Publications

LIRIAS4241469
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Publisher: Wiley
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LIRIAS4238025
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LIRIAS4240371
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Webinar in the 'Back2School' series, organised by the Belgian Institute on Illumination.

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LIRIAS4240681
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LIRIAS4233956
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A glimpse on the artificial light at night research being performed at the Light & Lighting Laboratory of KU Leuven.

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LIRIAS4158657
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Publisher: Taylor & Francis (Routledge)
Accepted

LIRIAS4163913
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We aimed to investigate the potential added value of postmortem MRI (PMMRI) in sudden unexpected infant death (SUID) cases referred to our center between September 2020 and June 2023. Ultimately, 19 SUID cases underwent PMMRI alongside standard autopsy procedures, which included technical examinations such as postmortem CT (PMCT). Four radiologists, two with prior PMMRI experience, provided structured reports following consensus. For each case, the responsible forensic medicine specialist documented the cause of death before and after reviewing the PMMRI report. Additionally, they assessed the overall impact of the PMMRI report and had the opportunity to provide written comments. The results of our study indicate that none of the PMMRI reports altered the prior determined cause of death, which included cases of infection, asphyxia, and sudden infant death syndrome (SIDS). However, we observed a moderate impact in one case and a low impact in 10 cases. The moderate impact arose from the PMMRI report identifying hypoxic-ischemic changes, where histologic examination of the brain was perceived as normal. Conversely, in the 10 cases with a low impact, the PMMRI reports supported the autopsy findings, specifically indicating brain injury and intra-alveolar cellular infiltrates. In conclusion, our study suggests that while PMMRI may not be pivotal in determining the cause of death in SUID cases, it could aid in detecting hypoxic-ischemic changes and reinforcing brain and lung observations. However, distinguishing genuine lung pathology from postmortem changes using PMMRI remains challenging. Further research is warranted to clarify the role of PMMRI in forensic SUID investigations.
Publisher: Springer (part of Springer Nature)
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LIRIAS4223353
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Publisher: Taylor & Francis (Routledge)
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LIRIAS4205960
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Publisher: Elsevier
Published

LIRIAS4202803
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The growing demand for advanced lighting systems with LEDs, has led to the development of spectrally tunable systems. These systems have huge potential for human centric lighting (HCL), which considers both the visual and circadian effects of light. However, HCL systems face a trade-off between circadian and visual effectiveness. This dissertation introduces an optimization framework to establish the theoretical boundaries of this trade-off. These results reveal that while higher CCTs enhance circadian effects, significant variation exists even at fixed CCTs, and current HCL systems fall short of these limits. To close this gap, an optimization model for quantum dot (QD) LEDs was developed, which allows to approach these limits more closely than existing systems. Furthermore, one simulated QD-LED was experimentally fabricated, demonstrating the potential of the optimized LEDs for practical HCL applications. In addition to spectral control, the emitted spatial light distribution must match the illuminated scene. In order to adjust the spatial light distribution, adaptive spotlights with adjustable beam patterns are available, but they require manual adjustment. This thesis introduces an automated adaptive spotlight, combining computer vision and a Raspberry Pi microcontroller to detect and dynamically illuminate objects. Such spotlights rely on single high-brightness LEDs, limiting them to illumination of a single object. As an alternative, micro-LED arrays provide a more advanced solution to dynamically shape the light by activating or deactivating individual LED pixels. However, the wide emission angle of the individual LEDs makes it difficult to design efficient projection optics, due to the large numerical aperture (NA) that is required. This research proposes a differentiable design method for such projection optics with integrated phase elements. These phase elements are practically realized as Fresnel lenses, which enables advanced imaging designs with high NA and field-of-view, while maintaining sufficient transmission efficiency for general lighting applications. Using such advanced projection optics, LED arrays can generate custom light distributions, which could be automated when paired with a microprocessor and camera. However, optical imperfections and alignment issues complicate precise illumination. This dissertation presents an optimization method that calibrates far-field projection patterns for each LED pixel and computes their optimal activation using least-squares optimization, showing significant improvements over traditional driving methods. Despite their flexibility, LED arrays are limited by the luminous output per direction and imperfections in the projection optics. Freeform optics offer an alternative, by redistributing light from a single source. In particular, spatial light modulators (SLMs) can mimic freeform optics, as so-called programmable freeform optics, by applying pixelated phase shifts to alter the incoming wavefront. However, current SLMs have limited phase modulation ranges, leading to chromatic dispersion and limiting broadband light use. This research addresses these challenges by designing ultra-thin, oscillating freeform topologies via differentiable raytracing, minimizing phase wrapping and chromatic effects. This results in a strong reduction in chromatic effects when compared to previous applications with such programmable freeform optics. Designing these freeform topologies via raytracing enables precise irradiance tailoring but is computationally intensive, limiting real-time adaptability. To address this, a deep learning framework was additionally developed to predict freeform optics for a targeted irradiance in fixed settings. The trained network generates thin, oscillating freeform topologies, suitable for SLM implementation, demonstrating the potential of deep learning in nonimaging optical design and opening opportunities for future innovation.

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LIRIAS4202770
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Phase elements can improve the performance and reduce the size of imaging systems, thanks to the additional degrees of freedom that are offered by the independent phase gradient on top of a refractive/reflective surface. Possible implementations include diffractive elements or metasurfaces, but these suffer from diffractive dispersion. Similar optical functionality however can be provided by thin, curved Fresnel optics, which solely rely on refraction. In this study, a differentiable raytracing framework is presented that offers precise and rapid optimization of curved Fresnel surfaces, by modeling them as a combination of a distinct geometrical and refractive surface, both differentiable with respect to the imaging merit function. The method is demonstrated by designing a compact imaging and projection lens, both with high numerical aperture. The paper analyzes the impact of Fresnelizing the optimized "theoretical" surfaces on both the imaging performance and transmission efficiency. It furthermore shows how the system performance can be enhanced through dedicated post-processing steps, emphasizing the practical relevance of compact Fresnel optics.
Publisher: Optical Society of America (OSA)
Published online

LIRIAS4183079
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Publisher: SAGE Publications
Published online

LIRIAS4200440
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Time Difference of Arrival (TDoA) based Visible Light Positioning (VLP) and Optical Wireless Positioning (OWP) offer considerable potential over conventional Received Signal Strength (RSS) methods. The current state of the art in TDoA OWP is however limited concerning experimental assessments, mainly focusing on simulations. This work presents experimental TDoA measurements in a proof-of-concept setup. The measurement results demonstrate a 95th percentile accuracy of 5.3 cm in a 50×50 cm area, in the absence of reflections. The susceptibility to reflections is examined experimentally, demonstrating that the performance deterioration can easily be minimized by performing a recalibration.
Publisher: Institute of Electrical and Electronics Engineers
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LIRIAS4206629
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LIRIAS4202378
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In this abstract we report on our work towards integrating photovoltaic (PV) solar cells into a Concentrated Solar Power (CSP) configuration. First we introduce the motivation for such a concept and the targeted demonstration, utilizing more efficiently the available solar energy, through spectrum splitting (and capturing also diffuse light as a side effect). Then we discuss the implementation and evaluation in 1- cell laminates, followed by some preliminary reliability testing on these. Finally, we discuss the upscaling through fabrication, evaluation and implementation in large-area, reflective, parabolic, bifacial CSP+PV elements that were integrated and tested in an existing CSP plant.

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LIRIAS4202081
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LIRIAS4202080
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LIRIAS4180658
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This PhD thesis presents research conducted within the framework of the SBO project "Quantum Dots for On-Chip Luminescent Downconversion (QDoCCO)". The project's main goals were the development of novel InP quantum dots, the investigation of innovative encapsulation methods for these nano-particles,<br />and the creation of spectrum-by-design LED demonstrators using InP quantum dots. This final goal, the realization of spectrum-by-design demonstrators, using accurate optical simulations for their design, forms the core scientific focus of this thesis. This research rationale stems from the transformative role of LEDs in the lighting and display industry due to their high energy efficiency and spectral design capabilities. However, conventional LEDs still face important challenges like the green gap and the limited color rendering of typical phosphor-converted white LEDs. These limitations can be mitigated by using quantum dots as luminescent materials. Quantum dots (QDs), in particular InP-based QDs, offer distinct advantages over traditional phosphors due to their narrow and tunable emission spectra, nanoscale size, short decay time and high absorption coefficient. Despite these advantages, achieving stable and efficient on-chip QD-LEDs remains challenging because the durability of QD-LEDs depends on the QD stability, and their efficiency is affected by QD re-absorption losses.<br />The primary research objectives of this thesis were to develop optical models to simulate QD-on-chip LED behavior using InP QDs, and to design and fabricate spectrum-by-design demonstrators for applications such as high color rendering lighting and optimal circadian lighting, and efficient displays with high color gamut. The main research contributions include: &bull; Optical models for QD-LEDs : Two optical models for simulating QDs, varying in the number of input parameters and the precision of their outcomes, were proposed in this thesis. &bull; High-CRI white LEDs with InP QDs : Efficient, high CRI white LEDs were demonstrated by combining InP QDs with traditional powder phosphors. This research highlights the potential of InP QDs to replace cadmium-based QDs in high-CRI applications. &bull; Monochromatic InP QD-LEDs : Over 50% color conversion efficiency and wall-plug efficiencies above 30% for pure green, amber, and red QDLEDs using InP QDs with nearly perfect photoluminescence quantum yield, were achieved. Furthermore, a comparison of the efficiency of such monochromatic QD-LEDs when excited with violet light (~405 nm) and blue light (~450 nm) was performed.<br />&bull; Spectrum-by-design light sources : This research demonstrates the potential of InP QDs to customize LED spectra for specialized applications like circadian lighting and display lighting. In conclusion, this thesis demonstrates the feasibility of using InP-based QDs for efficient, high-CRI white LEDs and spectrum-by-design solutions. The developed models and demonstrators offer valuable insights for future research and commercial applications in lighting and display technologies.

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LIRIAS4181091
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The external quantum efficiency of light-emitting diodes (LEDs) emitting in the spectral range from 510 to 600 nm is much smaller than blue- or red-emitting LEDs. The current strategy to overcome this so-called green gap is the color conversion of the emitted light by an efficient violet or blue III-nitride-based LED. Colloidal quantum dots (QDs) are promising candidates thanks to their narrow, size-tunable emission spectrum. Single-color QD-on-chip LEDs consisting of a blue pump LED with either green-, amber- or red-emitting QDs have already been reported. In this paper, the question is investigated whether a shorter excitation wavelength, i.e. violet light, offers an interesting alternative to a blue pump LED. After describing the main determining factors for the overall LED efficiency, the paper focuses on the color conversion efficiency, and whether excitation with violet instead of blue decreases re-absorption losses significantly. The performance of green-emitting InP/Zn(Se,S)/ZnS QD-LEDs with either a violet (408 nm) or blue (445 nm) LED is experimentally compared; resulting in a slightly higher color conversion efficiency for the violet-pumped QD-LEDs. The impact of the photoluminescence quantum yield of the QDs, the pump absorbance, and resulting color saturation, on the color conversion efficiency, was further investigated with ray-tracing simulations of green- and amber-emitting QD-LEDs with a violet or blue pump LED. Our findings emphasize the role of the excitation wavelength in optimizing the performance of QD-on-chip LEDs and serve as an important guideline for deciding which pump LED to use.
Publisher: Optica Publishing Group
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LIRIAS4206626
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Publisher: Society for Imaging Science and Technology
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