Advanced Methods for Field-Sequential-Color LCDs with Associated Power Reduction Advantages

Using stencil-field-sequential-color methods with field rates as low as 120 Hz in conjunction with local color backlight dimming can effectively suppress color break-up. With the addition of a color-filterless LCD with an intelligent LED backlight and a non-polarized LC cell, optical throughput can be increased by a factor of 10, while at the same time requiring a lower material cost/count, resulting in an environmentally friendly display.*

by Han-Ping D. Shieh and Yi-Pai Huang

HIGH IMAGE QUALITY, low power consumption, and low material costs are all important factors for display devices if they are to thrive in the mainstream of the future. However, conventional red-green-blue color-filter LCDs are still low in optical throughput and have an imperfect "dark" state. For a typical 32-in. LCD panel, the use of polarizers and color filters produces a net optical throughput of about 5–10% to yield a front-of-screen image (Fig. 1), while the polarizer and color filter represent 10% and 19% of the material cost, respectively. If the next generation of LCDs is to meet market expectations and ever-stricter government regulations with regard to environmental friendliness, lowering power consumption is essential.

 

Fig_1a

Fig_1b

Fig. 1: Low light efficiency in a CCFL-backlit LCD can be attributed to the low efficiency of the optical components.

 

The Beginnings of an "Eco-Friendly" Display Using FSC

As far back as 1985, a high-light-efficiency field-sequential-color (FSC) LCD without the use of a color filter has been demonstrated to reduce power consumption.1 In this demonstration, the authors, by rapidly displaying red (R), green (G), and blue (B) field images time sequentially, created a full-color image by temporal color synthesis, as illustrated in Fig. 2. Consequently, fast-response RGB light-emitting diodes (RGB-LEDs) were applied to an LCD backlight system to replace conventional CCFLs. Without a color filter, FSC-LCDs are capable of high light efficiency, wide color gamut, low material cost, and a screen resolution that is possibly three times higher than that of RGB-LCDs.

However, in order to commercialize the FSC-LCD, a serious visual artifact must be overcome: color breakup (CBU), which occurs when relative velocities exist between the screen objects and the human eye, as shown in Fig. 2.2 During eye movements, the separated R, G, and B frames of an image degrade image quality and cause viewer discomfort. CBU suppression has been implemented in digital light-processing (DLP) projectors by increasing the field rate to 540 Hz or higher. Although LED backlights can be switched very rapidly, a slow LC response time of several milliseconds still limits the implementation of FSC in large-sized FSC-LCDs.

 

Fig_2

Fig. 2: In the FSC-LCD mechanism, color images are created by rapidly and time sequentially flashing the display primaries (top). Below, FSC display color breakup during eye movement is shown at right.

 

Stencil-FSC Methods

For practical applications, the field rate of FSC systems is limited to 240 Hz or lower. Stencil-field-sequential-color (stencil-FSC) methods using commercial OCB (optically compensated bend) (for 4- and 3-field), or even MVA (multi-domain vertical alignment), IPS (in-plane switching), and TN (twisted-nematic) (2-field) LC modes to effectively suppress CBU have been demonstrated for large-sized TFT-LCDs, as shown in Fig. 3(a). The stencil-FSC method incorporates local color-backlight-dimming technology at a low field rate of 240 Hz, which significantly reduces CBU effects. (For more on this methodology, see the article "2009 JSID Outstanding Student Paper Award" in the May/June 2010 issue of Information Display.)

Conventional FSC-LCDs compose a full-color image by using three high-luminance primary-color (R, G, and B) field images. When the eyes perceive the three high-luminance images sporadically, CBU is easily seen and reduces image clarity. Therefore, it is better for the major luminance to be in a single field, with much lower luminance in residual images. A multi-color field therefore can be used to show the most image luminance instead of a conventional mono-primary image. As a result, the low-luminance residual field images are only used to modify the color details.

4-Field Stencil-FSC Method

By using 4-field stencil-FSC, each primary color has two fields to display information, including the first field and the residual primary fields. Therefore, 4-field stencil-FSC could easily maintain image fidelity and suppress CBU by more than 50% and be made almost imperceptible with 24 x 24 backlight divisions. This method was implemented by researchers in 2009 on a 32-in. FSC-LCD TV to yield a high dynamic contrast of 26,000:1, a power consumption of less than 35 W, and a wide color gamut of 114% NTSC.3-5

To yield a multi-color image in a color-filterless LCD, local color backlight dimming [also referred to as high dynamic range (HDR) technology] was utilized in 2008, as described in the paper, "Dynamic Backlight Gamma on High-Dynamic-Range LCD TVs".6 An LC panel using the RGB-LED backlight was studied as a dual-panel display with different spatial resolutions: a low-resolution backlight module and a high-resolution LC panel. The backlight displayed a low-resolution color image and the color-filterless LC cell preserved high-resolution monochrome-image details. By combining these two panels, a multi-color image was generated, as shown in Fig. 3(b).

 

Fig_3a

(a) Stencil-FSC field images

Fig_3b

(b) Multi-color image

Fig. 3: (a) Shown are a target image of girl (©Microsoft), along with field images using the 240-, 180-, and 120-Hz stencil-FSC methods, respectively, and (b) a multi-color image yielded by a low-resolution RGB-LED backlight and a high-resolution color-filterless LC cell.

 

3-Field Stencil-FSC Method

For further hardware implementations, researchers reduced the number of field images from four to three, as described in the referenced 2010 article. The green-field content was moved to the first field because the human eye is most sensitive to green color. Using 32 x 24 backlight divisions, CBU could be suppressed by more than 40%, making CBU almost imperceptible and yielding high image fidelity.7

2-Field Stencil-FSC Method

The 2-field sequential method utilizing a high-resolution LC panel and a low-resolution RGB-LED backlight system to generate two field images – a red-blue and a green-blue field – is illustrated in Fig. 4. By sequentially displaying these two field images at 120 Hz, a full-color image can be generated without visible CBU. 40 x 40 backlight divisions can render pleasing imagery with less color difference (ΔE00< 3) due to the human eye being less sensitive to the blue image.8, 9 Therefore, current commercial LC modes, such as TN, MVA, or IPS, can be used for 2-field stencil FSC.

 

(a) Fig_4a

(b) Fig_4b

(c) Fig_4c

Fig. 4: (a) A two-field driving scheme is used to display two field images sequentially, which are integrated by the human visual system to form a frame image. This process is decomposed into (b) the first field and (c) the second field on an LCD with a spatially modulated color backlight.

 

Further Perspective on an Eco-Display

To reduce the power consumption even more, the optical throughput of the LC cell and backlight system needs to be further improved. For example, a pair of polarizers absorbs around 55% of the total light throughput. However, in order to eliminate the polarizers, new LC modes would be needed. If such an LC mode were available, it could be combined with the inherent high dynamic range of an LED backlight to produce a very efficient display system. A 4-in-1 style R-G-B-W backlight system such as that shown in Fig. 5(a) could take advantage of white LEDs that have efficiencies of more than 100 lm/W.

Considering practical viewing situations, backlighting luminance can be reduced in a dark environment, and the emitting angle of the backlight can be directed at a smaller angle for a single viewer, as illustrated in Fig. 5(b). Using all the above-mentioned factors with the stencil-FSC color-filterless LCD, the optical throughput of the LCD can be increased by a factor of 10, and the power consumption can be reduced to only 25% of a current FSC-LCD (as shown in Table 1). For a 42-in. LCD-TV, the power may be reduced from 200 W to less than 30 W.

 

(a)     Fig_5a          (b)     Fig_5b

Fig. 5: (a) At left is an RGBW 4-in-1 LED and (b) at right, a single-viewer and environment-controlled backlight.

 


Table 1: Conventional RGB-FSC and stencil-FSC methods are compared.
 
Conventional
RGB-FSC
Stencil-FSC
 
240 Hz
180 Hz
120 Hz
Field Rate (Hz)
180
240
180
120
BL Divisions
Global
24 x 24
32 x 24
45 x 80
*Color Difference (Avg ΔE00)
0.07
0.8
4.1
*CBU (%)
100
58.6
59.1
37.8
**Relative Optical Power of Backlight with RGBW LED (%)
100
62.6
51.3
24.6

*: With 70 test images
**: Based on the same brightness; with 110 lm/W white-light LED>

 

Conclusion

Stencil-FSC methods with field rates as low as 120 Hz can effectively suppress CBU. The stencil-FSC method yields a high image contrast of 26,000:1, an average power consumption of less than 35 W, and a wide color gamut of 114% NTSC for a 32-in. RGB-backlight LCD. A low-field-rate stencil-FSC could be achieved by using commercial LC materials and MVA, IPS, or TN modes. To further reduce the power consumption and material counts, the authors are actively working on an FSC-LCD without a polarizer that is powered by an intelligent backlight system. A factor-of-10 increase in optical throughput and lower material counts/costs indicate that a stencil-FSC LCD with an intelligent backlight system is a potent possibility for an eco display – perhaps even one running on a simple battery.

References

*The May/June 2010 issue of Information Display included an announcement regarding a paper relating to this topic, "Color-Breakup Suppression and Low-Power Consumption by Using the Stencil-FSC Method in Field-Sequential LCDs" by Fang-Cheng Lin, Yi-Pai Huang, Ching-Ming Wei, and Han-Ping D. Shieh. That paper, which is referenced in this article, received the 2009 JSID Outstanding Student Paper Award.

1H. Hasebe and S. Kobayashi, "A full-color field-sequential LCD using modulated back-light," SID Symposium Digest 16, 81-83 (1985).

2S-P. Yan, et al., "A Visual Model of Color Break-Up for Design Field-Sequential LCDs," SID Symposium Digest 38, 338-341 (2007).

3Y. P. Huang, et al., "Adaptive LC/BL Feedback Control in Field Sequential Color LCD Technique for Color Breakup Minimization," J. Display Tech. 4(3), 290-295 (2008).

4F. C. Lin, et al., "Color-Filter-Less LCDs in Achieving High Contrast and Low Power Consumption by Stencil Field-Sequential-Color Method," J. Display Tech. 6(3), 98-106 (2009).

5F. C. Lin, et al., "Color Breakup Suppression and Low Power Consumption by Stencil-FSC Method in Field- Sequential LCDs," J. Soc. Info. Display 17(3), 221-228 (2009).

6F. C. Lin, et al., "Dynamic Backlight Gamma on High Dynamic Range LCD TVs," J. Display Tech. 4(2), 139-146 (2008).

7F. C. Lin, et al., "Color-Breakup Reduction by 180-Hz Stencil-FSC Method in Large-Sized Color-Filterless LCD-TVs," J. Display Tech.6(3), 107-112 (2010).

8Y. R. Cheng, et al., Distinguished Student Paper 18.4: "Two-Color Field-Sequential Method for Color-Filter-Free MVA-LCDs," SID Symposium Digest 40, 239-242 (2009).

9Y. K. Cheng, et al., "Two-Color Field-Sequential Method with Spatial and Temporal Mixing Method," J. Display Tech. 5(10), 385-390 (2009). •

 


Han-Ping D. Shieh is a Chair Professor with the Department of Photonics and Display Institute, National Chiao Tung University, Hsinchu, Taiwan, and a Changjang Chair Professor at Shanghai Tang University, Shanghai, China. Chang Jiang is Chair Professor at Shanghai Jiao Tong University, Shanghai, China. He can be reached at hpshieh@mail.nctu.edu.twYi-Pai Huang is an Associate Professor with the Department of Photonics and Display Institute, National Chiao Tung University, Hsinchu, Taiwan. He can be reached at boundshuang@mail.nctu.edu.tw.