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Abstract
Current mini-LED backlights improve high-dynamic-range liquid crystal displays (LCDs) by using tens of thousands of direct-lit sources for local dimming. However, relative thick profile and high power consumption are the inherent limitations while compared with edge-lit backlights. By synthesizing edge- and direct-lit advantages, we propose a novel hybrid mini-LED backlight equipped with a specially designed integrated light guiding plate (LGP) for large-area displays. This LGP is seamlessly spliced by multiple physically segmented sub-LGPs with a scattering dot array on the bottom and U-shaped grooves at the corners. Each sub-LGP is a single local dimming zone that can be independently controlled. Scattering dot distribution can be numerically calculated even from multiple edge-lit sources. High optical performance and satisfactory local dimming effect are verified and analyzed via both simulation and experiment. The experimental spatial illuminance uniformity and the light extraction efficiency reach 81% and 83% while the crosstalk can be well suppressed below 0.2% between adjacent local dimming zones. The significant advantages of our design towards state-of-the-art mini-LED backlights include the zero optical distance for an ultra-thin profile, low mini-LED amount for local dimming, high optical efficiency, and infinite extension of zone number, which is expected to have a broad application prospect in the near future.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Liquid crystal displays (LCD) and organic light-emitting diode (OLED) displays keep competing with each other over the past decade [1,2]. Presently, they have comparable performance on color gamut [3,4], resolution density, response time [5], and power consumption. However, LCDs still have advantages in peak brightness [6], lifetime [7], and cost, while the inherent advantages of OLEDs are true dark state [8], and an ultra-thin or even flexible profile [9,10]. Unlike OLEDs, true dark state is quite hard to achieve in LCDs, because each LC pixel cannot completely cut off the transmission light from a global dimming backlight [11]. The contrast ratio of traditional LCD could only reach around 5,000 : 1 with a global dimming backlight [8]. It is a simple but effective way to boost the contrast ratio by selectively modulating the backlight, which is to switch on the zones for full-color display image and turn off the other zones for black display image. This concept is called local dimming [12]. Each local dimming zone can be electronically divided and independently controlled according to the display image [13]. It is obvious that a local dimming backlight can help to improve the contrast ratio and reduce the power consumption [14], which is beneficial for high dynamic range (HDR) LCDs [15].
HDR displays require high peak luminance over 1000 cd/m2 and excellent dark state less than 0.01 cd/m2 [16,17]. Local dimming methods based on a traditional LED backlight have attracted increasing attention, because it is an effective way to reproduce nature scenes for HDR displays [18]. There are two typical LED backlight architectures, which are edge-lit and direct-lit backlights [19]. On one hand, edge-lit backlights require fewer number of LEDs and a thinner light guiding plate (LGP) [20], but local dimming is hard to achieve [21]. On the other hand, a direct-lit backlight can easily adjust local luminance for local dimming [22], but long optical distance for light mixing limits the backlight to become thinner. Therefore, modified backlights with multi-region LGPs [23,24] or other structural backlights [25,26], have been proposed to improve local dimming performance. For example, both inverse trapezoidal microstructures on LGP’s surface [27,28] and stacked multiple-layer LGPs [29] provide local dimming realization approaches.
Recently, mini-LED backlights have become a hot research topic [30,31], because their ∼100 µm size could provide finer local dimming control, and they also feature high peak luminance, HDR, true dark state, and thin form factor for LCDs [32]. There are two common approaches to obtain higher uniformity of mini-LED backlight. One is to increase the optical distance between the mini-LED backplane and the diffuser plate while fixing the mini-LED array arrangement [33]. Too long light mixing distance may bring a thick backlight profile, and deteriorate the local dimming effect. The other is to use more mini-LEDs to compensate the optical distance [34]. Larger number of mini-LEDs help to achieve precise local dimming control with a thin backlight profile, but the power consumption, driving circuit design, and heat dissipation would become potential problems. Therefore, it is critical to realize local dimming for current mini-LED backlight with a zero optical distance while exponentially reducing the mini-LED amount for lower power consumption.
To address this, we propose here an edge/direct-lit hybrid mini-LED backlight by using an integrated LGP for local dimming. Unlike previous researches, the edge-lit mini-LED is embedded in the U groove of the LGP, and the direct-lit homogenization is realized by the scattering dot distribution on the LGP’s bottom surface. Due to the hybrid architecture, zero optical distance can be realized between the LGP and the above display panels, and this hybrid structure exponentially reduces the mini-LED amount for local dimming.
2. Optical model
Figure 1(a) shows the optical model of the hybrid mini-LED backlight for local dimming. From bottom to top, the backlight unit is comprised of a reflector sheet, an integrated LGP, lower and upper diffuser sheets, and two crossed prism sheets. This LGP is seamlessly spliced by multiple physically segmented sub-LGPs with a rectangular shape and U-shaped grooves at the corners. The groove’s depth is lower than the sub-LGP’s thickness. The top-firing mini-LED sources are embedded into the grooves with the light emitting direction towards the center of the sub-LGP. Specifically designed scattering dot array is required on the LGP’s bottom surface to regulate the local illuminance. Each U-grooved sub-LGP is a single local dimming zone that can be independently controlled. Moreover, a reflector sheet is added on each side wall of the local dimming zone to prevent crosstalk between adjacent local dimming zones. The optical films can be added above the LGP for uniform spatial illuminance distribution and high light extraction efficiency (LEE). Here, the LEE is used to evaluate the optical efficiency of the proposed backlight, because the scattering dot array is designed for extracting the light trapped inside LGP. The LEE is defined as the output luminous flux of the backlight divided by the total luminous flux of mini-LEDs, which is the same as the optical efficiency. Meanwhile, the uniformity is the ratio of the minimum and the maximum illuminance within the local dimming zone.
Fig. 1. Schematic of (a) the hybrid backlight equipped with sub-LGPs as physical local dimming zones, (b) a single local dimming zone enabled by edge-lit mini-LEDs embedded in the U grooves of the sub-LGP, and (c) the cross-sectional view along the diagonal of the sub-LGP.
In order to eliminate the hot spots and dark region caused by edge-lit mini-LEDs, the optical transmission channels (OTCs) should be carefully designed at the four corners of the sub-LGP, which is discussed in the following section. As shown in Fig. 1(b), OTC-1 refers to a certain spatial distance between the upper surface of the groove and the LGP’s top surface, while OTC-2 represents the minimum width between the side wall of the U groove and the LGP. These channels allow for light transmission to the corner region and produces uniform spatial illuminance distribution for the local dimming zone. Figure 1(c) shows the cross-sectional view along the diagonal of the sub-LGP, in which the U-shaped groove can be clearly seen.
3. Design principle
As mentioned above, the mini-LEDs are embedded at the corners of the sub-LGP, and scattering dots are distributed on the sub-LGP’s bottom surface. Since the LGP’s thickness is comparable to the height of the scattering dots, the total reflection part of the light path can be ignored and only the light transmission path is considered for further process [35,36]. The light path is illustrated by the red solid line in Fig. 2.
Fig. 2. Definition of the illuminance at an arbitrary target point inside the local dimming zone illuminated by multiple edge-lit mini-LEDs.
In this work, each local dimming zone has a square shape with the size of a × a. It is noted that there has no close relation between the zone size and the mini-LED number, and the zone size can be flexibly adjusted as needed without limitation of the mini-LED number. The position of the mini-LEDs is defined by Si (i = 1, 2, 3, 4). The mini-LED’s spatial intensity satisfies the Lambert’s cosine law. The optical axis of mini-LED source is along the LGP’s diagonal. The coordinates of an arbitrary target position on this local dimming zone are defined as (x0, y0). θi (i = 1, 2, 3, 4) represents the angle between the optical axis of a certain mini-LED source and the line between the target position and the source. According to the trigonometric relation, the absolute value of θi can be expressed by
The light intensity of mini-LED source satisfies Ii (θi) = I0 cos(θi). Here, I0 is the normal intensity of the mini-LED source, and Ii is the intensity along the direction of θi. Considering the energy loss caused by the propagation inside the LGP, the Lambert-Beer law is introduced here:
where A is the absorbance of the LGP. T is the transmittance of the LGP that can be expressed by $T = \frac{{I(L )}}{{{I_0}}}$. Here, I (L) is the light intensity as travelling a certain distance L through the LGP. K is the molar absorption coefficient of the LGP, which is related to the inherent material properties of the LGP and the wavelength of the mini-LED. c is the concentration of the light-absorbing substance. By defining I (0) = I0 and I (∞) = 0 as the boundary conditions, the illuminance Ei (θi, Li) from a mini-LED satisfies the following equation according to the inverse square law of illuminance:In this work, polymethyl methacrylate (PMMA) is chosen as the LGP’s material [37], then we can define K = 0.02, ρ = 1.2 g/cm3, and μ = 100 g/mol. Figure 3 shows the numerical results of the illuminance distribution with regard to any coordinate position on the LGP. It is clearly seen that the illuminance near the corner is quite high, and gradually decreases away from the corner, because the source is set close to the four corners. In Fig. 3(a), rapid attenuation occurs while the local dimming zone size is x = 40 mm and y = 40 mm. As the size of the local dimming zone shrinks, the internal relationship between the illuminance and the position becomes more obvious from Fig. 3(b). This illuminance distribution is the calculation basis for defining the scattering dot array on the LGP’s bottom surface.
Fig. 3. Numerical results of the illuminance distribution within a single local diming zone. The local dimming size is (a) x = 40 mm, y = 40 mm and (b) 3 mm < x < 37 mm, 3 mm < y < 37 mm, respectively.
A position-varying parameter is needed to define the scattering dot density. From the discussion above, Eq. (4) has confirmed the functional relation between the illuminance and the coordinate position. Therefore, the illuminance is chosen as the independent variable for the following derivation. At the same time, the lowest illuminance in the local dimming zone is used as a standard to avoid an indistinguishable density value after calculation. Based on this, an exponential function is used to define the mathematical relationship between the illuminance and scattering dot density at any position of the local dimming zone.
The shape of each scattering dot is set to be half an ellipsoid. The radius is 0.12 mm, and the height is 0.06 mm. The projected area of the scattering dots in a unit region of the local dimming zone is defined as β, referring to the scattering dot density. The maximum scattering dot density is β0 = 1. R is a ratio coefficient ranging from 0 to 1, which can be used to define the scattering dot density at any position in a local dimming zone:
where $R = {e^{{E_{\min }} - E({{x_0},{y_0}} )}}$, Emin refers to the minimum illuminance, and E(x0, y0) is the illuminance at position (x0, y0) of the local dimming zone. Obviously, R is the illuminance ratio of the position (x0, y0) and the position having the maximum illuminance. Increasing scattering dot density and adjusting the dot arrangement can help to reduce the space for light mixing and achieve the output light homogenization.Combining Eqs. (4) and (5), the numerical relationship between the scattering dot density and the local dimming position can be obtained. Figure 4(a) and 4(b) illustrate the three-dimensional (3D) and two-dimensional (2D) maps of this relationship. The lowest scattering dot density is near the mini-LED source at the corner of the local dimming zone, because lower dot density represents less light extraction, which helps to balance the highest illuminance near the corners. The density increases rapidly as the coordinate position gradually moves to the center, and its changing trend agrees well with Eq. (4). The scattering dot density at any position is related not only to the illuminance from the edge-lit mini-LEDs, but also to the scattered light from other dots. That is to say, the density at this position is determined by the superposition of this two. Therefore, the scattering dot density will be gradually decreased towards the center, but still higher than that near the corners. The most central area of the local dimming zone has a relatively low illuminance, resulting in a relatively high scattering dot density. A more intuitive distribution like a “cross” pattern can be seen in Fig. 4(b). According to the distribution, the calculated radius distribution of the scattering dots can be converted, which is shown in Fig. 4(c). The “cross” pattern is in good agreement with the density distribution. This dot distribution is used for further simulation and experimental verification.
Fig. 4. Numerical calculation results of (a) three-dimensional (3D) and (b) two-dimensional (2D) maps of the relationship between the scattering dot density and the local dimming position. (c) Radius distribution of the scattering dots for uniform light output.
4. Optical characteristics
4.1 Analysis of U-shaped groove
Based on the scattering dot distribution above, the size of the U-shaped groove can be determined and the corresponding influence on the optical characteristics of the backlight is discussed. As shown in Fig. 1, the light entering the local dimming zone is related to the OTC’s thickness. However, by using a mini-LED, the LGP’s thickness can become much thinner, providing that the main parameters are reasonable, such as the refractive index, light transmittance rate, tensile strength, bending strength, thermal expansion coefficient, thermal conductivity, and other factors. Therefore, it is necessary to analyze the OTC’s parameters under a certain LGP’s thickness for good performance.
During the simulation, the edge-lit mini-LED is set as a Lambertian light source and its spread angle is within −60 ∼ + 60 deg. The mini-LED’s size is 400 µm × 200 µm. The dimension of the sub-LGP is 40 mm × 40 mm, equaling to the size of a single local dimming zone.
The thickness of OTC-1 and OTC-2 are two parameters to be determined. First, the OTC-1’s thickness was analyzed by setting the OTC-2’s thickness as a fixed value at 20 µm. Then, the OTC-1’s thickness was scanned from 600 µm to 700 µm by every 10 µm. From simulation, it is found that the optical performance including LEE and uniformity have a certain relationship to the OTC-1’s thickness. In Fig. 5(a), the LEE reaches the maximum of 92% while the OTC-1’s thickness is 700 µm. As mentioned earlier, the LEE has positive correlation with the OTC-1’s thickness, because this thickness determines the light entering the local dimming zone. As shown in Fig. 5(a), the maximum illuminance uniformity of 84.7% exists at the OTC-1’s thickness of 700 µm. This is consistent with our theoretical design that assumes all the emitting light will be collected into the local dimming zone.
Similarly, the initial OTC-1’s thickness was set to 700 µm to find the optimal OTC-2’s thickness. The OTC-2’s thickness is changed within the range of 20 µm ∼ 60 µm increased by every 5 µm. The simulation results are shown in Fig. 5(b), where the backlight maintains a high efficiency with the OTC-2’s change. The maximum LEE of 92% and the best uniformity of 84.7% are both found when the OTC-2’s thickness is 20 µm. The thinner thickness of the OTC-2 results in higher LEE and uniformity, because the U groove is closer to the LGP’s corner, and the light can be easily transmitted to the corner. The OTC’s optimal parameters can be determined jointly by the results in Fig. 5(a) and 5(b).
4.2 Spatial illuminance distribution
From the discussion above, the optimal thickness of OTC-1 and OTC-2 is 700 µm and 20 µm, respectively. The size of the U groove is 230 µm × 450 µm. After determining the OTC’s parameters, the spatial illuminance distribution of a single local dimming zone can be simulated and analyzed. Figure 6(a) and 6(b) shows 2D illuminance map with uniform spatial illuminance distribution and the optimal OTC for simulation, respectively. The simulated LEE and uniformity are 92% and 84.7% within the local dimming zone, exceeding the optical performance level of currently commercial LCD standard. The similar illustrations can be seen in Fig. 6(c) and 6(d). That means most of the emitted light from the edge-it mini-LED sources can be collected by the U groove, and after the total reflection propagation and scattering by the scattering dots, uniform light output can be achieved.
Fig. 6. Simulation results of a single local dimming zone with the U-grooved LGP and edge-lit mini-LEDs. (a) 2D luminance map; (b) Schematic of the optimal value of OTC from simulation; (c) Normalized luminance distribution curves in the horizontal and vertical direction; (d) 3D spatial luminance distribution.
It is noted that no scattering dots are distributed above the U groove area according to Fig. 4(c). This notwithstanding, we cannot see any dark area on the luminance map. One reason is that the mini-LED source has a large angular distribution, and part of the light directly escapes from the LGP’s top surface and illuminate the corner area. The other is that the optical functional sheets including two diffuser sheets and two prism sheets are inserted into the backlight, which can balance the illuminance distribution and shield the dark area on the LGP.
4.3 Local dimming performance
A number of local dimming zones can be seamlessly spliced to compose an integrated LGP. For verification, nine sub-LGPs are considered here. That means this backlight includes nine independent local dimming zones. Figure 7(a) shows the simulated spatial illuminance distribution when the total nine local dimming zones are working simultaneously, where the uniformity reaches 94.8%. For a more intuitive illustration shown in Fig. 7(b), the illuminance curves in horizontal and vertical directions only have little fluctuation, which proves that the backlight can provide uniform spatial distribution when nine local dimming zones are lit simultaneously.
Fig. 7. Simulation result of the local dimming performance of the hybrid backlight integrated by nine U-grooved sub-LGPs. (a) 2D illuminance map and (b) the correspond luminance distribution curves when the nine local dimming zones are working at the same time. (c) 2D illuminance map and (d) the correspond luminance distribution curves when only the central local dimming zone is active. (e) LEE and (f) uniformity with different number of active local dimming zones.
Each local dimming zone can be operated individually. As shown in Fig. 7(c), only the central local dimming zone is active, which has a uniform illuminance distribution. Total 91.96% of all the emitting light are collected inside the central zone, which can be seen in Fig. 7(d). Slight light leakage can be observed near the border of the central local dimming zone, because the output light has a large spread angle from the local dimming zone. More precise local dimming effect can be controlled by adjusting the zone size and shape, mini-LED distribution, and scattering dot distribution, and even the well-matched optical functional sheets.
The influence of the local dimming number on the optical performance are analyzed for this backlight, including LEE, illuminance uniformity, and crosstalk. Figure 7(e) illustrates the LEE change as the number of active local dimming zones increases. The backlight provides high LEE over 87% ensuring that the emitting light from edge-lit mini-LEDs can be efficiently utilized. Meanwhile, the illuminance uniformity maintains high stability, as shown in Fig. 7(f). As the active local dimming zones increases, the luminance uniformity keeps over 82% on average.
Crosstalk between the adjacent local dimming zones can be used to better evaluate local dimming performance. The crosstalk level can be defined as [38,39]:
where LCBW is defined as the central luminance when the central local dimming zone is not lit when the adjacent zones are active, as shown in Fig. 8(a). LCWB is defined as the luminance when the central local dimming zone is active when the adjacent zones are not lit, as shown in Fig. 8(b). LCBB is defined as the luminance when all the zones are dark, as shown in Fig. 8(c).According to Eq. (6), the range of the crosstalk value is between 0∼1. Lower crosstalk value means that fewer light is leaked into the adjacent local dimming zones, and most light is confined in the target local dimming zones. According to simulation results, both LCBW and LCBB are zero, indicating that this backlight can well suppress the crosstalk.
5. Experimental verification
5.1 Experimental results
The scattering dot distribution determines the optical output of the local dimming zone, which keeps the same in simulation and experiment. Screen printing technology was used to fabricate the dot array [4]. Figure 9(a) shows the microscopic image of the scattering dot array. A certain spatial distance ranging from 380 µm ∼ 690 µm is set to separate the adjacent dots. Figure 9(b) shows the profile of a single scattering dot measured by a 3D laser microscope (OLS4100), where the dot’s height is around 15 µm ∼ 21µm, and the width is within the range of 475 µm ∼ 535 µm.
Fig. 9. Experimental characterization of printed scattering dots and a single local dimming zone. (a) Microscopic image of the scattering dot array; (b) The 2D and 3D profile of a single dot. Actual luminous output of a single local dimming zone (c) without the printed scattering dots and (d) with the printed scattering dots; (e) The off-axis performance for the nine test positions labelled in Fig. (d).
The scattering dots are used to extract the total-reflection-propagating light trapped inside the LGP and regulate the emergent light for high spatial illuminance uniformity. A hybrid LGP corresponding to a single local dimming zone is prepared without printed scattering dots but equipped with edge-lit mini-LEDs in the four corners. Driving the multiple mini-LEDs is similar to conventional direct-lit mini-LEDs. The difference is that our mini-LED chip is embedded into the U-shaped groove, so that it needs a metal base to fix the mini-LED chip inside the groove, and lead wires to connect the driving and control board under the reflector sheet. In order to avoid dark areas, the reflector sheet was prepared some small holes for the lead wires to pass through. Actual luminous output of the LGP is shown in Fig. 9(c). Since no scattering dots are fabricated on the LGP’s bottom surface, we can see through the LGP and clearly observe the logo of “Fuzhou University”. The emitting light from four corners keeps total reflection propagation within the LGP. The light leaks out from the LGP’s side wall, resulting in high brightness observed from the four sides of the local dimming zone. However, the spatial illuminance uniformity can be significantly improved with the printed scattering dots and the optical functional sheets. As can be seen in Fig. 9(d), uniform luminous output is obtained and the logo cannot be observed from the local dimming zone. Based on the measured results by a luminance meter and an integrating sphere, the LEE and the spatial illuminance uniformity reach 83.10% and 81.5%, respectively. Off-axis performance is also important for the proposed design, so that the angular dependent luminance was measured and analyzed. Figure 9(e) shows the angular dependent luminance distribution of a single local dimming zone, where nine test points in Fig. 9(d) were measured. It can be found that the luminance meets the Gaussian distribution, and the off-axis performance of different test positions has high consistency. The light spread angle is within ± 50°, indicating that the light energy can be effectively collimated and concentrated into a small angle due to the two prism sheets.
The local dimming performance is evaluated with the prepared backlight. Four hybrid sub-LGPs are spliced side by side to form an integrated backlight with four independent local dimming zones. The size of each local dimming zone is 40 mm × 40 mm, and the integrated size is 80 mm × 80 mm. Figure 10 shows the experimental local dimming performance from the integrated backlight. Total 40 measured positions are set distributed on the luminous region of the backlight, especially at the boundary of adjacent local dimming zones. Different working states are operated to evaluate the optical performance.
Fig. 10. Local dimming performance and the corresponding spatial luminance distribution of the integrated backlight with four independent zones.
Figure 10(a) shows the local dimming performance while only the upper left zone is not lit. The corresponding luminance distribution curve can be seen in Fig. 10(b) that there is an obvious luminance change at the boundary of the adjacent local dimming zones. The A and B curves in Fig. 10(b) show the dramatic luminance change from the low illuminance region to the high illuminance region of the upper two local dimming zones. The C and D curves show the luminance change of lower two local dimming zones, in which the curve is relatively flat without obvious fluctuation, reflecting high illuminance uniformity of the region. The measured luminance uniformity is 81.83%. Similar local dimming performance is shown in Fig. 10(c) and 10(d), where the lower right zone is not working.
Figure 10(e) shows the optical performance while two local dimming zones on the diagonal line are active, i.e., the upper right zone and lower left zone. A brightness mutation is found between high-brightness zone to low-brightness zone at the boundary. The corresponding illuminance curve shown in Fig. 10(f) shows that the contrast is relatively high for this backlight. Based on the previous definition, the crosstalk is 0.187616 for this backlight. Similarly, only one local dimming zone at the up right corner is active, and the results are shown in the Fig. 10(g) and 10(h). The uniformity under this condition reaches 81%. The measured crosstalk is lower than 0.2%, which represents a high local dimming performance. The optical performance for different local dimming conditions is summarized in Table 1.
Table 1. Measured results of the prepared hybrid backlight.
5.2 Discussion
Light homogenization principles of the conventional direct-lit mini-LED backlight and the proposed design are discussed here. As shown in Fig. 11(a), the conventional mini-LED backlight employs a direct-lit mini-LED array and a diffuser plate to convert the mini-LED’s point-like sources into a uniform surface source. Direct-lit mini-LED illumination requires high-density mini-LED array or large optical distance for light mixing. However, in our proposed design, the light homogenization does not rely on the mini-LED’s density or optical distance, but is realized by the LGP and the scattering dot array on the bottom, as shown in Fig. 11(b). The arrangement and density of the scattering dots determine the output uniformity and efficiency of the backlight, which has been discussed in Sec. 3.
Fig. 11. Light homogenization principles of a conventional direct-lit mini-LED backlight and the proposed design.
The proposed hybrid backlight is large-area display oriented, such as TVs. Therefore, a conventional direct-lit mini-LED backlight for TV was built and compared via simulation. Here, the local dimming zone size is 40 mm × 40 mm, which is set the same as our design. Figure 12 shows the mini LED backlights with different parameters and the corresponding illuminance distribution. In Fig. 12(a), when only 2 × 2 mini-LED array is used and the optical distance is fixed at 1.5 mm, a complete illumination on the entire target surface is hard to achieve. It is noted that the parameters are the same as our design. Longer optical distance or more mini-LEDs is needed to improve the uniformity. The corresponding simulation results can be respectively seen in Fig. 12(b) and 12(c), where high uniformity is achieved when the optical distance is enlarged to 30 mm or totally 19 × 19 mini-LED array is used. The above results show that the mini-LED amount of our design can be greatly reduced for local dimming, and the thickness of 1.5 mm also has advantage in current large-area display backlights. Compared with a current benchmark TV product (TCL 75” 8-SERIES-75Q825), these advantages are also obvious. Moreover, by seamlessly splicing the sub-LGPs, the zone density can reach the same level of the current TV product.
Fig. 12. Illuminance distribution of the direct-lit mini-LED backlights with different optical distance or mini-LED array density.
Moreover, the proposed backlight has higher optical efficiency than conventional direct-lit mini-LED backlights. One reason is the multiple scattering or absorption between the diffuser plate and the reflector sheet, and the other is the light leakage at the edge of a single local dimming zone. The leakage not only causes the optical efficiency decease in this local dimming zone, but also brings some crosstalk. High optical efficiency together with low mini-LED amount is expected to reduce the power consumption of current large-area mini-LED backlights.
6. Conclusion
In this work, we propose a novel edge/direct-lit hybrid mini-LED backlight equipped with U-grooved sub-LGPs as physically segmented local dimming zones. By optimizing the OTC’s parameters and the scattering dots array, uniform spatial illuminance distribution and high light extraction efficiency can be both achieved for this backlight. At the same time, the integrated backlight, consisting of several independently controlled local dimming zones, can achieve satisfactory 2D local dimming effect and high contrast ratio. The feasibility of the backlight is verified by ray-tracing simulation and further experiment. The measured LEE and uniformity reach 83% and 81%, and the crosstalk can be well suppressed to be lower than 0.2% for different local dimming patterns. This wok only presents a new concept and preliminary verification, and the optical performance can be further improved by adjusting the mini-LED distribution, the local dimming size and shape, scattering dot distribution, etc. It is apparent that this backlight has some irreplaceable advantages, including the zero optical distance for an ultra-thin profile, low mini-LED amount for local dimming, high optical efficiency, infinite extension of zone number, and so on. With the rapid development of mini-LED backlight, it is believed that this design will have bright application prospects in the near future.
Funding
Science and Technology Projects of Fujian Province (2020H4021); Fuzhou Science and Technology Bureau (2020-Z-14); National Key Research and Development Program of China (2017YFB0404600).
Acknowledgments
The authors would like to extend their sincere gratitude to the colleagues of Top Victory Electronics (Fujian) Co., Ltd.
Disclosures
The authors declare no conflicts of interest.
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