Viewing Angle Enhancement for Visual Effect

For TN mode LCDs, the refraction ratio of liquid crystals depends on the applied voltage and wavelength, which usually have serious color shift in the vertical viewing angle. The LCD color shift degrades the image quality and detains purchase decision of consumers, caused by the phase retardation variance of the liquid crystals. The LCD color shift is dependent on a number of variables including transmittance, chroma, and viewing angle.

Figure 7-6 shows the color shift of VX912 at different viewing angles. From Figure 7-6(a), (b), and (c) we observe that the luminance reduces from the normal angle to the outside angle.

It is the most obvious in the vertical angle. The luminance variations will cause the contrast degradation.

(a) (b) (c)

(d) (e) (f)

Figure 7-6. Color shift of the panel of VX912 at different viewing angles.

Base on our backlight scaling algorithm, the image contrast can be enhanced while the viewing angle is not normal. The contrast degradation of the image on the TN mode LCDs can be improved. We can get better visual effect at different viewing angles. Figure 7-6 shows

the image without using backlight scaling algorithm and with backlight scaling algorithm while we tilted the two panels vertically. We can perceive the better image contrast of the right-side panel in Figure 7-7(b).

(a) Image without using backlight scaling algorithm

(b) Image with using backlight scaling algorithm

Figure 7-7. The panels were tilted about 30° vertical viewing angle.

7.3 Summary

We have implemented the backlight scaling algorithm for minimizing power consumption of RGB LED-backlight TFT-LCDs. The proposed algorithm was implemented by an FPGA board. The image quality can be preserved as the backlight intensity is dimmed.

For the benchmark images, up to 55% of power consumption can be reduced when the color difference constraint of ∆E_ab^* ≤2 was given. Moreover, the backlight scaling algorithm enhances the image contrast of TN-mode LCD at different viewing angles.

Chapter 8

Conclusion and Future Direction

8.1 Conclusion

Today, TFT-LCD is the most common display in the mobile electronic devices. It is also one of the components which contribute the most power consumption in the mobile electronics. The TFT-LCD backlight dominates the power consumption of the whole system.

For prolonging the battery lifetime of the mobile electronic devices, the power consumption have to be reduced.

Our goal is to reduce the power consumption of TFT-LCD backlight. The technique of backlight scaling is used to achieve the purpose. The technique of backlight scaling decreases the backlight intensity to conserve power consumption while preserving the visual quality.

The technique need to employ algorithm for implementing the backlight dimming and image processing. We proposed the Chromaticity and Luminance Scaling algorithm based on visual perception for minimizing power consumption of LED backlight TFT-LCDs. The CLS algorithm consists of two phases. The chromaticity scaling is guided by the CIELAB color difference to scale the red, green and blue backlight individually. The luminance scaling is based on the prior Concurrent Brightness and Contrast Scaling algorithm, which was solidified by a series psychophysical vision experiments. We also conducted a psychophysical experiment, in which the interaction between perceived brightness and contrast was validated.

Another psychophysical experiment gave luminance algorithm a strong persuasion to adopt the CBCS algorithm.

Finally, we prototyped an experimental LED backlight platform for characterizing its power consumption in red, green, and blue. The proposed algorithm was implemented by an

FPGA board. For the benchmark images, up to 55% of power consumption can be reduced.

At the same time, reducing the power consumption of LED backlights can prolong the LED lifetime and reduce the LED color shift due to thermal effects. Moreover, the light leakage of TFT-LCD results in angular-dependent luminance reduction and color shift. The backlight scaling algorithm increasing panel transmittance can avoid the light leakage of normally-white TFT-LCDs. This process can reduce the angular-dependent color shift of TFT-LCDs. In addition, our algorithm can increase the dynamic contrast ratio, and the image quality can be preserve.

8.2 Future Direction

The core in the technique of backlight scaling is the algorithm. Although the interaction between perceived brightness and contrast has been validated, the psychophysical experiments will be continued. The psychophysical experiments require to be improved, and then the consideration of Bartleson-Breneman effect also will be included. The model of perceived image brightness and contrast will be created by the psychophysical experiments.

The psychophysical experimental results will be utilized to refine our algorithm. However, the surrounding-aware LCD is not commercially available yet. For implementation, the ambient light can be quantified by installing a photo sensor. Moreover, the experimental platform will be transfer to small-sized panels. The final goal is develop an Application Specific Integrated Circuit (ASIC) to replace the FPGA board. It can be fabricated as an adapter between a laptop computer and a TFT-LCD or built into the internal circuit of a TFT-LCD.

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