Chapter 1 Introduction
1.5 Organization of this thesis
This thesis is organized as follows: the principle of color science and the introduction of driving hardware are presented in Chapter 2. The CBU mechanism, color standard such as CIE1931, CIE 1976, and the definition of color different are described in this chapter. Additionally, this chapter also presents the working principle of hardware components such as an OCB panel, LCD drivers, LED drivers, and an FPGA control board. In Chapter 3, using color field arrangement and RGBW method for suppressing CBU are demonstrated. The concepts of driving are distributed and
the experimental results are shown in this chapter. To suppress the issue of fixed driving principle, the exchange of color sequence RGBC/RGBY is proposed. The system architecture and the implementations of a timing controller are described in this chapter. In Chapter 4, the observations and the evaluation index of CBU are presented. With dynamically adjusting color sequence, the CBU is effectively reduced.
The dynamic color backlight, an extended method, is proposed. This adaptive FSC-LCD is the first sequential driving display in the world adapted by image contents for further mitigating CBU. In Chapter 5, the large-sized FSC-LCD application and the local adaptation are discussed. The perceived threshold of boundary-free image and recommendation of the profile of local backlight are also presented. Finally, the summary of dissertation and future works are given in Chapter 6.
Chapter 2
CBU Mechanism and Driving Technologies
The sequential driving in the FSC-LCD is an effective mechanism without requirement of color filters in the conventional LCD. However, it faces a serious issue:
color breakup (CBU). In order to suppress CBU, the basic concepts such as the eye movement response and the CBU phenomenon need to be understood. The color difference (ΔE) between the original and the CBU images is introduced and utilized to evaluate CBU phenomenon. In addition, the working principles of LC driving technologies such as the fast response LC, drivers, and circuits are introduced in this chapter.
2.1 Physiology of eye movements
Human eye is a complex visual system, and its structure is shown in Fig. 2-1.
Light passes through the cornea and the pupil controlled by the iris to adjust the incident magnitude of light. Then the lens focuses the light on the retina, and there are two kinds of receptors to detect intensity and color of light. The rod cells are sensitive to light intensity; the cone cells are sensitive to color. The receptors detect light and convert it into electrochemical signals. Finally, the signals pass through the optic nerve to the brain, and humans can perceive images [58]. The CBU phenomenon in the FSC-LCD has strong dependence on perception.
Fig. 2-1 Structure of human eye.
Fig. 2-2 (a)Perceived image and (b) saccade movement (black line)
The fovea lies near the projection point at the center of the retina. This region has the highest resolution and is 0.1% of retina area. In order to see objects clearly, humans need to move their eyes to focus on the fovea. In perception research, two major types of eye movements are mentioned: saccade and pursuit [59]. Saccade is a rapid, random movement while perceiving static objects. The movement moves around objects to focus on the fovea and gathers correct visual information [60]-[63].
The test image is shown in Fig. 2-2 (a), and the eye movement for the static image is lined with black in Fig. 2-2 (b). In order to recognize the woman, the observer’s eye moves around her face to collect information. Generally, saccade is spontaneous, and the speed happens quickly at about 200 degree/sec [64].
The other type of eye movements is pursuit. Pursuit is a smooth and predictable movement when perceiving dynamic objects. When perceiving a moving object, the human eye will follow at the same velocity of the object to focus on the fovea and catch clear images. Compared to saccade, pursuit velocity is much slower, and some research proposes the velocity is about 90 degree/sec [ 65 ]. Moreover, pursuit movement (pursuit latency) means the delay in eye pursuit, which is defined as the difference in beginning motion time between the target and the eye. According to literatures, the pursuit latency is about 100 ms to 150 ms [66].
2.2 Mechanism of CBU
By displaying R, G, and B fields sequentially faster than the time resolution of the eye, a full color image can be observed. If there is a relative movement between human eyes and moving object, the field images will be integrated separately on the retina, resulting in perception of the rainbow effect or CBU on the margin of the image. The CBU mechanism will be described in the following section.
2.2.1 Dynamic CBU
The dynamic CBU phenomenon always occurs at the edge of moving image on an FSC display. The mechanism of dynamic CBU is related to “Smooth Pursuit Eye Movement (SPEM)” of the visual system. Smooth pursuit is an eye movement that smoothly tracks a moving object in the visual system. The purpose of smooth pursuit is to perceive a moving object. Therefore, dynamic CBU can be perceived when three fields, R, G, and B, alternate slower than the SPEM.
A white image moves from left to right on an FSC-LCD and human eyes pursue this moving image as shown in Fig. 2-3. While the FSC-LCD displays a moving image from an initial pixel to the next one, R, G, and B fields are shown at the initial pixel, then these fields are presented at the next pixel. However, human eyes shift while displaying the individual field. Thus, R, G, and B fields separately project on the retina. After integrating color fields in the brain, the dynamic CBU is perceived.
Fig. 2-3 The scheme of CBU phenomenon in a conventional FSC-LCD.
(a)
(b)
Fig. 2-4 (a) The relationship between time and display position of a simulation scheme and (b) CBU image in a visual system.
We can simulate the integration of the visual system by using Matlab software, as shown in Fig. 2-4(a), the relationship between time and display location (horizontal direction). A white bar moves from left to right and the dotted lines represent the observer’s viewpoint shift. When the observer watches a moving image, observer will shift the viewpoint to trace the moving image. Consequently, the observer will recognize an image as shown in Fig. 2-4(b) [67].
2.2.2 Static CBU
For a stationary image, CBU is observed during or after a saccadic eyes movement. Saccade is an eye movement that randomly and rapidly moves to scan and perceive a target image. If there are several white images as shown in Fig. 2-5 eye will perceive part of the image that attracts the interest [68]. Human’s visual system will be sensitive to white bars in this case, as shown in Fig. 2-5(a), where line-A represents the eye saccade movement. Saccade eye movement is much faster than SPEM, the stationary image seems to break up into several field colors, as shown in Fig. 2-5(b).
Fig. 2-5 (a) Image in an FSC-LCD and path of a saccade. (b) Observed CBU during or just after the saccade.
2.3 Color difference (delta E)
In the study of the perception of color, one of the first mathematically defined color spaces was the CIE 1931 XYZ color space (also known as CIE 1931 color space), created by the International Commission on Illumination (CIE) in 1931 [69].
The CIE XYZ color space was derived from a series of experiments done in the late 1920s by W. David Wright [70] and John Guild [71]. The human eye has receptors (called cone cells) for short (S), middle (M), and long (L) wavelengths. Thus in principle, three parameters describe a color sensation. The tri-stimulus values of a
color are the amounts of the three primary colors in a three-component additive color model needed to match that test color. The tri-stimulus values are most often given in the CIE 1931 color space, in which they are denoted X, Y, and Z [72]. CIE XYZ is based on direct measurements of human visual perception, and serves as the basis from which many other color spaces are defined.
In the CIEXYZ color space, the tri-stimulus values are not the S, M, and L responses of the human eye, but rather a set of tri-stimulus values called X, Y, and Z, which are roughly red, green and blue, respectively. Two light sources, made up of different mixtures of various wavelengths, may appear to be the same color; this effect is called metamerism. Two light sources have the same apparent color to an observer when they have the same tri-stimulus values, no matter what spectral distributions of light were used to produce them.
Due to the nature of the distribution of cones in the eye, the tri-stimulus values depend on the observer's field of view. To eliminate this variable, the CIE defined the standard (colorimetric) observer. Originally this was taken to be the chromatic response of the average human viewing through a 2° angle, due to the belief that the color-sensitive cones resided within a 2° arc of the fovea. Thus the CIE 1931 standard observer is also known as the CIE 1931 2° standard observer.
The color matching functions are the numerical description of the chromatic response of the observer. The CIE has defined a set of three color-matching functions, called ,x(λ), y(λ) and z(λ), which can be thought of as the spectral sensitivity curves of three linear light detectors that yield the CIEXYZ tri-stimulus values X, Y, and Z. The tabulated numerical values of these functions are known collectively as the CIE standard observer (Fig. 2-6) [73]. The tri-stimulus values for a color with a spectral power distribution are given in terms of the standard observer by:
3)
where λ is the wavelength of the equivalent monochromatic light (measured in nanometers).
Fig. 2-6The CIE standard observer color-matching functions
Since the human eye has three types of color sensors that respond to different ranges of wavelengths, a three-dimensional plot of all visible colors is shown in Fig.
2-7(a). However, the concept of color can be divided into two parts: brightness and chromaticity. For example, the color white is a bright color, while the color grey is considered to be a less bright version of that same white. In other words, the chromaticity of white and grey are the same while their brightness differs.
The CIE XYZ color space was deliberately designed so that the Y parameter was a measure of the brightness or luminance of a color. The chromaticity of a color was then specified by the two derived parameters x and y, two of the three normalized values which are functions of all three tri-stimulus values X, Y, and Z:
6)
The derived color space specified by x, y, and Y is known as the CIE xyY color space and is widely used to specify colors in practice. The diagram represents all of the chromaticities visible to the average person. These are shown in color and this region is called the gamut of human vision. The gamut of all visible chromaticities on the CIE plot is tongue-shaped or horseshoe-shaped as shown in Fig. 2-7(b). The curved edge of the gamut is called the spectral locus and corresponds to monochromatic light, with wavelengths listed in nanometers. The straight edge on the lower part of the gamut is called the line of purples. These colors, although they are on the border of the gamut, have no counterpart in monochromatic light. Less saturated colors appear in the interior of Fig. 2-7(b) with white at the center.
Fig. 2-7 (a) A 3D figure of visible colors and (b)the CIE xyz.
white
In colorimetry, the CIELUV color space (Fig. 2-8) is a color space adopted by CIE in 1976, as a simple-to-compute transformation of the 1931 CIE XYZ color space, which is an attempt at perceptual uniformity [74][75]. It is extensively used for applications such as computer graphics which deal with colored lights. The non-linear relations are given below: [76]
7)
where Yn is equal to 100 as the tri-stimulus of reference white [77].
Fig. 2-8 The CIE Lu’v’.
By utilizing CIE1976, a more uniform chromaticity diagram can be established.
Therefore, the color difference is defined by ΔEin the CIELUV chromaticity diagram.
Consequently, the CBU phenomenon will be evaluated by the ΔE index between the CBU image and the original image to verify the proposed method [78]-[80]. We sum up the color difference between modified and original image pixel by pixel as an index, ΔEsum, for the evaluation of color separation.
.
where Lu’v’CBU and Lu’v’0 are color values of CBU and original image in the Lu’v’
color space.
2.4 Hardware involved in sequential driving
The fast response LC and display electronic circuit are key components in FSC-LCDs. The LC response time is essential for one temporal field with the rate higher than 180 Hz [22]. Otherwise, the display electronics should be capable of synchronic driving with the LCD panel and LEDs. An FPGA as a timing controller is designed to process the incoming video content into each color field data, as show in Fig. 2-9. The features of the hardware are described in this section.
2.4.1 OCB mode
The optically compensated bend (OCB) mode LC, owning a feature of high speed response, is used for LCDs, especially highlighting motion images [81]. In OCB mode, the pre-tilt angles in the top and bottom substrates are in opposite directions. The opposed pre-tilt angles have two important properties. First, the optical self-phase compensation effect is induced on LC directors. As a result, the viewing angle is wide and symmetric. Second, the bend directors in an on-state voltage eliminate the back-flow effect, resulting in a faster response time. As the
applied voltage increases, the LC directors change from splay to bend deformation, as illustrated in Fig. 2-10. Below the critical voltage (Vc 1.8V), splay state is more stable than bend state. On the contrary, above Vc, bend state is more stable as shown in Fig. 2-11. The OCB mode LC will be maintained in bend state by applying a bias voltage [82][83].
Fig. 2-9The display electronic circuit
Fig. 2-10The transition between splay and bend states in an OCB mode LC
Fig. 2-11The transition curve from splay to bend state in OCB mode LC[82]
2.4.2 Source and gate drivers
LCD source driver ICs with RSDS (Reduced Swing Differential Signaling) interface and single 12-bit differential bus for 24-bit data are utilized in the driving system. The source driver can receive 12-bit differential data from timing controller and can output 480-channel LCD driving voltage. The specifications are shown in Tab.
2-1 (a). There are some advantages of the RSDS interface, as shown below.
z Reduced bus width – enables smaller and thinner column driver boards z Low power dissipation – extends system run time
z Low Electromagnetic Interference (EMI) generation – eliminates EMI suppression components and shielding
z High noise rejection – maintains signal image z High throughput – enables high resolution displays
The timing diagram of RSDS data transition is shown in Fig. 2-12. We can see the differential signals are received at the rising edge of clock-P and clock-N. The 24 bit data can be received by using a 12 bit differential bus during one clock-P/N cycle.
Then the source driver can transform the received data, and control the LC by applying voltage to 480 output channels.
After the source driver output an electric signal in one line of the frame, the gate driver applies voltage to turn the thin film transistor (TFT) on. Thus, the source driver’s signal can store into each pixel in one line of the frame. In this way, the LC of each pixel can be controlled individually. In order to turn TFTs on, a start pulse signal is necessary to drive the gate driver output a high voltage. After a start pulse is triggered, output pins of gate driver will sequentially produce high-driving voltage pulses for the LCD panel. This gate driver can produce voltage through 240 channels to the LCD panel, The max clock frequency is 200 KHz, as shown in Tab. 2-1 (b).
Fig. 2-12 The timing diagram of RSDS.
Tab. 2-1 The specifications of (a) the gate driver and (b) the source driver for the TFT LCD panel
(a) Source driver Output 480 output channels
Input RSDS input interface for low EMI Resolution 8-bit resolution / 256 gray scale
(b) Gate driver Output 240 output channels input clock < 200 KHz
2.4.3 LED driver
The LED backlight system consists of LED drivers and 3-in-1 LEDs (R, G, and B). Each LED driver is a 16-channel constant-current sink driver. Each channel has an individually adjustable 4096-step grayscale by Pulse-Width Modulation (PWM) operation and 64-step constant-current sink (dot correction). The dot correction adjusts the brightness variations between LED channels and other LED drivers.
This LED driver has the capability of adjusting the output current for each channel (OUT0 to OUT15) independently. The driven method, called Dot Correction mode (DC mode), is used to adjust LED brightness. Each of the 16 channels can be programmed with a 6-bit word. Thus the channel output can be adjusted in 64 steps from 0% to 100% of the maximum output current. Dot correction data are entered for all channels at the same time. The complete dot correction data format consists of 16 x 6-bit, which forms a 96-bit wide serial output data to control the current of 16
channels independently. This LED driver also can adjust the brightness of each channel by using a PWM control scheme. It uses the 12-bit width digital signal per channel to result in 4096 ( ) different brightness steps, which is called grayscale mode (GS mode). The total grayscale data of 16 channels is 16 x 12 bit width, which forms a 192-bit wide data packet. Consequently, each LED current is controlled by the LED driver with DC mode operation. Each LED lighting period is controlled by a LED driver with GS mode operation. The serial data input timing diagram with DC and GS mode is shown in Fig. 2-13 and Tab. 2-2.
Fig. 2-13 Serial data input timing diagram of the LED controller.
Tab. 2-2 Tthe specifications of LED driver Input frequency 30 MHz (max) Output channels 16 output channels Dot correction
(DC mode)
6-bit (64 steps) for LED current
Grayscale PWM mode (GS mode)
12-bit (4096 steps) for LED grayscale
2.4.4 FPGA
A field-programmable gate array is a semiconductor device containing programmable logic components called "logic blocks". Logic blocks can be programmed to perform the function of basic logic gates such as AND, and XOR, or more complex combinational functions such as decoders or mathematical functions.
In most FPGAs, the logic blocks also include memory elements, which may be simple flip-flops or Static Random Access Memory (SRAM). In this thesis, the memory elements used in FPGA are Static Random Access Memory (SRAM) and Synchronous Dynamic Random Access Memory (SDRAM), which were used as data buffers and the frame buffers, respectively.
In order to define the behavior of the FPGA, the user provides a hardware description language (HDL), the Common HDLs are VHDL and Verilog. Then, the user uses an electronic design automation tool; a technology-mapped net-list is generated. This net-list can be fitted to the actual FPGA architecture by using a process called place-and-route. The user will validate the map, place and route results via timing analysis, simulation, and other verification methodologies. Once the design and validation process is complete, the binary file is generated to configure the FPGA.
2.5 Summary
The physiology of eye movement and CBU phenomenon have briefly discussed in this chapter. In order to recognize the stationary image, the observer’s eye moves around the object to collect information during eye saccadic motion. For a moving object, the human eye will follow at the same velocity of the object to catch clear images, which is called the pursuit. The relative movement between human eyes and
The physiology of eye movement and CBU phenomenon have briefly discussed in this chapter. In order to recognize the stationary image, the observer’s eye moves around the object to collect information during eye saccadic motion. For a moving object, the human eye will follow at the same velocity of the object to catch clear images, which is called the pursuit. The relative movement between human eyes and