## 1 Introduction  
Solid-state lighting considerably increased the possibilities and allowed for a re-interpretation of modern lighting design. The development and use of light-emitting diodes (LEDs), in contrast to conventional light sources, has yielded significant advantages in terms of lifetime, energy savings, environmental benefits, controllability, and spectral tunability [1, 2]. Over the past few years, scientific and technological progress in the design of LEDs, including significant improvements in the selection of suitable semiconductor material combinations, the topology of the lattice grid and quantum well structures, the chip packaging, and the coating materials [3, 4, 5, 6, 7, 8], has led to high luminous efficacies of \(150\,\mathrm{Im}\,\mathrm{W}^{-1}\) and beyond, depending on the desired level of color quality [9, 10, 11]. Typically, an increase in color rendering capabilities, e.g., expressed in terms of the CIE color rendering index (CRI) \(R_{\mathrm{a}}\)[12, 13, 14] or the ANSI/IES TM-30-20 \(R_{\mathrm{f}}\) metric [15], occurs at the expense of luminous efficacy so that an adequate trade-off between color quality considerations and energy savings has to be found [16]. Thus, numerous studies have been conducted pursuingthe goal of optimizing the spectral compositions of multi-channel LED light sources for achieving an optimal color rendering while still maintaining a sufficiently high level of luminous efficacy [17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30]. At the same time, such multi-channel LED solutions allow for a dynamic adjustment of the lighting conditions to satisfy the users' visual, emotional, and biological needs in support of positive human outcomes [31; 32; 33; 34; 35; 36]. Despite these recent technological and conceptual advances in LED and LED-based lighting design, properly dealing with mixed lighting conditions in environments where daylight entry through windows supplements the artificial lighting conditions still poses a defying challenge. Optimizing the latter in some sort of closed-feedback loop under the impact of a second, dynamically changing (daylight) light source defines a difficult regulation problem that so far has only been considered rudimentarily in the literature. A crucial aspect in this context is the proper monitoring of the indoor and outdoor lighting conditions with regard to the lighting parameters most relevant for an appropriate system control to always satisfy certain pre-defined visual and non-visual requirements. In experimental test settings, these kinds of data can be generated straightforwardly by using expensive spectroradiometers or other sophisticated measurement devices. However, in view of large-scale applications, where such \"laboratory\" devices are not an option due to budgetary constraints, the use of simple and inexpensive RGB color sensors to be applied for this purpose appears to be quite promising [37; 38; 39; 40; 41], yet requires a suitable processing of the sensor readouts in order to obtain meaningful measures that can eventually be used as the input for an adequate lighting control [42]. In this context, the present work focuses on the circadian aspects of lighting by proposing a novel sensor-based methodology to assess the circadian effectiveness of the prevalent lighting conditions caused by artificial and daylight light sources using RGB color sensors. As discussed by [PERSON] et al. [43], circadian effectiveness in lighting can be measured by the physiologically relevant circadian stimulus (CS) metric introduced by [PERSON] and [PERSON] [44]. In order to facilitate its application for the lighting practitioner, [PERSON] et al. [45; 46] recently published a family of computational approximation methods that can be used to calculate a lighting condition's CS based on a few easy-to-perform measurements using standard equipment. As will be shown in this work, the sensor readouts of an RGB color sensor, after some suitable processing, can likewise be used as the input for [PERSON] et al.