Abstract
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.
Highlights
Tunable light engines are gaining increasing attention after several research efforts have shown how they can be used to create dynamic spaces and mimic daylight patterns with respect to human circadian rhythms and physiology.[1,2] Solid-state lighting (SSL) is a mature technology that is fully compatible with current digital systems and information technology
The tunable light source developed in this work has 48 commercial monochromatic lightemitting diode (LED) arranged in 10 individual channels, essentially spread all over the visible part of the visible spectrum (400 to 700 nm, see Fig. 1)
The control system is executed in a microcontroller in the light engine
Summary
Tunable light engines are gaining increasing attention after several research efforts have shown how they can be used to create dynamic spaces and mimic daylight patterns with respect to human circadian rhythms and physiology.[1,2] Solid-state lighting (SSL) is a mature technology that is fully compatible with current digital systems and information technology. The role that light plays in the regulation of our approximately 24-h circadian rhythm is well accepted and understood.[4,5] It affects our body temperature,[6] attention,[7] hormonal secretion,[8] and sleep.[9] The discovery of a fourth type of retinal photoreceptor, the intrinsically photosensitive retinal ganglion cells (ipRGCs), in the 1990s was the missing link proving that light plays an image forming role and has an important nonvisual influence on our sleep-wake cycle.[10] ipRGCs are sensitive to light in a particular wavelength range, peaking at around 480 nm (melanopic region).[10] This explains why illuminance levels or colorimetric properties such as the correlated color temperature (CCT) or color rendering index[11] of light are important, but the whole spectral information of light, i.e., the spectral power distribution (SPD), needs to be considered
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