Chapter 1 Introduction
1.4 Organization
reduce cost. However, the LCF was designed for portable device.
The brightness and CR were limited by optical properties of Ch-LC as mentioned in section 1.2. Moreover, the reflective image was non-uniform due to the light source position.
In this thesis, the objective was to design an optical film for Ch-LCD E-banner.
The Mircrolens Array Film (MAF), Prism Array Film (PAF) and design method was proposed to solve the Ch-LCDs issues under indoor environments with a supplemental light source. The fabrication process can be applied to diamond tuning and roll-to-roll process, which can be easily applied to large billboards. Furthermore, the optical film should be able to apply to flexible display and maintain the performance when display is bent.
1.4 Organization
This thesis includes 6 chapters, as follows: In chapter 2, the ray-tracing method, BSDF, and Ch-LCD are introduced. In chapter 3, the fabrication method, and the instruments for measuring optical properties are described. In chapter 4, design method and simulations are presented. In chapter 5, the experimental results and discussions are described. Finally, conclusions and future work are presented in chapter 6.
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Chapter 2
Principles of Micro-optics of Optical Film and Cholesteric LCDs
In this chapter, several optical principles, such as Snell’s Law, radiometry, photometry, and bidirectional scatting distribution function (BSDF)[22], are described for designing the optical film. The ray-tracing method could simplify the light propagation behavior as a ray. However, the optical property of the cholesteric liquid crystal is difficult to describe by ray-tracing method. BSDF was adapted to characterize the optical property of the cholesteric liquid crystal, and BSDF were then measured. The measured BSDF data were input into the ray-tracing software, Lighttools[23] . For building the model of LC structure. The following is characterizing the working principle of cholesteric liquid crystal. Finally, a brief summary will be given.
2.1 Ray-tracing Method
Ray-tracing method is based on the geometric optics, Fresnel’s Law, and other principles. Geometric optics is used to describe the light propagation through a lens systems or optical instrument, allowing image-forming properties system to be modeled. There are many effects, such as dispersion, polarization, and thin film interference, can be integrated into a ray tracer in a straightforward fashion. The special case to consider is that of the interference of wavefronts which is approximated as planes. When the ray come close or even cross, the wavefront
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approximation breaks down. Thus, the diffraction at an aperture cannot be calculated by ray-tracing. The geometric optics is a convenient way to analyze the action of the optical systems. Therefore, some optical software, such as LighttoolsTM, ZeemaxTM, and TrazeProTM, apply the ray-tracing method to build the simulated environment.
2.2 Radiometry and photometry
The optical characteristic of Ch-LC was difficult to be described by ray-tracing method only. Accordingly, the BSDF based on radiometry and photometry is described and utilized to characterize the reflective characterization of Ch-LC.
2.2.1 Radiometry
The radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. The fundamental radiometric quantities are shown in Table 2-1.
Table 2-1. List of radiometric quantities
There are some quantities should be emphasized.
Q is the radiant energy of collection of photons whose energy of a single one is hν.
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E and M pertains to radiation incident on a surface and leave a surface, respectively.
A is the area element that is perpendicular to the light of sight. [Fig 2-1]
Fig 2-1. Schematic diagram of projection area Radiometry introduces the following equation,
weighted by a luminosity function that models human brightness sensitivity. The basic photometric unit of radiant power is lumen that is defined as a luminous flux emitted into a solid of one steradian by a point source whose intensity is 1/60 of 1 cm2 of a blackbody at the solidification temperature of platinum (2042K). One lumen corresponds by definition to 1/683 W of a monochoromatic light of λm=555nm which
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is the wavelength where V(λ) has its maximum value of unity as shown in Fig 2-2.
Therefore, the luminous flux emitted by a source with a radiant flux is given by 6 8 0
V ( ) ( )d (5)Fig 2-2. Human visual response function
Based on the radiometry and photometry, the bidirectional scattering distribution function (BSDF) which was introduced by Paul Heckbert in 1991[22] is often used to describe the scatter light distribution. In this thesis, the structure of cholesteric liquid crystal was very difficult and complex to build in the optical simulation software.
Therefore, the bidirectional reflectance distribution function (BRDF) was adopted to characterize the optics of the cholesteric liquid crystal. BRDF is one branch of BSDF family as shown in Fig 2-3.
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Fig 2-3. The Schematic of BSDF
The diagram of BRDF is shown in Fig 2-4, and the definition of BRDF is the horizontal planes, respectively. The reflected luminance and the incident luminance can be obtained by measuring instrument so that the BRDF is then obtained. Thus, BRDF can characterize the reflective specifications of samples that enable designers, manufacturers, and users to simplify and check the requirements.
In this thesis, the BRDF of cholesteric liquid crystal was measured by Conoscopes[26] in reflective mode. Then the measured data was imported to the optical software, LightTools, to build the optical models of LC structure.
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Fig 2-4. Diagram of the definition of BRDF[26]
2.3 Optics of Cholesteric LCDs
It has been some 40 years since cholesteric liquid crystal (Ch-LC) were considered as a candidate for an electro-optic flat-panel display. The cholesteric phase is a liquid crystal phase exhibited by chiral molecules or mixtures containing chiral components[7]. A cholesteric liquid crystal is similar to a nematic liquid crystal. It has, however, one property that is different from the nematic liquid crystal, in that it has helical structure as shown in Fig 2-5. The distance along the helical axis for the director to rotate 2π is called the pitch and is denoted by P0. For a given pitch, the optical characteristic of cholesteric liquid crystal is depending on the helical axis as shown in Fig 2-6. In the planar texture, the helical axis is perpendicular to the cell surface as shown in Fig 2-6 (a). The material reflects light centered at the wavelength given by λ0=n P0, where n is the average refractive index. If λ0 is in the visible light region, the cell has a bright colored appearance. In the focal conic texture, the helical axis is more or less parallel to the cell surface, as shown in Fig 2-6 (b). When the pitch is short, the choleteric liquid crystal can be regard as a layered structure, which is a multiple domain structure and the material is scattering. When the applied field is
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larger than a critical fieldEc, the helical structure is unwound with the liquid with the liquid crystal director aligned in the cell normal direction as shown in Fig 2-6(c), This texture is called the homeotropic texture. With the appropriate surface anchoring condition or dispersed polymer, both the planar texture and the focal conic texture can be stable at zero field.
Fig 2-5. Schematic diagram showing the cholesteric structure
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Fig 2-6. The structures of cholesteric textures
The operation principles of Ch-LCDs are illustrated in Fig 2-7. In the voltage-off state, the planar texture reflects brilliant colored light if the Bragg reflection condition is satisfied[27]. The circularly polarized light with the same handedness as the helical structure is reflected strongly because of constructive interference. On the contrast, the circularly polarized light with opposite handedness to the helical structure is not reflected because of destructive interference. If the incident light is unpolarized, the light will decomposed into right and left circularly polarized components with one component reflected and the other transmitted. The transmitted one is absorbed by the black paint coated over the rear substrate, as shown in Fig 2-7 (a). When the voltage is applied, the periodic helical structures are changed to focal conic. Thus, the Bragg
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condition is destroyed and a dark state is then obtained as show in Fig 2-7 (b).
Fig 2-7. The operation principle of Ch-LCD at (a) Voltage-off and (b) Voltage-on state.
Ch-LCDs have advantages of low power consumption, bistable structures, and color filter less. Therefore, Ch-LCDs are suitable for portable devices and E-banner[28], as shown in Figure However, the specular reflection and the surface reflection lower the viewing angle and contrast. Moreover, the color appearance is related to the incident angle from the light source, hence, restricting the practicability of Ch-LCD.
(a) (b)
Fig 2-8. (a) Matsushita sigma book[29] and (b) Magink E-billboard[30]
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2.4 Summary
The ray-racing method which is based on geometric optics was used to design the proposed method to improve the reflected image quality of reflective Ch-LCD and other reflective displays based on the specular reflection. The optical software, LightTools, was then used to build the simulated environment. Next, the BRDF of cholesteric liquid crystal was measured and imported to the software to simplify the simulation. The basic principle of Ch-LCD is also introduced in this chapter. The Ch-LCDs have the advantages of non-polarizer, non-color filter, and bendable.
However, the specular, surface reflection and color shift restrict the applications of Ch-LCD at indoor environment even with the additional light source to enhance the brightness. Therefore, in this thesis, the optical film for improving the brightness and widen the viewing range with additional light source at indoor is proposed.
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Chapter 3
Fabrication Technology and Measuring Instruments
The proposed microstructure was a prism structure with specific angles, suitable for diamond micromachining combining and was combined with the roll to roll process. Additionally, the Conoscope system can obtain Ch-LCD BRDF data and evaluate prism array film (PAF) performance. In the following sections, the fabrication process and measurement instruments are presented.
3.1 Fabrication-Diamond Turning
To fabricate the designed microstructures, the diamond turning with a roll to roll process was provided by ITRI, was utilized. A single point diamond tool (SPDT) can be used to fabricate a finished optical component on a precision machine under precisely controlled[31]. Although diamond turning application for optical components started in the 1960s, the technology attracted wider attention in the mid-70s. The process step is as the following:
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Fig 3-1. Diamond turning fabrication steps
Diamond turning is a multi-stage process. Initial stages of machining are carried out using a series of CNC lathes of increasing accuracy. A diamond-tipped lathe tool is used in the final stage of the manufacturing process to achieve a sub-nanometer level surface. The process finishes with sub-micrometer form accuracies. The surface finish quality is measured as the peak-to-valley distance of the grooves left by the lathe. The form accuracy is measured as a mean deviation from the ideal target form.
Finally, the quality and accuracy is monitored throughout the manufacturing process using equipment such as laser profilometers, laser interferometers, optical and electron-microscopes.
The best quality natural diamonds are used as a single-point cutting element CNC lathes
Diamond-tipped lathe tool
Sub-micrometer
Laser profilometers, etc.
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during the final stage of the machining process. A CNC SPDT rests atop a high quality granite base, which is placed on air suspension on a solid foundation to keep its working surface strictly horizontal, creating a quality micrometer surface. The machine tool components are placed on top of the granite base and can be moved with high degree of accuracy using a high-pressure air cushion or hydraulic suspension.
The machine element is attached to an air chuck that is separated from the electric motor which spins it to another air cushion.
The cutting tool is moved with nanometer precision using a combination of electric motor piezoelectric actuators, as illustrated in Fig 3-2[32]. The motion of the tool is controlled by a list of coordinates generated by a CAD model. The final surface is achieved with a series of decreasing depth cutting passes.
Fig 3-2. Schematic overview of ruling
Additionally, the inclination angle of the cutting edge can be controlled so that the degrees of freedom are enhanced as depicted in Fig 3-3. Therefore, the angle of prisms can be realized by tilting the cutting edge. Moreover, the optical film length can be expanded using a roller design. Cutting edge shape can be a cone, triangle, tetragon, and others.
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Fig 3-3. Diagram of diamond cutting
3.2 Measurement-Conoscope System
In order to measure the BRDF data of Ch-LCD and evaluate the variation of brightness, the conoscope was utilized in this thesis. The Conoscope system applies Fourier transform lens to transfer light beams and emitted from the test area to a CCD array. Therefore, the angular properties can be easily measured on the CCD sensor plane. The CCD array consists of various directional CCD sensors which detect brightness, color, and angular distribution of transmissive light. Besides, not only tramissive type but also reflective type can be measured through the operation mode.
The BRDF can be measured using the finger functionality of illumination as illustrated in Fig 3-4. Furthermore, the finger can also detect the spectrum and provide a photomultiplier function. For reflective mode shown in Fig 3-5, finger is at the Fourier transform plane to illuminate the samples. This collimated light strikes upon the sample surface and be reflected back the system. Thus, the angular distribution can be detected by the CCD-array. BRDF of samples are then be obtained by changing the finger position at the Fourier plane.
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Fig 3-4. Functionalities of finger in Conoscope system
Fig 3-5. Schematic of the Conoscope in reflective mode
The working principle of transmissive mode is depicted in Fig 3-6, where the first lens provides a Fourier transform image of the display surface. Each light emitted from the test area at incident angle, θ, will be focused on the focal plane at the same azimuth and at a position x=F(θ). The sample angular characteristics are thus measured simply and quickly, without any mechanical movement.
Test sample
Fourier lens
Fourier transform plane (Finger)
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Fig 3-6. Schematic of the Conoscope in transmissive mode
3.3 Summary
In order to simplify the simulation and evaluate the designed optical film performance, the Conoscope system, which can measure the test sample BRDF and reflected angular luminance distribution, was adopted. Then the measurement result was imported into simulation software to build the corresponding model.
Additionally, the process method for fabricating the PAF was introduced. The method of diamond turning provides high accuracy and high efficiency what are required for optical films.
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Chapter 4
Design Method and Simulation
The design method for solving the issues of Ch-LCDs specular reflection with the supplemental light source is described in this chapter: asymmetrical microlens array and prism array were used to refract and redirect the specific incident light to the desired viewing zones. By using simulation software, the simulation models of designed optical film were built and the results are shown in this chapter as well.
4.1 Design of Asymmetrical Microlens Array
As section 1.2 and 2.3 mentioned that the general issues of reflective type LCDs with metal-reflector and the characteristic of Ch-LC planar state were specular reflection and surface reflection, the optical film was an efficient way to solve these issues. Because our objective was to apply Ch-LCD to indoor billboard, the ambient light might not provide adequate light to illuminate. Thus, the supplemental light source was necessary. Our group has proposed MAMA-LCF for portable reflective displays to enhance the brightness and contrast ratio near normal viewing direction.
However, the design was not suitable for billboard application. As shown in Fig 4-1, MAMA-LCF was designed for the portable devices, thus, the viewing zone was narrow. Furthermore, the multi-direction of microlens was for ambient illumination.
For a specific light source, the enhancement of one direction will higher. The idea for new design continued using the asymmetrical microlens array due to the oblique incident light. The function of micro-lens was not just focusing, but to spread the reflected light to the desired viewing zone. Therefore, the slope set of microlens was
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the key point of this design. The schematic of Ch-LCD is shown in Fig 4-2 and the viewing angle is shown in Fig 4-3 according to the position of viewers and display, so the reflected light should redirect to the range, which is 0 to 25 degree.
(a) (b) Fig 4-1. Schematic of (a) MAMA-LCF and (b) MAF
Fig 4-2. Schematic of Ch-LCD e-banner
Ambient light
MAMA-LCF
Light source
MAF
Light source
2m 30cm
5°
14°
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Fig 4-3. The related position of the display and viewers
To evaluate the suitable slope that could redirect light to viewers, the relationship of incident angle, reflection angle and microlens slope, were analyzed as illustrated in Fig 4-4. Assume lens-A is where the light incidents on the optical film and lens-B is where the light exits. The incident angle is ρ and the reflection angle is σ with respect to the normal direction of the optical film. α and β are the angle between the lens curve slope and horizontal axis of lens-A and lens-B respectively. The Ch-LC reflective characteristic is near specular reflection that can be treated as a mirror. Thus, the relationship can be analyzed by ray tracing method as the following equation.
1s i n1 ( 1
nfilm sin( )) (7) s i n1(nfilm(1 )) (8)
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Fig 4-4. The condition of incident and reflection angle
The analysis result is shown in Fig 4-5. Because the application was for billboard, the uniformity was taken consider to avoid non-uniform reflected image due to the light source position. Therefore, the display was divided into three divisions. The color variation is the related reflection angle of different α and β. The mark areas in Fig 4-5 are the desired reflection angle according to the viewers, hence, the suitable prism angle range of each area can be obtained.
σ ρ
α β
Lens-A Lens-B Microlens
Ch-LCD
θ1
θ2
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(a)
(b)
α (degree)
β (degree)
σ (degree)
ρ σ
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Fig 4-5. The contour of reflection angle for light incident on (a) top, (b) middle, and (c) bottom of the display
Furthermore, the shape of curved surface was also considered. Assume the angle between the incident light wavefront and the normal direction of the substrate is , and the curved surface can be expressed as
(9) which is illustrated in Fig 4-6. Then its tangent vector is
(10) The wavefront unit vector of incident light is
(11) Therefore,the curve surface weighting with respect to the incident light wavefront is (12)
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According to Eq. (10), the higher the weighting is, the higher the related slope to the incident light. Therefore, the lens shape can be determined.
Fig 4-6. The weighting of curved surface to the wavefront of incident light
4.2 Simulation Model of Microlens Array Film
The simulation software, LightTools, was adopted to design the microlens array film (MAF) in this thesis. LightTools follows the ray-tracing method and can model real objects in simulation. The Ch-LCD BRDF which was measured using the collimated light source in Conoscope is shown in Fig 4-7. The profile of BRDF shows similar distribution compared with specular reflection. The peak at 50 degree was lower due to a bit of scatter by Ch-LC. Thus, the value around 50 degree was higher than the value around 40 and 60 degree. Besides, the simple configuration of Ch-LCD as depicted in Fig 4-8 was created using LightTools and the Ch-LC was substituted by the measured BRDF.
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Fig 4-7. Measured BRDF of Ch-LCD
Fig 4-8. Schematic diagram of Ch-LCD configuration
The lens shape, which was determined by the curved surface weighting with
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Fig 4-9. Schematic of microlens
Ray-tracing results were used to examine redirected light direction of mircolens,
Ray-tracing results were used to examine redirected light direction of mircolens,