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Abstract
A new mini light-emitting diode (mini-LED) backlight with reflective dots is proposed for high luminance uniformity, high contrast ratio, and low power consumption for use in mobile liquid crystal displays. The proposed backlight, comprising a small number of mini-LEDs, was verified as having high luminance uniformity and high light use efficiency, due to the optimized reflective dots, backlight thickness and light distribution of the mini-LEDs. Moreover, the light leakage to adjacent segments was reduced by cutting a slit between each segment, improving the light use efficiency per segment and suppressing halo artifacts.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, display technology has been used in a wide range of applications such as TVs, smartphones, watches, and in-vehicle devices [1]. The ever-increasing use of these popular devices has resulted in a demand for a higher contrast ratio and lower power consumption, especially in small displays. A high contrast ratio of 1,000,000:1 is desired, given the advances in high dynamic range. In small displays, organic light-emitting diodes (OLEDs) [2,3] are widely used, as they offer exceptional image quality and a thin profile; however, low reliability related to a short lifetime continues to limit these devices.
Liquid crystal displays (LCDs) have the advantages of high reliability and low power consumption over OLEDs. LCDs are non-emissive devices that display images by controlling the transmittance of light from the backlight. However, the contrast ratio of the displayed image is, at most, 5,000:1, due to light leakage during the black state, which is lower than that of OLEDs [4,5]. A conventional approach used to improve the contrast ratio of LCDs is the local dimming method [6–15], as shown in Fig. 1. Here, direct-lit backlights control the LED brightness with respect to that of the image, thus improving the contrast ratio and lowering power consumption [16–18].
Local dimming has been applied to large LCDs, such as TVs, but not to small LCDs such as smartphones. This is because the dimensions of conventional LEDs are large, and direct-lit backlights with large LEDs must retain a certain distance between the emitting surface and the diffuser plate, such that the irradiation areas of individual LEDs overlap for high uniformity [19,20], as shown in Fig. 2. This makes it more difficult to build thinner backlights as well as control each irradiation area precisely, resulting in problems such as halo defects [21–24]. In recent years, smaller mini-LEDs [25–31] (about 80–500 μm in size) have been developed and applied to mobile LCD backlights. However, several to tens-of-thousands of mini-LEDs are required to construct a direct-lit backlight with uniform luminance, which raises concerns about the increased cost of backlight manufacturing. This issue can be solved by widening the pitch of individual mini-LEDs to configure a backlight with a smaller number of mini-LEDs; however, this approach tends to reduce luminance uniformity and create hot spots [32].
Mini-LED backlights with a “bat-wing” light distribution [32] have been investigated, in an effort to widen the pitch of the mini-LED and achieve high luminance uniformity simultaneously. However, there is a limit to the pitch between individual mini-LEDs with respect to retaining the desired light distribution. Moreover, reducing the number of mini-LEDs to about 2000 is required for mobile applications to lower manufacturing costs [32].
One potential solution is to spread the light of each mini-LED uniformly within one segment of the local dimming method. In this paper, we introduce a new backlight structure with reflective dots [33]. The mini-LED backlight structure was designed, and the optical properties were evaluated through computational simulations.
2. Principle and structure of the proposed mini-LED backlight
Figure 3 shows the configuration of the proposed mini-LED backlight using a light guide plate with reflective dots. The light guide plate is placed on top of a blue-emitting mini-LED. Reflective dots and a reflector are placed on the top and bottom surfaces, respectively, of the light guide plate. A diffuser plate and a quantum dot (QD) sheet for color conversion are positioned above the light guide plate.
The proposed backlight can achieve a wide irradiation area with individual mini-LEDs and high luminance uniformity, based on the following principle: light emitted from the mini-LEDs is reflected by the reflective dots, and the light is guided inside the light guide plate. The light is then emitted through the gaps between dots. Therefore, the size and placement of the reflective dots must be configured in such a way as to create uniform irradiation, based on the light distribution of the mini-LEDs. Given that the luminous intensity of the mini-LEDs with a general light distribution is strongest in the normal direction, the reflective dots are densely arranged above the mini-LEDs to block and reflect the light. The size of the reflective dots becomes smaller toward the edge of the segment, and the pitch between dots becomes wider, as shown in Fig. 4, thus eliminating hotspots and realizing uniform irradiation over a broader area.
3. Design and performance of the backlight
For the proposed backlight structure, dimensions and placement of the reflective dots, the light distribution of the mini-LEDs, and thickness of the light guide plate are the important factors because these affect the optical properties of the backlight. Therefore, luminance uniformity and the light use efficiency of the backlight was studied using theoretical calculations to optimize those three factors.
3.1 Conditions for simulation
In this study, the size of a smartphone display was assumed to be 6 inches along the diagonal. The number of segments for the local dimming method must exceed 512 segments to achieve the HDR effect of different images in mobile LCDs [34]. Thus, the local dimming method was assumed to be controlled with 512 segments. In this case, each segment was a square with sides of 4.26 mm; thus, the light emitted from the mini-LED was spread uniformly over a 4.26 mm × 4.26 mm square.
The reflective dots were configured in a rectangular shape, and the light distribution of the mini-LEDs was Lambertian. In a Lambertian light distribution pattern, the luminous intensity at angle θ is expressed as the product of cos θ and the luminous intensity in the normal direction. Figure 5 shows the light distribution of the mini-LEDs. The thickness of the light guide plate was set to 0.8 mm, considering the thickness of the diffuser plate, such that the overall thickness of the backlight was less than 1 mm. The reflectance of the reflective dots and the reflector for normal incidence was set to 97% based on the reflectance of Ag, and 460 nm wavelength was used considering color conversion with a QD sheet. The incident angle dependence of reflectance was taken into account in all simulations. The parameters of the proposed backlight are shown in Table 1.
Table 1. Simulation parameters
In the proposed backlight configuration, the light guide plate is used to spread the light emitted from the mini-LEDs; thus, the pitch and the overlapped light of each LED are unrelatable factors with respect to the luminance uniformity of the backlight. Given that the light guide plate only needs a certain thickness to reflect the light from the mini-LEDs, the thickness of the entire backlight, i.e., the backlight profile, can be thinner. However, the light use efficiency of the backlight depends on the thickness of the light guide plate. Specifically, a thinner light guide plate requires more reflections to guide the light to the edge of the segment, leading to increased optical loss due to reflectance of the reflective surfaces in the light guide plate. Using a mini-LED with a wide light distribution can reduce optical loss in the light guide plate, but it also causes more light leakage to the adjacent segments. This reduces the light use efficiency per segment and causes halo defects. Therefore, the thickness of the light guide plate and the light distribution of the mini-LEDs must be designed to optimize the light use efficiency per segment.
3.2 Evaluation of luminance uniformity
In this study, we designed 3 × 3 backlight matrix consisting of nine segments using LightTools (Cybernet Systems) to verify the luminance uniformity of the proposed backlight, considering the overlapped light of each segment.
To calculate the luminance of the backlight, a light observing plane was placed 0.002 mm away from the diffuser plate. The size and placement of the reflective dots were optimized by theoretical calculations, such that the amount of incident light on the light observing plane was uniform. The size of the light observing plane was the same size as the 3 × 3 backlight matrix.
The light emitted within a measurement angle of a luminance meter was evaluated by scanning the light observing plane sequentially with the luminance meter, as shown in Fig. 6; the measurement angle of the luminance meter was set to 2 ° to evaluate the front luminance of the backlight. Luminance uniformity is defined by Eq. (1), where Lmin is the minimum luminance and Lmax is the maximum luminance on the light observing plane:
Figure 7 shows the results of the luminance distribution of nine backlight segments, in which the luminance is indicated by color, with brown (white) corresponding to the lowest (highest) luminance. The variation in luminance among segments was minimal, i.e., the luminance uniformity was 91.9%. The size of reflective dots after optimization was about 10–210 μm, and the pitch between dots was 3–180 μm. These results show that the proposed backlight can expand the irradiation areas of the mini-LEDs to create uniform irradiation via the optimized reflective dot configuration.
3.3 Optimization of the light use efficiency of the backlight
Light use efficiency affects the power consumption of the backlight. If the light use efficiency is low, the power required to achieve high luminance will be large, leading to higher power consumption. In direct-lit backlights with the local dimming method, the light use efficiency of the entire backlight is maximized by maximizing the light use efficiency per segment, lowering power consumption even when the backlight is configured with a small number of mini-LEDs.
As described in Section 2, optical loss due to reflectance from the reflective surfaces in the light guide plate and the amount of light leakage to the adjacent segments depend on two factors: the thickness of the light guide plate and the light distribution of the mini-LEDs. Therefore, the light use efficiency per segment is determined by the abovementioned factors.
The light use efficiency per segment and the light leakage to the adjacent segments were investigated while changing the thickness of the light guide plate and the light distribution of the mini-LEDs. The central segment of the nine segments was turned on. Various thicknesses of the light guide plate: 0.4, 0.6, and 0.8 mm, were evaluated. The thickness of the backlight was set to less than 1 mm in total, considering the thickness of the diffuser plate. The maximum light emission angle of the mini-LED was varied over the range of 30° to 90° for each light guide plate thickness. The maximum light emission angle of the mini-LED is the angular measure of light emission from the normal direction to the half-value angle.
The light use efficiency per segment is given by
The light leakage to the adjacent segments is defined by Eq. (3):
Figures 8(a)–8(c) shows the results for an 0.8-, 0.6-, and 0.4-mm light guide plate thickness, respectively. Regardless of the thickness of the light guide plate, the light use efficiency per segment and light leakage to the adjacent segments decreased as the maximum light emission angle of the mini-LED became narrower. On the other hand, when the maximum light emission angle was wider than a certain angle, the light use efficiency per segment also decreased and only the light leakage to the adjacent segments increased. When the thickness of the light guide plate was 0.8 mm, the light use efficiency per segment decreased beyond a maximum light emission angle of 70°, and the light leakage increased. When the thickness of the light guide plate was 0.6 and 0.4 mm, the light use efficiency per segment decreased as the maximum light emission angle of the mini-LED approached 75° and 80°, respectively, and the light leakage increased. This was because the number of light reflections in the light guide plate was smaller, whereas the amount of light leakage to the adjacent segments increased as the light distribution of the mini-LED broadened. Thus, for light guide plate thicknesses of 0.8, 0.6, and 0.4 mm, the optimum maximum light emission angle of the mini-LED was 70°, 75°, and 80°, respectively. Besides, luminance uniformity for each condition was over 80%. However, for all light guide plate thickness values, more than 20% of the light leaked to adjacent segments of the plate.
Fig. 8. Relationships of the maximum light emission angle of a mini-LED with respect to the light use efficiency per segment and light leakage to adjacent segments.
In a comparison of the light use efficiency per segment at the optimum maximum light emission angle, the light use efficiency per segment decreased as the light guide plate became thinner. This was because the number of light reflections in the light guide plate increased; thus, the optical loss according to the reflectance of the reflective surfaces increased as the light guide plate became thinner.
For the above reasons, the light guide plate thickness of 0.8 mm with the maximum light emission angle of the mini-LED of 70° was optimum, considering the thickness of the diffuser plate, such that the overall thickness of the backlight was less than 1 mm. Besides, the overall light use efficiency of the backlight was 65.2% when the nine backlight segments were turned on.
In the earlier simulations, the reflectance of the reflective surfaces for normal incidence was set to 97%. However, this reflectance might drop in real cases. Therefore, we examined the light use efficiency per segment with different reflectance: 97%, 95%, 93%, and 91%. The thickness of the light guide plate was 0.8 mm and the maximum light emission angle of the mini-LED was 70°. The size and arrangement of reflective dots were optimized for each condition in the same way as described in Section 3.2. Figure 9 shows the light use efficiency per segment decreased by 4% as the reflectance reduced by 2%. Besides, luminance uniformity for each condition was over 80%.
4. Suppression of light leakage to adjacent segments
As mentioned in Section 3.3, the proposed backlight had more than 20% light leakage to the adjacent segments, causing a decrease in the light use efficiency per segment. Moreover, the light leakage to the adjacent segments may produce halo artifacts, as mentioned in Section 3.1. Therefore, we cut a slit from the bottom surface of the light guide plate at the boundary of each segment to suppress the light leakage to the adjacent segments and improve the light use efficiency per segment. Slits can suppress the light leakage to the adjacent segments, as shown in Fig. 10, because the light entering a slit at an incident angle higher than the critical angle returns to the segment by total reflection. In this simulation, the slits were introduced when the thickness of the light guide plate was 0.8 mm and the maximum light emission angle of the mini-LED was 70°. The size and the arrangement of the reflective dots were optimized in the same way as described in Section 3.2, such that the luminance was uniform, even after cutting the slits.
In addition, the width and depth of the slit are important considerations. With regard to the slit width, making a slit at the boundary of each segment reduces the luminance of the slit area because the light emitted from the top surface of the segment boundary is reduced. Additionally, a wider slit width causes a greater reduction in luminance. Uniform luminance is achieved by optimizing the size and the placement of reflective dots, but it can lead to a reduction in the overall luminance. For this reason, the slit width should be as narrow as possible. In this study, the slit width was set to 60 μm.
With regard to the slit depth, a deeper slit reduces light leakage to the adjacent segments. Given that the slit depth cannot be deeper than the thickness of the light guide plate, we examined three slit depths: 200, 400, and 600 μm. The light use efficiency per segment and the light leakage to the adjacent segments for each condition were compared.
Table 2 shows that the light use efficiency per segment improved by about 4% and the light leakage to adjacent segments decreased by 4% on average as the slits became deeper by 200 μm. When the slit depth was 600 μm, the light use efficiency per segment improved by 13.2%, and the light leakage to the adjacent segments decreased by 13.7% compared to the case without slits. From the above results, we verified that the light leakage to adjacent segments was controlled by cutting a slit between individual segments; this improved the light use efficiency per segment. Figure 11 shows the cross-sectional luminance distribution before and after making slits, in which the deeper slits realized higher luminance at the edge of the segment and a top-hat luminance distribution for each segment.
Table 2. Difference in light use efficiency per segment and light leakage according to the variation in slit depth
5. Influence of slit introduction on luminance uniformity
To verify the influence of the slits on luminance uniformity of the backlight, the luminance distribution of the nine backlight segments was evaluated before and after cutting a slit between each segment. The width of the slits was 60 μm and the depth was 600 μm. The size and arrangement of reflective dots were optimized in the same way as described in Section 3.2 after cutting the slits. Light was emitted even above the slits, and a decrease in luminance above the slits was suppressed, as shown in Fig. 12. The evaluation of luminance and the calculation of luminance uniformity were performed in the same way as described in Section 3.2.
Figures 13 shows the luminance distributions before and after cutting the slits. As a result, high luminance uniformity was achieved even after the slits were cut: 92.9% before cutting in and 92.8% after cutting in. Thus, making slits at the boundary of each segment reduced the light leakage to the adjacent segments and improved the light use efficiency per segment, without degrading the luminance uniformity.
Fig. 13. Luminance distribution of nine backlight segments (a) before cutting slits (b) after cutting slits.
6. Conclusion
Using simulation modeling, this study demonstrated that the proposed backlight structure provides high luminance uniformity and a broader light distribution with a small number of mini-LEDs by optimizing the size and arrangement of reflective dots. The uniformity of the proposed backlight exceeded 80%. The light use efficiency of the backlight was maximized by optimizing the thickness of the light guide plate and the light distribution of the mini-LEDs. Moreover, the light leakage to adjacent segments was suppressed by cutting a slit between each segment, improving the light use efficiency per segment.
In future work, a prototype of the proposed backlight will be fabricated for an experimental demonstration.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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