Wide angle mini-LEDs combined with multifocal micro reflector cavity for thin portable device flashlight

Abstract

LEDs employed in portable device flashes are Lambertian light sources, additional optical components such as Fresnel lenses and reflector cups must be incorporated to adjust the light distribution. This results in a need for a certain module thickness or high alignment precision. This paper introduces a modification in the packaging structure of mini-LEDs by employing a first optical design to create wide-angle mini-LEDs (WA mini-LEDs) with a light emission angle of 180°. A multi-focal micro-reflector cavity (MF-MRC) is included in the packaging process, thus enabling the achievement of a slim flash module. For the prototype design, measurements were conducted using WA mini-LEDs combined with a 2 mm height MF-MRC. The full width at half maximum can be reduced to 56° while maintaining a uniformity of up to 60% across all measurement points. Additionally, the center illuminance is 3.24 times higher compared to Lambertian mini-LED light sources.

Keywords:

• Wide angle mini-LEDs
• flashlight
• first optical design
• multi-focal
• micro-reflector cavity
• full width at half maximum
• portable device

1. Introduction

With the continuous advancement of technology, mobile phones have transitioned from mere communication devices to portable and highly accessible smart devices.^[1,2] Therefore, there is a growing interest in smartphone development,^[3] and the user base of smartphones has been steadily increasing each year.^[4–6] Smartphones offer several unique benefits such as their compact size and convenient portability,^[7–8] providing numerous features such as communication, entertainment, and photography to meet the consumer’s desire for convenience and practicality.^[9–11] As a result, the daily usage rate of mobile phones has been steadily increasing.^[12–14] Although modern phones can replace traditional cameras in terms of functionality,^[15] current smaller-sized smartphones face limitations in sensor size, thus requiring the use of auxiliary LED flash in low-light environments to capture adequately bright photos.^[16]

The development of light-emitting diodes (LEDs) has experienced rapid progress over the past decade.^[17] The benefits of utilizing Mini/Micro LED in the Internet of Things (IoT) and Virtual/Augmented Reality (VR/AR) applications include their compact size, energy efficiency, and rapid data transmission capabilities.^[18–19] LEDs offer various advantages such as flexibility and low power consumption^[20–21] and are therefore widely utilized in everyday applications such as displays and lighting.^[22] Mini-LEDs and micro-LEDs have smaller packaging sizes compared to traditional LEDs and are thus more suitable for compact and lightweight requirements. Consequently, mini-LEDs and micro-LEDs are commonly employed as backlight sources for LCD modules^[23–24] and applications in nanosensors, which can be combined with artificial intelligence and integrated packaging at the chip level.^[25–26] However, while micro-LEDs demonstrate excellent performance, challenges such as yield and mass transfer still lack satisfactory solutions.^[27–28] Due to their Lambertian light distribution, LEDs are not suitable for direct application in areas like flashlights and spotlights, where targeted light focusing and specific light distributions are required.^[29] This narrow FWHM application also enables light to reach longer distances.^[30] To achieve this narrow beam application, additional first and second optical designs are necessary to adjust the light distribution according to specific requirements.^[31]

Several studies have sought to modify the light distribution of LEDs by incorporating additional first and second optical components. In the first optical design, researchers enhanced the optical performance of LEDs at small angles by modifying their packaging structure.^[32–34] The second optical design represents another primary approach for achieving narrow-beam LEDs.^[35] Some scholars have successfully achieved a narrow FWHM light distribution by utilizing Fresnel lenses to modify LED light distribution.^[36–38] Other scholars have applied Total Internal Reflection (TIR) principles to design lens curvatures, thus effectively controlling the emission angle of LEDs to achieve the desired light distribution while maintaining excellent optical performance.^[39–42] Several studies have also incorporated reflector cavities around LEDs to collimate the light and generate uniform parallel beams.^[43–46] The design utilizing these second optical components to adjust light distribution requires high alignment accuracy with the LED light source, which directly impacts the module’s assembly yield.

Numerous studies have focused on modifying LED light distribution through first and second optical designs. However, the second lenses themselves possess larger volumes, requiring additional space when implemented in light source modules. The first optical design also requires altering the light path by changing the structure. Nonetheless, very few studies have evaluated the feasibility of directly incorporating a reflective cavity into the one-time package.

Hence, this paper proposes a method of altering the packaging structure through a first optical design during the Mini-LED packaging process to achieve a wide-angle light distribution of 180 degrees. Additionally, a multi-focal micro-reflector cavity design is integrated into the packaging process to further modify the light distribution. With a full width at half maximum (FWHM) of 56°, the center illuminance is 3.24 times higher than that of a Lambertian light distribution LED, and the uniformity surpasses 60%. This paper presents a revision to the packaging configuration of mini-LEDs, utilizing an initial optical concept to develop wide-angle mini-LEDs (referred to as WA mini-LEDs) with a light emission angle of 180°. The packaging procedure incorporates a multi-focal micro-reflector cavity (MF-MRC), facilitating the creation of a compact flashlight module for flashlights in portable devices. Compared to traditional LED assembly with secondary optics, it eliminates alignment issues during assembly and reduces the thickness of the optical film assembly.

2. Methods

This paper introduces the utilization of wide-angle mini-LEDs (WA mini-LEDs) in combination with a multi-focal micro-reflector cavity (MF-MRC) design during the manufacturing process. Figure 1 presents the normalized light distribution pattern and light path diagrams of conventional mini-LEDs and WA mini-LEDs. For conventional mini-LEDs with a 120° emission angle, the light path diagram reveals that the reflector cavity requires a height of H₁ to collimate the emitted light. Due to the light field distribution, the reflector cavity needs to maintain a specific height. On the other hand, WA mini-LEDs adjust the light transmission and reflection ratio above the LEDs and guide the light sideways through a light guide layer, creating a butterfly-wing-like light field. In the light path diagram, the reflector cavity for WA mini-LEDs requires a height of H₂. Comparing the required heights of the reflector cavities for both types, it can be observed that the use of WA mini-LEDs results in a lower reflector cavity height (H₁ > H₂). Allowing the light to reach the reflector cavity at a lower position effectively reduces the required height, resulting in a thinner module.

Figure 1. Normalized light distribution curve and light trace diagram of traditional mini-LEDs and WA mini-LEDs.

We first simulated and analyzed the light distribution of LEDs using Lambert’s Cosine Law to determine which light distribution is more suitable for use in a smartphone flashlight source. Lambert’s cosine law can be used to simulate the light distribution of LEDs with various FWHMs, as shown in Equation (1)(1) $$\mathit{I}={{\mathit{I}}_0(\mathbf{\text{cos}}\mathbf{\theta })}^{\mathit{n}}$$(1) : (1) $$\mathit{I}={{\mathit{I}}_0(\mathbf{\text{cos}}\mathbf{\theta })}^{\mathit{n}}$$(1) where I represents the light intensity, I₀ is the light intensity at 0 degrees, and θ is the light emission angle. By adjusting the value of n, LEDs with different FWHMs can achieve varying FWHM values. This equation allows for obtaining the desired light distribution suitable for a flashlight source, as shown in Figure 2.

Figure 2. Light intensity distribution of mini-LEDs at different beam angles.

Next, we using DIALux evo 11.1 simulation software to analyze the light distribution of different FWHM light field patterns for flashlights. Simulations were conducted using the aforementioned light distributions for each angle from Equation (1)(1) $$\mathit{I}={{\mathit{I}}_0(\mathbf{\text{cos}}\mathbf{\theta })}^{\mathit{n}}$$(1) to analyse their uniformity and central illuminance. This analysis aids in determining the optimal emission angle for designing the reflector cavity. The detector is positioned at a distance of 1 m from the light source, with a detector area measuring 1 square meter. The simulated results of the illuminance distribution for each emission angle are depicted in Figures 3(a)–(h).

Figure 3. Illuminance distribution of mini-LEDs at a FWHM of (a) 50°, (b) 54°, (c) 56°, (d) 58°, (e) 60°, (f) 68°, (g) 70°, and (h) 120°.

The formula for calculating uniformity is shown in Equation (2)(2) $$\mathrm{\text{Uniformity}}\left(\mathrm{\%}\right)=100\mathrm{\%}*\frac{\mathbf{\text{Minimum}}\mathrm{ }\mathbf{\text{illuminance}}\left(\mathbf{\text{lux}}\right)}{\mathbf{\text{Maximum}}\mathrm{ }\mathbf{\text{illuminance}}\left(\mathbf{\text{lux}}\right)}\mathrm{ }$$(2) . (2) $$\mathrm{\text{Uniformity}}\left(\mathrm{\%}\right)=100\mathrm{\%}*\frac{\mathbf{\text{Minimum}}\mathrm{ }\mathbf{\text{illuminance}}\left(\mathbf{\text{lux}}\right)}{\mathbf{\text{Maximum}}\mathrm{ }\mathbf{\text{illuminance}}\left(\mathbf{\text{lux}}\right)}\mathrm{ }$$(2)

Based on the simulation results, as the emission angle of the module increases, the uniformity improves while the central illuminance decreases. To ensure a uniform light output for mobile phone flash applications, it is crucial to maintain an illuminance uniformity of over 60%. WA mini-LEDs with a 180° emission angle are employed to reduce the height of the multi-focal micro-reflector cavity. DIALux evo simulations were conducted to determine the optimal parameters. The results showed that, with a FWHM of 28° and 56°, the uniformity remained above 60% while achieving a central illuminance 3.24 times higher than that of the 120° FWHM. Next, the design of the MF-MRC was carried out. To ensure that the emitted light could be collimated through the reflector cavity and reduce the FWHM of the LEDs, we designed a multi-focal point reflector cavity with three sections of different curvatures. By controlling the emission angle using different focal point reflector cavities, an approximate FWHM of 56° was achieved. By directing the WA mini-LEDs through reflector cavities of varying heights and focal point curvatures, the original butterfly-shaped light distribution can be collimated, resulting in the adjustment and reduction in the FWHM. Table 1 summarises the uniformity for each FWHM, given an input luminous flux of 100 lumens.

Table 1. Illuminance and uniformity of mini-LEDs with different FWHMs.

FWHM (degree)	Input luminous flux (lm)	Center illuminance (lux)	Average illuminance (lux)	Min illuminance (lux)	Max illuminance (lux)	Uniformity (%)
50	100	101	72.5	53.5	95.7	56%
54	100	90.6	65.3	49.7	83.3	59%
56	100	84.4	62.5	48	78.8	61%
60	100	73.4	56.5	44.4	70.9	63%
64	100	68	51.7	41.5	64.2	65%
68	100	58.9	47.5	38.1	57.5	66%
70	100	57.2	45.5	36.7	54.3	67%
120	100	26	22.2	19.3	25.2	77%

LED is a non-ideal point light source, belonging to a type of surface light source. Therefore, this article is designed using a multifocal micro-reflector cavity. Figure 4 is used to illustrate the design concept of WA mini-LEDs combined with MF-MRC. In Figure 4(a) Schematic diagram of the multi-focus reflective cavity; Figure 4(b) schematic diagram of WA mini-LEDs combined with MF-MRC. The normalized light distribution curves are illustrated in Figure 4(c). The red curve represents the normalized light distribution curve of the MF-MRC with a 2 mm height, yielding an FWHM of 28° and 56°. Conversely, the black curve represents the normalized light distribution curve of the MF-MRC with a 2.5 mm height, exhibiting an FWHM of 22° and 44°. Notably, the design with a reflector cavity height of 2.0 mm achieves both uniformity and optimal central illuminance simultaneously.

Figure 4. Illustrate the design concept of WA mini-LEDs combined with MF-MRC. (a) Schematic diagram of the multi-focus reflective cavity, (b) schematic diagram of WA mini-LEDs combined with MF-MRC, (c) normalized light distribution curve of WA mini-LEDs combined with different height reflective cavities.

3. Results and discussion

An explanation of Figure 5 illustrates the differences between traditional mini-LEDs and WA mini-LEDs. Figures 5(a)–(c) respectively represent 2D ray tracing, 3D polar distribution, and Irradiance plot. It is evident that the emission from Traditional mini-LEDs is concentrated in forward emission, requiring a higher reflector cup height to adjust the light field pattern. On the other hand, WA mini-LEDs exhibit lower forward emission energy and stronger lateral emission energy. This characteristic contributes to reducing the height of the reflector cup, resulting in a thin and light multi-focal micro-reflector cavity. To achieve a 180° wide-angle design, WA mini-LEDs are manufactured by bonding to an aluminum nitride (AlN) substrate. The LED chip size is 0.508*0.508*0.15 mm. Firstly, white adhesive is applied around the chip to prevent blue light leakage from the sidewalls. Then, a fluorescent powder conversion layer with a thickness of 0.05 mm is formed on the top surface of the chip to convert the wavelength. The light guide layer, made of silicone with a thickness of 0.2 mm, is shaped above the fluorescent powder conversion layer to guide light for side emission. Finally, a diffuse reflection layer composed of silicone and TiO₂ with a thickness of 0.2 mm is stacked on top of the light guide layer to allow partial light diffusion and penetration, achieving wide-angle white light emission. Lastly, the LED chips are cut and separated to complete the production of wide-angle white light mini-LEDs.

Figure 5. Illustrates the differences between traditional mini-LEDs and WA mini-LEDs. (a) 2D ray tracing, (b) 3D polar distribution, and (c) Irradiance plot.

Traditional mini-LEDs WA mini-LEDs

Figure 6 illustrates the fabrication process of WA mini-LEDs combined with a multi-focal micro-reflector cavity. Firstly, the MF-MRC is positioned at the center of the mini-LED light source, and silicone is used to bond it with the mini-LEDs. After baking, the components are fixed, and the module is then separated by cutting, resulting in a slim mobile phone flash module. The configuration parameters of each layer of the fluorescent powder conversion layer (FPCL), light guide layer (LGL), diffuse reflection layer (DRL), and sidewalls. The FPCL is a mixture of Dow Corning OE-7662(100g)/YAG-Y959(15g)/YAG-Y468(15g)/R202 Nano-silica (1.3 g), the powder to gel ratio is 30: 100, the LGL is Dow Corning OE-7662 transparent glue, the DRL is Dow Corning OE-7662/titanium dioxide (TiO₂:25%), the reflectivity of 90%. The sidewall is Dow Corning OE-7662/(TiO₂: 30%), the reflectivity of 92%.

Figure 6. Manufacturing process of WA mini-LEDs combined with thin multi-focal micro-reflector cavity.

The physical prototypes of the WA mini-LEDs developed in this study, combined with MF-MRC measuring 2 mm and 2.5 mm in height, are depicted in Figure 7(a,b), respectively. The emission angles of the prototypes were measured, and their corresponding light distribution curves are presented in Figure 7(c). The red curve represents the normalized light distribution curve obtained with a 2 mm reflector cavity, displaying an FWHM of 28° and 56°. Conversely, the black curve represents the normalized light distribution curve achieved with a 2.5 mm reflector cavity, showing an FWHM of 23° and 46°. To assess the uniformity of the physical prototypes, a five-point measurement was conducted, as illustrated in Figure 7(d). The measurement was carried out on a plane measuring 1 m × 1 m and at a distance of 1 m from the light source. The illuminance uniformity was evaluated at the four corners and the center.

Figure 7. Image of the actual sample with a reflective cavity height of (a) 2 mm and (b) 2.5 mm; (c) normalized light distribution curve of WA mini-LEDs combined with actual samples with different reflective cavity heights; (d) schematic diagram of five-point uniformity measurement.

The measurement results were compared with the traditional design using Fresnel lenses, as shown in Table 2. Among the measurement points, the WA mini-LEDs designed in this study, combined with reflector cavities with heights of 2.0 mm and 2.5 mm, exhibited better illuminance uniformity than the results obtained using Fresnel lenses from other companies. The proposed design achieved a uniformity of 60% or above at all measurement points. The module exhibited the best uniformity when the reflector cavity height was 2.0 mm.

Table 2. Comparison of the uniformity of different designs of flashlight modules.

The illuminance value at point P1-5	LED With Fresnel lens(lux)	Height of MF-MRC 2.0 mm (lux)	Height of MF-MRC 2.5 mm (lux)
P1	9.5	9.8	10.2
P2	5.5	6.4	6.2
P3	5.6	6.3	6.3
P4	5.8	6.3	6.1
P5	5.8	6.4	6.1

This study proposes the first optical design to create WA mini-LEDs with an FWHM of 180° by modifying their packaging structure. The packaging process incorporates an MF-MRC, resulting in a slim and compact flashlight module. The prototype design combines WA mini-LEDs with a reflector cavity height of 2 mm, achieving a reduced FWHM of 56°. The uniformity at all measurement points exceeds 60%, and the center illuminance is 3.24 times higher than that of Lambertian LEDs. The objective of this design is to achieve a slim, uniform, and portable device flashlight module.

4. Conclusions

This article proposes the design of wide-angle mini-LEDs combined with a multifocal micro-reflector cavity for a thin portable device flashlight. WA mini-LEDs with a 180° FWHM are designed to reduce the height of the micro-reflector cavity. By combining them with an MF-MRC of 2 mm in height, the light emission pattern is adjusted to a 56° FWHM. Measurements conducted at a distance of 1 m show uniformity exceeding 60% at all points, with the center illuminance being 3.24 times higher than that of Lambertian LEDs. The contribution of this article is to eliminate alignment issues during assembly and reduce the thickness of the optical film assembly. Through the first optical design, the packaging structure of the mini-LEDs is modified, and an MF-MRC is integrated during the packaging process, allowing for the adjustment of the light emission pattern and device miniaturization. This design enables the fabrication of a slim, high uniformity, and portable device flashlight module, and also serves as a supplementary light source for photography and illumination purposes.

Consent form

All authors consent to the publication of this manuscript.

Authors contributions

Zhi Ting Ye and Chia Chun Hu are responsible for the structure and conception of the entire article. Zhi Ting Ye and Chia Chun Hu is responsible for the simulation data of the article. Zhi Ting Ye and Chang Che Chiu is responsible for the initial writing of the manuscript.

Nomenclature
LEDs	=	light-emitting diodes
AR	=	Augmented Reality (VR/AR)
AlN	=	aluminum nitride
VR	=	Virtual Reality
WA mini-LEDs	=	wide-angle mini-LEDs
IoT	=	Internet of Things
MF-MRC	=	multi-focal micro-reflector cavity
FWHM	=	full width at half maximum

Disclosure statement

The authors declare that they have no competing interests.

Data availability statement

The data presented in this study are available on request from the all authors.

Additional information

Funding

This work was supported by the National Science and Technology Council of Taiwan (NTSC 112-2622-E-194-004).