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Abstract
Mini–light-emitting diode (Mini-LED) backlight units (BLUs) in combination with high dynamic range technology can reduce energy and ensure high contrast and luminance. However, the number of LEDs used in mini-LED BLUs is considerably larger than the number of partitions in local dimming, resulting in low cost effectiveness. We proposed a design combining edge-light mini-LEDs and light-guiding microstructure lenses to reduce the number of light sources required in displays considerably. A 16-inch prototype was produced for experiments. The length, width, and thickness of the liquid crystal display module were 351.87, 225.75, and 1.709 mm, respectively. For edge-light mini-LEDs with a pitch of 8.6 mm, the average luminance was 18,836 nits for an input power of 22.5 watts, the uniformity was 85%, the uniformity merit function was 10.13, and the contrast ratio was 60,000:1. Thus, a zero-optical-distance (ZOD) mini-LED backlight for extra-thin, large-area notebook LCDs was produced.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With advancements in technology, displays have been increasingly used in consumer products such as TVs, mobile phones, and notebook computers. As display technology continues to improve, consumer requirements for the thinness, luminance, contrast, and color saturation of these displays are gradually increasing [1–4]. Several emerging display technologies use light-emitting diodes (LEDs), such as organic LED (OLED) and quantum dot (QD) LED (QLED) displays; these are becoming more common in display applications that require high performance because of their excellent performance in terms of color gamut, luminance, and contrast. These new displays are replacing traditional liquid crystal display (LCD) screens [5–8]. However, LCDs are still less expensive and have longer lifetimes than do newer screens; thus, mainstream displays are mostly still LEDs. However, LCDs have slow response times, low color saturation, and relatively low photo-electric conversion efficiency [9–11]. On the basis of the light source arrangement, LCD backlight modules can be categorized as direct lit or edge lit. Direct-lit backlight modules have higher optical efficiency and can more easily achieve local dimming. However, a larger optical distance (OD) and thus greater thickness are required for these modules. Edge-lit backlight modules require fewer LEDs and are thinner than direct-lit backlight modules. However, achieving two-dimensional local dynamic dimming is challenging on these displays, resulting in relatively poor contrast performance [12–15]. OLED displays have excellent color gamut space performance and contrast because each pixel can be switched independently. However, material aging and high luminance both require high power input, which reduce the service life of these displays [16–18]. QLED displays produced using QDs and blue LEDs have a wide color gamut, high color purity, and high quantum efficiency. For these advantages, QLED display are increasingly applied as backlight modules, replacing conventional blue LEDs combined with phosphor coloration; and emerging as the state of the art in high color space displays [19–21]. In future high-end displays, high dynamic range (HDR) is a key factor affecting the contrast ratio, and excel-lent HDR displays require superior bright and dark states for each pixel [22,23]. Mini-LEDs have become the backlight source for a new generation of LCDs. In LCDs, the bright and dark states of the backlight can be controlled through local dimming to greatly improve the contrast and achieve HDR [24–26]. In addition, the packages of mini-LEDs are smaller than those of traditional LEDs, and they can thus be used to produce thinner and lighter displays. With the use of flip-chip blue LEDs in mini-LEDs, they can withstand higher current density injection because of their lower thermal resistance. The general light-emitting angle of mini-LEDs is approximately 120°. However, to achieve a thin, large-area light source, numerous chips are required for high uniformity [27–29]. Scholars have proposed changing the light angle of LEDs by optical design, thereby both reducing the number of LEDs required and increasing uniformity [30,31]. The following section introduces several studies on this topic.
Ye et al. proposed a light source module that uses a high-reflectivity film to optimize the light distribution and increase the light-emitting angle of the light source, thereby achieving high uniformity [32]. Feng et al. proposed a design method based on total internal reflection to create ultrathin mini-LED backlight systems by using optical films with microstructures [33]. Ye et al. proposed a light source module with high efficiency and uniformity without a dot pattern; the module was produced with a fully printed diffusion reflection on the bottom of a light guide plate with an optimized design [34]. To increase the uniformity, efficiency, and weight of the design, they also proposed using hollow light guides to reduce weight by using a mini-LED light pattern and reflective slope on the end wall of the module; a hollow light guide with high uniformity and efficiency was thus created [35]. Kikuchi et al. proposed a mini-LED backlight structure with reflective mirror dots for mobile LCDs by using the pitch between the LEDs and the structure of reflective mirror dots to reduce the halo effect and increase uniformity; they also reduced module thickness. However, the light efficiency was only 58.8% of the original efficiency [36]. Ye et al. proposed using mini-LEDs in a design involving multiple three-dimensional (3D) diffuse reflection cavity arrays to control the light field shape and create a high-luminance and high-uniformity backlight source [37]. Zhu et al. proposed using a diffused transmission freeform surface to increase the light extraction efficiency and uniformity [38]. Sun et al. proposed a total internal reflection (TIR) structure based on the ray-mapping method; the design increased uniformity and light extraction efficiency [39]. Lu et al. proposed a 32 × 32 mini-LED array solidified on a polyethylene terephthalate (PET) transparent flexible substrate; the design had a wide color gamut and high uniformity at room temperature [40]. Chen et al. proposed a design involving mini chip-scale, packaged LEDs to change the light field shape and increase the light output angle; the design was combined with a QD film and diffuser to achieve high uniformity and luminance efficiency [41]. Several scholars have attempted to optimize the uniformity, color saturation, and luminance of traditional direct-lit and edge-lit flat light sources. However, few studies have produced ultrathin, large-area displays with these traits. Therefore, we proposed a design for edge-light (EL) mini-LEDs combined with a light-guiding microstructure (LGMS) lens and localized scattering (LS) film to reduce the number of light sources required for a high uniformity merit function (UMF) value. The design enables the production of ZOD Mini-LEDs backlight with light-guiding microstructure lens for extra-thin, large-area backlight module and would be advantageous for applications such as thin and light notebook displays that require high contrast. Compared to previous research, this design achieves a brightness of 1200 nits for the LCM (Liquid Crystal Module), and an increased pitch/OD ratio of 10.13, resulting in higher brightness and uniformity merit function values.
2. Methods
The 3D drawing software Solidworks (Dassault Systèmes, Vélizy-Villacoublay, France) and optical simulation software Light Tools (Synopsys, Mountain View, CA, USA) were used to optimize the design of the modules. In order to achieve ultra-thin, large-area, high- luminance, ultra-high contrast flat panel display module. The design structure consists of a mini-LED array, light board, reflectors, LGMS lenses, LS films, two diffusers, and two brightness enhancement films (BEFs; Fig. 1(a)).
Fig. 1. (a) EL mini-LEDs, LGMS lens, and LS film as flat light source, (b) package structure of EL mini-LED top, bottom and side view, (c) light distribution curve of EL mini-LEDs.
The EL mini-LED package has length LCPG, width WCPG, and height HCPG of 0.8, 0.8, and 0.35 mm, respectively. The light-emitting chip had a flip-chip structure; the chip pad length LP and width WP of 76 µm. The package structure of an EL mini-LED is displayed in Fig. 1(b).
Figure 1(c) presents the light distribution curve of the EL mini-LEDs; the central light intensity is 40%, and the curve represents the light distribution of the horizontal section.
The goal of the proposed design was to increase the value of the uniformity function. The TIR mechanism was used to cause the majority of the light source energy to be emitted sideways through guidance by the LGMS lens. Fig. 2(a) presents the EL mini-LEDs combining the LGMS lens and LS film module. For the lateral light, the microstructure of the LGMS lens is used to achieve uniform light output. TIR is used to expand the light output range through the LGMS lens, and the laser dot microstructure on the lower layer of the LGMS lens destroys TIR and controls the light output. For the upper layer of the lens, the circle prism microstructure was used to adjust the light angle to further increase overall luminance. For the forward light source, the LS film controls the light transmittance and reflectance at different angles through partial reflection and partial penetration of the concentrated light to increase uniformity. The proposed design can create a wider light-emitting area for a fixed thickness, reducing the required number of mini-LEDs and increasing uniformity.
Fig. 2. Proposed design (a) presents the EL mini-LEDs combining the LGMS lens and LS film module, (b) LS film structure, (c) laser dot microstructure, (d) laser dot density distribution calculation for tangent laser dots.
The EL mini-LED sidelight is transmitted to both sides through the LGMS lens design to increase the light transmission distance. The LS film directly above the light source also eliminates hot bright spots through diffuse reflection; this also reduces the module thickness and the required number of light sources. The distance between the centers of adjacent light sources is represented by Pitch, and the distance from the top of the LGMS lens to the detector is represented by OD.
Light Tools was used for optical simulations. For the BEF, the refractive index of the substrate layer (PET) was 1.66, the refractive index of the prism layer (UV Glue) was 1.59, and the vertex angle was 90°. The material of the LGMS lens was methyl methacrylate polymer (PMMA) with a refractive index of 1.49. The diffuser sheet was Lambertian, with a diffuse transmittance of 50% and diffuse reflectance of 50%. The light board surface was a Lambertian diffused surface with a reflectance of 90%. The main wavelength of the light source was 550 nm, the output power was normalized to 1 W, and the number of rays in the simulation was 50 million. The simulation parameters are listed in Table 1. To simplify the simulation, 4 × 4 arrays were simulated. The front and side views of the simplified simulated design are presented in Fig. 2(b). The EL mini-LEDs were bonded to the surface of the light board, and an LGMS lens was placed on the side of the light source. The bottom layer comprised laser dot micro-structures. The circle radius and depth of the laser dot microstructures are RLD and HLD, respectively; Fig. 2(c) presents the 3D structure. The pattern on the bottom was divided into several 0.2 mm × 0.2 mm areas for random number operations. For this arrangement, distance DLD between the dots was assumed to be random and greater than 2RLD [Fig. 2 (d)]. The LS film is employed for the forward emission of diffused and reflected light sources, while the LGMS lens enables lateral conduction of light from the side.
Table 1. Simulated and measured parameters of the LGMS lens.
The UMF is used to calculate the relationship among the uniformity of the plane light source, the pitch of the light source, and the thickness of the module. The UMF formula is given by Eq. (1). Larger UMF values indicate that the module can be thinner or the number of light sources can be lower for the same plane light source area.
The max coverage rate of laser dots is given by Eq. (2); RLD is the radius of the laser dots, and DLD is the distance between the centers of adjacent laser dots. The method for calculating laser dot density in this paper involves keeping the size of laser dots within a unit area constant while calculating density by varying their spacing, unit in percentage.
In the upper layer of the LGMS lens, a circle prism microstructure is used to adjust the light output angle. The pitch between each vertex (Vpitch) is 112.5 µm. In order to improve light output efficiency, the microstructure design was optimized by changing the depth and vertex angle. The circle prism’s internal thread angle is θ1, and the external thread angle is θ2. Let x and z represent the distance and depth of the LGSM lens respectively. The coordinate point position of Pn(xn,zn) can be calculated through the following Eqs. (3) to (6), where z3n and z3n-2 is a constant 0.3, n = 1,2,3,4…. The structure of the circle prism of the LGMS lens is presented in Fig. 3(a).
Fig. 3. (a) Circle prism structure of the LGMS lens, (b) LGMS lens structure top view and side view, (c) depth of LGMS lens circle prism microstructure, (d) laser dot microstructure with a center density of 2% and an edge density of 10%,20%,30%, (e) presents the laser dot microstructure with three different edge densities.
The 3D unequal views of LGMS lens is presented in Fig. 3(b). Fig. 3(c) presents top and side views, respectively. Distance Dpe from center point Cp0 to the edge is 4.3 mm, and distance Dpc from the center to each corner is 6.08 mm. Fig. 3(d) presents a graph of the change in depth of the circle prism microstructure; depth increases from 0.01 mm at center Cp0 to 0.025 mm from edge Dpc. The distribution of the laser dot microstructure on the bottom of the LGMS lens was optimized to increase the light output efficiency and uniformity. Fig. 3(e) presents the laser dot microstructure with three different edge densities; the central density is 2%, and the edge densities are 10%, 20%, and 30%, respectively.
After optimization, laser dot microstructure radius RLD of the lower layer of the LGMS lens and depth HLD were 10 and 3.8 µm, respectively. The lowest density of 21.13% is close to center point Cp0, and the highest density is 46.92% at edge Dpc. The density distribution is presented in Fig. 4(a) is the two-dimensional (2D) density distribution map, Fig. 4(b) is the density distribution for center Cp0–edge Dpe of the laser dots, and Fig. 4(c) is the density distribution for center Cp0–corner Dpc.
Fig. 4. (a) 2D density distribution map, (b) density distribution for Cp0−Dpe, (c) density distribution for Cp0−Dpc.
Figure 5(a) presents the simulation results for the EL mini-LED combined with the LGMS lens unit without optical films. After adjustment of the LGMS lens, the full width at half maximum (FWHM) of the light-emitting angle was 70.1 degrees, the uniformity (UM) was 11.9%, and the BLUs central luminance was 20,692 nits. In order to increase uniformity, an LS film was designed for placement directly above the EL mini-LED to eliminate the hot spot directly above the light source. The average reflectance and transmittance of LS film were 36.88% and 61.8%, respectively. Fig. 5(b) presents the simulation results for the design with LGMS and optical films as a BLU. The FWHM of the light-emitting angle converged to 52.1 degrees. The average luminance was 18,762 nits, and the uniformity increased to 85.6%.
Fig. 5. Simulated EL mini-LED BLU of light polar diagram and luminance distribution, (a) LGMS lens unit without LS films, (b) LGMS lens unit with LS films.
A summary of the simulation optimization data for the LGMS lens. The optimized θ1 is 25°, θ2 is 65°, and the laser dot edge density is 20%. The optimized design had superior light output efficiency and was used as the microstructure design for the LGMS lens.
3. Result and discussion
In order to expand the output range of the light source and achieve uniform light output, the LGMS lens is used. Fig. 6(a)-(i) presents a sectional view of the sample measurement of the laser dots. A Keyence VK-9510 (Keyence Corporation, Osaka, Japan) was used to measure the microstructure sample; Fig. 6(a) and Fig. 6(b) presents top and side views, respectively, of the central area of the laser dot; the diameter and depth are 20.9 and 3.8 µm, respectively. Fig. 6(d) and Fig. 6(e) presents top and side views, respectively, of the midsection, with a diameter and depth of 20.9 and 3.9 µm, respectively. Fig. Fig. 6(g) and Fig. 6(h) presents top and side views, respectively, of the edge region, with a diameter and depth of 20.7 and 3.9 µm, respectively. The output energy of the light can be controlled by adjusting the density of the laser dot microstructures in different regions. Laser dots with higher density or closer to the edge broaden the range of the light output and increase uniformity. Fig. 6(c), Fig. 6(f), and Fig. 6(i) presents the center, middle, and edge density distributions of the laser dots. Fig. 6(j)presents measurements of the LGMS lens microstructure. The microstructure Vpitch of the circle prism, θ1, and θ2 were 113.5, 60.5°, and 40.7°, respectively. The production process of LGSM lenses involves the following steps: first, shaping is performed using a diamond blade. Next, a UV resin is used for molding, and the surface is coated with Au through sputtering. Subsequently, the Ni Stamper is formed into a mold using the Electroforming process, and the final step is the completion of LGMS through the Injection process.
Fig. 6. Laser dot and LGMS lens microstructure measurements, (a) Central area top view, (b) depth measurements of laser dots in the central area, (c) density distribution, (d) Middle region top view, (e) depth measurements of laser dots in the middle region, (f) density distribution, (g) Edge region top view, (h) depth measurements of laser dots in the edge region, (i) density distribution, (j) measurements of the LGMS lens microstructure.
The parameters of the LGMS lens obtained from simulation and measurement are presented in Table 1.
Figure 7(a) presents the simulated (blue line) and measured (red line) light distribution curves. The simulation and measurement results are highly consistent. The measurements of the optical properties of the LS film; its scattering intensity distribution is displayed in Fig. 7(b). The bidirectional scattering distribution function (BSDF) is presented in Fig. 7(c); the blue line is the bidirectional reflectance distribution function (BRDF), and the red line is the bidirectional transmittance distribution function (BTDF). Fig. 7(d) presents a sample of the produced LS film.
Fig. 7. (a) simulated and measured light distribution of LGMS lens unit, (b) measurements scattering intensity of optical properties of LS film, (c) BSDF plot with BRDF and BTDF, and (d) LS film sample.
In order to reduce the number of mini-LEDs and maintain high UMF, an ultra-thin, large-area and high- luminance flat light source module is designed. Fig. 8(a) and Fig. 8(b) show the ZOD EL mini-LED combined with LGMS lens and LS film to design a 16-inch prototype. The length, width, and total thickness of the backlight module were 344.68, 215.42, and 1.669 mm, respectively. The thickness of the LCD module was 2.319 mm. The pitch of the EL mini-LEDs, OD, and UMF were 8.6, 0.849, and 10.13 mm, respectively.
Fig. 8. Prototype of 16-in ZOD EL Mini-LEDs backlight module, (a) front view, (b) Unequal views, (c) light polar diagram measurement results, (d) simulated and measured light distribution, (e) diagram of 81-point uniformity measurement for prototype module.
Figure 8(c) and Fig. 8(d) presents the measurement results for the prototype module. Fig. 8(c) presents the light polar diagram; the light-emitting angle FWHM was 52.3°. Fig. 8(d) displays the measured and simulated light distribution, which are similar. Fig. 8(e) presents the 81-point uniformity measurement for the prototype module. Red points indicate measurement points, and the 9 × 9 array is evenly distributed on the flat light source module. W0 is the width, and L0 is the length. The average value was calculated from 90% of the area (0.9W0 – 0.9L0) by excluding the outermost periphery of the module within the selected detection range.
The 81-point uniformity calculation method is given by Eq. (7).
The 81-point uniformity measurement was performed with an input voltage, total input current, and total input power of 6 V, 3.75 mA, and 22.5 W, respectively. BLUs of the average luminance, center luminance, and uniformity were 18,836 and 19,521 nits and 85.3%, respectively.
Figure 9 illustrates a comparison of the design proposed in this article with traditional edge-lit mini-LED BLU in terms of light leakage under black screen conditions. The length, width, and thickness of the liquid crystal display module were 351.87, 225.75, and 1.709 mm, respectively. Contrast analysis is also an important parameter of NB LCD. Fig. 9(a) presents an LCD with a traditional edge-lit light guide plate. For an input voltage, input current, and total power of 12.8 V, 2.15 A, and 27.52 W, respectively, LCD of the minimum luminance value was 0.5 nits, the maximum luminance value was 1200 nits, and the contrast ratio was 2400:1. Fig. 9(b) presents an LCD display produced using the ZOD EL Mini-LEDs and LGMS lens module. For an input voltage, input current, and total power of 6 V, 3.75 A, and 22.5 W, LCD of the minimum luminance value was 0.02 nits, the maximum luminance value was 1200 nits, and the contrast ratio was 60,000:1. Fig. 9(c) presents the halo distribution curve for the black and white areas. The width of the halo at half the relative intensity was 9 and 13 mm for the proposed and conventional displays, respectively. ZOD EL Mini-LEDs and LGMS lens for NB LCD module effectively reduced the halo and substantially increased contrast.
Fig. 9. Prototype of 16-in ZOD EL Mini-LEDs module for NB LCD, (a) LCD with conventional edge-lit light guide plate, (b) LCD with ZOD EL Mini-LEDs and LGMS lens module, (c) Halo distribution curve for black and white areas.
4. Conclusions
We proposed an optimized design in which an LGMS lens is used with EL mini-LED light sources to create a Zero-Optical-Distance, high-luminance module with a large area. A circle prism microstructure on the top of the LGMS lens was combined with a laser dot microstructure on the bottom to increase the light-emitting area. A 16-inch prototype was produced, and the thickness was only 1.709 mm; the pitch of the EL mini-LEDs was 8.6 mm, the average backlight module luminance was 18,836 nits at an input power of 22.5 watts, the LCD luminance was 1200 nits, the uniformity was 85%, the uniformity evaluation function reached 10.13, and the contrast ratio was excellent at 60,000:1. The proposed design has excellent potential for use in applications displays that require high contrast, thinness, luminance, and color saturation, such as automotive monitors and gaming notebook computers.
Funding
National Science and Technology Council (112-2622-E-194-004).
Acknowledgments
This research was supported by the Ministry of Science and Technology, The Republic of China under the Grants NSTC 112-2622-E-194-004. The authors would like to thank department of R&D division of Darwin corporation for the measurement support.
Disclosures
The authors have no conflicts to disclose.
Authorship contribution statement. Zhi Ting Ye and Chia Chun Hu are responsible for the structure and conception of the entire article. Zhi Ting Ye, Chia Chun Hu and Yen Lung Chen are responsible for the simulation data of the article. Zhi Ting Ye and Chang Che Chiu are responsible for the initial writing of the manuscript.
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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