Chapter 1 Introduction
1.2 Principle of 3D vision
Three-dimensional information is formed by complex activities in the brain and obtained by the visual system. A lot of related researches have been made in this field [2]. There are several clues to depth perception by the visual sense as shown in Fig.
1.2 [3-4].
Convergence
Convergence is effective when the distance between eyes and objects is within 20 m. Convergence rapidly loses its influence on the visual system as the distance increases because the convergence angle becomes smaller.
Binocular Parallax
Binocular parallax occurs when there are other objects in front of or in the rear of the object on which the right-left eyes are focused. If the degree of this binocular
sensation of depth before and after the object. The binocular parallax is valid and precise in distinguishing the depth difference, i.e. the difference in the distance between several objects. It plays the important role in understanding the relative position of objects within 10 m.
Accommodation and Motion Parallax
Accommodation is only effective when the observation distance is no more than 3 m. Depth perception obtained by motion parallax provides important information to depth perception.
Others
Visual information received by one eye is not the same as that through the other eye, but we perceive these data as a single image. Namely, binocular vision which appears to be based on interaction of the right and left images is an important function in the design of a 3D display. Besides, the distance between the pupils of the human eyes is approximately 65 mm which is adopted when considering the viewing condition.
Fig. 1.2 Depth clues and display factors.
1.3 Introduction to 3D Display Technologies
The first of 3D display went back to the early 1800s. However, most applied 3D technologies were proposed after the middle of 20th century. The 3D displays have their widespread applications today. Generally speaking, the 3D Display can be divided into stereoscopic displays and auto-stereoscopic displays.
1.3.1 Stereoscopic displays
Stereoscopic displays are needed to wear a device, such as polarized glasses, which ensures the left and right eye’s views are received by the correct eye. Many stereoscopic displays have been proposed [5-6]. Most of them have widely used in
close to, some devices to separate the left and right eye’s views. Those requirements limit the widespread attraction of stereoscopic displays as personal displays.
1.3.2 Auto-stereoscopic displays
Auto-stereoscopic displays do not require the user to wear any device to separate the left and right views and instead send them directly to the correct eye. This removes a crucial obstacle to the appealing of 3D display.
Various auto-stereoscopic technologies have been proposed. The principles of several technologies [7] are discussed in the following.
Volumetric type
There are many kinds of technologies to form the volumetric type display. The common point is to produce the object in the real space. One is to draw 3D profiles on a scattering medium with a scattering laser beam [8], as shown in Fig 1.3(a). Another is to project or scan layered images on a spatial designed screen to create a volumetric image profile [9-10], as shown in Fig. 1.3(b). The other is to induce psychological effects with use of a super-large image projection screen [11], as shown in Fig 1.3(c).
Holography
Holography is utilized the laser beam to form the illumination beam and reference beam. Thereafter, the interference fringes can be displayed by the superposition of rays from each object point [12], as shown in Fig 1.3(d).
Integral imaging
Integral imaging is composed of a micro-lens array, the pickup device, and the display device. By means of micro-lens array, the pickup device can pick up several images with the different angles. By combining these pick up images, the 3D images can be revealed, as shown in Fig 1.3(e).
Stereo pair
The stereo pair type is including spatial-multiplexed and time-multiplexed display.
These displays are utilized the spatial or time multiplexed sequence of many different viewing images to form the 3D perception. The spatial-multiplexed displays including parallax barrier and lenticular lens are shown in Fig 1.3(f). (The time-multiplexed display is introduced below)
Method (b) Method (c)
Method (a)
Fig. 1.3 Examples of 3D methods: (a) volumetric 3D display system with rasterization hardware; (b) a solid-state multi-planar volumetric display; (c) DSHARP - a wide screen multi-projector display; (d) color images with the MIT holographic video display; (e) The concept of integral imaging; (f) stereo pairs type (this figure shows the spatial-multiplexed type.)
There are two ways to generate the sense of depth. The first one is to simulate the objects in the real space, as mentioned by methods (a) to (d). The other is to directly
Method (d)
Method (e)
Method (f)
Display device Pickup device
Reconstructed image Elemental image
Object
Lens array
Display Pickup
send pairs of parallax images to each eye respectively, as mentioned by method (e) and (f).
1.3.3 Comparisons between Various 3D Methods
According to the 3D image qualities, system size and cost, each 3D method has their advantages and disadvantages as shown in Table 1.1[19]. The major drawback of stereoscopic display is needed to wear a device. Moreover, the volumetric display often has the drawbacks of bulky and 3D effect limitation. Furthermore, the holographic display has the poor feasibility due to the requirement of ultrahigh technical support. The concept of integral imaging is similar to the stereo pair but has the drawback of low image resolution. Among above-mentioned the 3D methods, the overall evaluations of the stereo pair type are the most appealing, not only has compatibility with the current 2D display technology but also maintain the image qualities. Besides, the stereo pairs display has higher feasibility than the other 3D methods. Therefore, the stereo pair display is widely applied for most of available auto-stereoscopic displays.
Table 1.1 Comparisons between various 3D displays.
Auto-stereoscopic Stereoscopic
Volumetric Holographic Stereo Pair
Natural depth ㄨ-△ ㄨ-△ △-○ △-○
1.3.4 Stereo Pair Type
Based on the image display methods, the stereo pair type can also be divided into two types: spatial-multiplexed (or time-parallel) and time-multiplexed types. The spatial-multiplexed type displays the stereo pairs at the same time and the time-multiplexed type displays the stereo pairs sequentially, as shown in Fig. 1.4.
Both of them require viewing zone forming optics to send the stereo pairs to the correct eye. The viewing zone is a region where the viewers can see the whole images displayed on the screen. There are two viewing zones to match each eye, than forming a complete 3D vision. In order to form the 3D perception, each display system is needed a specific optical power.
Fig. 1.4 Principles of the spatial-multiplexed and time-multiplexed types.
For the spatial-multiplexed method, the well-known examples are the flat panel with a lenticular screen or a parallax barrier, as shown in Fig. 1.3(f). The usage of the lenticular screen or the parallax barrier is to separate the images displayed on the panel to form the parallax images. However, lower resolution is the major drawback of spatial-multiplexed type display because the parallax images are revealed on the panel at the same time. Furthermore, the parallax barrier decreases the image
brightness.
For the time-multiplexed method, its development was restricted in early periods because the fast response time display was not available. In 2001, a 3D display using field-sequential LCD with light direction controlling backlight was proposed [20], as shown in Fig. 1.5. Its viewing zones were formed by a lenticular sheet and a LC shutter. The direction of light can be controlled by switching the LC shutter. Another method was proposed in 2003 [21]. Its viewing zone is formed by a lightguide and a double-edged prism sheet, as shown in Fig. 1.6. After emitted from bottom lightguide, light penetrates into the double-edged prism sheet and is reflected to the certain direction due to total internal reflection (TIR). By switching two light sources sequentially, pairs of parallax images are perceived by each eye and thus form the 3D vision. In 2004, another similar design for time-multiplexed 3D display was proposed by K. W. Chien [22], as shown in Fig. 1.7. In 2005, a novel idea is proposed by Y. M.
Chu [23]. The parallax images are formed by the dual directional backlight system including two stacked lightguides with their own light sources respectively, as shown in Fig 1.8. Each lightguide can yield the incident light to one eye. By switching the light sources sequentially, the stereo pairs can be perceived by eyes respectively, then form the 3D image. More detailed discussion is in chapter 2.
Fig. 1.5 A 3D display using field-sequential LCD with light direction controlling backlight (2001) [20].
Lenticular Lens LCD
Left eye Right eye
Stripe patterned back light
Lens
Fig. 1.6 Dual directional backlight for stereoscopic LCD (2003) [21].
Fig. 1.7 3D mobile display based on sequentially switching backlight with focusing foil (2004) [22].
Fig. 1.8 3D mobile display based on dual directional stacked lightguides (2005) [23].
Light source (for right eyes) Prsim sheet
Light guide plate Light source
(for left eyes)
The attractive feature of the time-multiplexed 3D display is resolution comparable with 2D display technology, yet is impracticable in spatial-multiplexed type display. In addition, spatial-multiplexed type display has the issues of complex design and critical alignment. For the previous work on the time-multiplexed type display, alignment is an issue for the lenticular screen and LC shutter in the first design and the patterns on the double-edged prism in the second design. Furthermore, the light efficiency was reduced by the blocking of the LC shutter in the first design.
In the second and the third designs, inadequate light efficiency resulted from the grooves of the lightguide which were too flat to guide light effectively. In the last design, moiré pattern is formed by the periods of color filter and two stacked lightguides.
1.3.5 Summary
Various 3D display technologies have been discussed above. These 3D displays can yield acceptable 3D images, but most of them can not provide the solution completely, except stereo pair type. Compared with other 3D display technologies, the stereo pair type has many advantages including good image qualities, compact size, high feasibility, and compatibility with current 2D display technology.
Among several kinds of above-mentioned stereo pair type, the time-multiplexed method has an inherent advantage of no decreased image qualities and no alignment issue.
1.4 Motivation and Objective of this Thesis
3D display technology is expected to be the next crucial display technology and shall play an important role in the future. Among them, the time-multiplexed type
and the comparable image qualities with 2D display.
In these years, several researches have been devoted to the time-multiplexed type display. A complete time-multiplexed 3D display consists of driving circuits, fast switching LC panel and directional backlight. The requirements and developments of the first two parts are similar to those of the flat panel display technology. Therefore, this thesis only considers the directional backlight system. In the above introduction, Y. M. Chu has proposed a novel dual directional backlight system including two stacked lightguides and their own light sources for the time-multiplexed type display.
However, moiré pattern caused by the periods of color filter and grooves of lightguide decreases the viewer’s comfort. Besides, the image crosstalk is presented when the panel is scanning the parallax images.
As a result, the objective of this thesis is to mitigate the moiré pattern effect, to improve the optical performance, and to bring up the method to alleviate image crosstalk, based on dual directional backlight system designed by Y. M. Chu.
1.5 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 ⇒ θy =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
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