Luminous Exitance Research Articles

Unveiling the effect of Ce3+ doping concentration in YAG:Ce single crystals towards high luminance laser lighting

Doping concentration of a phosphor is key parameter, affecting multiple properties such as light absorption, quantum yield, luminescence decay, thermal conductivity, and thermal quenching, etc. These properties could directly influence the luminous efficacy, the saturation threshold, and the emitting area of the phosphor when irradiated by laser, thus determining the peak luminance of a phosphor-converted laser lighting device. But surprisingly no systematic investigation has been done to research the effect of phosphor doping concentration on the luminance performance of laser lighting. Here we report the YAG:Ce single crystal phosphors with different Ce3+-doping concentrations (between 0.05 at% and 0.30 at%) and thicknesses (0.15 mm–0.45 mm). The relationship between Ce3+ concentration, phosphor thickness, and the colorimetric/photometric parameters (luminous efficacy, luminous flux, luminance) are built and elucidated. It is found that the thinner sample with higher doping density tends to be saturated at lower laser power density (314.1 W/mm2) and the device shows lower luminance. This result is also verified by thermal simulation that the corresponding sample shows highest working temperature. Remarkably, the optimal sample with thickness of 0.45 mm and Ce3+ density of 0.30 at% presents a record high luminous exitance of 33414 lm/mm2 and a record high luminance of 10636 Mcd/m2, indicating potential for use in special lighting, projector, and optical communication fields.

Performance improvement of reflective phosphor film via adjusting the thickness realizing bright and efficient laser lighting.

Phosphor-in-glass-film (PiG-F) has been extensively investigated, showing great potential for use in laser lighting technique. Thickness is apparently a key parameter for PiG-F, affecting the heat dissipation, absorption, and reabsorption, thus determining the luminous efficacy and luminescence saturation threshold (LST). Conventional studies suggest that thinner films often have lower thermal load than that of the thicker ones. Unexpectedly, we found that the Lu3Al5O12:Ce (LuAG:Ce)-based PiG-F with a moderate thickness (78 μm) yielded the optimal LST of 31.9 W (14.2 W·mm-2, rather than 28.0 W (12.3 W·mm-2) for the thinnest one (56 μm). This unexpected result was further verified by thermal simulations. With the high saturation threshold together with a high luminous efficacy (∼296 lm·W-1), an ultrahigh luminous flux of 7178 lm with a luminous exitance of 2930 lm·mm-2 was thus attained. We believe the new, to the best of our knowledge, findings in this study will substantially impact the design principles of phosphors for laser lighting.

Pore-existing phosphor-in-glass film realizing ultra-efficient and uniform laser lighting

The development of phosphor-converted laser lighting has encountered a significant problem: the difficulty of homogeneously mixing the laser light with the luminescent light. Previous studies reveal that enhancing the scattering of the phosphor can improve color mixing but often sacrifices luminous efficacy and limits the peak luminance. To address this, a pore-existing YAG:Ce-based phosphor-in-glass film is prepared using the screen-printing technique. Interestingly, it is found that the pore size is highly correlated with the thickness, with thicker samples having larger pores. When excited by blue laser, a typical sample with a thickness of 60 μm exhibits an ultra-high luminous efficacy of 261 lm/W and a high saturation threshold of 20.5 W (9.1 W/mm2), resulting in a peak luminous exitance of 1244 lm/mm2@3839 lm. Furthermore, the color mixing was evaluated via angular color temperature distribution, showing uniform light emission with a maximum correlated color temperature (CCT) variation of only ∼800 K.

Red‐Emitting Cordierite Ceramic Enabling General Healthy Warm White Laser Lighting

AbstractLaser‐driven phosphor‐converted white‐laser diodes are supposed to be next‐generation high‐brightness lighting sources for their unparalleled photoelectric efficiency and light quality, yet a significant obstacle hitherto has resided in the shortage and serious laser‐induced luminescence saturation of red emitters. Here, a hot‐pressing synthesis route to obtain high‐performance red‐emitting cordierite (Mg2Al4Si5O18:Eu2+) ceramic is developed, which enables the in situ formation of bulk specimens with high crystallinity (>90%) from the counterpart glass powders. A preeminent saturation threshold of 21 W mm−2 is realized in a red‐emitting Mg2Al4Si5O18:Eu2+ ceramic film converter. More importantly, the luminous exitance reaches 209 lm mm−2, which is 5 times that of commercial red phosphor‐converted‐LED, indicating breakthrough progress in the comprehensive light‐emitting performance of red converters. This work innovates the solid‐state synthesis of laser bulk converters from glass to meet the demanding requirements of advanced warm white laser lighting.

Ce3+-based phosphor converter enabling laser lighting to attain both high CRI and high luminous efficacy

Eu2+ doped-CaAlSiN3 possesses broad red emission, enabling a phosphor-converted lighting device to achieve a high color rendering index (CRI) and proper color temperature. However, CaAlSiN3:Eu2+ exhibits relatively slow decay (∼1 μs) and intense re-absorption of luminescence from green/yellow emitters, thereby causing optical saturation and reducing the luminous efficacy. Here, we fabricated a novel phosphor converter comprising Lu3Al5O12:Ce3+ and Y1.3Gd1.6Al5O12:Ce3+ powders. The typical sample, when excited by a blue laser, exhibited a high luminous efficacy of 231 lm/W and a high saturation threshold of 22.2 W/mm2, resulting in a high luminous exitance of 695 lm/mm2. Importantly, the phosphor converter produced a broad emission band that included sufficient cyan and red components, resulting in a full width at half maximum (FWHM) of 134 nm and a high CRI of 81. With this excellent balance between CRI and luminous efficacy, the reported phosphor converter can significantly expand the range of laser lighting applications.

Novel strategy to optimize luminance for phosphor-converted laser lighting

Phosphor-converted white laser diodes (pc-wLDs) are a new development in high-luminance white light sources. However, the optical design and material evaluation methods of the pc-wLD system have been facing a long-neglected problem which is associated with the lack of understanding the selecting principle of the incident laser spot area (ILSA). The ILSA design affects the optical/thermal load on the phosphor(s) and the emitting area and beam quality of the luminaire. Here, we present a comprehensive evaluation of canonical phosphors (Y3Al5O12:Ce3+ and (Sr,Ca)AlSiN3:Eu2+) on the optical/thermal properties, using an excitation laser with wide range laser power (0.1 W–16 W) and ILSA (0.023 mm2–4.780 mm2). The relationship between the lighting parameters (luminous efficacy, luminous flux and luminous exitance), temperature increase and the ILSA is elucidated, showing that the optimal ILSAs for different phosphors can be significantly different: the Y3Al5O12:Ce3+-based light source attains the highest luminous exitance with ILSA of 0.41 mm2; in contrast, the (Sr,Ca)AlSiN3:Eu2+-based light source attains highest luminous exitance with ILSA of 0.023 mm2. These new finding will not only impact the material selection and optical design of pc-wLD, but also provide new strategy to optimize the lighting parameters of pc-wLD.

High color rendering and high-luminance laser lighting using all inorganic nitride phosphor films.

Despite the huge advances that have been made in the development of ultra-high luminance laser lighting, achieving high color rendering properties in such systems at the same time remains a challenge. Recent studies show that in most cases, the luminous efficacy (LE) of laser lighting is compromised to improve the color rendering index (CRI). In this study, a possible solution to this problem has been proposed by preparing phosphor-in-glass (PiG) films comprised of the yellow-emitting phosphor (LSN:Ce3+) and the red-emitting phosphor (CASN:Eu2+). The composite material synthesized in this study exhibited outstanding optical and thermal properties. A uniform white light with a high CRI of 80.0 and a high LE of 185.9 lm W-1 was achieved by optimizing the yellow/red ratio and the emission peak position of the blue laser. Furthermore, it was found that this design enabled the phosphor to restrict the light emission area effectively, thus attaining a high luminous exitance of 1302 lm mm-2. With their superior optical performance, the PiG films can be regarded as promising color converter candidates for future high-quality laser-based white light sources.

The environmental impact of the new version of the Interior Lighting European Norm in Lighting and Circadian Design

The new version of the European Standard of Interior Lighting (EN 12464-1) considers LED technology for energy saving and minimizing environmental impact by giving new criteria for quantitative and qualitative lighting, including those that concern the “non-image-forming effects” (NIF) of light. In addition, it takes into account normal aging as an assumption for proposing increased light levels in indoor workplaces. In real applications, this seems to create lighting conditions that do not fully support human well-being based on the criterion of WELL Building Standard v2 (2021) for Circadian Lighting Design. Therefore, in offices, it is proposed to increase the light levels by at least one step from the minimum provided by the EN Standard. In order to examine this hypothesis, a typical office space is examined. In addition to the main requirements, the Mean Ambient Illuminance (Ēamb) and the Mean Room Surface Luminous Exitance (Mrs) are calculated according to the EN Standard but also the Equivalent Melanopic Lux (EML) and the Melanopic Equivalent Daylight Illuminance (M-EDI) as proposed by the WELL Standard. The comparison between EN 12464-1 and Well Standard has shown that the qualitative lighting design criteria are stricter in WELL than in EN Standard.

Design of a β -SiAlON:Eu based phosphor-in-glass film with high saturation threshold for high-luminance laser-driven backlighting

β-Si6−zAlzOzN8−z:Eu2+ (β-SiAlON:Eu) possesses an intense and narrow emission band peaked at ∼530 nm, which makes it a highly suitable green-emitting phosphor for LCD backlight. However, it can hardly be used in high-power laser displays or projectors because of the laser-induced saturation effect. The aim of the present study is to develop a β-SiAlON:Eu based phosphor-in-glass (PiG) film that can withstand high-power density laser irradiation. A facile blade-coating method was employed to fabricate the PiG film. The morphology, quantum efficiency, luminescence spectra, and laser-pumped colorimetric properties of the β-SiAlON:Eu PiG films were investigated. Under excitation of 450 nm, a typical PiG film showed internal quantum efficiency of 58%, which is an acceptable value. The laser-pumped photometric properties of the PiG films were measured in reflection mode. When irradiated with a blue laser, the PiG film attained a high luminous efficacy of 109 lm/W. Furthermore, its saturation threshold reached over 26.4 W/mm2, thus achieving a high lumen density of 1970 lm/mm2 and a high luminous exitance of 1052 lm/mm2. With the above excellent properties, the β-SiAlON:Eu PiG film has great potential for use in high-power and wide-color gamut laser displays or projectors.

High-Performance III-Nitride Light-Emitting Diode Stripes

In this work, a high-performance stripe-shaped light-emitting diode (LED) with a size of $7500\times 300\,\,\mu \text{m}^{{2}}$ is reported. With the optimized electrode design, the current can be evenly injected into the elongated chip, achieving a uniform light distribution without the use of external optics. Compared with the conventional cubic LED, the light output performance of the stripe-shaped LED (S-LED) at high current levels is significantly improved, which is mainly due to the enhanced heat dissipation. White light emission is also demonstrated by covering the chip with the yellow phosphor film. With the advantages of reduced luminous exitance, uniform illumination, and improved light output, the S-LEDs are not only suitable for various lighting applications but can also be used as backlight units for display panels.

Emitting area limitation via scattering control in phosphor film realizing high-luminance laser lighting

One major benefit of laser lighting is the possibility to achieve very high luminance. In phosphor-converted laser lighting systems, a blue (pump) laser can be focused into a very small spot. However, after excitation of the phosphor, the white-light-emitting area usually increases considerably, which reduces the luminance parameter substantially. Herein, we design and investigate a highly scattering YAG:Ce/glass composite film with a porous microstructure. Both the glass/phosphor interfaces and the introduced pores act as scattering centers, which can confine the emission area effectively. The relationship between the spot size and the microstructure (porosity, phosphor-particle size, thickness) is elucidated. Under excitation with blue laser, the composite film shows a uniform white-light emission with high luminous efficacy (230 lm/W) and high saturation threshold (> 40 W/mm2), thus achieving a high luminous exitance of ∼1239 lm/mm2. With above excellent properties, the designed composite films show great potential for use in high-luminance laser lighting.

Phosphor material dependent spot size limitations in laser lighting.

In laser lighting, a major benefit over other lighting techniques is the possibility to achieve very high luminous exitance. Focusing the exciting laser to a very small spot size on the phosphor, however, does not necessarily provide a very small emitting area for the white light. In this study we investigate experimentally and numerically the relationship between the white light spot size and the incident blue laser spot size. We show that the specific phosphor material properties have significant impact on this relationship and on the achievable minimum spot size. This constitutes a limitation on the minimum spot size achievable in laser lighting and has important implications in applications.

Daylight and Architectural Simulation of the Egebjerg School (Denmark): Sustainable Features of a New Type of Skylight

This article discusses the performance of a new skylight for standard classrooms at the Egebjerg School (Denmark), which was built ca. 1970. This building underwent important reforms under a European project to which the authors contributed. This research aimed to create a new skylight prototype that is useful for several schools in the vicinity, since there is a lack of educational facilities. The former skylights consisted of plastic pyramids that presented serious disadvantages in terms of sustainability matters. During the design process, the priority changed to studying the factors that correlate daylighting with energy and other environmental aspects in a holistic and evocative approach. Accordingly, the new skylight features promote the admittance and diffusion of solar energy through adroit guidance systems. In order to simulate different scenarios, we employed our own simulation tool, Diana X. This research-oriented software works with the effects of direct solar energy that are mostly avoided in conventional programs. By virtue of Lambert’s reciprocity theorem, our procedure, which was based on innovative equations of radiative transfer, converts the energy received by diffusive surfaces into luminous exitance for all types of architectural elements. Upon completion of the skylights, we recorded onsite measurements, which roughly coincided with the simulation data. Thus, conditions throughout the year improved.

Packaging of InGaN stripe-shaped light-emitting diodes.

We demonstrate the fabrication of InGaN/GaN stripe-shaped light-emitting diodes (LEDs) in flip-chip packaging (FC-LED) and vertically mounted packaging (VM-LED). Compared to conventionally packaged LEDs, these packaging schemes enhance light output and emission divergence in ways favorable for general lighting applications. The FC-LED can sustain efficiency at high current operations due to effective heat sinking, while the VM-LED excels at light extraction efficiency due to the exposure of two large emission surfaces. Together with the properties of low luminous exitance and emission uniformity, the stripe-shaped LEDs are ideal for the assembly of luminaires. An LED light tube comprising a continuous linear array of 10 stripe-shaped LED chips has been assembled. The optical performance of the light tube is compared to another light tube assembled with conventional square-shaped LED chips (with and without external diffuser) by confocal microscopy. It is found that emission uniformity of the stripe-shaped LED tube is significantly improved, with a threefold increase in illumination area, without efficiency loss associated with diffusers.

Development of a Colour Quality Assessment Tool for indoor luminous environments affecting the circadian rhythm of occupants

The colour of indoor luminous environments affects the circadian rhythm and awakening of humans. If the blue colour of a luminous environment perceived by an observer's circadian action factor (CAF), which represents the impact of light on the hormones and biorhythm of an observer, is high, the secretion of melatonin is suppressed. To design an appropriate indoor luminous environment, the final colour of a space perceived by an observer must be accurately predicted and its effects on the observer must be considered. Therefore, this study developed a Colour Quality Assessment Tool (CQAT) that utilizes the spectral reflectance factor of interior finishes and the spectral power distribution (SPD) data of luminaires to accurately calculate the luminous exitance radiated by interior finishes and the colour quality components of the SPD, luminance, CAF, colour coordinates, and correlated colour temperature (CCT) based on the colours of the interior finishes and light sources. In addition, the colour quality was evaluated considering the angle between the interior finishes and the view of an observer from a specific direction and position. The colour quality of different interior finishes was assessed using the CQAT, and the results showed that the luminous environment CCT perceived by observers differed by 1882 K (approximately 31%) or more and that the CAF differed by 0.16 (approximately 14%) or more based on the colour of the interior finishes.

Evaluation of the Photon Transmission Efficiency of Light Guides Used in Scintillation Detectors Using LightTools Code

Background: To optimize the photon transmission efficiency of light guides used in scintillation detectors, LightTools code, which can construct and track light, was used to analyze photon transmission effectiveness with respect to light guides thickness. Materials and Methods: This analysis was carried out using the commercial light guide, N-BK 7 Optical Glass by SCHOTT, as a model for this study. The luminous exitance characteristic of the LYSO scintillator was used to analyze the photon transmission effectiveness according to the thickness of the light guide. Results and Discussion: The results of the simulations showed the effectiveness of the photon transmission according to the thickness of the light guide, which was found to be distributed from 13.38% to 33.57%. In addition, the photon transmission efficiency was found to be the highest for light guides of 4 mm of thickness and a receiving angle of 49°. Conclusion: Through such simulations, it is confirmed that photon transmission efficiency depends on light guide thickness and subsequent changes in the internal angle of reflection. The aim is to produce an actual light guide based on these results and to evaluate its performance.

InGaN light-emitting diode stripes with reduced luminous exitance.

InGaN light-emitting diodes of stripe geometries have been demonstrated. The elongated geometry facilitates light spreading in the longitudinal direction. The chips are further shaped by laser-micromachining to have partially-inclined sidewalls. The light extraction efficiencies of such 3D chip geometries are enhanced by ~12% (~8% according to ray-trace simulations), leading to a reduction of junction temperatures. The effective emission area is also increased four times compared to a cubic chip. The stripe LEDs are thus more efficient emitters with reduced luminous exitance, making them more suitable for a wide range of lighting applications.

A simple tool to evaluate the effect of the urban canyon on daylight level and energy demand in the early stages of building design

Daylight is a restricted resource in urban contexts. Rooms situated in an urban context often have a significant proportion of the sky and the sun blocked out by the urban building mass. The reduced direct daylight potential makes daylight reflected from outdoor surfaces an important daylight source to the room. It is therefore important to be able to take into account the daylight reflections from the urban environment in early design stage. This paper describes a simplified method that uses a combination of ray-tracing, the luminous exitance method and the concept of the urban canyon to represent daylight levels in rooms situated in an urban setting. The method is implemented in the daylight algorithm of an existing building simulation tool capable of making rapid integrated daylight and thermal simulation. Comparisons with the more sophisticated lighting tool Radiance show a maximum relative error of 17% but it is often much lower. The accuracy of this approach is therefore considered to be adequate for the early stages of the building design process. The results from integrated daylight and thermal simulations are presented to illustrate how the tool can be used to investigate the impact of urban canyon parameters on indoor environment and energy performance.

Quantitative analysis of microfluidic mixing using microscale schlieren technique

In this study, we discuss the employment of microscale schlieren technique to facilitate measurement of inhomogeneities in a micromixer. By mixing dilute aqueous ethanol and water in a T-microchannel, calibration procedures are carried out to obtain the relation between the concentration gradients and grayscale readouts under various incident illuminations, concentrations of aqueous ethanol solution, and knife-edge cutoffs. We find that to broaden measuring range with minimal error, the luminous exitance should be tuned to have a reference background with an average grayscale readout of 121, and dilute aqueous ethanol solution with a mass fraction of 0.05 should be used along a 50 % cutoff. For concentration gradients greater than 6.8 × 10−3 or below −2.5 × 10−2 μm−1, the calibration curves show great linearity. Correspondingly, the discernable limit of our microscale schlieren system is 2.3 × 10−5 μm−1 for a positive refractive index gradient and −8.6 × 10−5 μm−1 for a negative refractive index gradient. Once the relation between concentration gradients and grayscale readouts is known, the concentration distribution in a microfluidic can be reconstructed by integrating its microscale schlieren image with appropriate boundary conditions. The results prove that the microscale schlieren technique is able to provide spatially resolved, noninvasive, full-field measurements. Since the microscale schlieren technique is directly linked to the measurement of a refractive index gradient, the present method can be easily extended to other scalar quantifications that are related to the variation of refractive index.

A hybrid ray-tracing and radiosity method for calculating radiation transport and illuminance distribution in spaces with venetian blinds

This paper presents a hybrid ray-tracing and radiosity method for processing luminous flux in spaces equipped with horizontal venetian blinds. The method considers both diffuse and specular characteristics of the blinds and aims to establish a balance between computational speed and accuracy. Direct components are treated with ray-tracing techniques employing a shining factor for the blinds to split between specularly and diffusely reflected components. The specular components are traced inside the blind cavity and inside the room while the direct-diffuse components inside the blind cavity are processed in a two-dimensional radiosity calculation until the final diffuse flux departing the cavity is determined. Diffuse-to-diffuse transmission is considered using a traditional radiosity method. Each room surface is divided into sub-surfaces and given an initial luminous exitance, after accounting for directly traced portions. Then a three-dimensional radiosity method is employed for the entire room to compute illuminance distributions on each sub-surface and on the work plane. Comparison between the current model and results obtained with full radiosity showed that significant errors can be introduced by improper modeling of blind specular components for almost all profile angles and slat angles.An in-depth analysis of the transmission process with different profile angles and shining factors showed the potential of blinds with diffuse characteristics to transmit more light than blinds with highly specular surfaces for certain angles. Further analysis for profile angle close to 45°, showed that for certain commonly used slat angles, a second reflection occurs at the bottom side of the upper slat, redirecting direct light towards the work plane, with serious potential glare consequences. The model contributes to rapid and accurate assessment of illuminance/solar radiation distribution in spaces with venetian blinds and related potential lighting energy savings when electric lighting controls are utilized. Analysis of the number of inter-reflections as a function of profile and slat angles with simultaneous consideration of different reflectances and shining factors is particularly important for providing guidelines and recommendations for venetian blinds optimized design and control.