CBU Suppression Methods

If the CBU can be well suppressed at a practical level, FSC LCD will become a very promising display. At CBU suppression, many researches and experiments were reported recently to optimize the driving method for CBU suppression. These proposals could be categorized as (1) field rate increasing [21-22], (2) multi-primary color fields [23-24], and (3) motion compensation [25]. The following section will describe these methods in detail.

2.5.1 Field Rate Increasing

CBU can be perceived in relative motion between the image and observer’s eyes.

However, the refresh frequency of R, G, and B fields is the main issue causing CBU in FSC displays. Thus, CBU in FSC displays is suppressed by increasing the refresh frequency with the decreasing of flashing time [26]. Some researchers have performed experiments in which observers adjusted the refresh frequency of FSC displays until CBU was just invisible. The threshold of refresh frequency is proposed as 1200Hz in order to avoid the perception of CBU

[27-30]. This threshold is extrapolated from measurements and derived using mathematical models. CBU was measured as a function of the stimulus’s luminance, contrast, and retinal velocity. A nonlinear regression equation that describes the observer’s mean threshold refresh frequency, so that the equation constitutes design rules that can be used to evaluate the adequacy of FSC LCD field rate for specific applications.

The CBU width can be influenced by changing field rate. The relation between the time and the location of moving image on single frame is shown in Fig. 14. Double field rate repeats a set of RGB color fields in a frame. Compare the double field rate with the single field rate, the CBU width of double field rate becomes almost a half of the single field rate as shown in Fig. 15.

Fig. 14 Relation between the time and the location of moving image on single field rate [26].

Fig. 15 Relation between the time and the location of moving image on double field rate [26].

According to the above discussions, increasing the refresh frequency will improve the

CBU phenomenon. However, these methods require fast scanning time of TFT and fast response time of LC, which is not practical in conventional LCD.

2.5.2 Multi-primary Color Fields

Color sequential projection displays also exhibit the phenomenon of CBU. It is considered a distributing artifact with negative marketing impact. Therefore, D. Eliav et al.

perform a psychophysical experiment to compare the visibility of CBU in a three-primary (RGB) projection display, which operating at a higher frame rate, and five-primary (RGBCY) displays operating at lower frame frequency [31]. They assumed that green in the RGB display and yellow in the RGBCY display is the brightest color field. The RGB color fields are transformed into three opponent color channels, i.e. Achromatic, Red-Green, and Yellow-Blue channels. Each of them is filtered spatially with the corresponding response. For RGBCY display, they measured the XYZ tristimulus values to estimate the opponent signals.

For the RGB display, they use the same chromaticity as the RGB components, and adjust the relative luminance to obtain the equal white points at equal brightness. Subsequently, they integrate the opponent color signals of all fields to mimic a low-pass temporal response. The results show a strong modulation in the Red-Green channel, with a weak Yellow-Blue modulation on the edges of the white strip in the RGB displays, and then the mostly Yellow-Blue modulation is seen in the RGBCY displays. That is consistent with the observation that red and green fringes are seen in the RGB displays, the blue and yellow fringes in the RGBCY displays. The modulation pattern is determined by the order of primary colors and the tracking of the brightest field. An optimization of color order may suppress the CBU. According to the experimental results, the RGBCY displays produce less CBU than RGB displays, even RGBCY displays at lower frame rates. The simulation results on white or whitish magenta solid rectangles moving towards right on a black background are shown in Fig. 16. The CBU in multi-primary color fields is slighter than that in conventional FSC LCD.

However, these color fields still require the fast scanning of TFT and fast response time of LC, which are not practical.

Fig. 16 Simulation results on white or whitish magenta solid rectangles moving towards right on a black background [31].

2.5.3 Motion Compensation

CBU are varied with colors of object, background, and moving direction. This means dynamic CBU can be anticipated by knowing above conditions. The method called Adjust of Color Element on the Eves (ACE) has been developed by N. Koma and T. Huchida [32] to suppress the negative effects of CBU with moving objects. The schematic plot of motion compensation is shown in Fig. 17. RGB images are displayed at different points, which have the same moving speed using velocity method. When the eyes always track the moving object, RGB sub-images are focused on the same point on the retina, hence CBU will not be perceived.

Fig. 17 Schematic plot of motion compensation [32].

However, this driving method is only effective in particular cases. That is motion compensation, which has a problem with the eye tracing in opposite direction as shown in Fig.

18 and two moving objects are displayed at the same time [33]. Moreover, most of video contents are too complex for taking into account all motion vectors in a real situation.

Fig. 18 A crossing object is moving from left to right at speed S and the observer’s eye is tracing from right to left at speed M [33].