We explore methods that efficiently replicate arbitrary spectra with both high precision and accuracy using multichannel light-emitting diode (LED) lighting systems. It is well known that LED-based light sources deteriorate over time and change their spectral output with varying operating junction temperatures. A simple open-loop approach to the spectral matching problem would bring about unbearable spectral and color inaccuracies. In the literature, different solutions have been studied that make use of integrated spectrometers as closed-loop feedback elements that warrant spectral awareness and self-correction. However, the prohibitive cost of small spectrometers (that generally involve CMOS-based gratings) constitutes a high barrier that prevents their integration into final lighting products. We demonstrate how a cost-effective colorimeter can be used not only to preserve the color point of the target spectrum but also to keep the spectral matching error extremely low (relative spectral error <10 % ). With the proposed system and methods, we obtain relative color differences between target and emitted spectra below Δu ′ v ′ < 0.002, always with spectral shape preservation.
KEYWORDS: Light emitting diodes, Light sources and illumination, Detection and tracking algorithms, Algorithms, Spectrometers, Control systems, Monte Carlo methods, Light sources, Solid state lighting, Optical engineering
Spectrally tunable light sources for general lighting have recently attracted much attention as versatile solutions that can be used in humancentric lighting implementations provided with excellent color rendering and increased user perception. However, temperature and age-dependent color shifts and flux variations in the light-emitting diode (LED) emission are nonresolved challenges that need to be overcome in order to be used in final applications. We demonstrate two strategies that can be used to efficiently and precisely generate arbitrary spectral power distributions (SPDs) using multichannel LED engines. First, we introduce different methods to match a given SPD and select an algorithm (simulated annealing) in virtue of its speed (in the milliseconds range) and accuracy (color shifts Δu ′ v ′ < 5 × 10 − 4). Then, we propose a closed-loop feedback control (PID) to compensate for spectral shifts due to temperature changes or lumen decay of the LEDs. Both methods can be used independently, but only a combination of them (which uses the output of the first method as an initial guess for the second) offers fast computational times and high spectral accuracy and precision. Computation times are important because these algorithms are intended to be executed on dedicated microprocessors integrated in the LED modules, often sharing scarce memory and processing resources. The results presented here are aimed to be universal and hold for different implementations of the light engine and any number of LED channels.
In this work, we show a spectrally tunable LED lighting system that uses the SPDs of the different channels of the light engine to optimize for different parameters, i.e. flux, illuminance, efficacy, CRI-Ri, TM30-15-Ri, Damage Potential, melanopic flux, PPF/YPF, CLA/CS, etc. Our results show how this tool and their associated methods can be used to design complex lighting applications in a meaningful and objective manner.
In this work, we explore methods to create arbitrary spectra efficiently with high precision and accuracy using multichannel LED light engines. We propose an optical feedback controller using integrated optical sensors, namely spectrophotometers and colour sensors, for real-time monitoring of the emitted light and for effective spectral corrections. Our results show that the such kind of close-loop systems can be used to obtain relative spectral errors and ▵uv' between target and emitted spectra significantly below threshold values, ▵uv' < 0.002 in all cases.
We present a multimode longitudinal pumping scheme for integrated rare-earth doped waveguide amplifiers which
allows an efficient use of low cost multimode pump sources. The scheme is based on evanescent pump light coupling
from a multimode low loss waveguide, which is gradually transferred to a single mode Si-nc sensitized Er3+ doped active
core. Population inversion is ensured along the whole amplifier length, thus overcoming the main limitation of
conventional single mode pump butt-coupling in case of strongly absorbing active materials. Great flexibility in
controlling the pump power intensity values within the active core is also provided.
We propose this pumping scheme at 477 nm for Si-nanocluster sensitized Erbium doped waveguide amplifiers, in
which top pumping by LED arrays is limited by the low pump intensity values achievable within the active region.
The coupling between the multimode waveguide and the active core has been numerically studied for slab waveguide
structures using a 2D split-step finite element method.
Numerical simulation results, based on propagation and population-rate equations for the coupled Er3+/Si-nanoclusters
system, show that high pump intensities are indeed achieved in the active core, ensuring good uniformity of the
population inversion along the waveguide amplifier.
Although longitudinal multimode pumping by high power LEDs in the visible can potentially lead to low-cost integrated
amplifiers, further material optimization is required. In particular, we show that when dealing with high pump intensities,
confined carrier absorption seriously affects the amplifier performance, and an optimization of both Si-nc and Er3+
concentrations is necessary.
A Conductive Atomic Force Microscope (C-AFM) has been used to investigate the nanometer scale electrical properties of Metal-Oxide-Semiconductor (MOS) memory devices with Silicon nanocrystals (Si-nc) embedded in the gate oxide. This study has been possible thanks to the high lateral resolution of the technique, which allows to characterize areas of only few hundreds of nm2 and, therefore, the area that contains a reduced number of Si-nc. The results have demonstrated the capability of the Si-nc to enhance the gate oxide electrical conduction due to trap assisted tunneling. On the other hand, Si-nc can act as trapping centers. The amount of charge stored in Si-nc has been estimated through the change induced in the barrier height measured from the I-V characteristics. The results show that only ~20% of the Si-nc are charged. These nanometer scale results are consistent with those obtained during the macroscopic characterization of the same structures. Therefore, C-AFM has been shown to be a very suitable tool to perform a detailed investigation of the performance of memory devices based on MOS structures with Si-nc at such reduced scale.