Luminance Saturation Research Articles

Fabrication of LuAG:Ce–Al2O3 eutectics via laser‐heated pedestal growth technique for high‐power laser‐driven lighting

AbstractThermally robust and highly stable bulk luminescent materials are essential for advancing high‐power laser‐driven lighting. In this study, we report a yellow–green LuAG:Ce–Al2O3 eutectic, synthesized using the laser‐heated pedestal growth (LHPG) technique. The emission intensity of the eutectics reaches a maximum at an Al2O3 content of 20% due to the enhanced light scattering. Additionally, owing to the high thermal conductivity of Al2O3, the prepared LuAG:Ce–Al2O3 eutectic exhibits low thermal quenching, with only a 5% loss in luminescence observed at 150°C, along with a high luminance saturation threshold of approximately 15.8 W·mm−2. When irradiated under blue laser excitation at 7.9 W, the prepared LuAG:Ce–Al2O3 eutectic demonstrates a luminous flux of 1917 lm and a luminous efficacy of 242.4 lm·W−1. These results highlight that the potential of LuAG:Ce–Al2O3 eutectics as luminescent materials for high‐power laser‐driven lighting applications.

Ce:(Lu,Sr)3(Al,Si)5O12 transparent ceramics for high-power white LEDs/LDs with ultra-high luminance saturation threshold

“Heavy atom substitution” was utilized to improve the thermal behaviors of Ce:LuAG transparent ceramics via Sr2+/Si4+ doping, exhibiting high luminous efficiency of radiation (261.98 lm W−1) and high luminance saturation threshold (∼65 W mm−2).

Orange-red-emitting (Sr,Ba)3SiO5:Eu2+ phosphor-in-glass film for enhanced color rendering of laser lighting

Nitride red-emitting materials exhibit severe luminance saturation and are thus not suitable for achieving enhanced color rendering in laser lighting applications. The silicate luminescent material (Sr,Ba)3SiO5:Eu2+ displays excellent stability and can emit orange-red light when excited by blue laser light, making it a promising candidate for realizing high-performance laser lighting. In this study, (Sr,Ba)3SiO5:Eu2+ phosphor-in-glass (PiG) films were prepared on a sapphire substrate, and the influences of preparation temperature, mass ratio of phosphor to glass frit (PtG ratio), and film thickness on the application performance of PiG film were analyzed. The (Sr,Ba)3SiO5:Eu2+ PiG film with a PtG ratio of 1:1 and a film thickness of ∼50 μm prepared at 475 °C exhibited optimal comprehensive luminescence performance. This film exhibited a luminous saturation threshold of 5.98 W/mm2, a maximum luminous flux of 372.3 lm, and a luminous efficiency of 62.3 lm/W at the luminous saturation threshold. The maximum luminous flux of the (Sr,Ba)3SiO5:Eu2+ PiG film was 77.6% higher than that of the CaAlSiN3:Eu2+ nitride PiG film. Finally, through the incorporation of green-emitting YAGG:Ce3+ phosphor into the (Sr,Ba)3SiO5:Eu2+ PiG film, laser-driven white light with a luminous flux of 531.7 lm and a high color rendering index (CRI) of 83.4 was realized.

Pixelated Phosphor Converter for Laser‐Driven Adaptive Lighting

AbstractWith the advent of artificial intelligence and unmanned driving, it is an inevitable trend to provide laser‐driven adaptive lighting with super‐brightness, high penetration, and high spatial resolution; one of the best solutions is to pump pixelated phosphor converters by laser diodes (LDs). However, achieving high‐performance pixelated phosphor converters remains a great challenge due to the large size of commercial phosphor particles. In this study, promising pixelated phosphor films are proposed to be fabricated by producing arrayed holes in the Ag substrate with the laser direct writing technology and then filling the holes with the mixture of phosphor particles and the heat‐resisting inorganic glue. A strong thermal transfer field grid is built in Y3Al5O12:Ce3+‐pixelated‐film@Ag, which can withstand a high luminance saturation threshold of over 18.08 W mm−2 pumped by blue LDs. The maximum luminous flux density is ≈1300 lm mm−2, and the luminance is 3.36 × 107 cd m−2 of the white pixel light with excellent uniformity. Further, based on this fabrication strategy, the full‐color‐pixelated‐film@Ag is successfully prepared for the first time. The realization of laser‐driven white pixel light and full‐color pixel light shows an interesting application in intelligent adaptive lighting, and this study paves an avenue for designing pixelated phosphor converters.

Method of Calculation of a Road Tunnel Illumination

The study proposes a method of electrical lighting calculation for a road tunnel, which allows estimating locations of selected luminaires providing the required road surface luminance distribution in the areas of vehicle driver adaptation while driving through a tunnel during daytime. The method is based on the adopted concepts of saturation curve and luminance saturation zone which are used for building of a step curve approximating the set zonal distribution of luminance. The basic terms of the method and the calculation algorithm as well as an example of calculation of luminaire locations in the tunnel luminance zones illustrated by comparison of estimated and pre-set luminance distributions are presented.

Illuminant Adaptive Wideband Image Synthesis Using Separated Base-Detail Layer Fusion Maps

In this study, we present a wideband image synthesis technique for day and night object identification. To synthesize the visible and near-infrared images, a base component and a detailed component are first decomposed using a bilateral filter, and the detailed component is synthesized using a local variance map. In addition, considering the difference in the near-infrared image characteristics between daytime and nighttime, the base components are synthesized using a luminance saturation region map and depth and penetration map using a joint bilateral filter. The proposed method overcomes the partial over- or under-exposure caused by sunlight and infrared auxiliary light, which is experienced variously in wideband imaging, and improves the identification of objects in various indoor and outdoor images compared with that achieved by existing methods by emphasizing detailed components.

Effect of wall parameters on impinging combustion and soot emission characteristics of heavy-duty diesel engine at low temperature

The effect of wall parameters on combustion and soot emission characteristics remains to be revealed so far, especially in the case of spray impingement at low temperatures. Therefore, the visualization experiments are carried out in a constant volume combustion chamber using the Mie scattering and direct photography techniques, and the two-color method was used to extract the flame temperature and soot volume. The results show that increased wall distance and wall angle contribute to fast and stable ignition, but result in more soot production. In addition, the lower the ambient temperature, the more prominent the above characteristics are. At Tamb = 820 K, as the wall distance increases from 40 mm to 60 mm, the liquid spray impingement changes to the vapor impingement, so the ignition delay shortens, and the flame area and natural luminance increase significantly. At lower Tamb = 770 K, a complete misfire is observed at Lw = 40 mm but the ignition remains stable at Lw = 60 mm. The variation of the ignition characteristic parameters with the wall angle is similar to that of the wall distance. Under Pinj = 40 MPa, as the wall angle increases from 0° to 70°, the time to reach luminance saturation advances from 0.60 ms to 0.27 ms. Under higher Pinj = 100 MPa, a complete misfire occurs at θ =0° but bright flames are observed at θ =30–70°. With the increase of wall distance, the mean flame temperature increases due to reduced wall cooling. Coupled with the expanded flame area and combustion duration, the high-temperature region (>1800K) of soot distribution and KL factor increase significantly, especially in the range of Lw = 40–50 mm.

Thermally Robust Orange‐Red‐Emitting Color Converters for Laser‐Driven Warm White Light with High Overall Optical Properties

AbstractLaser‐driven solid‐state lighting with super‐brightness and compactness is recognized as a promising alternative to white light‐emitting diodes. However, it is quite difficult to achieve laser‐driven warm white light with long‐term stability due to the shortage of robust orange‐ or red‐emitting laser phosphors. In this work, an efficient and thermally robust orange‐red (La,Y)3Si6N11:Ce3+ (LYSN)‐BN phosphor‐in‐glass (PiG) film is proposed to realize warm white light, where h‐BN acts as an optical medium to enhance light scattering, a heat sink to reduce the temperature, and a protective layer to prevent LYSN from oxidation. The “LYSN+15 wt.% BN” PiG film shows an internal quantum efficiency of 70% and a luminance saturation threshold of 12.82 W mm−2, much higher than those without h‐BN (i.e., 57.5% and 7.63 W mm−2). A warm white light lamp fabricated by combining “LYSN+15 wt.% BN” PiG film with blue laser diodes, showing tunable correlated color temperatures (2800–5000 K), has a maximal luminous flux of 740 lm and a luminous efficacy of 133.5 lm W−1, which promises high collimation and penetration lighting in the rainy or foggy weather. By designing a composite orange‐red‐emitting phosphor converter, this work lays a foundation for realizing high quality laser‐driven warm white lighting source.

Sandwich structured phosphor-in-glass films enabling laser lighting with superior optical properties

YAG:Ce3+ PiG film cannot produce high color-rendering laser lighting due to the deficiency in red spectral component, but adding nitride-based red phosphors usually leads to dramatic decrease of brightness and efficiency as a result of their obvious luminance saturation. In this work, a sandwich structured PiG (SS-PiG) film was created using green-emitting YAGG:Ce3+ and orange-emitting GdYAG:Ce3+ phosphors, where two types of PiG films were separately co-fired on each surface of a sapphire substrate. The color-rendering index of SS-PiG film can reach 81.4 upon blue laser excitation, improved by 24.3% when compared with that of YAG:Ce3+ PiG film (65.5). Driven by blue laser diodes with a flux density of 7.69 W mm-2, the SS-PiG film shows a luminous emittance of 1362 lm mm-2, which is 179% higher than traditional multilayer structured PiG film (489 lm mm-2). The SS-PiG film enables to enhance both of color rendition and luminance of laser-phosphor-converted lighting.

Phosphor converters for laser driven light sources

Due to the “efficiency droop” effect of light-emitting diodes (LEDs) under high drive currents, laser diode (LD) driven light sources are considered to replace LED lighting for the next-generation solid-state-lighting technology. In this Perspective, the history of lighting technology and the intrinsic advantages of LDs lighting are outlined. We provide a comparative overview of several typical LD phosphors, such as single crystal, transparent ceramic, phosphor-in-glass, and film thereof, as well as surface texture modification of film-type phosphor plates. The relationship between the illumination characterizations (luminance saturation, conversion efficiency, extraction efficiency, illuminating directionality, and color spatial uniformity) with thermal conductivity, pores, defects, second phases, and surface roughing or patterning is discussed. Finally, the current challenges and prospects are highlighted to provide guidelines for future research on LD phosphors.

Highly thermal conductive red-emitting AlN-CaAlSiN3:Eu2+ composite phosphor ceramics for high-power laser-driven lighting

Thermally robust red-emitting phosphor ceramics are urgently required for laser lighting and displays with high luminance and better color saturation. The most promising CaAlSiN3:Eu ceramics have a low thermal conductivity of 4.2 W m−1 K−1 and a small luminance saturation of 0.5 W, making it hard to be used under high power laser irradiation. In this work, we incorporated AlN into the CaAlSiN3:Eu ceramic to produce red-emitting AlN-CaAlSiN3:Eu composite phosphor ceramics by spark plasma sintering. The fully densified phosphor ceramics have the highest thermal conductivity reported so far (53.5 W m−1 K−1), which is about 13 times higher than the reported one. The luminance saturation of the composite ceramics occurs at a high threshold of 4.2 W under blue laser excitation, enabling them to be used for high power laser lighting. This work provides an idea of tackling the microstructure of nitride phosphor ceramics and of preparing thermally robust red-emitting color converters.

Zonal projection based on binocular structured light for localized luminance saturation of intraoperative liver surface

Zonal projection based on binocular structured light for localized luminance saturation of intraoperative liver surface

On the luminance saturation of phosphor-in-glass (PiG) films for blue-laser-driven white lighting: Effects of the phosphor content and the film thickness

To investigate the luminance saturation in high-power blue-laser-driven solid state lighting, the Y3Al5O12: Ce (YAG)-based PiG films was co-fired with a high thermally conductive sapphire substrate. When the PtG ratio or the thickness of the PiG film increases, the luminance saturation becomes worse and both of the luminous flux and luminous efficacy decrease. With an additional sapphire substrate coated with an anti-reflection layer on one side and a blue-pass filter on another side attached to the PiG film, the film shows an improvement in luminous flux and efficacy, and produces the white light with a luminous flux of 1709 lm, a luminous efficacy of 211 lm W−1 and a correlated color temperature of 6602 K under the maximum (10.3 W mm−2) blue laser light excitation. It indicates that the PiG film could be applied in high power laser-driven solid state lighting when its microstructure is carefully tailored.

A search for extra-high brightness laser-driven color converters by investigating thermally-induced luminance saturation

Clarification of thermal saturation helps to guide the design of high-brightness yellow and green components for laser lighting and displays.

Saturation-aware human attention region of interest algorithm for efficient video compression

We propose a saturation-aware human attention region-of-interest (SA-HAROI) video compression method that performs a perceptual adaptive quantization algorithm on video frames as a function of the distribution of their luminance, motion vector, and color saturation. Our work is an application of a psycho-visual study that demonstrated that human attention automatically enhanced perceived saturation. Consequently, the adaptive quantization phase of our compression algorithm is characterized by a luminance and saturation-aware just noticeable distortion (JND) function. After running multiple experiments on 18 videos with various resolutions ranging from QCIF to 4 K, results showed that our method achieves higher compression than that of both the H.264/AVC JM and the HEVC HM while maintaining subjective quality. We observed that in comparison to both implementation of the standards (JM and HM), for an IPPP coding structure, the performance of our algorithm culminated with HD and 4 K videos yielding a bit rate reduction averaging 15% and an encoding time reduction of about 20% in certain cases. Finally, after comparing our method to other similar techniques, we concluded that saturation is a significant parameter in the improvement of video compression.

Nitride phosphors as robust emissive materials in white flat field emission lamps

Nitride phosphors, showing very promising photoluminescence properties, are now attracting great attention as down-conversion luminescent materials in white light emitting diodes (wLEDs). Their interesting cathodoluminescence properties, such as high stability and reliability against electron bombardment, also enable them to be used in white flat field emission lamps (FELs). Dual-band white FELs were prepared by using blue AlN:Eu2+ and yellow Ca-α-sialon:Eu2+ in this work. These FELs show no luminance saturation and stable chromatic coordinates with increasing anode voltage and current. Driven at 7 kV and 100 μA, the white FEL with the AlN:Eu2+ to Ca-α-sialon:Eu2+ ratio of 0.65 to 0.35 has the luminance of 3,412 cd/m2, a color temperature of 3,430 K, and a color rendering index of 76. Higher color rendition of 93-95 can be obtained by using the phosphor blend of AlN:Eu2+, β-sialon:Eu2+ (green) and CaAlSiN3:Eu2+ (red). It indicates that nitride phosphors could be used as robust emissive materials in highly reliable white FELs.

Cathodoluminescence Properties of Blue Emitting Eu2+‐Doped AlN‐Polytypoids for Field‐Emission Displays

Eu2+‐doped AlN‐polytypoids (8H, 15R, 12H, and 21R) were successfully synthesized by nitrogen‐gas‐pressure sintering. The phosphors show intense blue emissions under the electron beam excitation. All the polytypoid phosphors exhibit relatively a smaller degradation in luminance and a higher thermal stability in comparison to the oxide counterparts. Among the polytypoids, 12H has no luminance saturation, and shows a brightness of 40 cd/m2 at 3 kV and 100 μA. These results indicate that Eu2+‐doped AlN‐polytypoids could also be used as blue phosphors for FEDs.

An evaluation of organic light emitting diode monitors for medical applications: Great timing, but luminance artifacts

In contrast to the dominant medical liquid crystal display (LCD) technology, organic light-emitting diode (OLED) monitors control the display luminance via separate light-emitting diodes for each pixel and are therefore supposed to overcome many previously documented temporal artifacts of medical LCDs. We assessed the temporal and luminance characteristics of the only currently available OLED monitor designed for use in the medical treatment field (SONY PVM2551MD) and checked the authors' main findings with another SONY OLED device (PVM2541). Temporal properties of the photometric output were measured with an optical transient recorder. Luminances of the three color primaries and white for all 256 digital driving levels (DDLs) were measured with a spectroradiometer. Between the luminances of neighboring DDLs, just noticeable differences were calculated according to a perceptual model developed for medical displays. Luminances of full screen (FS) stimuli were compared to luminances of smaller stimuli with identical DDLs. All measured luminance transition times were below 300 μs. Luminances were independent of the luminance in the preceding frame. However, for the single color primaries, up to 50.5% of the luminances of neighboring DDLs were not perceptually distinguishable. If two color primaries were active simultaneously, between 36.7% and 55.1% of neighboring luminances for increasing DDLs of the third primary were even decreasing. Moreover, luminance saturation effects were observed when too many pixels were active simultaneously. This effect was strongest for white; a small white patch was close to 400 cd/m(2), but in FS the luminance of white saturated at 162 cd/m(2). Due to different saturation levels, the luminance of FS green and FS yellow could exceed the luminance of FS white for identical DDLs. The OLED temporal characteristics are excellent and superior to those of LCDs. However, the OLEDs revealed severe perceptually relevant artifacts with implications for applicability to medical imaging.

Explanation for Luminance Saturation of PDP Phosphor

Luminance saturation is primary determined by whether the decay time is on the order of microseconds ( μs) or milliseconds (ms), that is, whether or not electric dipole moment transitions are allowable. For prohibited or magnetic transitions, for which electric dipole moment transitions are not allowable (phosphors with ms-order decay times), there is no linear relationship between decay time and luminance saturation. In order to explain the luminance saturation of phosphor with ms-order decay time in plasma display panel we proposed non-radiative transitions as a cause of luminance saturation and derived simulation formulas based on them. The results from the simulation were in good agreement with those from the experiments.

19.4: Luminescence Characteristics of YAl3B4O12: Tb3+ Phosphor for PDP Application

AbstractPhotoluminescence(PL) characteristics under vacuum ultraviolet(VUV) region of Tb3+ doped yttrium aluminium borate phosphor were investigated for Plasma Display panel(PDP) application. The single‐phase green emitting Y1−xTbxAl3B4O12(YAB: Tb3+) powder phosphors were prepared with various Tb3+ ion concentration. As a result, the intensity around 147nm of excitation spectrum in YAB: Tb3+ decrease slightly with increase of Tb3+ ion concentration due to concentration quenching behavior, but on the contrary, the intensity of 172nm increase with increase of Tb3+ ion concentration due to attributing the 4f8 → 4f7 5d1 transition of Tb3+ [1]. However in the result of gas discharge chamber test which has the same gas mixture as real PDP, relative PL luminance of YAB: Tb3+ increase with increase of Tb3+ ion concentration due to effect of increasing absorption at 172nm. Besides, YAB: Tb3+ with high Tb3+ ion concentration (x=0.7) has higher VUV luminance saturation property than low Tb3+ ion concentration (x=0.4) since high Tb3+ ion concentration phosphor has more activators than lower one which can react with VUV photon in unit volume.