Chapter 5 Application - Planer Lighting by Remote Phosphor Sheet
5.4 Simulation
For the LED backlit use, a 5x5 blue LED chips array was placed above a reflector and covered with the YAG-phosphor layer, as shown in Fig. 5-6. We import the measured BSDFs into the commercial software LightTools TM to accomplish the influence of YAG-phosphor on the whole BLPL system. In order to keep the uniformity and luminance as the first merit, the module gap (h) and the interval of blue LED chips (p) were modulated from 4 to 20 mm. Here the luminance uniformity is defined as
,min
which is the luminance ratio of the positions with the minimum luminance and the maximum luminance at the normal viewing direction. The simulated luminance uniformity is shown in Fig. 5-7 in comparison with a conventional white LED direct-emitting backlit (covered by diffuser plate).
It is found that the operating region is much wider in BLPL system than the conventional backlit due to BLPL structure includes a strong scattering function conducted by YAG-Phosphor layer in the optical path. Even without additional diffuser or diffusing plate, high uniformity is able to be achieved under ultra-slim configuration. As shown in Fig. 8, as spacing between the LED chip and diffusing structure is reduced to 10 mm (cross line (b) and (e) in Fig. 5-7), the optimized light-redistributed mechanism on the flat YAG-phosphor layer makes BLPL system achieved 82% uniformity associated with 10-mm LED interval, whereas the conventional white LED backlight with 20% uniformity with the identical LED interval, the results exhibit the advantage of remote phosphor for uniformity issue in large-area backlit applications.
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Fig. 5-6 The scheme of the BLPL structure for simulation
(a) (b)
Fig. 5-7 The simulated uniformity of (a) the BLPL system and (b) LEDs array with varied LED pitch and system gap.
Fig. 5-8 The comparison of the uniformity between the BLFL and the LEDs array
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Off-axis color deviation, which is a major optical issue of a backlight system, was considered in the BLPL system. Based on the uniformity optimization, the 10-mm mixing gap was chosen as the appropriate parameters for the slim backlight design.
Thus, the chromaticity at the center of the simulation model (as shown in Fig. 5-9) was obtained, and the color differences (Δu’, Δv’) versus different viewing inclinations θ were evaluated (the chromaticity quantities are introduced in Appendix).
Fig. 10 represents the color deviations Δu’v’ of the 0°-30° and 0°-60° viewing inclinations, respectively. According to the results, the LED arrangements with 8-10 mm interval have relatively low color difference.
Fig. 5-9 Setup of the BLPL system in the simulated environment.
0.021 0.046
Fig. 5-10 The color difference (Δu’v’) with a fixed module gap (10mm) and varied LED pitches (4-20mm).
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From the previous simulation, the 10-mm light mixing space with 8-10 mm LED interval were chosen as the appropriate geometrical parameters of the BLPL system for the purpose of the slim backlight design. However, the radiated blue light has a relatively narrower angular distribution than the radiated yellow light. Therefore, the BLFL system exhibits a yellowish phenomenon in large viewing direction. This issue can be suppressed by using the commercial optical films with a lenticular configuration.
Fig. 5-11 The experimental results of (a) (c) the BLPL system and (b) (d) the direct LEDs array
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5. 5 Experiment
Based on the BTDFs of YAG-Phosphor laye
slim BLPL system was demonstrated. Compared with a conventional backlight with a commercial diffuser (80% Haze) under the sa ixing space, the experimental results are shown in Fig. 5-11. T e of BLPL system achieved 9800 nits and the 86% uniformity, w
only 5600 nits and 18% uniformity. Thus, BL
y. Thus the BLPL system generated at lighting and performed higher uniformity than the conventional direct-emitting hite LEDs. However, such specific optical property makes BLPL r and optimized LED layout, a 7-inch
me LED interval and m he emitted luminanc
hereas the conventional backlight had PL system indeed showed the potential for fabricating the ultra-slim backlight system for the large-sized LCD-TV applications.
5.6 Summary
The optical properties of the blue light excited planar lighting (BLPL) system had been discussed in this paper. In BLPL system, the YAG-phosphor acts as a diffuser film and the wavelength converter simultaneousl
fl
backlight with w
system hard to be simulated by conventional simulation software. By using the characterization of the YAG-phosphor layer, a methodology for the purpose of modeling the BLPL system was proposed. According, a prototype slim format BLPL system had been demonstrated. The small-sized BLPL system achieved 86%
uniformity and 9800nits with 10-mm backlight module thickness and without using any diffuser film or plate, while the conventional backlight system had only 20%
uniformity and 5600nits with the same backlight module thickness. Consequently, BLPL system is indeed the potential technology for developing the future backlight system with high brightness and ultra-slim module thickness.
73
[2] D. Feng, Y. Yan, X.Yang G. Jin and S. Fan, “Novel Integrated Light-Guide Plates pl. Opt., vol. 7, 111-117 (2005).
[3] Y.-H. Lu and C.-H. Tien, “Novel Direct-LED-Backlight Unit Using Grooved ht-Guide Plate,” SID Int. Symp. Digest Tech. Papers 37(2), 5.7 References
[1] Munisamy Anandan, “Progress of LED backlights for LCDs,” Journal of the SID 16/2, pp. 287-310 (2008).
for Liquid Crystal Display Backlight,” J. Opt. A: Pure Ap
Hexagonal Lig 1513-1516 (2006).
[4] Y. Ito, T. Tsukahara, S. Masuda, T. Yoshida, N. Nada, T. Igarashi, T. Kusunoki, and J. Ohsako, “Optical Design of Phosphor Sheet Structure in LED Backlight System,” SID Int. Symp. Digest Tech. Papers 39(2), 866-869 (2008).
[5] H.-T. Huang, C.-H. Hung, Y.-P. Huang, C.-H. Tien, C.-C. Tsai, and H.-P. D.
Shieh, “UV Excited Flat Lighting (UFL) System for LCD-TV Backlight Application,” SID Int. Symp. Digest Tech. Papers 39(2), 862-865 (2008).
[6] Chung-Hao Tien and Chien-Hsiang Hung, “An iterative model of diffuse illumination from bidirectional photometric data,“ Optics Express 17 , 723-732 (2009).
[7] Y.-C. Lo, J.-Y. Fang, Y.-P. Huang, H.-P. D. Shieh, G.-S. Yu, and T. Chiang, “A Novel Patterned Diffuser for High Uniform and High Bright LCD Backlights,” Int.
Display Manufactory Con. ’07, 642-645 (2007).
[8] Michael Bass, Eric W. Van Stryland, David R. Williams, William L. Wolfe, 2
Handbook of Optics, vol. , (McGraw-Hill, New York, 1995).
[9] Francois X. Siilion, and Claude Puech, Radiosity and Global Illumination, (Morgan Kaufmann Publishers Inc., San Francisco, 1994).
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Chapter 6
Conclusions and Future Works
6.1 Conclusions
Radiometry and photometry have been widely used for various purposes, such as the optical efficiency evaluation, uniformity analysis, and optical property prediction for various optical systems. The studies of the luminair, flat panel display, solar energy, and indoor illumination are assisted by this technology. As the rapid progress of the lighting application, the classification between the light source and object in the triangle in Fig. 1-1 is gradually indeterminate, such as the photo-fluorescent material.
The angular modulation and wavelength conversion of this material should be both considered. However, the conventional definitions of radiometry can’t completely describe such complicated phenomena.
In this thesis, we study the measurement instruments, mathematical relationships, and calculation methodologies of radiometry for lighting applications. Based on the energy balance equation (Eq. 1.1), the proposed characterization and calculation methodology are verified by practical cases, respectively.
6.1.1 BSDF measurement
For the BTDF measurement, we designed a light source module to produce the collimated beam at various incident angles. Here the collimated beam is produced by a LED combined with a total internal reflection (TIR) lens, and two apertures with 2-mm diameter are used to limit the beam divergent angle within +/- 1 degree. This LED can slide on an arc track to provide variable incident angle θ. The conoscopic
75
system is used to record the angular spreading function. Through the definition, the BTDF of a scattering film can be recorded and calculated.
6.1.2 Dichromatic BSDF
For the phosphor-converted light-emitting diodes (pcLEDs), the interaction of the illuminating energy with the phosphor would not just behave as a simple wavelength-converting phenomenon, but also a function of various combinations of illumination and viewing geometry. We presents the dichromatic BSDF to characterize the converting and scattering mechanisms of the phosphor layer in the pcLEDs by the measured bidirectional scattering distribution functions (BSDFs). A commercially available pcLED with conformal phosphor coating was used to examine the validity of the proposed model. The close agreement with the measurement illustrates that the proposed characterization opens new perspectives for phosphor-based conversion and scattering feature for white lighting uses.
6.1.3 Energy balance equation calculation
The energy balance equation is an integral formula. We propose a calculation methodology for including the photometric raw data sets into the diffuse illumination design process. The method is based on computing the luminance distribution on the outgoing side of diffusing elements from measured bidirectional scattering distribution functions (BSDFs). The calculation procedure includes the linear superposition and the correcting feedback. As an application example, the method is verified by a commercially available diffusing sheet illuminated by a 32-inch backlighting module. Close agreement (correlation coefficient = 98.6%) with the experimental measurement confirmed the validity of the proposed procedure.
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6.1.4 Application - Planar Lighting by Remote Phosphor Sheet
A novel direct-emitting LED backlit for LCDs was demonstrated. Unlike the conventional white LED schemes for display applications, proposed blue light excited planar lighting (BLPL) exploits blue LED chip to remotely excite the YAG-phosphor film and thus render a uniform planar source, where the YAG-phosphor acts as the diffuser film and wavelength down converter simultaneously. Based on the diffusing characterization of YAG-phosphor layer, we examined the optical properties of the BLPL system in viewpoints of uniformity, luminance and mixing capability.
Consequently, a prototype 10-mm-thickness BLPL module was demonstrated with 86% uniformity and 9800 nits without using any diffuser film or light guiding plate.
6.2 Future Works
6.2.1 Physical study of BSDF
Although the measurement instruments, mathematical relationships, and calculations have been discussed in this thesis, the physical description of the relationship between the object properties and the BSDF curves should be further studies. These object properties include the surface roughness, refraction index, particle size, geometrical configuration, and so on, which introduce phase changes on the incident wave. The light cannot be treated as being reflected or transmitted only at the interface of materials, but rather the incident light interacts with all the atoms and molecules in the object. As we mentioned in Chapter 2, the BSDF provides a general and useful description of the far-field light distribution. Therefore, we will study on the relation of the physical mechanisms with the BSDF curves.
6.2.1 Polychromatic BSDF
Photo- fluorescent technology have been widely used in many applications,
including cold cathode fluorescent lamp (CCFL) as the most popular light source, pcLED as next generation white light illuminants, and plasma emission devices for information displaying, as Fig. 6-1 shows. In additional to the dichromatic white mixing scheme we proposed, the methodology can be extended to a general form subject to the multiple excitation radiance,
0 0 0
( , , , )
( , , , ) ( m( , , , ) m m( , , , )),
i i t t
fs i i t t fe i i t t fs i i t t
m
λ λ λ λ λ λ
ρ θ φ θ φ
ρ − θ φ θ φ ρ − θ φ θ φ ρ − θ φ θ φ
= +
∑
+ (6.1)where λ0 is the wavelength of the light source, such as the blue light from the GaN LEDs. The index m represents m-th emitted spectral peak which depends on the light source and the converting properties of the fluorescent material. For instant, the phosphor in a CCFL simultaneously emits the red, green, and blue spectral peaks by the ultraviolet ray excitation. In this case, the multiple excitations of the three spectral peaks can be modeled by the polychromatic BSDF for various configuration designs [1].
Fig. 6-1 the applications of phosphor conversion schemes
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6.2.2 Application – Dual Side Backlighting by Remote Phosphor Sheet
Dual-side displays are suitable for the public information display (PID) application. Due to the high uniformity of the planer lighting with remote phosphor sheet [2], the dual-side backlighting can be realized by the remote phosphor concept.
As the configuration proposed in Fig. 6-2, the remote phosphor sheets and free-form surface reflectors for precise light distribution control [3] are expected to perform dual-side planar lighting with uniform brightness and compact structure. The proposed measurement instrument, characterization, and calculation methodology are expected to assist the structure optimization.
Fig. 6-2 Cross-section of proposed backlight of dual-side display.
6.3 References
[1] Hsin-Tao Huang et al, “UV Excited Flat Lighting (UFL) System for LCD-TV Backlight Application”, SID Int. Symp. Digest Tech. Papers 57.1 (2008).
[2] Bo-Wen Xiao et al, “Optical Properties of Visible-light Excited Phosphor Sheet (VEPS) System,” SID Int. Symp. Digest Tech. Papers 68.4 (2009).
78
79
[3] Hao-Wen Chuang et al, “Front Lighting Design for Reflective Display: Free-form Surface Reflector for Uniform Illumination,” Proc. of International Display Manufacturing Conference (IDMC’09)
Appendix
Colorimetry
Colorimetry is the science relating color comparison and matching. As mentioned in Chapter 1, for visible light, the optical radiations within wavelengths ranging from 380 nm to 780 nm, the photometric quantities have provided measures to describe the amount of energy. However, in human visual system, the optical radiations arouse not only intensity response (brightness) but also chromatic response (chromaticity).
Therefore, in this thesis, colorimetry is imported to specify the chromatic performance of backlight units. The CIEXYZ and CIELUV color spaces, which have been developed for denoting colors numerically, are described in the following paragraphs.
A.1 CIEXYZ
The CIE XYZ system, created by the International Commission on Illuminance (CIE) in 1931, is one of the first mathematically defined color systems that specify colors numerically [1]. The human eye has receptors for short (S), middle (M), and long (L) wavelengths. Thus in principle, three parameters describe a color sensation.
The tristimulus values of a color are the amounts of three primary colors in a three-component additive color model needed to match that test color[2]. In the CIE XYZ system, the tristimulus values are called X, Y, and Z. The tristimulus values for a color with a stimulus Ψ( )λ can be derived from the color matching functions, the numerical description of the chromatic response of standard observer[3] (see Fig.
A-1), according to the following equations:
80
( ) ( )
vis
X =k
∫
Ψ λ x λ λd (A.1)( ) ( )
vis
Y =k
∫
Ψ λ y λ λd (A.2)( ) ( )
vis
Z =k
∫
Ψ λ z λ λd (A.3)where k is a constant and the integral is taken in the visible light wavelength. The
( )
y λ is set so that is identical to the spectral luminous efficiency function V(λ) mentioned earlier. Thus the tristimulus value Y directly expresses a photometric quantity.
Fig. A-1 Color matching functions x( )λ , y( )λ , and z( )λ in the CIE XYZ color system.
Basing on CIE XYZ system, a color could be specified by utilizing the tristimulus values X, Y, and Z in a three-dimensional color space, called CIEXYZ color space. Besides, for convenient descriptions of colors, a color space specified by x, y, and Y, known as CIExyY color space, was derived[4]. The x and y are defined as following equations:
81
x X
X Y Z
= + + (A.4)
y Y
X Y Z
= + + (A.5)
Z 1
z = X Y Z = −
+ + x− y (A.6) The z coordinate could be omitted by providing Y parameters which is a measure of the luminance of a color. Accordingly, the chromaticity description of a color could be expressed more conveniently in a two-dimensional plane, which is called CIE xy chromaticity diagram and be widely used in practice (see Fig. A-2).
Fig. A-2 xy chromaticity diagram of CIE XYZ color system.
However, the xy chromaticity diagram is highly non-uniform and has been found to be a serious problem in practice [5]. The color difference between two colors could not be calculated by using CIEXYZ color space or xy chromaticity diagram. Therefore, a uniform color space, the CIELUV color space, is proposed to replace the non-uniform CIEXYZ color space.
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A.2 CIELUV
The CIELUV color space adopted by CIE in 1976 is an attempt to define an encoding with uniformity in the perceptibility of color difference [6]. Such a uniform color space is based on a simple-to-compute transformation of the 1931 CIEXYZ color space[7,8]. For the non-linear relations from CIEXYZ color space to CIELUV color space, the three-dimensinal orthogonal coordinates adopted in CIELUV color space are defined as follows[9]:
83
where u’ and v’ is the coordinates of two-dimensional u’v’ chromaticity diagram (Fig.
A-3) defined as Eq. A.10 and A.11, Yn, un’, and vn’ are the tristimulus value and the chromaticity coordinates u’ and v’ of reference white, respectively.
' 4
Basing on the uniform CIELUV color space, the color difference of two colors could be calculated. The color difference Δu’v’ between two colors (u1’,v1’) and (u2’,v2’) at the u’v’ chromaticity diagram is defined as[10]:
In this thesis, Eq. A.12 is imported to judge the chromatic performance of the backlight units.
Fig. A-3 u’v’ chromaticity diagram of the CIELUV color system.
A.3 Color Temperature and Correlated Color Temperature
Color temperature expresses the chromaticity of a given radiation by the temperature of the black body having the same chromaticity as that of the radiation.
For radiation whose chromaticity is not exactly equal to that of a black body, correlated color temperature (CCT) is defined as the temperature of the black body whose chromaticity is nearest to that of the radiation. The absolute temperature scale (in kelvin) is used for describing these temperatures. However, it does not necessarily indicate that the light source itself is heated to this temperature. The line connecting the chromaticity points of the series of absolute temperatures of black bodies is called the Plankian locus. For chromaticity not on the Planckian locus, the correlated color temperature can be obtained on the CIE 1960 uv chromaticity diagram by drawing a line from the chromaticity point of the radiation in such a manner that it crosses the Planckian locus at a right angle, and determining the temperature corresponding to the cross point. These lines, which are called isotemperature lines, can be obtained for a series of correlated color temperatures and converted into xy coordinates. The results are shown in Fig. A-4.
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Fig. A-4 Planckian locus and iso-temperature lines.
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A.4 Reference
[1] T. Smith and J. Guild, “The C.I.E. Colorimetric Standards and Their Use,”
Transactions of the Optical Society 33 (3), pp. 73-134 (1931)
[2] R. W. Hunt, Measuring Colour, 3rd Edition, Fountain Press, England, pp. 39-57 (1998)
[3] A. C. Harris and I. L. Weatherall, “Objective Evaluation of Colour Variation in the Sand-burrowing Beetle Chaerodes Trachyscelides White by Instrumental Determination of CIELAB Values,” Journal of the Royal Society of New Zealand, 20(3) (1990)
[4] N. Ohta and A. R. Robertson, Colorimetry: Fundamentals and Applications, John Wiley & Sons, pp.76 (2005)
[5] N. Ohta and A. R. Robertson, Colorimetry: Fundamentals and Applications, John Wiley & Sons, pp.115 (2005)
86
[6] N. Ohta and A. R. Robertson, Colorimetry: Fundamentals and Applications, John
Wiley & Sons, pp.119 (2005)
[7] M. D. Fairchild, Color Appearance Models, Reading, Massachusetts, Addison-Wesley (1998)
[8] D. H. Alman, R. S. Berns, G. D. Snyder, and W. A. Larson, “Performance Testing of Color Difference Metrics Using a Color-Tolerance Dataset,” Color Research and Application, vol. 21, pp.174-188 (1989)
[9] J. Schanda, Colorimetry: Understanding the CIE System, Wiley Interscience, pp.
61-64 (2007)
[10] TCO'06 Media Displays, http://www.tcodevelopment.com/, (2006)
Vita
Name: 洪健翔
Birthday: Jan. 19 1982
Address: 台北市內湖路二段 103 巷 100 弄 11 號五樓 Education:
Sep. 06’ – Jun. 10’: National Chiao Tung University, Hsinchu, Taiwan.
Ph. D. in Institute of Electro-Optical Engineering.
Sep. 04’ – Jun. 06’: National Chiao Tung University, Hsinchu, Taiwan.
Master in Display Institute.
Sep. 00’ – Jun. 04’: National Sun Yat-sen University, Kaohsiung, Taiwan.
Bachelor in Physics.
Experience:
Sep. 06’ – Jan. 07’: Short-term visiting student at Industrial Technology Research Institute.
Dec. 09’ – May 10’: Intern at Advanced Optoelectronic Technology, Inc.
Publications
Journal Papers
1. Chung-Hao Tien, Chien-Hsiang Hung, and Chi-Hung Lee “Aberrations Measurement of Fiber-End Microlens by Free-Space Microoptical Ronchi Interferometer”, IEEE Photon. Tech. Let., vol. 18, no. 16, pp.1768-1770, 2006.
2. Chung-Hao Tien and Chien-Hsiang Hung, “Micromachined Polarization Beam Splitter with Adjustable Leak Ratio for Optical Pickup,” IEEE Photon.
Tech. Let., vol. 19, no. 15, pp.1109-1111, 2007.
3. Chung-Hao Tien and Chien-Hsiang Hung, “An iterative model of diffuse illumination from bidirectional photometric data,” Optics Express, vol. 17, no.
2, pp. 723-732, 2009.
4. Chung-Hao Tien and Chien-Hsiang Hung, “Microlens Arrays by Direct Write Ink-Jet Printing for LCD Backlighting Applications,” IEEE/OSA J. Display Technology, vol. 5, no. 5, pp. 147-151, 2009.
5. Chien-Hsiang Hung and Chung-Hao Tien, “Phosphor-converted LED Modeling by Bidirectional Photometric Data,” Optics Express, vol. 18, no.103, pp. A261-A271, 2010.
International Conference Papers
1. Hsin-Tao Huang, Chien-Hsiang Hung, Yi-Pai Huang, Chung-Hao Tien, C. C.
Tsai, Han-Ping D. Shieh, “UV Excited Flat Lighting (UFL) System for LCD-TV Backlight Application,” SID Symposium Digest Tech Papers, vol. 39, No.57.1, 2008.
2. Chien-Hsiang Hung, Tsung-Han Yu, Jung-An Cheng, Chi-Hsien Chang, Yi-Hau Hsiau, Chung-Hao Tien, “Microlens Array by Ink-Jet Technology for LCD Backlight Applications,” SID Symposium Digest Tech Papers, vol. 39, No.P-97, 2008.
3. Bo-Wen Xiao, Chien-Hsiang Hung, Hsin-Tao Huang, Yi-Pai Huang, Chung-Hao Tien, Chuang-Chuang Tsai, and Han-Ping D. Shieh, “Optical Properties of Visible-light Excited Phosphor Sheet (VEPS) System,” SID Symposium Digest Tech Papers, No.68.4, 2009.
3. Bo-Wen Xiao, Chien-Hsiang Hung, Hsin-Tao Huang, Yi-Pai Huang, Chung-Hao Tien, Chuang-Chuang Tsai, and Han-Ping D. Shieh, “Optical Properties of Visible-light Excited Phosphor Sheet (VEPS) System,” SID Symposium Digest Tech Papers, No.68.4, 2009.