Passive anti-sunlight glare traffic signs with non-axisymmetric freeform lens design.

This paper proposes a passive optical brightening element design, a non-axisymmetric freeform lens (NAFL), arranged and assembled on a traditional traffic sign. NAFL is the first optical design which can effectively solve the traffic problem that direct sunlight affects the driver's inability to look directly at the traffic sign. The NAFL can converge the sunlight behind the traffic sign and diverge forward to 150 meters away. In this way, the NAFL array combinations on the traffic sign can directly rely on sunlight as image information pixels. According to the simulation, the optical efficiency of the NAFL can be as high as 81.5%. Besides, the angular tolerance is also analyzed to evaluate the working hours of the NAFL. Finally, we made the prototype and proved that such passive brightening components could effectively improve the traffic sign's visibility in harsh sunlight.

Single reflector design for integrated low/high beam meeting multiple regulations with light field management.

This paper proposed an effective multi-objective design procedure, called light field management, for a single multi-segment reflector that can simultaneously project low beams and high beams for bicycle and e-bike applications. Furthermore, two different regulations can be met, including the K-mark and the ECE Class B regulations. Through light field management, the etendue and flux density of each segment can be effectively managed, so that the design successfully meets the multiple regulations. In the experimental verification, two mockup samples including a plastic reflector with aluminum coating and an aluminum reflector were fabricated to verify the validity of the design. The experiment showed that the contrast across the cutoff line reached 100 and above, where the brightest point for low beams reached 200 lux and the whole pattern reached 250 lux. The supreme behavior of the head lamp shows that the proposed design procedure is valuable and helpful to an optical designer.

Combining near-field hyperspectral imaging and far-field spectral-angular distribution to develop mid-field white LED optical models with spatial color deviation.

The integration of spatial distribution of light intensity and color in the midfield is instrumental for LED optical design. On the basis of this rationale, we proposed an accurate and convenient method for developing white LED optical models. Near-field hyperspectral images and far-field spectral-angular distributions were integrated to illustrate changes in spatial light intensity and color distribution in the mid-field, to the exclusion of the absorption, conversion, and scattering of phosphors. The corresponding optical models were developed for three LED samples under different packaging conditions. Their normalized cross-correlation values for spatial light intensity and correlated-color-temperature distribution between simulation and measurement averaged as high as 0.995 and 0.99 respectively, which validated the accuracy and feasibility of the proposed method.

Ideal luminous efficacy and color spatial uniformity of package-free LED based on a packaging phosphor-coated geometry.

This paper presents the optical simulation of the luminous efficacy of radiation (LER) and color spatial uniformity (CSU) of a package-free white LED conducted to boost the LER and simultaneously improve the CSU. According to the simulation results, the main effect on the LER and CSU was the change in the geometrical ratio of phosphor coating. Regardless of the packaging size, when the ratio of the top coating to the sidewall coverage of the phosphor layer thickness was in the range from 0.9 to 1.1, the maximum LER and optimal CSU can be simultaneously obtained. Besides, effectively increasing the volume of the overall packaging dimension to be 30 times the size of the chip can enable a package-free LED to achieve 90% saturated maximum LER without affecting the CSU.

Miniaturized LED primary optics design used for short-distance color mixing.

Color-tunable LED light fixtures generally change colors by controlling LEDs of multiple colors. This type of light source requires additional secondary optics and light-mixing distances to deliver color-mixing functions and perform high color uniformity. However, the color-mixing elements increase the optics size, resulting in more difficulties in making tiny lighting fixtures. Therefore, in this study, we introduce a LED primary optics design method that retains standard LED package size while featuring a color-mixing chamber. This method combines a lens having a rotational symmetry with a freeform profile and a zigzag structure by using double total internal reflection to disperse light uniformly. In contrast to a typical hemispherical lens, our design effectively lowers the weighted average color difference from 0.03 to 0.0035, and maintains optical efficiency of at least 90% without using any optical diffuser.

Efficient speckle-suppressed white light source by micro-vibrated and color-mixing techniques for lighting applications.

In this paper, we have demonstrated an efficient speckle-suppressed white light source generation when a blue laser diode illuminates on a micro-vibrated phosphor paper. Both micro-vibrated and color-mixing techniques are used in this system. With only micro-vibrated technique, the speckle contrast can be reduced from about 50% to 7.4% for the scattering blue image for a 16-ms integration time. Together with color-mixing technique, mixing speckle contrast is defined for laser diode pumped phosphor and almost speckle-free result is achieved. For color temperature lower than 5000 K, almost speckle-free mixed white can be obtained even without vibration technique.

Effect of chip spacing on light extraction for light-emitting diode array.

GaN LED array in packaging is important for the demand of high optical flux. In order to tighten the whole packaging size, the spacing among LED chips becomes an important factor in the packaging design. This study is to investigate the change of the light extraction when a GaN LED chip array packaging is applied. The shielding effect with various spacing for the GaN LED array with or without silicon encapsulation is obtained. We apply the Monte Carlo ray-tracing method in the simulations to analyze the optical behavior of the two major types of the GaN LED array. The shielding effect is more dominant for bare chip packaging. When a silicone thin dispensing layer is applied, the shielding effect is not obvious because of more light extraction, the neighbor dies play an important role in photon recycling.

Color gamut characteristics of diffractive-light guides of near-eye augmented reality glasses.

We delve into the distinctive color gamut characteristics resulting from color dispersion of surface relief grating (SRG) and wavelength degeneracy of volume holographic optical element (VHOE) in a diffractive light guide. While a laser-like spectrum achieves an impressive 194% sRGB color gamut for both cases, it proves unsuitable for VHOE light guides due to limitations in breaking the field of view (FOV) of the display. Conversely, a broad-band light source, such as LEDs, offers continuous FOV but reduces the common color gamut to 50% sRGB. We then present a newly designed VHOE light guide capable of achieving the common color gamut of 130% sRGB using two multiplexed holograms of each color, closely matching the 133% sRGB achieved by an SRG light guide. This article presents the first theoretical methodology to elucidate color performance of diffractive light guides utilizing VHOEs with holographic multiplexing, affirming their suitability for crafting high-quality near-eye display.

Design of a high-efficiency collection structure for daylight illumination applications.

In developing a high-quality natural light illumination system (NLIS), the primary considerations include how to increase system efficiency and broaden its applications. This paper describes the conception, design, and analysis of a daylight collector that presents the combined advantages of excellent efficiency and a compact size. The collector structure consists of extendable two-channel collecting units, a planar light guide, and a central coupler to improve light collection efficiency and increase surface area. In this study, two types of daylight collectors are proposed for illumination applications with different light patterns. With these collectors, the NLIS can now provide sufficiently powerful light for indoor illumination.

Hyperspectral screen-image-synthesis meter with scattering-noise suppression.

The screen image synthesis (SIS) meter was originally proposed as a high-speed measurement tool, which fused the measured data from multiple sample-rotational angles to produce a whole-field measurement result. However, it suffered from stray light noise and lacked the capability of spectrum measurement. In this study, we propose an SIS system embedded with a snapshot hyperspectral technology, which was based on a dispersion image of the sparse sampling screen (SSS). When a photo was captured, it was transformed and calibrated to hyperspectral data at a specific sample-rotational angle. After the hyperspectral data in all sample-rotational angles were captured, an SIS image-fusion process was then applied to get the whole field hyperspectral data. By applying SSS to the SIS meter, we not only create a screen image synthesis hyperspectral meter but also effectively address the issue of stray-light noise. In the experiment, we analyze its correctness by comparing the hyperspectral value with a one-dimensional spectrum goniometer (ODSG). We also show the 2D color temperature coefficient distribution and compare it with the ODSG. Experimental results also demonstrate the feasibility in terms of both spectrum distribution meter and color coefficient temperature distribution meter.

Active thermal-fuse for stopping blue light leakage of white light-emitting diodes driven by constant current.

In this study, we proposed and demonstrated a circuit design for solving problems related to blue light leakage (e.g., eye damage) when phosphor-converted white light-emitting diodes (pcW-LEDs) overheat. This circuit only needs a positive thermal coefficient thermistor, resistor, and diodes in series and parallel; thus, it can easily be integrated into components. Simulations and corresponding experimental results show that this method can accurately suppress the overheating component's injection current and allow for LEDs to work normally after returning to the operating temperature. It thus allows the user's eyes to be actively protected, e.g., to avoid exposure to the bluish light when overheating occurs. In addition, the quenching of luminous flux is a signal to remind the user to replace the LED. The proposed method is low-cost, effective, simple, and useful for increasing the quality of LED lighting and biological safety.

GaN-based mini-LED matrix applied to multi-functional forward lighting.

In this paper, we propose and demonstrate to use of a single reflector with 68 segments to project vehicle low beam and high beam with the use of a GaN-based mini-LED matrix, which is a 5 × 6 LED die array. The design of the reflector is based on light field technology in considering etendue from the light source across the segments. The group of the segments with smaller etendue from the LED dies in the bottom 2 rows are used to project low beams. When the other LED dies are turned on, the reflector will project light upward and form the high beam. The selection of the turn-on LED dies in the mini-LED matrix can adjust the width of the illumination pattern so that an adaptive low/high beam can be performed. Besides, to extend the functionality of the headlamp module, we propose to dispense IR phosphor on LED dies in the high-beam zone of the GaN-based mini-LED matrix. Thus the vehicle can emit IR high beam, which can be imaged through a camera and can be incorporated with machine vision for an autonomous vehicle without using a complicated adaptive headlight to avoid glare. The proposed multi-function in spatial and spectral domains will be helpful to various applications with use of a mini-LED matrix.

Precise optical modeling for silicate-based white LEDs.

In this paper, as to our best knowledge, we propose and demonstrate the first precise phosphor modeling scheme to simulate the chromatic performance of white LEDs with silicate phosphors. The phosphor model is useful to accurately simulate the power ratio of the blue and yellow lights emitted by the white LEDs and is important in white LED package.

Light extraction analysis of GaN-based light-emitting diodes with surface texture and/or patterned substrate.

Light extraction analysis of GaN-based light-emitting diodes (LEDs) with Monte Carlo ray tracing is presented. To obtain high light extraction efficiency, periodic structures introduced on the top surface and/or on the substrate of various types of LED are simulated, including wire bonding, flip chip and Thin GaN. Micro pyramid array with an apex angle from 20o to 70o is shown to effectively improve the light extraction efficiency. In addition, for an LED encapsulated within an epoxy lens, the patterned substrate with pyramid array is found to be a more effective way to increase light extraction efficiency than the surface texture.

Analysis of position-dependent light extraction of GaN-based LEDs.

The light extraction efficiency of GaN-based LEDs as a function of the position of the light source over the active layer is studied. Several parameters, including chip dimensions, absorption coefficients and package are analyzed on the basis of a Monte-Carlo ray tracing simulation. The light extraction efficiency of a Thin-GaN LED is studied with respect to a sapphire-based LED, including the surface texture.

Precise optical modeling for LED lighting verified by cross correlation in the midfield region.

A novel LED modeling algorithm for precise three-dimensional light pattern simulation is proposed and demonstrated. We propose to use normalized cross correlation to verify the validity of the simulation in one-dimensional intensity patterns as well as two-dimensional irradiance patterns in various midfield distances and to provide feedback to achieve a successful model. The model is demonstrated to obtain an average of 99% in normalized cross correlation between the simulation light pattern and experimental measurement for a truncated inverse pyramid LED.