Chapter 1 Introduction
1.4 Organization
This thesis is organized as follows. In Chapter 2, multi-field driving scheme, some auxiliary solutions provided longer time for LC response, and a prior art, two-field method with color filter, will be discussed. In Chapter 3, proposed method, two-color field sequential method without color filter, will be detailed. The concept and the algorithm of proposed method are described. Also, the evaluation index for color difference, and CBU suppression method will be introduced. In Chapter 4, the experimental procedure for establishing color model to predict correct driving signals, and some results of colorimetric accuracy are shown. According to experimental results, some discussions will be presented. In Chapter 5, some optimization results will be shown. Finally, the conclusions and future works will be given in Chapter 6.
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Chapter 2
Prior Methods
The FSC-LCD was proposed to increase optical throughput of color LCDs.
However, a serious issue, CBU, cause poor viewing quality. Many field sequential methods for suppressing CBU were proposed during recent years [8] [9]. In order to implement in hardware, some auxiliary solutions to extend time for LC response are introduced. Finally, the two-field method with color filters will be given.
2.1 Multi-field Driving Scheme
A multi-field driving scheme was proposed for suppressing the CBU phenomenon, such as increasing field rate method and insertion multi-primary color field method. Take double frame rate, RGBRGB sequence for example. When the field rate increased, the CBU width becomes thinner than RGB method, as shown in Fig. 2-1(b), and human eye can get less sensitive to the CBU phenomenon. The second method, insertion multi-primary color filed method, was proposed by the Tatsuo Uchida group. As the simulation results shown in Figs. 2-1 (c) and (d), the RGBCY, and RGBW methods reduce the chrominance difference between each field, so these methods suppress CBU efficiently. However, the mentioned methods all face a huge challenge, the limitation of LC response time. The RGBW method timing chart is illustrated in Fig. 2-2. Comparison between RGB method and RGBW method, the LC response time of RGBW method (tLC’) is the one-quarters shorter than the RGB method (tLC). The time for LC response becomes shorter when the field number increases and the time are too short to be practically implemented in hardware.
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Therefore, some auxiliary solutions for providing longer time to LC response will be given in the next section.
RGB
Fig. 2-1 (a) The conventional RGB method and (b) the double frame rate method. (c) The RGBCY sequence and (d) the RGBWmax sequence of insertion multi-primary color field method.
≈ TFT
Scanning
B/L
Flashing tBL’ t
B
Fig. 2-2 The timing charts of (a) the RGB method and (b) the RGBW method.
2.1.1 Multi-division Backlight
The multi-division backlight technique is used to provide more time for LC response. This method is divided display into many areas, and the multi-division
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backlight technique timing chart is illustrated in Fig. 2-3. Eq.2-1 describes the time distribution for scanning time, LC response time, and backlight flashing time. Where, N is number of the divided area, tTFT is the scanning time for the whole panel, and tBL
is the back-light flash time. Comparing Eq.2-1 with Eq.1-1, the TFT scanning time becomes shorter with increase N. Thus, the more divided areas can save longer time for LC response. Taking a 60Hz frame rate conventional RGB sequence, XGA panel as an example. Each field rate of RGB method is 5.67ms, if the divided area is 10 (N=10), the LC response time is computed to be 2.57ms which is longer than the conventional FSC method (1ms) without using the divided area technique. Therefore, the multi-division backlight technique enhanced LC response time efficiently.
frame time
t
Fig. 2-3 The timing chart of the divided display area method.
tBL tLC N
tTFT f
3
1 = + + (Eq.2-1)
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2.1.2 Overdrive Scheme
The overdrive technique artificially boosts LC response time by increasing the voltage used to make liquid crystals change state [10]. Currently LCDs have a drawback, blurred phenomenon, which happens during rapid movement due to low response LC modes. Thus, the fast response LC mode, optically compensated bend (OCB) mode was proposed for TV applications [11]. The OCB configuration is shown in Fig. 2-4, and it has advantages such as fast response and a wide viewing angle.
However, the LC response time in the conventional TFT drive is longer than that in a static drive, due to the dielectric anisotropy of liquid crystal material. Therefore, the capacitively coupled driving method (CC driving method) was proposed to improve LC response time [12]. The CC driving method utilizes the capacitively coupled voltage which is applied to the pixel electrode through a storage capacitor formed between the preceding scanning line and the pixel electrode. The response times with CC driving method are enhanced twice or more than conventional TFT-LCD driving method. Unfortunately, the OCB mode has not yet been commercialized for large size LCDs. The further way to increase possibility of FSC-LCD implementation is to reduce the field number. The two-filed method will be detailed in the next section.
Fig. 2-4 The OCB-LCD configuration [11]
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2.2 Two-field Method with Color Filters
A spatial-temporal display method was proposed by Louis D. Silverstein for enhancing display image quality [13]. This spectrum sequential method combined spatial color mixing with color filters and temporal color mixing by using sequential backlight. The number of primary color sub-pixel elements was reduced from three to two. Two primary sub-pixel elements arranged in a two dimensional checkerboard mosaic and the third primary color was presented through temporal synthesis. A type of spatial-temporal display configuration is described in Fig. 2-5 . This configuration combines a switchable illumination source, an LCD panel, and pixel mosaic color filters which consist of a checkerboard pattern of magenta (M) and cyan (C). Where, the light source switches between yellow (Y) and blue (B) spectrum power distribution, and illumination sources can be provided by CCF fluorescent or LEDs.
When the Y illuminant is activated during the first temporal field, the output image consist a checkerboard pattern of red and green sub-pixels. The B illuminant active in the second temporal field yields the output display in homogenous B sub-pixels. The two filed images display sequentially to generate a full-color image. Moreover, the spatial-temporal display method provides many degrees of freedom for choosing color combinations of temporally switched illuminants and color filter mosaics to achieve different display performance objectives.
In Philips research [14], they compared the three types display methods. One is the display without color filter with R, G, B primary sequential fields; the second type display is the display with three color filters, and using two color fields display sequentially to yield full-color image; the third type display with two color filters, and using two color fields display sequentially for displaying colorful target image.
Table. 2 lists the comparisons of four kinds display methods. The display without
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color filters can gain up to three times luminance and resolution than conventional LCD. The two field method with three color filters has highest color gamut (127%) compared with other three display methods and slighter CBU visibility than the two field method with two color filters. Two kinds of two field methods reduced CBU visibility compared to the three field method due to reduce the contrast of color bar.
The two field method with 120 Hz field rate allows longer time for LC response and promotes the light efficiency from 33% to 50%. Additionally, two field methods can reduce CBU visibility effectively.
LCD Panel Pixel Mosaic (M/ C) Illumination Source
(Y/B) CCFL
LED Or
LCD Panel Pixel Mosaic (R/ G) Illumination Source
(Y on /B off)
LCD Panel Pixel Mosaic (B) Illumination Source
(Y off /B on)
Fig. 2-5 The spatial-temporal display con configuration [13].
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Table. 2 The comparisons of four types display [19].
2.3 Summary
The FSC-LCDs have three times optical throughput than conventional LCDs.
However, the serious issue for the operations is limitation of LC response time. Some auxiliary solutions were proposed for increasing LC response time, such as multi-division backlight technique and overdrive technique. However, these methods cannot provide enough LC response time for large size display. Louis D. Silverstein proposed a spatial-temporal two field method to reduce field number and save longer time for LC response. This two field method still needs color filters which will sacrifice the light throughput. Therefore, we proposed the two-color-field sequential method without color filters which can not only reduce the LC response time but also can promote the optical throughput of FSC-LCDs to 100%.
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Chapter 3
Two-color-field Sequential Method
Two-color-field sequential method for color filterless LCD was proposed to further reduce the field rate, and provide the longest time for LC response. Thus, many commercial LC modes such as TN, IPS, and MVA modes can achieve the response time. The concept and algorithm of proposed method will present in section 3.1. The following section described the two indexes for evaluating the accuracy of colorimetric reproduction. Finally, the CBU examination will be given.
3.1 Two-color-field Sequential Method
3.1.1 Concept
The concept of proposed two-color-field sequential method is displaying two color-mixing fields with double frame rate to generate a full-color image. The proposed two-color-field sequential method is different from other two-field methods in regard to color filters. The comparison of driving scheme between conventional three field method and proposed two-color-field sequential method is illustrated in Fig.
3-1. The conventional FSC method displays red, green, and blue field images time sequentially with triple frame rate to yield a full-color image. The proposed two-color-filed sequential method flashes two color-mixing fields with two-thirds filed rate of conventional method to generate a full-color image. Two-color-field sequential method uses the least field number, and provides longest time for LC response. Thus, the LC modes can be utilized in commercial LC modes, such as TN,
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MVA, or IPS. Moreover, the proposed two-color-field sequential method is a type display method for color filterless LCD, so it can take full advantages of optical efficiency enhancement of the temporal color-mixing methods.
B-field G-field
R-field
RB1-field GB2-field Target image
t Field 2
Field 1 Field 3
Field 1 Field 2
Three-field method
Two-color-field method
Fig. 3-1 Two driving-scheme types, a typical three-field method and the proposed two-color-field sequential method, are illustrated, by field decomposition of color fields to generate a full-color image.
3.1.2 LCD Structure
In order to display a color-mixing field in proposed method for LCD without color filters. We propose to incorporate the local color dimming backlight technique [15] to substitute for the equivalent function of the special color filters. The local color dimming backlight technique is a kind of backlight controlling technique, which can locally control LED signals per color per division. This technique was also called high dynamic range (HDR) technique, and the flowchart is illustrated in Fig. 3-2. First of all, normalized target intensity of each pixel was got by input digital LC signals in full on white backlight. Then, LED signals were computed based on target image content by local controlled backlight algorithm, such as maximum, root, or average of signals in each segment [16]. Following, the point spread function was convoluted the LED
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signals in each segment to get backlight distribution. In step 5, the LC intensity in each pixel was calculated by target intensity divided backlight distribution. Finally, the LED and LC driving signals were got by inversing transfer functions of LED and LCD individually. This algorithm can be used into two different processes, one is the forward process and the other is backward process. The forward process follows at step 3 compute the backlight distribution firstly, and then in step 6 get compensated LC signals. However, sometimes the compensated LC signals will be got firstly, thus the process will be inversed to calculate LED driving signals. Using the local dimming backlight system can dim or boost backlight intensity in each segment depend on image content. Thus, the HDR technique has low power consumption, high contrast ratio, and high color saturation advantages. Moreover, the local controlled backlight technique can be utilized to achieve color-mixing fields of the two-color-field sequential method.
I 1
I : target intensity of single pixel (normalized) r1: transfer function of LED
p1: point spread function r2: transfer function of LCD
Forward: Step 1⇒ Step 3⇒ Step 6 Backward : Step 1⇒ Step 6⇒ Step 3
Fig. 3-2 The local dimming system flowchart.
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3.1.3 Algorithm
The procedure starts at transforming the digits of an input image into the corresponding tri-stimulus values of each primary channel as the target information.
The transformation is accomplished through a suitable color model [17], as detailed in the next section. Then, the red and the green, for example, are chosen as the first and the second primaries (Fig. 3-3). After being dealt with by the forward process of local color dimming backlight technique, the red information in the first field is achieved by the sets of LED signals (dBL-R) and LC signals (dLC1), while the green one in the second field by the sets of dBL-G and dLC2. The forward process features the sequence of the first deriving the light-emitting diode (LED) signals and then computing the LC compensation signals to achieve the target information; the backward process reverses the forward one [18].
As for the blue one in the first field, the blue target information is dealt with via the backward process, based on dLC1, to deduce the blue LED signals (dBL-B1) for the first field. In general, the blue information, resulted from the combination of dLC1 and dBL-B1, may be different from the blue target information. Therefore, the difference between the original and the resulted blue information in the first field is set as the new blue target information for the second field. Following the same procedure, the set of blue LED signals (dBL-B2) for the second field is obtained.
In practice, the accuracy of the third color reproduction depends on backlight layout, light spread function (LSF), and image content. The algorithm should be iterated with additional optimization process to improve color presentation accuracy. In addition, the decision of the third primary is not unique. The blue information, in the aforementioned example, is selected since the human vision system is less sensitive to blue information. Besides, a primary with least significant content is a useful option to
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increase reproduction accuracy.
1stfield image 2ndfield image
Target image Rdimming B/L
(dBL-R) Gdimming B/L
(dBL-G)
+ +
Local dimming algorithm Local dimming algorithm
(dBL-B2 ) (dBL-B1 )
B1dimming B/L B2dimming B/L
1stfield LC(dLC1 ) 2ndfield LC(dLC2 )
2ndfield B/L 1stfield B/L
forward process reverse process
Fig. 3-3 The algorithm flowchart of the two-color-field sequential method.
3.2 Colorimetric Reproduction
In order to predict the accurate color information in display system, the colorimetric characterizations of displays were determined. In 1998, Fairchild and Wyble, recognizing the fundamental differences between liquid-crystal and cathode-ray-tube technologies, develop a successfully LCD color model [19]. The color model configuration is described in Eq.3-1. The process is divided into two main stages. The first non-linear stage of the model is built up three one-dimensional look-up tables (LUTs) of the radio-scales in each channel, as shown in Eq.3-1-1. The LUTs described the relationship between the digital input signals used to drive a display and the radiant output produced through LC cells by accounting for non-linear optoelectronic transfer function, OETF. Where, d defines digital counts and R, G, and B are radio-scales for red, green, and blue channels, respectively. The radio-scales’
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ranges were conformed from 0 to 1. The second linear stage, as shown in Eq.3-1-2, represented the correlations between radio-metric scalars and the resultant tri-stimulus values. The flare term, [Xk, Yk, Zk]T accounts for the radiant output at black level in LCDs, since the liquid crystal having a minimum transmittance factor above zero.
Moreover, the “max” subscript defines each channel’s maximum output and subscript
“kmin” defines the black-level radiant output. Furthermore, this color model performs non-linear optimization to minimize the mean CIEDE2000 color difference of test colors sampling the display’s gamut in complex models. However, this color model is not suitable used in color dimming backlight system.
0 ≤ R,G,B ≤ 1
Therefore, the color model should be modified to incorporate the color backlight intensity. Then, the equation was transformed, as shown in Eq.3-2 [17], where, L is the normalized backlight intensity in each color which can be obtained by the convolution computation with LED signals and light spread function of each LED.
The subscript “r, g, and b” defines red, green, and blue colors respectively. Summarily, this color model was suitable to be used for predicting color information in our proposed system.
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3.2.1 Color Difference Formula
Color difference formula is used to evaluate the color difference between two static images. In our research, the color difference index would be used to verify the colorimetric reproduction accuracy. Since 1976, the International Commission on Illumination (CIE) recommended two color-difference formula for industrial applications, the CIELAB and CIELUV formula [20]. The modification formula, CIEDE2000 [20], includes not only lightness, chroma, and hue weighting functions, but also an interactive term between chroma and hue differences for improving the performance for CIELAB color difference indexes. There are four steps include in CIEDE2000 calculation. In the first step, calculate the CIELAB as shown in Eq.3-3-1, the parameters L＊ represents lightness, a＊ approximate redness-greenness, b＊ approximate yellowness-blueness, and C＊ab chroma. Then, compute a’, C’, and h’, follow Eq.3-3-2, in this step, bar C＊ab is the arithmetic mean of the C＊ab values for a pair of samples. The third step, obtain ΔL’, ΔC’ and ΔH’ values between standard and sample in a pair as shown in Eq.3-3-3. At last step, calculate the color difference values using CIEDE2000 formula in Eq.3-3-4. The parameters SL, SC, and SH are the
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weighting functions for lightness, chroma, and hue differences, respectively. kL, kC, and kH values are the parametric factors to be adjusted according to different viewing parameters, for the lightness, chroma, and hue components, individually. RT function is intended to improve the performance of color-difference equation for fitting chromatic differences in the blue region. The color difference formula CIEDE2000 considers more color conditions, and it will be more suitable for evaluating the color differences.
(Eq.3-3-1)
(Eq.3-3-2)
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(Eq.3-3-3)
(Eq.3-3-4)
3.2.2 Spatial-CIELAB (S-CIELAB)
The CIE color difference formulae are developed to measure the color difference between color patches with small color difference after moderate chromatic adaptation.
However, this value does not give satisfactory results in human visual system;
because of the point-by-point computation result in complex image is always larger than observer’s visibility. Therefore, X.Zhang proposed extension of the CIELAB color metric, Spatial-CIELAB(S-CIELAB) to measure color reproduction errors in images [21]. The Spatial-CIELAB flowchart is illustrated in Fig. 3-4. In the first step, transform the input images into a device independent space, CIE 1931 XYZ tri-stimulus values. The second step, put tri-stimulus values into opponent-color space, AC1C2.These channels were determined through series psychophysical experiments testing for pattern-color separability [22], where A denotes the luminance channel and C1, C2 are chrominance channels. The opponent channels are a linear transform CIE
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1931 XYZ as shown in Eq.3-4. The third step, operate the opponent sensitivity signals to frequency domain by Fourier transformation. These three independent channels can be spatially filtered, using filters that approximate the contrast sensitivity function (CSF) of the human visual system. Each channel can accomplish using multiplications in the frequency domain. Moreover, a three parameter exponential model, described
1931 XYZ as shown in Eq.3-4. The third step, operate the opponent sensitivity signals to frequency domain by Fourier transformation. These three independent channels can be spatially filtered, using filters that approximate the contrast sensitivity function (CSF) of the human visual system. Each channel can accomplish using multiplications in the frequency domain. Moreover, a three parameter exponential model, described