Applying to the Backlight Module

Both-sided beam shaping diffuser is able to be adopted for the divided areas in the scanning BLM. The material of the film is PMMA, and the substrate thickness is 125um. We designed that the lens pitch is 100um and the sag is 24um. Then, two films are put together as a result the thickness of the film is the focal length of 250um, as shown in Fig. 31. The tolerance of light divergent angle of the diffuser we designed can be approaching 15°. Fig. 32 illustrates the direct type BLM within both-sided beam shaping diffuser. The distance between the detector and the both-sided micro-lens array diffuser is 120f, which is 30 mm. The light source close to the film illuminates imperfect collimated light with divergent angle 15° and the beam size is 10mm. One light source whose beam size is 10mm can produce a square beam shaping pattern whose length is 24mm. when combining 9 light sources, the length of the pattern is 54mm, as shown in Fig. 32(b), and we can get the uniformity achieved to 86%.

Fig. 31 Two one-sided lens array films put together

Fig. 32(a) The profile of the direct type BLM (b) the beam shaping pattern (c) 9 LED light sources

Chapter 4

High Gain Diffuser Film

4.1 Light Diffusion and Collection

Optical films are usually employed to promote light uniformity and collimation in the conventional backlight module (BLM). As shown in Fig. 33(a), after emitting from the light guide, light propagates through diffuser and is scattered into all directions of the hemi spherical space. Then the scattered light is redirected by two orthogonally arranged brightness enhancement films (BEFs) and collected to the normal direction. In order to reduce the production cost of the BLM, many novel devices were developed to reduce the usage of optical films or combine their functions [14~16]. In this chapter, a novel optical layout, high gain diffuser film with surface relief of lens array, performing dual functions of light collection and diffusion was proposed to combine BEFs and diffusers, as shown in Fig. 33(b).

Fig. 33(a) The conventional backlight module (b) lens array film replacing BEFs and diffuser in the BLM

4.2 Film Structures

On the lens array film, the lens surfaces can be divided into five types, oblate ellipsoid, sphere, ellipsoid, paraboloid and hyperboloid, respectively. Any form of the aspheric or spherical surface can be described by aspheric equation, which is expressed as blow:

where c is the conic constant and r is the vertex radius of curvature. The surface type depends on the conic constants, as shown in the Table 2 and Fig. 34.

Table 2 Conic constants and surface types

c (conic constant) Surface type

(a) c > 0

Oblate ellipsoid

(b) c = 0

Sphere

(c) -1 < c < 0

Ellipsoid

(d) c = -1

Paraboloid

(e) c < -1

Hyperboloid

Fig. 34 Surface types depending on conic constants in Table 1

Fig. 35 exhibits the hyperboloidal, paraboloidal and spherical surfaces, in which the length l is fixed and the vertex radius of curvature r decreases, where aspect ratio (AR) is defined as below:

AR h

=

l

. (13)

Fig. 35 indicates that AR increases as r decreases. The prismatic structure of BEF can be regarded as a hyperboloidal surface shown as Fig. 35(a)(iii), where c = -2 and r is infinitesimal. In the other words, the feature shape is the asymptotes with slope angle of 45°. Therefore, the maximum AR of the hyperboloidal surface with c = -2 is 0.5. In Fig. 35(c), the spherical surface with c of 0 also has the maximum AR of 0.5, which is a hemi-spherical surface (r = l/2). However, the paraboloidal surface does not have the limit of AR because of its parallel asymptotes.

Fig. 35 Examples of spherical and aspheric surfaces as r decreases

The lens shown as Fig. 36(a) is an aspheric lens model, where l is the length on the x-y plane and h is the height along the z axis. Aspect ratio (AR) was defined as the ratio of h to l. The lens surface is sculptured into a square shape to make 100%

fill-factor on the substrate, as shown in Fig. 36(b), where p is the pitch of the square

lens.

Fig. 37(a) is the schematic illustration of a lens array film periodically arranged with square shape. In order to reduce Moiré effect caused by the interference between its periodic structures and TFT array, the lens pitches are randomized as a result the lenses are not square as the periodic arranged one, as shown in Fig. 37(b).

Fig. 36(a) An aspheric surface lens (b) an aspheric surface lens sculptured into square shape

Fig. 37(a) The lens array film arranged by periodic square lens array (b) the lens array film subject to random lens pitches

4.3 Simulations

4.3.1 Periodic Arrangement

The optical function of a periodically-arranged lens array is primarily provided for light collection. The increase of the on-axis intensity along the normal direction is able to explain the light-collecting ability of a film. In general, BEF containing prismatic features is regarded as the best light-collecting component which enhances on-axis intensity achieved to 152%. And conventional diffuser not only scatters light into all directions but also offers about 123% on-axis intensity enhancement, as shown in Fig. 4.

To compare the functions of the lens array film with BEF and diffuser, a backlight was modeled by the optical software, in which the light guide plate was assumed to emit light with Lambertian distribution. In the following angular intensity distribution figures, the x-axis is the viewing angle and the y-axis is the relative radiant intensity.

The dotted line represents the Lambertian distribution light extracting from the light guide plate, in which the maximum intensity is normalized to be 1. In the following simulations, the lenses were classified by conic constants, and the parameter is aspect ratio (AR).

A hyperboloidal lens model whose profiles shown as Fig. 38(a) is formed when conic constant c = -2, in which p = 100um and l = 100 2um. Furthermore, the maximum AR is 0.5. Fig. 38(b) indicates the simulation results of hyperboloidal lens array films (HLAF) with c = -2. The “square symbol” represented by HLAF with AR of 0.5 has the highest relative intensity of 1.41 at viewing angle 0°. As the decrease of AR, the radiant intensity is lower.

Fig. 38(a) The profiles of hyperboloidal surfaces (b) The angular intensity distributions of HLAF with different ARs

Fig. 39(a) The profiles of paraboloidal surfaces (b) The angular intensity distributions of PLAF with different ARs

A paraboloidal surface is formed when c = -1. Fig. 39(b) indicates the simulated performance of angular intensity distribution of paraboloidal lens array films (PLAF) with AR of 0.4, 0.6 and 0.8, whose profiles are shown in Fig. 39(a). The paraboloidal surface does not have the maximum value of AR. As the AR increases to exceed 0.6, the on-axis intensity does not increase. The “square symbol” represented by PLAF with AR of 0.6 has the highest relative intensity of 1.55 at viewing angle 0°.

A spherical surface is formed when c = 0, whose profiles are shown in Fig. 40(a).

The maximum value of AR is 0.5. Fig. 40(b) indicates the simulated performance of angular intensity distribution of spherical lens array films (SLAF). The “square symbol” represented by SLAF with AR of 0.5 has the highest relative intensity of 1.22 at viewing angle 0°. As the decrease of AR, the radiant intensity is lower.

Fig. 40(a) The profiles of spherical surfaces (b) The angular intensity distributions of SLAF with different ARs

Fig. 41 The enhancements in the range of conic constants of -2.5~0.5 and the corresponding ARs

The conic constants of the examples mentioned above are -2, -1 and 0. Then, we will show the simulation results in the range of -2.5~0.5. In Fig. 41, the y-axis is the maximum enhancement of on-axis intensity and the corresponding AR. The x-axis is the conic constants. When conic constants are in the range of -1.4~-0.6 within AR of 0.5~0.7, these lens array films perform the high enhancement of about 1.55. Moreover, the paraboloidal surface located in the range is the applicable lens structure for light collection. Consequently, paraboloidal lens array film with periodic arrangement, which has better on-axis enhancement than conventional diffuser and single sheet of BEF, as shown in Fig. 42, is able to be regarded as a light-collecting component.

Fig. 42 The angular intensity distributions of PLAF compared with conventional diffuser and BEF

4.3.2 Random Arrangement

Moiré effect in the conventional BLM is due to the interaction of the periodic structures of BEFs and the LCD pixels. In order to reduce Moiré effect, randomization of micro-features can alleviate such undesired characteristics [4].

Therefore, PLAF with periodic lens array is reformed as the random arrangement of the lens pitches. PLAF subject to random lens pitches was proposed as the high gain diffuser which diffuses and collimates light simultaneously.

Considering the fabrication of PLAF, the vertex radius of curvature was fixed at 20um. According to the equation (1) and (2), the pitch of PLAF with AR of 0.6 is 68um. The smallest pitch of fabrication is 5um. The randomizing pitches are in the range of 5~68um within uniform distribution. Fig. 11 indicates light-collecting effects among conventional diffuser and PLAFs by the angular intensity distribution.

periodically-arranged PLAF but 9% increase compared with conventional diffuser.

The random arrangement sacrifices the decrease of on-axis enhancement for the reduction of Moiré effect. Furthermore, the random range was not optimized.

Fig. 43 The angular intensity distributions of PLAFs and conventional diffuser

The diffusing ability can be explained by full width at half maximum (FWHM) of light diffusion in bidirectional transmittance distribution function (BTDF). Fig. 44 shows the BTDF of conventional bead-coating diffuser and PLAF subject to random lens pitches. At incident angle 0° shown by the “black solid lines”, FWHM of diffuser is 24° and randomly-arranged PLAF is 36°. In accordance with the comparison, the random arrangement has better diffusing ability than conventional diffuser.

Additionally, BTDF explains the light-collecting ability. To take the “triangular symbol” of incident angle 60° for example, the output peak is located at 40° in BTDF of conventional diffuser. It means that the diffuser causes 20-degrees shift for 60-degrees incident light. However, randomly-arranged PLAF has 29° deviation.

Consequently, PLAF subject to random lens pitches possesses both of the light-collecting and diffusing abilities.

Fig. 44(a) BTDF of conventional diffuser and (b) randomly-arranged PLAF

Transmittance is defined as the ratio of the energy passing through the film to the total energy of the light source. In the simulation model, a plane light source was assumed to emit light with Lambertian distribution, and no reflector is below it. Table 3 shows transmittance of PLAF and BEF. The transmittance of periodic PLAF is similar to single sheet of BEF.

Table 3 Transmittance of BEF and PLAF

Single sheet of BEF

Two sheets of

BEFs Periodic PLAF Random PLAF Transmittance

(%) 52 28 51 61

Chapter 5

Conclusion

Diffusing film is defined while the propagating light redistributes. A film is regarded as a diffusing film whatever functions it possesses when light passes through, such as light collection, diffusion or beam shaping. In general, the micro-structures of diffusing film control the propagating direction of light for specified distribution. In this study, two optical functions of the films with surface relief of lens array were proposed and applied to backlight module (BLM).

Beam shaping diffuser with Both-sided lens array was adopted for the divided areas in the scanning BLM. We designed that the lens pitch is 100um and the sag is 24um. The thickness of the film is the focal length of 250um. The tolerance of divergent angle of the incident light is able to be approaching 15°. We got the uniformity achieved to 86%.

The novel optical film, PLAF, was introduced to perform dual functions of diffusion and light collection. PLAF with periodic paraboloidal lens array enhances on-axis intensity achieved to 155% along the normal direction. On the other hand, PLAF subject to random lens pitches performs 134% on-axis intensity and 50%

FWHM increase compared with conventional diffuser in BTDF. Therefore, randomly-arranged PLAF can be regarded as the high gain diffuser which is substituted for BEFs and diffusers in the BLM. In Appendix, the study “High Gain Diffusing Film with Surface Relief of 2D Paraboloidal Lens Array” was accepted and published in International Display Workshops (IDW) 2007, Sapporo, Japan.

References

[1] Masakazu Uekita, Yutaka Mineo, and Hitoshi Masaki, “A Novel Protective Diffuser for Backlit LCDs,” SID ’00 DIGEST, pp. 1036-1039, 2000.

[2] Huang-Chen Guo, Cheng-Lin Yang, Ying Tsung Lu, Pong Lai, Wann-Diing Tyan, and Chang-Sheng Chu, “Novel Diffuser for LCD Backlight Application,” SID ’05 DIGEST, pp. 575-577, 2005.

[3] R. C. Allen, L. W. Carlson, A. J. Ouderkirk, F. M. Weber, A. L. Kotz, T. J. Nevitt, C. A. Stover, and B. Majumdar, “Brightness Enhancement Film,” U.S. Patent No.

6111696, 2000.

[4] Hwi Kim, Yong Jun Lim, Byungchoon Yang, Kyongsik Choi, and Byoungho Lee,

“Geometrical Analysis of Optical Transmission Characteristics of Prism Sheet Layers,” Optical Engineering, Vol. 44(12), 128001, 2005.

[5] Eugene G. Olczak, Masako Yamada, Dennis J. Coyle, and Daniel R. Olson, “A Moiré-Free Platform for LCD Backlighting,” SID’06 DIGEST, pp. 1336-1339, 2006.

[6] Tasso R. M. Sales, “Structured Microlensarrays for Beam Shaping,” Optical Engineering, Vol. 42(11), pp. 3084-3085, 2003.

[7] Tasso R. M. Sales, Stephen Chakmakjian, Donald J. Schertler, and G. Michael Morris, “LED illumination Control and Color Mixing with Engineered Diffusers™,” Proc. of SPIE, Vol. 5530, pp. 133-140, 2004.

[8] A. A. S. Sluyterman and E. P. Boonekamp, “Architectural Choices in a Scanning Backlight for Large LCD TVs,” SID’05 DIGEST, pp. 996-999, 2005.

[9] Yuan-Jung Yao, Yen-Hsing Lu and Chung-Hao Tien, “Tandem Light-Guide with Prismatic Micro-Bumps Structures for Field-Sequential-Color Backlight Module,”

IDMC’07, pp. 409-412, 2007.

[10] Kälil Käläntär, Tadashi Kishimoto, Kazuo Sekiya, Tetsuya Miyashita and Tatsuo Uchida, “Spatio-Temporal Scanning Backlight Mode for Field-Sequential-Color Optically Compensated-bend Liquid-Crystal Display,” Journal of the SID 14/2, pp.

151-159, 2006.

[11] Ming-Chin Chien, Cho-Chih Chen, Yen-Hsing Lu and Chung-Hao Tien,

“Region-Partitioned LED Backlight Design for Field Sequential Color LCD,”

SID’07 DIGEST, pp. 441-444, 2007.

[12] Yen-Hsing Lu, Yu-Kuo Cheng and Chung-Hao Tien, “A Localized Partition Approach for High-Dynamic-Range Display,” SID’07 DIGEST, pp. 449-452, 2007.

[13] Koichi Akiyama, Matsumoto, “Light Device and Projector Equipped with the Same” U.S. Patent No. 7150535, 2006.

[14] Liwei Lin, T.K. Shia, and Chun-Jung Chiu, “Fabrication and Characterization of

IC-Processed Brightness Enhancement Films,” IEEE, pp. 1427-1430, 1997.

[15] A. Nagasawa, T. Eguchi, Y. Sanai, and K. Fujisawa, “A Novel Backlight System with the Unified Component,” IDW/AD’05, pp. 1285-1288, 2005.

[16] Joo-Hyung Lee, Jun-Bo Yoon, Joon-Yong Choi, and Sang-Min Bae, “A Novel LCD Backlight Unit Using a Light-Guide Plate with High Fill-factor Microlens Array and a Conical Microlens Array Sheet,” SID’07 DIGEST, pp. 465-468, 2007.

High Gain Diffuser Film with Surface Relief of 2D Paraboloidal Lens Array

Shun-Ting Hsiao*, Po-Hung Yao** and Chung-Hao Tien*

*Department of Photonics & Display Institute, National Chiao Tung University, Taiwan

**Department of Photonics & Institute of Electro-Optical Engineering, National Chiao Tung University, Taiwan

ABSTRACT

A novel micro-structure diffuser film, parabol-oidal lens array film (PLAF), was provided for the backlight module of liquid-crystal display (LCD).

PLAF, which enables both of light-collecting and diffusing abilities, has the similar functions to BEFs and diffusers. PLAF with periodic paraboloidal lens array enhances on-axis intensity achieved to 155%

along the normal direction. In addition, PLAF sub-ject to random lens pitches performs 134% on-axis intensity and 50% full width at half maximum (FWHM) increase compared with conventional dif-fuser in bidirectional transmittance distribution function (BTDF).

1. INTRODUCTION

Optical films are usually employed to promote light uniformity and collimation in the backlight module (BLM). As shown in Fig. 1(a), after emitting from the light guide, light propagates through the diffuser film and is scattered into all directions within the hemi-spherical space. Then most scat-tered luminous is redirected by the light-collecting films toward the normal direction.

In the conventional BLM, two orthogonally ar-ranged brightness enhancement films (BEFs) containing prismatic features are treated as the light-collecting films to enhance the on-axis inten-sity along the normal direction [1, 2]. Besides, bead-coating and micro-structure embossing dif-fusers are generally applied in the BLM to increase the uniformity [3, 4]. In order to reduce the produc-tion cost, lots of studies were aimed to integrate various functions [5~7]. In this study, a novel opti-cal layout, paraboloidal lens array film (PLAF), performing dual functions of diffusion and light collection is proposed to combine and replace BEFs and diffusers in the BLM, as shown in Fig.

1(b).

2. FILM STRUCTURES

A paraboloidal surface is one form of aspheric surfaces, and the mathematical expression of an aspheric surface is expressed as:

2 2 2

where c is the conic constant and r is the vertex radius of curvature. We are able to obtain the ex-pression of the paraboloidal surface by setting c = -1. In addition, when c as 0 and -2, the spherical and prismatic surfaces are obtained, respectively, as show in Fig. 2. The vertex radius of curvature of the prismatic surface is infinitesimal.

Fig. 1 Backlight modules of LCD

(a) The conventional backlight module includes light guide, diffuser and light-collecting films (BEFs). (b) PLAF replaces the BEFs and diffuser in the BLM.

The lens shown as Fig. 3(a) is a paraboloidal lens model, where l is the length on the x-y plane and h is the height along the z axis. Here the as-pect ratio (AR) is defined as below:

AR h

= l . (2) The paraboloidal surface is sculptured into a square shape to make 100% fill-factor on the sub-strate, as shown in Fig. 3(b), where p is the pitch of the square lens.

Fig. 4(a) is the schematic model of PLAF ar-ranged with periodic square lens array through the optical software. In order to reduce Moiré effect caused by periodic structures, the lens pitches are randomized as a result the lenses are not square as the periodic arranged one, as shown in Fig.

4(b).

3. SIMULATIONS

3.1 Periodic arrangement

The optical function of PLAF with periodic lens array is primarily provided to collimate light. The increase of the on-axis intensity along the normal direction is able to explain the light-collecting abil-ity of a film. In general, BEF containing prismatic features is regarded as the best light-collecting component which enhances on-axis intensity achieved to 152%. And conventional diffuser not only scatters light into all directions but also offers about 110% on-axis intensity enhancement.

To compare the functions of PLAF with BEF and diffuser, a backlight was modeled by the opti-cal software, in which the light guide plate was assumed to emit the lambertian distribution. In the following angular intensity distribution figures, the x-axis is the viewing angle and the y-axis is the relative radiant intensity. The dotted line repre-sents the lambertian distribution light extracting from the light guide plate, in which the maximum intensity is normalized to be 1.

Fig. 3 Paraboloidal surfaces

(a)The surface of the lens model is paraboloidal, and (b) the lens is sculptured into square shape.

Fig. 4 Paraboloidal lens array

(a) PLAF is arranged with periodic square lens array and (b) subject to random lens pitches.

Fig. 2 Spherical and aspheric surfaces When conic constant c is 0, the surface is spherical. And when c is set as -1 and -2, the aspheric surfaces are paraboloidal and prismatic, respectively.

Fig. 5 The angular intensity distributions of PLAFs with different ARs.

Comparing PLAFs with AR = 0.4, 0.6 and 0.8, the “square symbol” represented by PLAF with AR of 0.6 has the highest relative on-axis inten-sity of 1.55.

Fig. 5 indicates the simulated performance of angular intensity distribution of PLAFs with differ-ent ARs. The “square symbol” represdiffer-ented by PLAF with AR of 0.6 has the highest relative inten-sity of 1.55 at viewing angle 0°. Considering the fabrication of PLAF, the vertex radius of curvature was fixed at 20um. According to the equation (1) and (2), the pitch of PLAF with AR of 0.6 is 68um.

Fig. 6 shows the simulated performance of PLAF with AR of 0.6, hemi-spherical lens array film, BEF and conventional diffuser. In the simulations, BEF has grooves with prism angle fixed at 90° and the pitch of the grooves is 50um. The shape of the spherical lens is square like the arrangement of PLAF, and the pitch is 100um. As shown in Fig. 6, PLAF with AR of 0.6 has higher relative intensity of

Fig. 6 shows the simulated performance of PLAF with AR of 0.6, hemi-spherical lens array film, BEF and conventional diffuser. In the simulations, BEF has grooves with prism angle fixed at 90° and the pitch of the grooves is 50um. The shape of the spherical lens is square like the arrangement of PLAF, and the pitch is 100um. As shown in Fig. 6, PLAF with AR of 0.6 has higher relative intensity of

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