Chapter 5 Local Adaptation and Boundary Issue
5.4 Proper light distribution formula
The proper light distribution in a segment is required to achieve whole backlight profile as uniform as possible when each segment switched on the same level. On the other hand, the boundaries should not be visible while 2D dimming. Moreover, the distributions should be substantially local for maximizing contrast [92][93]. So far, the Lorentz with σ=1.2 could be a good choice. In order to find whether other light distributions are suited for 2D-dimming, a simple model is analyzed as follows.
First, the origins are set in the center of each segment, and the pitch between two origins is set to 1. We choose three points located in origin (A), middle point of segment edge (B), and crossing point of segments(C), respectively, as shown Fig. 5-9.
These three points are significant for the light distribution. At point A, overlap light is contributed by four closest light sources and itself. For B, it is contributed by two closest light sources and other four sources. Similarly, four closest light sources contribute to the point C. Tab. 5-2 shows the relation between distance and number of contributed sources. Contributions from the distances larger than square root of 1.25 are cut out. Later we will show that this cut-off is justified.
Fig. 5-9 Three points in the model for light source contribution analysis
Tab. 5-2 The relation between distance and contribution of light sources
In uniform condition, these three points should have the same intensity, which can be expressed as:
)
where I(r) is a function of light profile. The square of distance is approximately inversion proportional to contributed number of sources in the first item on point B and C (2/0.25: 4/0.5). The possible form I(r) is assumed to be
A
,
where r is distance from origin, k determines the width of profile, and a is the power factor. One is added in denominator to avoid infinity at origin and normalize the contribution to one. Summation of the intensity differences between each two points is used to optimize a and k. In Fig. 5-10(a), the intensities of these three points are calculated with an extended 7x7 segments, whose cut-off distance about 3.5. While k around 0.8 to 1.2 and a around 2.5 to 2.8, these intensities are approximated. When the segments is increased to 15x15 or higher, a and k are converged on 2.2 and 1.0 as shown in Fig. 5-10(b). Convergence is achieved when the number of segments is larger than 15 x 15.
Fig. 5-10 Summation of differences and parameters in (a) 7x7 array (b) 15x15 and 31x31 array
This form of light distribution has a long tail, thus it degrades the contrast enhancement and worsens the localized ability. In order to improve contrast of backlight image, we add another power factor b as
,
to sharpen the profile of segment. In one condition of b equal to 2, the intensity difference is converged when segment array is 7x7 or higher. The parameter (a, k) are (2.3, 1.4) as shown in Fig. 5-11(a). We further increase b to 3, and modify again the (a, k) values to (2.2, 1.5) as shown in Fig. 5-11(b). With higher b such as 4, the gradient of backlight image will exceed the boundary-free criterion. Surprisingly, the parameter (a, b, k) equal to (2.2, 3, 1.5) so matches the Gaussian distribution with width factor equal to pitch of segment as shown in Eq. 5-7 and Fig. 5-12. The full-on and 2D-dimming backlight images of camera man are shown in Fig. 5-13. Gaussian distribution has the shortest tail of these three profiles, thus the contrast enhancement is the highest. The boundary issue can be avoided because the gradient of image is less than threshold. Moreover, the most uniform backlight image of these three ones can be achieved.
Fig. 5-11 Summation of differences and parameters with sharp factor (a) b=2 (b) b=3
Fig. 5-12 Similarity between a, b, k model , Gaussian, and Lorentz distributions
Fig. 5-13 (a) 2D-dimming (b) full-on backlight image of Gaussian distribution
-6 -4 -2 0 2 4 6
0.0 0.2 0.4 0.6 0.8 1.0
Position/ pitch
a,b,k model Gaussian Lorentz
Image Image
5.5 Summary
The human contrast sensitivity function was analyzed to determine the threshold of boundary-free perception in 2D-dimming backlight. The gradient of backlight image was used to evaluate the visibility of boundaries. From a perception study, it was found that the gradient of backlight image should be lower π/100 of background luminance. This value appears to be the threshold for the visibility of boundaries. The Lorentz distribution of backlight profiles was examined on images over different spatial scales, ranging from coarse to fine detail. We focus on perceived image quality of dimmable backlight with an economical number of segments for 2D-dimming backlight. The effects of size and profile of segment were studied based on human visual properties. Considering the boundary perception and contrast enhancement, the size of 2 degrees was found to the recommended profile of each segment. The gradient is close to 100% below the boundary-free criterion and the average backlight contrast is 3.5. Simultaneously, the proper light profile in each segment was derived for high uniformity of three significant points. Considering a better localized ability and high contrast enhancement, the Gaussian function was found to be the most suitable profile of each segment in 2D-dimming backlight.
Chapter 6
Conclusion and Future Work
6.1 Conclusion
The field sequential color liquid crystal display (FSC-LCD) has emerged as a new branch of LCD application. Sequential driving LED backlight represents a potential technological breakthrough in terms of optical efficiency. Because of its needless color filter and low material cost, the FSC-LCD has become the key technology for reducing the power dissipation and the resource consumption.
FSC-LCDs rapidly flash the primaries time-sequentially such that the colors are mixed by means of temporal integration in the eye. The lacking sub-pixels and color filters result in high transitivity and large aperture ratio. The primary chromaticities are determined solely by the LEDs which enable wider gamut and scalable number of primaries. Furthermore, the impulse driving of the backlight ensures high moving image quality.
However, color breakup (CBU) is the most disturbing artifact, which occurs in FSC-LCDs. The CBU reveals itself in the appearance of multiple color images of stationary object during saccadic eye motion, or along the edges of moving objects when tracking the objects with the eye. Although increasing the frame to several thousand Hz can completely eliminate CBU [94], it is highly unlikely that affordable panels with frame rates higher than 240 Hz will be widely available in the foreseeable future.
In this thesis work, we have successfully relaxed the requirements on 240 Hz frame rate and reduced CBU significantly by modulating the backlight. The proposed methods, such as color field arrangement and gray level redistribution, were implemented in the timing controller circuit. Additionally, the index of color difference has been utilized to evaluate the CBU. Compared to the CBU perception with previously sequential driving methods, the newly developed methods have greatly improved the image performance of FSC-LCDs. Tab. 6-1 shows the dynamic control algorithm is the first sequential driving LCD in the world adapted by image contents. For LC availability, field processing loading, and power consumption, this adaptation method also presents its competitiveness.
Tab. 6-1 The comparison between normal RGB, 3 prior arts described in Chapter1, and the proposed adaptation method
6.1.1 The consistent color fields in mobile-sized FSC-LCDs
On a 5.6-in LCD platform, we have demonstrated the color field arrangement (CFA) method, which modifies the consecutive color order to resolve CBU issue. The integration of three consecutive frames on retina was compensated as a gray level image because of the viewpoint through the same ratio of each primary color. Blurring margins of test patterns were observed in the synchronized camera experiment for the movement of eyes tracking. Comparatively, the perceived images with the typical 3-field RGB driving strategy have obvious multicolor margins, the notable CBU phenomenon.
The 4-CFA method with consistent orders of RGBR, GRBG, and BRGB in three consecutive frames was implemented and confirmed experimentally on the CBU suppression. The physical evaluation results on dynamic CBU adhere to the prediction for the moving images. Furthermore, the field frequency of CFA method is 1.3 times faster than that of typical one, resulting in the slighter static CBU for stationary images. Generally, increasing field frequency can effectively mitigate the static CBU.
However, the LC response time is still a crucial limitation to apply the sequential driving on LCDs.
We presented the 4-field RGBW method by flashing a gray image in a white field and displaying the color residuals in remaining fields. This allows the image energy to be better focused in the initial field, thus reducing the intensities of the red, green, and blue fields thus suppressing CBU within the field frequency of 240 Hz.
The minimum one of the RGB gray levels per pixel is assigned as the gray level in the white field. For a pure white image, this method displays the image only in the white field, thus CBU can be totally eliminated. For other colors, at least one gray level of color fields will be zero. Perception tests have confirmed that the color separation is markedly reduced.
6.1.2 The adaptive B/L for CBU optimization in laptop-sized LCDs
The consistent color orders of field such as CFA and RGBW methods can not fulfill all kinds of image for static CBU reduction, especially in cyan, yellow, and magenta images. Remaining two primary colors result in an obvious color separation during the observation.In order to overcome this restriction, we presented the intelligently adaptive backlight. According to the incoming video content, the exchange of RGBC/Y color sequence dynamically mitigates the CBU phenomenon to lighten the most sensitive green field. The redistribution of LC gray levels on these two color sequence is assessed by the gamma curve between gray levels and transmittances to maintain the white balance. We implemented the RGBC/Y method on a 32-in LCD platform. The brightness of a white image can reach at 400 nits at total power consumption of 50W, a half of power of typical CCFL backlight module [55]. From the CBU evaluation index, 27% to 56% of suppression ratio in the specific color pattern was obtained.
This redistributed method has been extended to determinate a suitable color in the fourth field. With RGBC/Y sequence, the fourth color is fixed as the cyan or yellow one, resulting in still one primary color with higher values of gray levels. An approach is to flash an adaptive image in a dominated field (D-field) and use the remaining fields to display the color residuals. Typical FSC displays with consistent color order like a rotating color wheel in a projector are unable to obtain an adaptive color field except the modulation of LED backlight.
The color backlight in D-field that induced the minimum color difference between CBU images and original ones was proposed for the CBU minimization. The simulation results of test images show that the 2x4 sampling period of image and 3-bit gray levels of color backlight can adequately simplify the calculation of color
difference, which is necessary in a real time application. Most importantly, an adaptive feedback control algorithm was described to locate the optimal D-field color efficiently. 8 sets of color backlight were evaluated at each frame and more accurate sets were used at following frame. Consequently, the proposed DRGB method concentrates the majority of image intensity into the D-field and owns the lowest color difference compared to those of typical RGB and RGBC/Y method. This average CBU suppression ratio around 70% of typical RGB driving is obtained. The color separation at the edges of objects in test images was essentially improved and agreed with the observation.
6.1.3 The recommended light profile for TV-sized FSC-LCDs
Adaptive dimmable backlights consisting of local addressable LEDs allow the D-field color to be optimum for each segment. For highly-colored images, the mass of image intensity in primary color fields may remain even through the entire backlight is modulated in the D-field. Nevertheless, color addressable backlights are capable to locally module the D-field along horizontal and vertical segments to resolve the issue of global adaptation, especially for large-sized LCDs. Moreover, these backlights generate only the amount of light required to correctly depict the video content while dimming underneath dark areas. The power consumption can be further reduced while simultaneously improving the black level [95]-[99].
To combine this highly-potential backlighting with FSC-LCDs, we have investigated the optical profiles on segments to influence the perceived image quality.
The brightness variation at the boundary between segments should be indistinguishable in the backlight image. The gradient of backlight image was used to evaluate the visibility of boundaries. According to contrast sensitivity function, we identified π/100 of background luminance as the threshold for the boundary
perception. Below this threshold, boundary was not sensible.
Several images with different spatial scales, ranging from coarse to fine detail, were examined. Considering the boundary perception and contrast enhancement, the size of 2 degrees was found to the optimal segment profile. The gradient is close to 100% below the boundary-free criterion. At a viewing distance of a factor of 3 longer than screen diagonal, the segment size of 2 degrees results in about 10x10 as the recommended and economical number of segments. For a closer viewing distance, more segment numbers are needed. In addition, the proper light profile in each segment was derived for high uniformity of three significant points. Considering the localized ability and contrast enhancement, the Gaussian function was the recommended profile of each segment in local adaptive backlights.
In conclusion, this dissertation explores the sequential driving and the evaluation of newly developed CBU reduction techniques. The practical circuit control and optical profile design has been examined on full-scale LCDs. In this thesis, we have demonstrated that the adaptation to image content has a great potential to attain high image quality and low power consumption, the essential capacities of FSC-LCDs.
6.2 Future work
The adaptive backlight can be viewed as a natural extension of the consistent color in the sense that the fourth field is modified instead of only white, yellow, or cyan. The adaptive field allows for a higher focusing of image energy in time, while reducing the visual saliency of individual fields. So far, the adaptive color has only been tested in combination with global LED backlights that allow fields to change color only as a whole backlight. Although this global adaptation was verified to satisfactorily reduce CBU, we anticipate that the performance will greatly improve when combined with a local color dimming LED backlight to enable a far superior
adaptation to the incoming video content. Fig. 6-1 shows the original image and individual color fields of global and local adaptive backlights. The two characters are presented by themselves color adaptation as shown in Fig. 6-1(c). Simulated results of non-adaptation (conventional RGB), global and local adaptation methods support the prediction of CBU reduction, as illustrated in Fig. 6-2. Furthermore, three or even two local color dimming fields can be applied for a slower response time with typical LCs instead of fast response ones. The algorithm for the cases of fewer color fields can be expectably reported on a normal twisted nematic (TN) mode LCD in the near future.
In the scope of whole LCD system, we have discussed and concluded that the major consuming devices in LCD embodiment are polarizers and color filters. In this thesis, the sequential driving as a feasible approach provides to leave out the color filters. Nevertheless, the polarizer still absorbs about 50% incident light, thus much lower the output light efficiency. Therefore, liquid crystal capable of having spontaneous polarization can be used for excluding the polarizer in LCD embodiment.
Liquid crystalline conjugated polymers are potential candidates as inexpensive, easy to process polarized back lights for liquid crystal displays [100]. The device efficiency and brightness can be modified by the property of alignment layer. This polarized liquid crystal shall be an appealing topic to explore in the future.
In addition to enhancing the optical efficiency of components, the power consumption of light source is anticipated as low as possible. The highly efficient white LED can be introduced into the backlight besides RGB LEDs as shown in Fig.
6-3. The white LED is proposed to contribute major intensity in the color-mixed field.
The efficiency of RGB mixing white of 40 lm/W is markedly lower than that of single white LED of 100 lm/W [101]. Consequently, the combination of white and RGB LEDs is viewed as a further reduction of power consumption.
Fig. 6-1 The (a) original image and individual color fields of (b)global and (c)local adaptive backlights.
(a) non-adaptation (b) global adaptation (c) local adaptation (a) non-adaptation (b) global adaptation (c) local adaptation
Fig. 6-2 The CBU images with (a) non-, (b) global, and (c) local adaptation.
Fig. 6-3 The schematic of added white LEDs into the backlight module.
In a world that is increasingly aware of its ecological footprint and energy issue, the sequential driving technology has wide-spread acceptance to upgrade the efficiency of LCD applications. As the power-saving, or called green, technique become a common view of human beings, the sequential driving prototypes have been widely demonstrated and greatly capable of regard for both image quality and power consumption. The newly developed algorithms mitigate the effects of color breakup, and allow sequential driving LCDs to be attractive in practical scopes covering research for mobile devices, signage, monitor, and TV. Therefore, consumers can benefit from appealing multi-media; meanwhile, lower the injury that is causing to our planet.
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