Organization of this thesis

The thesis is organized as following: The principle and design of the dual directional Backlight and the issues of this panel including moiré effect and the image crosstalk is presented in Chapter 2. In Chapter 3, the essential technologies including the fabrication process of the directional light-guide and driving system are introduced. The major instruments used to measure the performance of directional lightguide are also described. The simulated results are presented in Chapter 4. The experimental results, including the fabricated directional light-guide, dual directional backlight, and the evaluated results, will be in Chapter 5. Finally, the applications of the proposed dual directional backlight and the conclusions of this thesis will be presented in Chapter 6 and Chapter 7, respectively.

Chapter 2

Design of Moiré-Free Dual Directional B/L System

2.1 Introduction

The moiré effect and the ways to solve moiré effect will be presented in this chapter. In addition, the concept of dual directional backlight system proposed by Y.

M. Chu in 2005 will be mentioned. The micro-groove structures are patterned on the bottom of the lightguide to redirect the incident side light into the desired direction due to total internal reflection (TIR). After stacking up two identical directional lightguides with the opposite alignment, a dual directional backlight system is formed and thus can be applied to time-multiplexed 3D display. Moreover, to add the concept of moiré effect solutions into the design of dual directional backlight system, the moiré-free dual directional backlight system can not only apply to the time-multiplexed display and also solve the moiré effect to decrease the viewers’

discomfort.

2.2 Moiré Effect

Moiré effect occurs when two or more images are nonlinearly combined to create a new superposition image, as shown in Fig. 2.1 [24]. Moiré patterns are patterns that do not exist in any of the original images but appear in the superposition image, for example as the result of a multiplicative superposition rule. Moiré phenomena are frequently encountered in daily life. For example, the moiré pattern is formed by two or more screens superimposition [25]. In this thesis, the moiré pattern is also observed

filter.

Fig. 2.1 An example of a moiré effect that occurs due to the superposition of two identical periodic layers with a small angle difference.

2.3 Moiré Pattern Analysis

Two models for moiré pattern analysis are reviewed. The geometric analysis method operates in the image plane, and the spectral analysis method [26-27] based on the Fourier transform operates in the frequency plane. Detailed descriptions of these models are listed in the following.

2.3.1 Geometric Analysis method

The geometric analysis method is usually applied to analyze the geometric characteristic of moiré pattern. This method which is regardless of the intensity distribution can merely obtain the pattern and locations.

The way to analyze is to write down two functions of the pattern and replace the zero into an ordinal number, such as, m1, m2 as shown in the following equation:

2 2

1

1(x,y,z) m , F (x,y,z) m

F = = (2-1) The moiré pattern can be obtained by solving the following equations including two

ordinal numbers:

Here, it was assumed that the spectral spectrums of the grating transmittance are pseudo square waves, i.e. the distribution of transmittance and location as shown in Fig 2.2. The harmonic terms will be generated after Fourier series expansion of the transmittance function. It is needed to consider all the moiré pattern combinations of fundamental frequency term and harmonic terms while analyzing the moiré pattern.

The general formula can be written as:

2 observe the smallest frequecy pattern. Therefore, it should take the largest period moire pattern into consideration.

Fig. 2.2 Transmittance distribution vs. location － the spectral spectrums of the grating transmittance are pseudo square waves.

2.3.2 Spectral Analysis Method

The spectral analysis method based on the Fourier transform is usually applied to analyze the frequency and intensity of moiré pattern. The way to analyze is described in the following.

image is given by the product of the reflectance functions of the individual images: Among the above equation, any rN can be rewritten to the Fourier series as the following equation:

If the moiré pattern is formed by the symmetric one-dimension structure, the above equation can be simplified as:

= ∫

If the moiré pattern is formed by the symmetric two-dimension structure, equation (2-5) can be simplified as:

∫

The advantage of Fourier series presentation is that frequency and intensity can be perceived respectively, as shown in the equations (2-6) and (2-7). The frequency can be observed by the coefficient of the function term. The intensity can be observed by the coefficient of am,n term. In addition, each function term can be presented by the two vectors: frequency vector and intensity vector. By these two vectors superposition, the frequency and alignment of moiré pattern can be obtained, as shown in Fig 2.3.

Fig. 2.3 The expression of function in the Fourier domain diagram.

2.4 The Shape of Moiré Pattern

In the following, two cases related to this thesis work will be introduced. In this thesis, the periods of structures are all linear distribution. Therefore, we take two cases with the linear structures into consideration. The first case by geometric analysis is two periods of structures are identical or very close. The second case by spectral analysis is a general condition with the variables of periods’ difference and angle θ (The included angle θ between two structures is shown in Fig. 2.1)

2.4.1 Close Periods of Two Structures

While Identical Period, Included Angle vs. Period of Moiré Pattern

We assume the periods of two structures are equal to d. The direction of one structure is parallel to X axis, the other one has an angle θ with X axis, as shown in Fig. 2.1. Thus the ordinal function can be written as:

dm1

x= (2-8) sin 2

cos y dm

x θ − θ = (2-9) dq

m m d y

x(1−cosθ)+ sinθ = ( ₁− ₂)= (2-10)

When θ →0 ⇒ θ_y =dq, ( ) θ q d

y = (2-11) From equation (2-20), when θ goes to zero, the period of moiré pattern (d/θ) is increased with decreasing θ.

Identical Alignment, Periods’ of Structures vs. Period of Moiré Pattern

We assume the periods of two structures are d and (d＋△), the ordinal equation can rewritten as: From equation (2-14), when the periods’ difference between two structures is very small, the period of moiré pattern is increased with decreasing the periods’ difference, as shown in Fig. 2-4.

Fig 2.4 Periods difference of two structures vs. moiré pattern periods. When the periods of two structures are very close but not the same and have the identical alignment.

2.4.2 General case

Since the small difference periods between two structures may amplify the period of moiré pattern. Therefore, the closed periods are not usually applied to design the structures of the lightguide. The general condition including relatively large difference and angle θ of two structures will be considered.

From the spectral analysis, we assume there are two periodic pseudo square waves of spectral spectrums. The fundamental frequency of one is f1 and the angle θ with X axis is α; and the fundamental frequency of the other one is f2 and the angle θ with X axis is zero. The ratio of two frequencies can be obtained as shown in equation (2-16).

By the vector addition (as shown in equation (2-17)), the frequency of moiré pattern can be obtained as shown in equation (2-18). In addition, the period of moiré pattern can be derived as shown in equation (2-19).

1 projection of moiré pattern on Y axis. Kn is the harmonic term of each frequency.

Only the lower order harmonic terms take into consideration because the intensity of higher order harmonic terms is much weaker than the lower terms. In the equation (2-18), f is the sum of fu and fv. In equation (2-19), T is the period corresponding to f.

The angle θ is assumed as zero, i.e. α is equal to zero. By equation (2-18), the diagram of q1 vs. Tu can be obtained. Among the calculation, fu is merely available by the minimum term. The relationship of moiré pattern period and period’s ratio of two structures is shown in Fig 2.5. The period of moiré pattern shows the relative maximum value when q1 is equal to 1 and 2, respectively. Besides, the minimum period of moiré pattern is about double even triple to that of minimum periods of two original structures.

Fig 2.5 The relationship of moiré pattern periods and period’s ratio between two structures. (The ratio of moiré pattern is the multiples of the minimum period between two original structures)

Ratio of Periods, Included Angle vs. Moiré Pattern Period

By equations (2-16), (2-17), (2-18) and (2-19), the angle θ is a non-zero value.

The relationships among these parameters are shown in Fig 2.6 where the numbers beside contour lines means the multiple of moiré pattern period. (The lager period of two structures is set to one unit.) The moiré pattern period can be reduced by

adjusting the angle θ [28,29], if the ratio of structures can not fit the results of Fig 2.5.

Fig 2.6 The relationship of ratio of periods, angle θ, and moiré pattern period.

2.5 Design of Directional Lightguide

After the introduction in chapter 1, the time-multiplexed type display has been chosen as the objective of this thesis. The configuration of this time-multiplexed display is shown in Fig 2.7. The backlight system is composed of two lightguides and two light sources. There are numbers of micro-groove structures patterned on the bottom of each lightguide which can yield the incident light to individual eye due to TIR, so called dual directional backlight system. The light source is located on the side of lightguide which faces the inclined surface of the micro-groove structures. By the switching the light sources sequentially to match the images on the panel, the

the shape of micro-groove structures is shown in Fig 2.8. The lightguide is made of PMMA (n=1.49). Each micro-groove is a right-angled triangle, and the inclined angle was designed to be 38°. In addition, we defined some parameters here. Groove gap is the distance between two grooves. Groove width is the length of micro-groove in the X axis. Groove pitch is the sum of groove gap and groove width.

Fig 2.7 The configuration of time-multiplexed display proposed by Y. M. Chu.

Fig 2.8 The shape of micro-groove structures and the parameters related to groove.

In order to provide a uniform image, there are two general schemes to design the micro-groove structures distribution. One is to fix the groove width, then tuning the groove gap; the other one is to fix the groove gap (or groove pitch), then tuning the groove width. Of course, it is feasible to tune these two parameters simultaneously, but it increases the difficulty to design the uniform lightguide. The previous design by

Y. M. Chu has chosen the first way to keep uniformity. However, there is an apparent moiré pattern issue. In this thesis, both of two ways will be discussed in detail in chapter 4. In addition, the discrete micro-groove structures may alleviate the moiré effect slightly by breaking the moiré pattern period. The concept of discrete micro-groove structure was mentioned as one solution for moiré pattern by Y. M. Chu [30], as shown in Fig 2.9. The simulation results will be revealed in the chapter 4.

Besides, moiré pattern is changed while the different alignment of color filter, since the moiré pattern in this thesis is formed by two lightguides and color filter. Also, the two cases of color filter alignment including parallel alignment and vertical alignment between lightguide and color filter, as shown in Fig 2.10, will be discussed in chapter 4.

Fig 2.9 The method to suppress moiré pattern (a) continuous and (b) discrete distribution of micro-groove structures.

(a) (b)

(a) (b)

Fig 2.10 Schematics of alignment with color filter and two lightguides. (a) parallel alignment and (b) vertical alignment

2.6 Summary

The principle of moiré effect has been introduced. Also, the design of micro-groove structures for time-multiplexed display by Y. M. Chu has been presented. The methods for suppressing moiré pattern, such as discrete micro-groove structures distribution, redesigning continuous micro-groove structures distribution and utilizing parallel and vertical alignment between two lightguides and color filter will be simulated and discussed in chapter 4.

Chapter 3

Essential technologies and Instruments

3.1 Introduction

The fabrication technology of the dual directional lightguides will be introduced in this chapter. In general, diamond turning and plastic injection molding [31] are applied to fabricate the lightguide. By considering cost and time efficiency, diamond turning was chosen as the fabrication technology in this thesis work.

The other crucial part of the time-multiplexed display, the driving circuit system, will be described in detail in the following. Then, the measurement instruments such as optical microscope (OM) and Conoscope will be illustrated.

3.2 Diamond Turning

Diamond turning [32], a kind of micro-mechanical machining, is based on the micromachining technology to fabricate the desired structure. The track and shape of the diamond knife are completely transferred to the machined surface, as shown in Fig 3.1. The profile accuracy and surface roughness which strongly depend on qualities of the machinery, diamond knife, workpiece material, and the environment may achieve 1 μm and 0.1 μm respectively in the best condition [33]. Besides, the suitable dimension of the structures by diamond turning is between a few to hundred micrometers.

Diamond turning has the advantages of low cost and high efficiency for small quantity. Therefore, it is appreciated to be applied on the prototype fabrication.

Fig 3.1 Fabrication process of diamond turning.

3.3 Driving System

Image crosstalk is a familiar issue for time-multiplexed display. In general, in order to form the 3D perception, the fast switching of two light sources and scanning images to match each other sequentially are needed. However, at the moment the light source switches, the panel retains the last image. As a result, the image crosstalk is formed as shown in Fig 3.2. The fast response LC panel can reduce the effect of crosstalk. In addition, the redesigned driving system is needed.

Fig 3.2 Driving scheme for time-multiplexed display. Image crosstalk is happened when the light source is switched.

A novel idea to design the driving system is adopted in this thesis. As it is mentioned before, each frame time is composed of a left-eye sub-frame and a right-eye sub-frame but causing the image crosstalk. Here, each frame is comprised of

two left-eye sub-frames and two right-eye sub-frames. Among each frame time, the light source is turned off in the first left-eye sub-frame and the first right-eye sub-frame. Therefore, the panel has the adequate time to scan the image but decreases the crosstalk because the backlight is turned off, as shown in Fig 3.3.

The frame time is used to be set as 60 Hz to make sure the image is not glimmer.

Therefore, in this case, each sub-frame can be predicted as 240 Hz. However, the charging time of each row on the panel is 25 μm. By the resolution of 128×160, each sub-frame rate can be raised to 190 Hz. The charging time of each row on the panel is about 30 μs which is just a little larger than the minimum of charging time. Thus, each frame rate can be obtained as 47.5 Hz which is acceptable. In each frame time, the left-eye image is scanned twice by panel in sequence. The left-eye light source is turned on in the second sub-frame time to reveal the left-eye image. Then, the right-eye image is scanned twice by panel in sequence. In the third sub-frame time, the panel is scanning the right-eye image but causes no crosstalk because the light source is turned off. Finally, the right eye light source is turned on to reveal the right-eye image.

The concept of this driving scheme is alike to insert the black image. Therefore, the brightness which is lower than the general 2D panel will be discussed in chapter 4.

The proposed driving system which can effectively solve image crosstalk integrated with fast response LC and the moiré-free dual directional backlight system proposed in this thesis can form a complete time-multiplexed display for mobile phone.

3.4 Measurement Instruments

After the fabrication of lightguides, its geometric characteristic and optical performances are very important to verify the simulation results. For the geometric analysis, optical microscope (OM) is utilized to observe the shape of micro-groove structures and to measure the size of lightguides.

The optical performances, such as angular distribution and uniformity of emitted light, are measured by Conoscope. Conoscope is a measurement system which utilizes optical lens based on Fourier transform to transfer the light beams transmitted (or reflected) from the platform of different angles to the CCD array, as shown in Fig. 3.4.

Each light beam transmitted from the platform with a incident angle θ will be focused on the focal plane at the same azimuth and at a position x=F(θ). Therefore, the angular properties of the sample are measured in a simple procedure without any mechanical movement. Besides, there are collimated and diffuse light sources depending on users’ requirements. Furthermore, moiré pattern regardless of the intensity distribution can be observed by Conoscope with the geometric analysis.

By equipped with a fast photometer system and a high sensitivity spectrometer, the functions of Conoscope are extended to compose of not only the simultaneous measurement of luminance and chromaticity versus viewing direction, evaluation of the data yields, i.e. luminance contras ratio, grey-scale inversion and reduction, color shift and many more parameters, but also the spectra and temporal luminance

variances.

Fig 3.4 Schematics of Conoscope.

3.5 Summary

The diamond turning of fabrication technology for lightguides has been introduced. Also, the double scanning driving system for resolving image crosstalk was proposed. The measurement of optical performances for lightguide is utilized by Conoscope whose principle has been introduced.

Chapter 4

Simulation Results and Discussions

4.1 Introduction

Based on the principle described in chapter 2, the simulation models are established for evaluating the performance of the dual directional backlight system. In this chapter, some parameters are obtained by Y. M. Chu [30] and others will be discussed respectively before the simulation. Then, some simulation models for maintaining the optical performances such as uniformity, angular distribution and suppressing moiré pattern will be discussed respectively. Finally, the moiré-free dual directional backlight system for time-multiplexed mobile display can be derived.

4.2 Simulation Software

The optical simulator Advanced Systems Analysis Program (ASAP^TM), developed by Breault Research Organization (BRO) was used to optimize the dual directional backlight system and simulate its angular distribution and light distribution on top surface of the backlight.

4.3 Simulation Model of Moiré-Free Dual Directional Backlight System

In order to evaluate the whole effect of 3D optical performances, a complete simulation model is needed. There are two main simulation models in the following simulation. One is parallel alignment between color filter and two lightguides as shown in Fig 4.1, the other one is vertical alignment between color filter and two lightguides as shown in Fig 4.2. The common points of these two simulation models

have two dual lightguides which are composed of numbers of micro-groove structures and the light source located to face the inclined surfaces of micro-groove structures.

Then, a detector which is to detect the optical performance including angular distribution and uniformity is set above two lightguides. Finally, a color filter which is to observe the moiré pattern with the lightguides by the geometric analysis positioned

Then, a detector which is to detect the optical performance including angular distribution and uniformity is set above two lightguides. Finally, a color filter which is to observe the moiré pattern with the lightguides by the geometric analysis positioned