Precise optical modeling for silicate-based white LEDs

Monolithic Multicolor Emissions of InGaN-Based Hybrid Light-Emitting Diodes Using CsPbBr3 Green Quantum Dots

To address the increasing demand for multicolor light-emitting diodes (LEDs), a monolithic multicolor LED with a simple process and high reliability is desirable. In this study, organic-inorganic hybrid LEDs with violet and green wavelengths were fabricated by depositing CsPbBr₃ perovskite green quantum dots (QDs) as the light-converting material on InGaN-based violet LEDs. As the injection current was increased, the total electroluminescence (EL) intensities of the hybrid LEDs increased, whereas the light-converted green emission efficiency of the CsPbBr₃ QDs decreased. The maximum green-to-violet EL spectral intensity ratio of the hybrid LEDs with CsPbBr₃ QDs was achieved with the injection current of <10 mA. Moreover, the EL spectral ratio of the green-to-violet emission decreased at an injection current of 100 mA. The light-conversion intensity of the CsPbBr₃ QDs decreased linearly as the junction temperature of the hybrid LEDs was increased with increasing injection current, similar to the temperature-dependent photoluminescence degradation of CsPbBr₃ QDs. In addition, the junction temperature of the hybrid LED was minimized by pulse injection to suppress the thermal degradation of QDs and increase the light conversion efficiency to green emission. Therefore, the overall emission spectrum color coordinates of the hybrid LEDs exhibited a red shift from violet to blue in the low-current region and a blue shift toward violet as the green emission of the QDs was decreased above 10 mA.

Improving light output by micro-TiO2 scatters in pc-WLED encapsulants

Phosphor-converted white light emitting diodes (pc-WLEDs) are used worldwide for an extensive amount of applications. The device is a complex combination of various components that introduce various technical issues: materials, electrical, chemical, thermal, and so on. All of these combined to obtain a targeted optical characteristic. While most of the pc-WLEDs are sufficient for basic illumination performance, there are still many issues to improve the pc-WLED performance. In this work, we deal with the incorporation of micron size particles of titanium oxide (TiO₂) in silicone encapsulant that contains yttrium aluminum garnet (YAG) phosphor in remote phosphor pc-WLED. Based on the light output and the scattering spatial distribution measurements of the phosphor plates, we have found that several essential performance indices, like the color uniformity, the efficiency, and the amount of phosphor for the pc-WLEDs, can be adjusted by tuning the amount of TiO₂ particles and thus be optimized. With a comprehensive model using a Monte-Carlo ray tracing process combined with the Mie scattering theory, two TiO₂ loading conditions are revealed. The first one is the sparse condition that the TiO₂ particles act as the scattering particles such as to increase the output flux to improve the efficiency of YAG. The second one is the dense condition that the TiO₂ particles act more as barrier particles such so to decrease the output flux.

Directional emission of white light via selective amplification of photon recycling and Bayesian optimization of multi-layer thin films

Over the last decades, light-emitting diodes (LED) have replaced common light bulbs in almost every application, from flashlights in smartphones to automotive headlights. Illuminating nightly streets requires LEDs to emit a light spectrum that is perceived as pure white by the human eye. The power associated with such a white light spectrum is not only distributed over the contributing wavelengths but also over the angles of vision. For many applications, the usable light rays are required to exit the LED in forward direction, namely under small angles to the perpendicular. In this work, we demonstrate that a specifically designed multi-layer thin film on top of a white LED increases the power of pure white light emitted in forward direction. Therefore, the deduced multi-objective optimization problem is reformulated via a real-valued physics-guided objective function that represents the hierarchical structure of our engineering problem. Variants of Bayesian optimization are employed to maximize this non-deterministic objective function based on ray tracing simulations. Eventually, the investigation of optical properties of suitable multi-layer thin films allowed to identify the mechanism behind the increased directionality of white light: angle and wavelength selective filtering causes the multi-layer thin film to play ping pong with rays of light.

Optical Design of an LED Lighting Source for Fluorescence Microscopes

In this study, we reveal an LED light source model applied in fluorescence microscopes. This optical model is composed of a confocal total internal reflection lens array system (CTLAS) with a nine-LED array. The CTLAS optical system that we designed consists of a total internal reflection (TIR) lens array and a confocal system. The electrical power of the nine-LED array is 7.9 watts, which is lower than traditional light sources, such as the original 120-watt halogen lamps used in fluorescence microscopes (Zeiss, Axio Imager 2). We have successfully applied the CTLAS system to an Axio Imager 2 fluorescence microscope to observe the vascular bundle organization, modified with Cy3 fluorescence molecules, and have found that in the process of system assembly, the fabrication errors of optical lenses could have a critical effect on the CTLAS system. The results of our experiment show that, in order to achieve the same illuminance as that of the halogen lamp, the displacement error tolerances of the lateral x-axis and the longitudinal z-axis must be controlled within 1.3 mm and 1.7 mm, respectively.

Novel Method for Estimating Phosphor Conversion Efficiency of Light-Emitting Diodes

This study presents a novel method for estimating the phosphor conversion efficiency of white light-emitting diodes (WLEDs) with different ratios of phosphors. Numerous attempts have been made for predicting the phosphor conversion efficiency of WLEDs using Monte Carlo ray tracing and the Mie scattering theory. However, because efficiency depends on the phosphor concentration, obtaining a tight match between this model and the experimental results remains a major challenge. An accurate prediction depends on various parameters, including particle size, morphology, and packaging process criteria. Therefore, we developed an efficient model that can successfully correlate the total absorption ratio to the phosphor concentration using a simple equation for estimating the spectra and lumen output. The novel and efficient method proposed here can accelerate WLED development by reducing costs and saving fabrication time.

Full spectral optical modeling of quantum-dot-converted elements for light-emitting diodes considering reabsorption and reemission effect

Quantum dots (QDs) have attracted significant attention in light-emitting diode (LED) illumination and display applications, owing to their high quantum yield and unique spectral properties. However, an effective optical model of quantum-dot-converted elements (QDCEs) for (LEDs) that entirely considers the reabsorption and reemission effect is lacking. This suppresses the design of QDCE structures and further investigation of light-extraction/conversion mechanisms in QDCEs. In this paper, we proposed a full spectral optical modeling method for QDCEs packaged in LEDs, entirely considering the reabsorption and reemission effect, and its results are compared with traditional models without reabsorption or reemission. The comparisons indicate that the QDCE absorption loss of QD emission light is a major factor decreasing the radiant efficacy of LEDs, which should be considered when designing QDCE structures. According to the measurements of fabricated LEDs, only calculation results that entirely consider reabsorption and reemission show good agreement with experimental radiant efficacy, spectra, and peak wavelength at the same down-conversion efficiency. Consequently, it is highly expected that QDCE will be modeled considering the reabsorption and reemission events. This study provides a simple and effective modeling method for QDCEs, which shows great potential for their structure designs and fundamental investigations.

Optical modeling based on mean free path calculations for quantum dot phosphors applied to optoelectronic devices

We proposed an optical simulation model for the quantum dot (QD) nanophosphor based on the mean free path concept to understand precisely the optical performance of optoelectronic devices. A measurement methodology was also developed to get the desired optical characteristics such as the mean free path and absorption spectra for QD nanophosphors which are to be incorporated into the simulation. The simulation results for QD-based white LED and OLED displays show good agreement with the experimental values from the fabricated devices in terms of spectral power distribution, chromaticity coordinate, CCT, and CRI. The proposed simulation model and measurement methodology can be applied easily to the design of lots of optoelectronics devices using QD nanophosphors to obtain high efficiency and the desired color characteristics.

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.

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.

Optical design of dental light with high performance and low power based on white LEDs

This paper presents an optical design of a dental light that meets the regulation of ISO 9680. The designed light pattern on the target is an elliptical shape with uniform illumination. Moreover, the real module contains four optical modules, and is operated at 9 W and performs a maximum illuminance of 42,010 lx. In order to reduce the correlated color temperature (CCT) variation, we replace the original white light-emitting diode with a new one, which has an extremely low angular CCT derivation. Accordingly, the CCT variation is reduced to 232 K from 1323 K of the original module.

Modeling of reflection-type laser-driven white lighting considering phosphor particles and surface topography

This paper presents a model of blue laser diode (LD)-based white lighting coupled with a yellow YAG phosphor, for use in the proper design and fabrication of phosphor in automotive headlamps. First, the sample consisted of an LD, collecting lens, and phosphor was prepared that matches the model. The light distribution of the LD and the phosphor were modeled to investigate an effect of the surface topography and phosphor particle properties on the laser-driven white lighting systems by using the commercially available optical design software. Based on the proposed model, the integral spectrum distribution and the color coordinates were discussed.

Packaging efficiency in phosphor-converted white LEDs and its impact to the limit of luminous efficacy

In this paper, we start from the study on the packaging efficiency of the phosphor-converted white LED via a new way to measure and calculate the blue light from the blue die to the encapsulation lens. Then we try to estimate the limit of luminous efficacy of a pcW-LED with Type VII structure. In the calculation, with the EQE of 81% of the blue die and Stokes loss, we obtain the optimal limit of luminous efficacy. The largest one reaches 300 lm/W, and occurs for green-white light with CCT from 4000K to 5000K. More practical limit is calculated in considering phosphor quantum loss and geometry loss, and the practical limit of luminous for CRI around 60 is around 210 lm/W, and for CRI larger than 80 is around 175 lm/W. Luminous efficacy will be sacrificed to obtain higher CRI. In order to know the real optical flux on the illuminated target, we introduce the optical utilization factor (OUF). Three application cases are discussed. The OUFs for light bulb, automotive head lamp, and street light are 90%, 60% and 45%, respectively. In considering human factors, it is interesting to find that a light source with lower luminous efficacy can perform higher illumination luminous efficacy (ILE). Therefore it is important to use ILE rather than LE when a light source is practically applied to lighting.

Improvement of phosphor modeling based on the absorption of Stokes shifted light by a phosphor

We have found that the emission spectrum of phosphors measured in the powder state differs from that measured for a single phosphor. When the emission spectrum of the powder state is adopted in an optical simulation, the simulated optical properties e.g., the correlated color temperature, color rendering index, and chromaticity coordinates, show a remarkable discrepancy from those of the fabricated LED package. However, the discrepancy is significantly improved when the emission spectrum from a low concentration of phosphor in a silicone binder is employed. We suggest that the discrepancy originates from the absorption of Stokes shifted light by a phosphor.

Effects of current crowding on light extraction efficiency of conventional GaN-based light-emitting diodes

Current crowding effects (CCEs) on light extraction efficiency (LEE) of conventional GaN-based light-emitting diodes (LEDs) are analyzed through Monte Carlo ray-tracing simulation. The non-uniform radiative power distribution of the active layer of the Monte Carlo model is obtained based on the current spreading theory and rate equation. The simulation results illustrate that CCE around n-pad (n-CCE) has little effect on LEE, while CCE around p-pad (p-CCE) results in a notable LEE droop due to the significant absorption of photons emitted under p-pad. LEE droop is alleviated by a SiO₂ current blocking layer (CBL) and reflective p-pad. Compared to the conventional LEDs without CBL, the simulated LEE of LEDs with CBL at 20 A/cm² and 70 A/cm² is enhanced by 7.7% and 19.0%, respectively. It is further enhanced by 7.6% and 11.4% after employing a reflective p-pad due to decreased absorption. These enhancements are in accordance with the experimental results. Output power of LEDs with CBL is enhanced by 8.7% and 18.2% at 20 A/cm² and 70 A/cm², respectively. And the reflective p-pad results in a further enhancement of 8.9% and 12.7%.

Phosphor-converted LED modeling using near-field chromatic luminance data

A new method of creating a source model of a phosphor-converted white LED is demonstrated. It is based on a simple phosphor model, of which some key parameters have been obtained from measuring the near-field chromatic and luminance characteristics of a complete LED. The accuracy of the model is verified by measurements and simulations on an LED with a ball lens as primary optic.

High uniformity in angular correlated-color-temperature distribution of white LEDs from 2800K to 6500K

In this paper, to our best knowledge, it is the first time to present a precise simulation and detailed design of angular correlated color temperature (CCT) distribution of white LEDs covering a range of CCT from 2800K to 6500K. An optimized design of packaging structure with a silicone lens covering a phosphor dome performed an extreme small angular CCT deviation of 105K in the simulation and 182K in a corresponding real sample for a white LED with the CCT near 6500K.

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).

Analysis of the far-field region of LEDs

Versatility in the design of optical systems is one of the key features of light-emitting diodes (LEDs) that has attracted considerable attention. In the analysis of systems using LEDs, it is useful to know if the distance is far enough from the LED to allow the radiation pattern to be simulated by the point source approach. We propose three far-zone conditions for LED light modeling: the far-field distance, and for practical purposes the quasi far-field and minimum far-field distances. Different types of LEDs have different far-field ranges. We analyze these differences by modulating key parameters like geometrical structure of encapsulating lens, chip size, chip shape, chip position, and package errors. We find that far-field region considerably depends more on the shape of both lens and chip than all other parameters.