8900) was used to obtain a back scattered electron image of theCe:YAG–GC, where precipitated Ce:YAG crystals can be clearly
distinguished from the glass-matrix phase. The back scatteredelectron images were analyzed using image-processing software(MITANI Corporation, WinROOF) to obtain the average geometric cross-section, A, and the area ratio of Ce:YAG crystals.
The average particle size of the Ce:YAG crystals was calculated
from the geometric cross-section, A. The area ratio wasemployed as the number density, D,4) of the Ce:YAG crystals.
All of the spectral power distributions, P(λ), were measured
in an integrating sphere with a 10-inch diameter (Labsphere Inc.,LMS–100), which was connected to a CCD detector (OceanOptics, USB2000) with a 400-μ m core optical fiber. The current
for exciting the blue LED was fixed at 20 mA. A standard
halogen lamp (Labsphere, SCL–600, 450 lm) was used for thecalibration of this measurement system. Total luminous flux, Φv,of the luminescence was calculated from the measured spectral
power distribution, P(λ), using
780 nmΦv = Km∫ V (λ ) P (λ ) d λ(1)380 nmwhere V(λ) is the relative eye sensitivity and Km is the maximum
luminous efficacy at 555 nm, 683 lm/W. Then, the luminous efficacy, η, was obtained by dividing total luminous flux, Φv, by theelectrical power on the blue LED.
The light scattering coefficient, μ, was derived from LambertBeer’s law:

μ =−logTd(2)where T is the transmittance and d is the thickness of Ce:YAG–GC plate. Transmittance, T, was measured by a spectrophotom©2010 The Ceramic Society of Japan

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Journal of the Ceramic Society of Japan 118 [2] 128-131 2010

eter (Shimadzu, UV3100–PC).
The refractive index, n, of Ce:YAG crystals and the glassmatrix phase was derived from the following equation:n = ( r + 1) / (1 − r )(3)where r is the reflectance at 550 nm that was measured by thereflective film thickness monitor (Photal, FE–3000).

3. Theory
The light scattering phenomenon of one spherical particle isexplained by the Mie theory when the particle size is almost thesame or larger than that of the wavelength of visible light. The
basis for applying the Mie theory was a publication by G. Mie5)
in 1908, and a detailed description was presented by H. C. vande Hulst.6) According to the Mie theory, light scattering efficiency, Q, is explained by the following equations as a functionof ρ when the ratio of n1/n0 is less than two:

Q=2−44sin ρ + 2 (1 − cos ρ )ρρnρ = 2 x( 1 − 1),n0x=2π an0λ(4)(5)

processing software. The geometric cross-section, A, and the particle size increased from (a) to (d), while the area ratio showed
little change, remaining at almost a constant value.
Emission spectra of these samples with a thickness of 0.4 mm
are shown in Fig. 2. The sharp emission band around 460 nm isdue to the transmitted blue light from the exciting blue LED, andthe broad emission band around 550 nm is due to the fluorescentyellow light from the Ce3+ ions in the YAG crystals. Both emission intensities increased with increasing heat treatment temperature or holding time, from (a) to (d). The Ce:YAG particle size
dependencies of luminous efficacy, η, and light scattering coefficient, μ, are shown in Fig. 3. The luminous efficacy, η,
Table 1. Geometric Cross-Section, Particle Size, and Area Ratio ofCe:YAG Crystals for Each Heat-Treatment Condition
Heat-treatment
condition

Geometric cross-section, A(× 10–12 m2)

Particle size(μ m)Area ratio(%)(a) 1380°C-9 h1441430(b) 1440°C-9 h

247

1834(c) 1440°C-36 h

485

2532(d)1440°C-72 h

688

3032

where n0 and n1 are the refractive indices of medium and scatterer, respectively, and a is the radius of scatterer, and λ is thewavelength of the incident light.
Light scattering cross-section for one particle, σ, is explained
by

σ=Q×A

(6)

where A is the geometric cross-section of one scatterer.
The calculated light scattering coefficient, μs, is explained by

μs = σ × D(7)where D is the scatterer number density.

4.

Results

4.1 Luminous efficacy, η, and light scattering coefficient, μ, of Ce:YAG–GCBack scattered electron images of Ce:YAG–GC are shown inFig. 1. Precipitated Ce:YAG particle sizes increased from (a) to(d) with increasing heat-treatment temperature or holding time.
Table 1 shows geometric cross-section, A, particle size, andarea ratio of Ce:YAG crystals, which were derived using image-Fig. 1. Back scattered electron images of Ce:YAG–GC, heat-treatedwith the following heat-treatment profiles: (a) 1380°C 9 h, (b) 1440°C 9h, (c) 1440°C 36 h, (d) 1440°C 72 h.

Fig. 2.

Emission spectra of Ce:YAG–GC with a thickness of 0.4 mm.

Fig. 3. Luminous efficacy, η, and light scattering coefficient, μ, ofCe:YAG–GC with Ce:YAG particle size.

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Fujita et al.: Influence of light scattering on luminous efficacy in Ce:YAG glass-ceramic phosphor

increased with increasing Ce:YAG particle size while the lightscattering coefficient, μ, decreased. The relationship between ηand μ is shown in Fig. 4, where an approximately linear relationship was observed.

4.2 Light scattering efficiency, Q, and calculated
light scattering coefficient, μs

coefficient, μs, calculated as the product of σ and D, decreased
with increasing Ce:YAG particle size.

4.3 Comparison of the light scattering coefficients
between the observed and calculated values
In Fig. 7, the observed light scattering coefficients, μ, are com-

The light scattering efficiency, Q, of Ce:YAG–GC was plotted
and indicated by square marks on the theoretical value shown bya solid line in Fig. 5. In the process of calculation for Q, 380 nm
and 780 nm were used as the incident light, λ, for each Ce:YAGparticle radius, a. The refractive indices of the glass-matrix phaseas a medium, n0, and the Ce:YAG crystals as scatterers, n1, were
fixed at 1.641 and 1.800, respectively. Q showed almost a constant value of 2.
Figure 6 shows the interdependence between the particle sizeof the light scattering cross-section, σ, the scatterer number density, D, and the calculated light scattering coefficient, μs. Relativevalues were plotted against the standard values at a particle sizeof 14 μ m. The value of σ increased with increasing Ce:YAG particle size, while D decreased. As a result, the light scattering
Fig. 6. Calculated relative light scattering cross-section, σ, number
density of scatterer, D, and light scattering coefficient, μs, with Ce:YAGparticle size.

Fig. 4. Relationship between luminous efficacy, η, and light scattering
coefficient, μ.

Fig. 5. Light scattering efficiency, Q. Solid line: theoretical curve
based on Mie scattering; open squares: experimental plots of Ce:YAG–
GC for typical values of wavelength.

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Fig. 7. Comparison between the observed light scattering coefficient,μ, and calculated light scattering coefficient, μs, with Ce:YAG particlesize.

Fig. 8. Luminous efficacy, η, and calculated light scattering coefficient, μs, with Ce:YAG particle size.

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Journal of the Ceramic Society of Japan 118 [2] 128-131 2010

pared with the calculated light scattering coefficients, μs.
Although the actual values are different, the same tendency wasconfirmed in that the light scattering coefficients decreased withincreasing Ce:YAG particle size.
Figure 8 shows the dependencies of the luminous efficacy, η,and the calculated light scattering coefficient, μs, on the Ce:YAGparticle size. The luminous efficacy, η, of Ce:YAG–GC increased
with increasing Ce:YAG particle size, while the calculated lightscattering coefficient, μs, decreased.

5. Discussion
5.1 Influence of light scattering on luminous efficacy
The luminous efficacy, η, of Ce:YAG–GC increased withincreasing Ce:YAG particle size, while the light scattering coefficient, μ, decreased, as shown in Fig. 3. Generally, a decreasein light scattering coefficient reduces light scattering loss. Therefore, the increase in luminous efficacy, η, in Ce:YAG–GC can be
attributed to the decrease in light scattering loss due to thedecreasing light scattering coefficient, μ, with increasing
Ce:YAG particle size.
The theoretical light scattering coefficient, μs, generally
showed a good agreement with the observed value, μ. Therefore,
it seems appropriate that the light scattering phenomena ofCe:YAG–GC be explained by the Mie theory. For the differencesin actual values between μ and μs, several reasons can be considered. One reason for the differences may be due to the nonspherical shape of the Ce:YAG crystals, because the Mie theoryis generally applicable to spherical particles. Another reason
might be an overestimation of transmittance for the Ce:YG–GC
samples, resulting in a small light scattering coefficient. Also,
since Ce:YAG–GC is translucent and exhibits very low transmittance less than 1%, some measurement error might be included.
The reason for the decrease in the light scattering coefficient,μ, with increasing Ce:YAG particle size, was considered on thebasis of Mie theory. The light scattering efficiency, Q, ofCe:YAG–GC showed almost a constant value, 2, as shown in Fig.
5. This result suggests that one Ce:YAG particle scatters the incident light in twice the area of its geometric cross section, A.
Since A increases with increasing Ce:YAG particle size, it is consistent that the light scattering cross section, σ = Q (const. = 2)
× A, also increases.
On the other hand, the number density, D, of the scatterers(Ce:YAG crystals) decreased with increasing Ce:YAG particle
size. The area ratio of Ce:YAG crystals exhibited little change asshown in Table 1; therefore, the volume fraction of Ce:YAGcrystals also must change in the same manner. In the small vol-ume fraction change, it is consistent that the scatterer number
density should decrease with increasing scatterer particle size.
Because the light scattering coefficient, μs, can be calculated
as the product of σ and D, the decrease in μs is attributed to thedecrease in the scatterer number density, D, rather than theincrease in the light scattering cross section, σ, with increasing
Ce:YAG particle size.

5.2 Estimation of the change in luminous efficacy
from the calculated light scattering coefficient
The relation between the luminous efficacy, η, and the lightscattering coefficient, μ, is almost proportional, as shown in Fig.
4. Therefore, the possible change in luminous efficacy ofCe:YAG–GC can be estimated for larger particle sizes by the tendency of the calculated light scattering coefficient, μs as shownin Fig. 8. In the small Ce:YAG particle-size region, the influenceof light scattering coefficient, μs on the luminous efficacy, η, ismuch larger than that in large particle-size region. From this consideration, it is suggested that the size of the Ce:YAG crystals has
already been optimally adjusted for higher luminous efficacy.

6.

Conclusion

This study evaluated the influence of light scattering on theluminous efficacy of Ce:YAG–GC phosphor. The luminous
efficacy of Ce:YAG–GC phosphor increased with increasing
Ce:YAG crystal size while the light scattering coefficientdecreased. The reason for the increase in luminous efficacy canbe attributed to the decrease in light scattering loss due to thedecreasing light scattering coefficient, μ, with increasing
Ce:YAG particle size.
From the calculated light scattering coefficient it appears thatthe size of Ce:YAG crystals has already been optimally adjusted
for higher luminous efficacy.
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