## 2 Materials and Methods  
### Circadian Stimulus and Its Approximation  
The hormone melatonin has proven to be an excellent and widely used biomarker for assessing how light affects the human circadian system [48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82]. In two independent studies, [PERSON] et al. [58] and [PERSON] et al. [59], for example, consistently found that the action spectrum for light-induced nocturnal melatonin suppression peaks at \(460\,\mathrm{nm}\) with an approximately \(110\,\mathrm{nm}\) broad absorption band at half-maximum sensitivity. Even though the results of both studies were in good agreement over much of the probed wavelength regime, an obvious discontinuity was found in the datasets at about \(505\,\mathrm{nm}\), i.e., at a wavelength that would appear as a unique green stimulus causing the spectrally opponent retinal blue versus yellow (b-y) color mechanism to signal zero. As argued by [PERSON] et al. [44; 83; 84], no single retinal photoreceptor model is capable of adequately describing these observations on the spectral dependencies of melatonin suppression. Instead, all known photoreceptors, including rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGGs), must be taken into account by explicitly considering their neuroanatomical, neurophysiological, and operational characteristics in order to properly model the experimental findings of human circadian phototransduction [85; 86]. Under these prerequisites, [PERSON] et al. [44], by pooling the available nocturnal melatonin suppression data, were able to develop the most complete model of the spectral sensitivity of the human circadian system that, in contrast to single-photopigment-based approaches (e.g., the calculation of \(\alpha\)-opic quantities [87; 88; 89], equivalent melanopic lux [90; 91], etc. ), is capable of adequately describing spectral opponency effects [62; 63; 73]. Based on their model formalism, [PERSON] et al. introduced a new measure called circadian light CL\({}_{\text{A}}\), which basically gives the model-weighted irradiance. In order to quantify the circadian effectiveness of arbitrary lighting conditions, they further defined the CS metric, which represents the functional relationship between the CL\({}_{\text{A}}\) measure and its theoretically provoked melatonin suppression in percent. The CS metric has already been applied successfully in various laboratory and field studies to predict non-visual effects in lighting [92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103; 104], confirming its physiological relevance. In order to facilitate the CS calculation for the lighting practitioner, [PERSON] et al. [45; 46] recently published a family of computational approximation methods. While the first method models CS as a function of illuminance \(E_{\
u}\) and the chromaticity coordinate \(z\) of the \(2^{\circ}\) standard observer, both measured vertically at eye level, the second method substitutes the latter by using the correlated color temperature (CCT) instead. Although performing CCT measurements is common practice in standard lighting design, which was the main reason for proposing the second approximation method, [PERSON] et al. [46] stated that the CS estimation based on this quantity becomes significantly inaccurate for lighting conditions with CCTs ranging from 3220 K to 3710 K and, therefore, is not recommended to be applied in this range. In contrast, an excellent agreement with [PERSON] et al. 's original CS model could be confirmed for the complete range of relevant CCTs when applying the first approximation method. The maximum prediction error \(|\Delta\text{CS}|_{\text{max}}\) calculated on a comprehensive database of measured SPDs representing both artificial and daylight light sources was found to be less than 0.058 [45].