Yeh-Wei Yu

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.

One-shot and aberration-tolerable homodyne detection for holographic storage readout through double-frequency grating-based lateral shearing interferometry

A simple method to decode the stored phase signal of volume holographic data storage with adequate wave aberration tolerance is highly demanded. We proposed and demonstrated a one-shot scheme to decode a binary-phase encoding signal through double-frequency-grating based shearing interferometry (DFGSI). The lateral shearing amount is dependent on the focal length of the collimated lens and the frequency difference between the gratings. Diffracted waves with phase encoding were successfully decoded through experimentation. An optical model for the DFGSI was built to analyze phase-error induction and phase-difference control by shifting the double-frequency grating longitudinally and laterally, respectively. The optical model was demonstrated experimentally. Finally, a high aberration tolerance of the DFGSI was demonstrated using the optical model.

High-directional light source using photon recycling with a retro-reflective Dome incorporated with a textured LED die surface

This paper demonstrates a novel retro-reflective dome that enhances the directionality of a light emitting diode (LED) by recycling photons reflected by a textured LED die surface. A simulation model is developed to describe both the photon recycling process within the dome and the role of specific pyramid patterns on the top surface of the LED die. Advanced simulations showed that a perfectly polished surface with 100% reflectivity potentially enhances the directionality of the dome by 340%, 250%, and 240% using reflective domes with 10°, 20°, and 30° light cones, respectively. In the experiment, the directionality of the domes exhibiting surface imperfections is enhanced by approximately 160%, 150%, and 130% using 10°, 20°, and 30° light cones, respectively. By incorporating a textured top surface on the LED die, the proposed dome effectively increases the directionality of the LED light source.

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.

Analysis of a lens-array modulated coaxial holographic data storage system with considering recording dynamics of material

In the first time, a simulation model with considering the recording dynamics of material is built and is used to simulate evolution of the grating strength of the recorded hologram in a coaxial volume holographic memory system. In addition, phase modulation by lens array in the reference is introduced and observed to perform better diffracted signal quality and higher shifting selectivity, in both simulation and experiment. The use of lens array is found to provide multiple advantages in volume holographic memory system. The new simulation model potentially can be used to precisely design the system to obtain higher diffracted signal quality, higher shifting selectivity, and reduction of M# consumption and increase of storage capacity.

Study of collinear VHS: point spread function and shift selectivity

We summarize our theoretical study of the collinear volume holographic storage system. Simple formulas with direct physical concepts are developed under paraxial condition with scalar diffraction theory and VOHIL model, which are much helpful in figuring out the characteristics on point spread function (PSF) and the shift selectivity. Accordingly, effective system design is possible.

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.

Design of coded structured light based on square-shaped primitives

This paper presents a new pattern design for encoding structured light for scanning 3D surface and the decoding process. The basic unit of the pattern is a code-block that contains one central label and eight square-shaped primitives that provide two functions: code-block indexing and point-positions for triangulating. The square-shaped primitive has one notch in a different position to represent 8 digit numbers, which are easily to convert label to number for indexing codeblock. One primitive also contributes five point-positions, centroid and four extreme, for triangulating and estimating 3D information. The design of code-block has eight 8-bit numbers allowing a higher error-tolerant and providing better resolution in 3D reconstruction. Experiment shows that the proposed pattern is suitable for indoor mapping and large scene scanning.