Precise Optical Modeling Research Articles

Analysis of Factors Affecting Optical Performance of GaN-Based Micro-LEDs with Quantum Dots Films

Optical performance in terms of light efficiency, color crosstalk and ambient contrast ratio were analyzed for blue GaN-based micro-light emitting diodes (micro-LEDs) combined with red/green quantum dots (QDs)-polymethyl methacrylate (PMMA) films. The thickness and mass ratio of QDs films are two critical factors in affecting the performance of micro-LEDs. Firstly, the precise optical modeling of QDs-PMMA films is established based on the double integrating sphere (DIS) testing system and inverse adding doubling algorithm (IADA) theory. Red and green QDs-PMMA films are composed of ZnCdSe/ZnS QDs and green ZnCdSeS/ZnS QDs, respectively. The fundamental optical parameters of QDs-PMMA films, including scattering, absorption and anisotropy coefficients, are obtained successfully. Secondly, based on these optical parameters, the Monte Carlo ray tracing method is applied to analyze the effect of a QDs-PMMA film’s thickness and mass ratio on the optical performance of micro-LEDs. Results reveal that the light efficiency first increases and then decreases with the increase of a QDs film’s thickness or mass ratio, owing to the scattering characteristics of QDs. Different from the variation tendencies of light efficiency, the crosstalk between adjacent pixels increases as the QDs-PMMA film’s thickness or mass ratio increases, and the ambient contrast ratio is kept stable when the thickness increases. The mass ratio variation of QDs film can change the optical performance of micro-LEDs more effectively than thickness, which demonstrates that mass ratio is a more important factor affecting the optical performance of micro-LEDs.

Design of a Bicycle Head Lamp Using an Atypical White Light-Emitting Diode with Separate Dies

To the best of our knowledge, this is the first demonstration of a design for a bicycle head lamp with a high-contrast cutoff line using an atypical white light-emitting diode (LED) with two separate dies. The precise optical model was created by setting the weighting factor on the emitting surface. The downward reflector was designed and fabricated to produce a high-contrast cutoff line in both short- and long-axis orientations, but a yellowish outer pattern was observed. A modified two-color optical model was created to describe the yellowish patterns in both orientations and explain the yellowish effect. Such an effect was caused by the larger coverage area of the phosphor than that by the blue dies. To reduce the yellowish effect near the cutoff line, a specific phosphor area was blocked in the experiment. The yellowish effect was greatly reduced, and the contrast across the cutoff line was enhanced. The presented technology is useful for designing a high-contrast light pattern with such an atypical white LED.

Precise Optical Modeling of Phosphor-Converted LEDs With Arbitrary Concentration and Thickness Using Bidirectional Scattering Distribution Function

Phosphor-converted light-emitting diode (pcLED) is one of the most promising methods to generate white light. The interaction of the excitation light with the phosphor in pcLED is a complicated phenomenon, combining light absorption, light conversion, and scattering. An effective and efficient way to predict the optical behavior of pcLED is using the bidirectional scattering distribution function (BSDF). In this paper, the BSDF for pcLEDs with arbitrary phosphor concentration and thickness was established. The measured integrating energy and angular distributions of BSDF were successfully fitted by the interpolation method and the ABg model, respectively, realizing the calculation of BSDF values for arbitrary phosphor configurations. The proposed model was applied to a remote pcLED using an arbitrary phosphor configuration in Monte Carlo simulation. The output radiant power and the angular distributions of the blue and yellow light show good agreements between experiments and simulations; the errors of radiant power were all less than 4%, validating the effectiveness of the model. Consequently, this study provides a simple and useful tool to precisely predict the optical performance of pcLEDs with arbitrary concentration and thickness, it is essential for white LED design and fabrication.

Precise optical modeling of quantum dots for white light-emitting diodes

Quantum dots (QDs)-based white light-emitting diodes (QDs-WLEDs) have been attracting numerous attentions in lighting and flat panel display applications, by virtue of their high luminous efficacy and excellent color rendering ability. However, QDs’ key optical parameters including scattering, absorption and anisotropy coefficients for optical modeling are still unclear, which are severely against the design and optimization of QDs-WLEDs. In this work, we proposed a new precise optical modeling approach towards QDs. Optical properties of QDs-polymer film were obtained for the first time, by combining double integrating sphere (DIS) system measurement with inverse adding doubling (IAD) algorithm calculation. The measured results show that the typical scattering, absorption and anisotropy coefficients of red emissive QDs are 2.9382 mm−1, 3.7000 mm−1 and 0.4918 for blue light, respectively, and 1.2490 mm−1, 0.6062 mm−1 and 0.5038 for red light, respectively. A Monte-Carlo ray-tracing model was set-up for validation. With a maximum deviation of 1.16%, the simulated values quantitatively agree with the experimental results. Therefore, our approach provides an effective way for optical properties measurement and precise optical modeling of QDs for QDs-WLEDs.

Precise Spatial-Color Optical Modeling in Phosphor-Converted White LEDs

White light LEDs are playing a more and more important role in display in various structures. Here, a new modeling algorithm is proposed for use in phosphor-converted white light-emitting diodes (pcW-LEDs); the algorithm can be used to accurately simulate color appearances, potentially enabling optical designers to remove yellowish/bluish spots in LED lighting. The proposed modeling method was applied to simulate a pcW-LED without an encapsulation lens, and subsequently, a pcW-LED with a hemispherical lens. Both simulations accurately predicted the blue and yellow light distribution. The model was further verified by applying a total internal reflector lens to the pcW-LED. In the midfield region, the blue and yellow light distribution exhibited large variations as the observation distance changed; this varying light pattern for both the blue and yellow lights can be accurately predicted using the proposed model. The developed optical model should facilitate designing a pcW-LED that features high-quality illumination and increased color uniformity.

Plane wave expansion method used to engineer photonic crystal sensors with high efficiency

A photonic crystal waveguide (PhC-WG) was reported to be usable as an optical sensor highly sensitive to various material parameters, which can be detected via changes in transmission through the PhC-WG caused by small changes of the refractive index of the medium filling its holes. To monitor these changes accurately, a precise optical model is required, for which the plane wave expansion (PWE) method is convenient. We here demonstrate the revision of the PWE method by employing the complex Fourier factorization approach, which enables the calculation of dispersion diagrams with fast convergence, i.e., with high precision in relatively short time. The PhC-WG is proposed as a line defect in a hexagonal array of cylindrical holes periodically arranged in bulk silicon, filled with a variable medium. The method of monitoring the refractive index changes is based on observing cutoff wavelengths in the PhC-WG dispersion diagrams. The PWE results are also compared with finite-difference time-domain calculations of transmittance carried out on a PhC-WG with finite dimensions.

Optical design of a Butterfly lens for a street light based on a double-cluster LED

We construct a precise optical model of the cluster LED and propose a design of a peanut form lens to achieve an optical utilization factor of 45% and uniformity of 1/2.7. Then we propose a new design of so-called Butterfly lens with unequal tilt design aimed to serve a double-cluster LED but with a tightened size. The optical utilization factor of the Butterfly lens is about 43.8%, the uniformity is around 1/2.7, and an averaged illuminance of 14lx, which could meet almost all types requested by IESNA. The design of the Butterfly lens is proven workable for a double-cluster LED.

Highly precise optical model for simulating light guide plate using LED light source

In this paper, a method is proposed for highly precisely simulating the optical behavior of a light guide plate, which has microstructures of rough texture and rugged edges thereon. By adopting the 'equivalent medium-immersed measurement' to establish precise optical characteristic of the light guide plate, the simulated results of luminance distribution are much accordant with experimental measurement. This method will be useful in luminance uniformity evaluation and hot-spot check in advance, and it will greatly contribute to save time and cost of product development.

Precise optical modeling of blue light-emitting diodes by Monte Carlo ray-tracing

Precise optical modeling of blue light-emitting diodes (LEDs) is constructed by reasonable optical parameters and Monte Carlo ray-tracing with the capability of precisely predicting light extraction and radiation pattern for both bare LED and packaged LED. Refractive indices and absorption coefficients of LED materials are determined by abundant references and comparisons between simulations and experiments. Surface roughness is considered in the optical model to improve the simulation precision. The simulation precisions are excellent for both bare blue LEDs (>96.5% for light extraction and >99% for radiation pattern) and packaged blue LEDs (>98.5% for both light extraction and radiation pattern).

LED beam shaping lens based on the near-field illumination

This paper presents a novel method in near-field beam shaping based on the precise optical modeling of a gallium nitride light-emitting diode (GaN LED). A Monte Carlo ray tracing simulation has been utilized to calculate the spatial photon distribution near the LED's top. By analyzing the ray data in near-field, the miniaturized lens profile is created and machined with aspherical surfaces and total internal reflection (TIR) Fresnel facets. The prototype lens reduce the viewing angle of the LED from 150 degrees to 17.5 degrees at full width half maximum (FWHM) while increasing the peak luminous intensity 10 times. The array of proposed lens with CSP LEDs exhibits feasibility of ultra thin uniform illumination in near-field.

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.

Precise optical model of multi-chip white LEDs

A single-chip white light LED is commonly modeled by considering the phosphor coating as a homogeneous Lambertian light source. However, this approach leads to an incorrect optical simulation of phosphor-coated multi-chip LEDs due to the presence of a previously unreported spatial distribution of emission spots across the phosphor layer. We introduce "weighting" factors based on position-dependent light strength across the phosphor surface in order to improve the model accuracy. Following the modeling algorithm in the mid-field region, we have built up a precise and practical optical model by using Monte Carlo ray tracing and weighting factors. We measure the LED radiation distribution at several representative distances to test the model performance. In all cases, the accuracy is higher than 99.5% in normalized cross correlation between the simulated pattern and experimental measurement.

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.

Electromodulated absorption spectroscopy of charge carriers in α-sexithiophene thin films

We have realized two types of metal-insulator-semiconductor devices with α-sexithiophene as the semiconductor: one with a polymeric insulator and one with a mineral one. Both electromodulated absorption spectra present a sharp peak at 1.50 eV, corresponding to an absorption cross section of 4 × 10 −17 cm 2 per injected charge ( ϵ = 10 4 1 mol −1 cm −1). By comparison with chemical-doping experiments we show that, in both devices, the charges are stored as radical cations (model polarons). The quantitative results are discussed with the help of a precise optical model of the multilayer structure.