Xuan-Hao Lee

High-performance LED street lighting using microlens arrays

An efficient LED lamp that illuminates the street with high quality is presented. The luminaire shows high optical efficiency, high optical utilization factor, low glare, and illuminates the street with high uniformity. The concept is simple but effective: a cluster of LEDs with TIR lenses are put inside a reflective box, which is covered with a microlens sheet; the reflective cavity improves efficiency by light recycling; each TIR lens collimates the LED light for the microlens array; and the microlens sheet uniformly distributes light only into the street. We verify its feasibility by Monte Carlo ray-tracing for the main types of road lighting arrangements: central, zigzag, and single-side pole positions.

Calculating model of light transmission efficiency of diffusers attached to a lighting cavity

A lighting cavity is a reflecting box with light sources inside. Its exit side is covered with a diffuser plate to mix and distribute light, which addresses a key issue of luminaires, display backlights, and other illumination systems. We derive a simple but precise formula for the optical efficiency of diffuser plates attached to a light cavity. We overcome the complexity of the scattering theory and the difficulty of the multiple calculations involved, by carrying out the calculation with a single ray of light that statistically represents all the scattered rays. We constructed and tested several optical cavities using light-emitting diodes, bulk-scattering diffusers, white scatter sheets, and silver coatings. All measurements are in good agreement with predictions from our optical model.

Bidirectional scattering distribution function by screen imaging synthesis.

Light-emitting diodes are common light sources in modern lighting. The optical distribution of an LED package and the bidirectional scattering distribution function (BSDF) of diffusing optical components are important factors in lighting design. This paper proposes an innovative method of measuring both the optical distribution of LEDs and BSDF quickly. The proposed method uses a 2-D screen and a camera to capture the illumination on a screen, and acquires the whole-field optical distribution by synthesizing the images on the screen in different angles. This paper presents theoretical calculations and experimental results demonstrating the construction of the BSDF.

Surface-structured diffuser by iterative down-size molding with glass sintering technology

In this paper, a down-size sintering scheme for making high-performance diffusers with micro structure to perform beam shaping is presented and demonstrated. By using down-size sintering method, a surface-structure film is designed and fabricated to verify the feasibility of the sintering technology, in which up to 1/8 dimension reduction has been achieved. Besides, a special impressing technology has been applied to fabricate diffuser film with various materials and the transmission efficiency is as high as 85% and above. By introducing the diffuser into possible lighting applications, the diffusers have been shown high performance in glare reduction, beam shaping and energy saving.

Noncontact and instant detection of phosphor temperature in phosphor-converted white LEDs

Phosphor-converted white light-emitting diodes (pc-WLEDs) have become a major light source in general lighting. To stabilize the photometric characteristics of pc-WLEDs, much effort has been made to manage the heat dissipation of the LED dies. The thermal problems of the phosphor parts, a critical reliability concern for pc-WLEDs, have recently attracted academic interest. This study proposed a practical approach for measuring phosphor temperature in an operating pc-WLED using a noncontact, instant detection method to remotely monitor the emission spectrum. Conventionally, an infrared camera or thermocouples have been used to measure temperature. An IR camera requires good calibration on the emissivity and is usually blocked by the lens or other components covered on the phosphors. Moreover, a thermocouple requires time to reach the thermal equivalence between the detector and the sample under testing, and this approach is destructive when used for inner detection. Our approach has advantages over the conventional methods because it is noninvasive, noncontact, and instant, and inner detection. The approach is also independent of the peak wavelength of pumping lights, the concentration and thickness of phosphor, and correlated color temperatures.

Stabilizing CCT in pcW-LEDs by self-compensation between excitation efficiency and conversion efficiency of phosphors

A novel method to stabilize the correlated color temperature in pcW-LEDs from their initial turn-on state to thermal equilibrium is proposed and demonstrated. Under the normal operation condition, it can stabilize the CCT of a pcW-LED by the positive matching of the blue LED peak wavelength to the phosphor excitation spectrum. When the operating temperature unavoidably becomes higher in the LED die quickly after the initial turn-on, the phosphor conversion efficiency degrades and the LED blue light performs red shift. With the positive matching, the red shift actually helps enhance the excitation efficiency of the phosphor to compensate the thermal quenching and efficiency degradation. Therefore, the ratio of the blue light to the yellow light can keep almost constant, as well as the CCTs. In the experiments, the CCT variation could be as small as from 7 K to 83 K in different cases. Finally, we introduce a new factor, the so-called guide number, which is used to count the total change of the enhancement in equivalent excitation efficiency and the relative reduction of the phosphor light emission. The guide number essentially helps in designing the matching blue LED die and phosphor pair for good CCT stabilization.

Design of LED Street Lighting Adapted for Free-Form Roads

Street lighting, which is one of the main players in energy waste and light pollution at night, still faces the challenge of efficient lighting of roadways with curved and twisted shapes. To meet this challenge, we propose an effective and efficient adaptive light-emitting diode (LED) luminaire. This LED lamp delivers a roadway-shape light pattern, which maximizes illumination performance. The light is efficiently and homogeneously directed only where is needed; which reduces glare, and improves both the eye comfort and the visual discrimination ability of car drivers and pedestrians. The proposed luminaire is very practical in that it only requires to replace the cover plate, which is a special microlens array sheet, to produce different shape light patterns. The adaptive mechanism is simple and effective: LED light is first collimated and then efficiently distributed on a freeform roadway by the special microlens sheet. We present an extensive analysis of the lighting adaptability of the proposed luminaire by Monte Carlo ray tracing. In particular, we studied the effect of the main microlens structural parameters in the shape and size of the delivered illumination distribution on the roadway. We present a design example, a prototype construction, and an experimental confirmation on a scale street lighting system. Simulations and experimental results show the advantages of adaptive luminaires over the traditional nonadaptive approaches.

Optical design of LED marine beacons

Two optical designs for marine beacon based on Direct In-line Package (DIP) LED is proposed and demonstrated. The luminous intensity of the marine beacon using DIP LED can achieve to the IALA recommendation’s requirement of 5 nautical miles. The measurement of color coordinates can also fit the IALA recommendation’s requirement. By the surface-structured TIR lens, we successfully keep the divergence angle to 68 degrees in the horizontal direction and converge to 8 degrees in the vertical direction.

CCT stabilization and phosphor temperature detection in pc-WLEDs

A novel method to stabilize the correlated color temperature in pc-WLEDs from their initial turn-on state to thermal equilibrium is proposed and demonstrated. Under the normal operation condition, it can stabilize the CCT of a pc-WLED by the positive matching of the blue LED peak wavelength to the phosphor excitation spectrum. In the experiments, the CCT variation could be as small as from 7 K to 83 K in different cases. In addition, this study also proposed a practical approach for measuring phosphor temperature in an operating pc- WLED using a noncontact, instant detection method to remotely monitor the emission spectrum. The approach is also independent of the peak wavelength of pumping lights, the concentration and thickness of phosphor, and correlated color temperatures. Firstly, a novel method to stabilize the correlated color temperature in pc-WLEDs from their initial turn-on state to thermal equilibrium is proposed and demonstrated. In the experiments, the CCT variation could be as small as from 7 K to 83 K in different cases. Secondly, this study also proposed a practical approach for measuring phosphor temperature in an operating pc-WLED using a noncontact, instant detection method. The approach is also independent of the peak wavelength of pumping lights, the concentration and thickness of phosphor, and correlated color temperatures.

A study of optical modeling and evaluation of color rendering property of a dual-phosphor system(Conference Presentation)

In this thesis, on the basis of the phosphor optical models, green and red phosphor mixture optical model has been well established. Under some specific green to red phosphor doping proportions, this model can be utilized to simulate the chromatic properties, spatial CCT distributions, and packaging efficiency. There are some benefits of applying the phosphor optical model, one is that the confusion about mixture or layer phosphor configuration can perform better could be solved. Another is that the comparison and analysis of these phosphor configurations can be made not only in experiment but also in simulation, and will be more details to be discuss in the simulation. There are several types of packaging structures in high color quality applications. Consequently, the importance of phosphor optical model cannot be overestimated. After few steps above and with the help of experimental analysis and optimized in simulation, a packaging structure with high color quality and high efficiency has been approved. Finally, this light source with high performance will be utilized in the luminaire to improve the color and energy saving properties.