Chapter 4 Experimental Demonstration
4.3 Experimental Demonstration II-60Hz/frame
The second experimental demonstration verified the Stencil-FSC method on a 32” FSC-LCD supported by C-company with local dimming backlight of 12*16 sub-regions and 180Hz field rate. The schemes of the FSC-LCD are presented in Table. 4-1.Because the field rate of the FSC-LCD was only 180Hz, and the frame rate must be set higher than 60Hz to prevent flickering, the panel could only display three fields sequentially in a frame. However, the Stencil-FSC method is a color sequence with four fields, so one of the fields must be rejected. Because human eye is less sensitive to blue compared to red or green, the blue field image can be rejected with less sensitivity to color distortion. Moreover, white and yellowish images were chosen to be the test images since the color contribution from blue field images were not important when utilizing the Stencil-FSC method. Therefore, two test images, Lily and Sunflower, were used in the experimental demonstration, and the field images are shown in Fig. 4-9.
Table. 4-1 Schemes of the 32” FSC-LCD 32-inch FSC-LCD
OCB-mode LC
1366 × 768 16 × 12 Divisions BL
48 × 24 (1152) LEDs Field rate 180 Hz (3-field)
(a)
(b)
Fig. 4-9 Test images of (a) Lily and (b) Sunflower and their field images
In the experimental demonstration, the backlight signal for 12*16 sub-regions was sent to the backlight system to get locally controlled color backlight, as in Fig.
4-10. The LC signal for the multi-color field, red-field, and green-field were implemented into the LC layers. Then, the multi-color field, red-field, and green field would light in sequence at 180Hz field rate, and a colorful image could be perceived as shown in Fig. 4-11. Finally, a moving camera was utilized to capture the CBU image and verify the Stencil-FSC method. The results are presented in Fig. 4-12 and Fig. 4-13, and verified that the Stencil-FSC method can suppress CBU effectively.
Moreover, as mentioned previously, the Stencil-FSC method not only suppressed CBU, but it also performed better compared to a conventional LCD and an LED based LCD, such as ultra-low power, high contrast ratio, and high NTSC. Therefore, the display performances were measured and summarized in the Table. 4-2. By utilizing the Stencil-FSC method on the FSC-LCD, The average power consumption was reduced to 44W which is only about 24% of a conventional LED based LCD. The result is better than the objective, 33%, because the locally controlled backlight was
applied with Stencil-FSC method. Moreover, the contrast ratio was enhanced to 5900:1 in the normal image, Sunflower, which was ten times larger than conventional CCFL-LCDs. The contrast ratio can even be increased to 27000:1 in high contrast image, Lily. The NTSC can also be increased to 114% compared to 72% in conventional CCFL-LCDs. Therefore, the Stencil-FSC can suppress CBU effectively and get the attractive performances, and they have been verified by the experimental demonstration on the 32” FSC-LCD.
(a) (b)
Fig. 4-10 The color locally controlled backlights of (a) Lily and (b) Sunflower
(a) (b)
Fig. 4-11 The Stencil-FSC images of (a) Lily and (b) Sunflower
(a) (b)
Fig. 4-12 The CBU image of Lily using (a) the Stencil-FSC method and (b) the conventional RGB color sequence
(a) (b)
Fig. 4-13 The CBU image of Sunflower using (a) the Stencil-FSC method and (b) the conventional RGB color sequence
Table. 4-2 The measured performance of CCFL-LCD, LED-LCD, and FSC-LCD with the Stencil-FSC method
114 % 114 %
72 % NTSC
Power consumption of a 32” conventional LED-based TV: ~180W
CCFL-LCD FSC-LCD
(RGB_ Driving ) Stencil-FSCLCD
P (W) 105 67 44
CR 579 : 1 442:1 5,973:1
114 % 114 %
72 % NTSC
Power consumption of a 32” conventional LED-based TV: ~180W
CCFL-LCD FSC-LCD
(RGB_ Driving ) Stencil-FSCLCD
P (W) 105 67 44
CR 579 : 1 442:1 5,973:1
4.4 Summary
The approximation for the real backlight intensity distribution has been completed by Fourier Transformation process, and when the D0 parameter in Gaussian low-pass filter equals 0.0023, the backlight intensity distribution is most similar to the real one on the 180Hz field rate, locally controlled backlight, 32” FSC-LCD supported by C-company. By using the hardware approximation, two experimental demonstrations of Stencil-FSC method have been completed on the conventional LCD and the 180Hz field rate, 32” FSC-LCD. By the results, the CBU suppression of the Stencil-FSC method has been verified. Moreover, the FSC-LCD with the
Stencil-FSC method could achieve attractive performances, such as average power consumption of 44W, which is only 24% of conventional LED-LCD, 114% NTSC, a Contrast ratio of 27000:1 in high contrast image.
Chapter 5
Optimization of Color Breakup Suppression
In previous chapter, a demonstration of 32” FSC-LCD with Stencil-FSC method was presented. However, the hardware parameters used in the demonstration were limited, thus the CBU suppression was not the best performance. In order to suppress CBU more effectively, the optimization of the hardware parameters were calculated, and these optimized results can be recommended to the FSC-LCD producers in the future.
The optimization works were done by nine test images with different detail content and color complexity. Three major parameters in Stencil-FSC method:
Number of backlight divisions, light spread function of backlight, and the dimming ratio in backlight colorful process, was analyzed to achieve best performance.
5.1 Classification of Test Images
In order to make the optimizations to be reliable and general results, the test images must be chosen carefully so that all kinds of images can be included in the test images. Therefore, images are chosen and classified by two defined indexes, detail complexity and color complexity.
5.1.1 Detail complexity
Detail complexity is used to evaluate how complex the detail is, and it can be defined easily by finding the edge component of the image. How to find the edge component in the image is a popular and well developed work in digital image
process[30] and utilizing image gradient is a common and effective method. Image gradient is defined by Eq. 5-1, and it is defined in the direction achieving maximum gradient. R, G, and B mean the gray level signal of red, green, and blue in an image; x and y indicate the horizontal and vertical direction, respectively; θstands for the direction causing the maximum gradient.
(5-1)
Generally speaking, because the edge component occur at the position causing a larger gradient, the edge can be found easily by set a threshold of the gradient as shown in Fig. 5-1. Finally, a summation of the edge parts is made, and the value is used to define the detail complexity. Larger value means the image has more detail complexity; contrarily, the smaller value indicates the image has less detail complexity. Examples of two images, lily and color balls, are presented in the Fig. 5-2.
The summation value of the simpler image, Lily, is 9437, and the value of the more complex image, Color balls, is 50648. Therefore, the detail complexity can be evaluated effectively by the summation value of edge component.
(a) (b)
Fig. 5-1 (a) Target image, and (b) Edge image gotten by calculating image gradient
2
ΣEdge=9437 ΣEdge= 50648 (a) (b)
Fig. 5-2 (a) Image, Lily, with less detail complexity and the edge summation=9437 (b) Image, Color balls, with more detail complexity and the edge
summation=50648
5.1.2 Color complexity
The other index, color complexity, is utilized to analyze the color abundance, and the concept of entropy is used to evaluate it. The index, entropy, is usually used in image compression to define the uncertainty of source[24], and the equation is Eq. 5-2.
P (i) means the probability of appearance of the i component, and entropy can stand for the dispersive level of each component.
=
∑
i
i P i
P
Entropy ()*log( ( )) (5-2) The concept is applied to determine the color complexity. At first, a color space, CIELAB, is divided into 20*20 regions such as Fig. 5-3(a). Then, the probability of color appearance in each color region is calculated as shown in Fig. 5-3(b). Finally, the entropy can be calculated by Eq. 5-2. The lager entropy means the image has more color complexity, and smaller entropy means the image has less color complexity contrarily. The example images, Lily and Color balls, are used again to verify the index as shown in Fig. 5-4. The image, Lily, is a single color image so the entropy is only 0.88. On the other hand, the entropy of the multi-color image, Color ball, is 3.41.
Thus, the entropy index can evaluate the color complexity correctly and will be applied in the following optimization.
(a)
(b) (c)
Fig. 5-3 (a) Target image, (b) CIELAB color space divided into 20*20 regions, and (c) probability distribution of color appearance
Entropy=0.88 Entropy=3.41 (a) (b)
Fig. 5-4 (a) Image, Lily, with single color and the color entropy=0.88 (b) Image, Color balls, with multi-color and the color entropy=3.41
Consequently, two index, detail complexity and color complexity, were utilized to classify images, and nine test images with different level of detail and color complexity were used in the following optimization as shown in Table. 5-1. In the table, images are more color complexity from right to left; images are more detail complexity form bottom to top. By these test images with different content, reliable and general optimization results can be gathered completely.
Table. 5-1 Nine test images with different detail and color complexity Color
Detail
Detail
ΣEdge High Detail
ΣEdge Mid Detail
ΣEdge Low
High
96863 90040 118142
Color
Entropy 3.40 2.71 1.91
Mid 34446 35330 35324
Color
Entropy 3.14 2.57 1.89
Low 18451 15868 15575
Color
Entropy 3.34 2.63 1.63
5.2 Backlight Division and Light Spread Function
5.2.1 Light spread function vs. Backlight division
Stencil-FSC method was accomplished on an FSC-LCD with locally controlled backlight, and the first multi-color field is composed by the minimum LC signal of R, G, B and the color backlight. Therefore, the number of backlight divisions and the light spread function of each backlight sub-region are two important parameters about CBU suppression intuitively. In previous hardware demonstration, the number of backlight divisions was 12*16, and the light spread function was the Fourier transformation of the Gaussian low-pass filter with D0 equal to 0.0023 as presented in
suppression, these two parameters have to be further optimized by simulation.
The relation between number of backlight divisions and light spread function were discussed at first. The color difference, ΔE, between target image and CBU image was used to evaluate CBU as mentioned in 2.2, and take the test image, aborigine, for example. The ΔE with different divisions number and D0 parameter are presented in Table.5.2 , and the D0 parameter was recorded when it achieves the minimum ΔE in each backlight divisions number. For example, when backlight division number is 48*48, D0 equal to 0.01 has minimum ΔE, thus, it means the D0=0.01 will suppress CBU best when using 48*48 backlight division. By gathering the statistics of nine test images, the relations between the number of backlight divisions and the D0 parameter causing minimum CBU are presented in Fig. 5-5.
According to the results, we can conclude that: more local light spread function (D0~0.01) is suitable for lager division number; more dispersive light spread (D0~0.001) is suitable for less division number. The conclusion makes sense, and it can be understood easily by referring to Fig. 5-6 and Fig. 5-7. In the two figures, the backlight signals with different numbers of backlight divisions, ex: 2*4 and 48*48, and their backlight distributions of different light spread functions, D0=0.001 and D0=0.01, are presented. When numbers of backlight divisions is small, ex. 2*4, local backlight distribution will cause boundary issue so the dispersive backlight distribution is more suitable for the backlight division as shown in Fig. 5-6. On the contrary, when numbers of backlight divisions is large, ex. 48*48, local backlight distribution can display an backlight image similar to the target image. Therefore, more color detail can be perform in the first multi-color field, and CBU suppression is better compared to the dispersive one as shown in Fig. 5-7. Therefore, more local backlight distribution is suitable for larger backlight division number.
Table. 5-2 ΔE caused by CBU with different division numbers and D0 parameters of the test image-Aborigine
Aborigine
Division No. D0
a
(Horizontal)
b
(Vertical)
B/L No. 0.001 0.003 0.005 0.007 0.01 min
48 48 2304 5.44*106 5.43*106 5.36*106 5.26*106 5.12*106 5.12*106 24 48 1152 5.42*106 5.42*106 5.35*106 5.27*106 5.14*106 5.14*106
… … … … … … … …
4 4 16 5.26*106 5.32*106 5.33*106 5.33*106 5.33*106 5.26*106 2 4 8 5.23106 5.26106 5.26106 5.26106 5.26106 5.23
Local B/L
Dispersive B/L
Fig. 5-5 Relation between the number of backlight divisions and the D0 parameter causing minimum CBU
(a)
2*4 D0=0.001 D0=0.01
(b) (c) (d)
Fig. 5-6 (a) Target image-Aborigine, (b) backlight signal with 2*4 division, (c) backlight distribution with D0=0.001, and (d) backlight distribution with D0=0.01
48*48 D0=0.001 D0=0.01
(a) (b) (c) Fig. 5-7 (a) Backlight signal with 48*48 division, (c) backlight distribution with D0=0.001, and (d) backlight distribution with D0=0.01
5.2.2 Optimization of backlight divisions
In previous section, we found the most suitable light spread functions (D0) for different backlight divisions (Fig. 5-5). In the following, the optimized backlight divisions will be discussed. The number of backlight division was analyzed according to the LED array in the backlight module of the FSC-LCD provided by C-company.
The total LED number is 1152, and they are arrayed in 24*48. The backlight divisions are limited, and it must be the common divisor of 24 and 48. Therefore, from 2*2 to 24*48 backlight divisions were utilized to the optimization work. Then, the relation between the number of backlight divisions and ΔE caused by CBU were analyzed, and the results are presented in Fig. 5-8. The results indicate larger number of backlight divisions will result in less CBU, and the CBU phenomenon is saturated when the division number is larger than 24*24. This tendency is more obvious when the extra data of division of 48*48 is put in the diagram. The results mean if backlight division is larger than 24*24, it cannot perform effective CBU suppression but will increase the complexity and cost of hardware. Consequently, the most efficient and optimum backlight division is 24*24.
75.0 80.0 85.0 90.0 95.0 100.0 105.0
0 500 1000 1500 2000 2500
Re la tive CBU (% )
BL division NO.
aborigine basketball mountain
bird butterfly church
lotus Azalea coast
24*24
Fig. 5-8 Relation between the number of backlight divisions and ΔE caused by CBU
5.3 Optimization of Dimming Ratio
In previous optimization, the 24*24 backlight division and light spread function with D0 equal to 0.01 can get best CBU suppression. Next, an optimization about dimming ratio in backlight colorful method will be described. As mentioned in 3.2.1, more colorful multi-color field can be gotten by backlight colorful method with enhancing the minimum LC signal, and the dimming ratio is the key parameter determining how colorful the multi-color field is. Lower dimming ratio will cause more colorful backlight and multi-color field, and higher dimming ratio will cause less colorful backlight and multi-color field as Fig. 5-9. Therefore, the dimming ratio will be an important parameter for CBU suppression. The relation between the ΔE caused by CBU and dimming ratio is presented in Fig. 5-10. The results indicate the lower dimming ratio will cause less CBU.
(a)
(b)
(c)
(d)
(e)
Fig. 5-9 (a) Stencil-FSC field images without utilizing backlight colorful method and (b) Stencil-FSC field images with dimming ratio=10%
0.9 0.92 0.94 0.96 0.98 1
0 20 40 60 80 100 120
Re la ti ve CB U
Dimming ratio(%)
aborigine basketball mountain
butterfly bird church
lotus Azala coast
Fig. 5-10 Relation between the ΔE caused by CBU and dimming ratio
However, the dimming ratio cannot be decreased infinitely to suppress CBU because it will cause the other issue, image distortion, as mentioned in 3.2.1. By utilizing the backlight colorful method, minimum backlight signal is dimmed to enhance the minimum LC signal. However, if the dimming ratio is too small, the minimum LC signal will be enhanced too much that the signal cannot display completely by four fields of Stencil-FSC method. Some pixel images will be distorted, and the image quality will be degraded. Therefore, the CBU suppression and the image distortion will be a trade-off, and the optimization between the two performances will be made in the following.
At first, distortion ratio is defined to evaluate the distortion phenomenon, and it equation is presented in Eq. 5-3[31]. Distortion ratio stands for the probability of distortion pixel in a test image.
100%
# # ×
= Total pixel pixel Distorted ratio
Distortion (5-3) As discussed before, the lower dimming ratio can suppress CBU more, so an analysis was made to discuss about the critical dimming ratio can suppress CBU and not cause unacceptable image. The relations between distortion ratio and dimming ratio of nine test images are presented in Fig. 5-11. The results show that when the dimming ratio becomes larger than 10%, the distortion ratio will be less than 0.5%. However, when the dimming ratio become smaller than 10%, the distortion ratio will increase rapidly, and image quality will be degraded seriously. Consequently, the lower dimming ratio can suppress CBU more, but the dimming ratio lower than 10% will cause serious image distortion. Consequently, the optimized dimming ratio could be set as 10%.
0 0.5 1 1.5 2 2.5 3
0 10 20 30 40 50 60 70 80 90 100
D is tor ti on ra ti o (% )
Dimming ratio (%)
aborigine basketball mountain
bird buterfly church
lotus Azala coast
Dimming ratio=10%
Fig. 5-11 Relations between distortion ratio and dimming ratio of nine test images
5.4 Discussions
By optimization works presented in the last sections, optimized hardware parameters in Stencil-FSC method were gathered. Number of backlight divisions is 24*24, light spread function is related to Gaussian low-pass filter with D0 equal to 0.01, and dimming ratio in backlight colorful method is 10%. These optimized parameters will achieve better CBU suppression compared to the demonstration mentioned in chapter 4. Now, a quantification discussion will be made about CBU caused by conventional RGB color sequence and Stencil-FSC method with optimized parameters, and the results are shown in Fig. 5-12 and Fig. 5-13. In Fig. 5-12, theΔE values of CBU caused by RGB color sequence and Stencil-FSC method are presented.
In the following, an index, CBU suppression, is defined to evaluate how many percentage of CBU caused by RGB color sequence is suppressed. The equation is presented in Eq. 5-4, and the results are shown in Fig. 5-13
%
×100
⎟⎟⎠
⎜⎜ ⎞
⎝
⎛ −
= −
RGB
FSC Stencil RGB
CBU CBU n CBU
Suppressio
CBU (5-4)
According to the results, Stencil-FSC method can suppress 41%~98% CBU compared to that of RGB color sequence in the nine test images, and the average CBU suppression is equal to 62%. Therefore, Stencil-FSC method with optimized parameters can solve the CBU issue successfully on an FSC-LCD.
0
Aborigine Bird Lotus Basketball Butterfly Azalea Mountain Church Coast
CB U_ Δ E (10^ 6)
RGB Stencil‐FSC
Test images
Fig. 5-12 ΔE of CBU caused by RGB color sequence (blue bars) and ΔE of CBU caused by Stencil-FSC method with optimized parameters (red bars).
41
Aborigine Bird Lotus Basketball Butterfly Azalea Mountain Church Coast
CBU Suppression (%)
Test imgaes
Fig. 5-13 CBU suppression of nine test images
5.5 Summary
Optimization works of Stencil-FSC method have been done in the chapter for achieving best CBU suppression. Nine test images were chosen by two indexes, detail complexity and color complexity, in order to gather general and reliable results. The optimized results of hardware parameters: number of backlight divisions is 24*24, light spread function which related to Gaussian low-pass filter with D0 is equal to 0.01, and dimming ratio in backlight colorful method is 10%. The optimizations can be recommendation parameters for FSC-LCD in the future. By utilizing the optimized parameters, Stencil-FSC method can suppress 62% CBU caused by conventional RGB color sequence on average.
Chapter 6 Conclusions and Future work
6.1 Conclusions
An LCD using field sequential color (FSC) technique displays field images in time sequence and generates full-color images using the temporal color mixing method. By using the FSC technique, the LCD does not need color filters (CFs) and is based on a LED backlight system, the FSC-LCD has several attractive advantages, including higher light efficiency/lower power consumption, easier manufacturing of higher image resolution, higher color saturation, and CFs cost saving. Therefore, the FSC-LCD has high potential as commercial applications in display market. However, FSC face a serious issue; color breakup (CBU), when there is a relative velocity between the viewers’ eye and observed images. The CBU will degrade image quality and cause viewers discomfort. In recent years, although many methods have been proposed to suppress CBU, they may be not only limited by LC response time but also cause image distortion. Thus, CBU is not suppressed effectively. Thus, the FSC-LCD has not been widely utilized in display applications yet.
In this thesis, we proposed the “Stencil Field Sequential Color (Stencil-FSC) method” with four fields to suppress the CBU phenomenon in the FSC-LCD. A rough
In this thesis, we proposed the “Stencil Field Sequential Color (Stencil-FSC) method” with four fields to suppress the CBU phenomenon in the FSC-LCD. A rough