# Measurement of Circadian Effectiveness in Lighting for Office Applications
showed that the level of nocturnal melatonin suppression caused by a homogeneous full-field illumination of the same 100 photonic lux at the cornea was not significantly different from what would be expected when averaging over the four hemi-fields. Finally, [PERSON] et al. [29; 30] based on theoretical considerations showed that, by assuming a nonuniform distribution of the retinal photoreceptors contributing to non-visual effects, lighting conditions of equal vertical illuminance and equal melanopic irradiance at the cornea may still cause very different integrated receptor signals that eventually result in distinct non-visual outcomes. Even though the literature provides clear evidence that information on the spatial light distribution is crucial for a proper prediction and comprehensive modeling of non-visual light-induced effects, it is still unknown how light contributions from different directions and with different incident angles should be weighted to come up with a reliable overall estimate of the circadian effectiveness in practical lighting applications. Despite this lack of knowledge, it seems expedient, though, to think about how to perform spatially resolved light measurements that may account for this kind of information. As a first step, the purpose of the present work therefore is to provide a novel methodological framework for in-field light measurements that allows for an estimate of the circadian effectiveness of a lighting situation for variable viewing angles and different field of view definitions. At a later stage, i.e., once the ongoing research on how to specifically weight light contributions from different directions has made significant progress, the proposed framework can easily be extended and adapted accordingly to comply with the latest findings. Here, \(Mc_{\lambda}\) is the spectral sensitivity of melanopsin [71] corrected for crystalline lens transmittance [72; 73], \(E_{\lambda}\) is the corneal spectral irradiance of the light source measured vertically at eye level, \(S_{\lambda}\) is the S-cone fundamental as defined by [PERSON] [74], \(mp_{\lambda}\) is the macular pigment transmittance [75], \(V_{\lambda}\) is the photopic luminous efficiency function [76] determined by the sum of L- and M-cone responses, and \(V^{\prime}_{\lambda}\) is the isotopic luminous function [76]. Note that all known photoreceptors contribute to Equation (1). For \(B_{\lambda}\leq 0\), i.e., when the output of the b-y mechanism signals 'yellow' because of the spectral nature of the perceived light source or generates no response in case of a unique green stimulus of 507 nm, the spectral sensitivity of the circadian system is determined by the melanopsin-containing ipRGCs only. If, on the other hand, the b-y mechanism signals 'blue', i.e., when \(B_{\lambda}>0\), the respective output from the cone network, which, from Equation (2), is given by the difference between macular-corrected S-cone responses and the likewise macular-corrected \(V(\lambda)\) function (\(\pm\)the sum of L- and M-cone responses), adds to the output of the ipRGCs. This overall circadian response is further modified by a rod-dominated shunting inhibition of the dendrites of the depolarizing ipRGCs that is mediated through all amacrine cells and expressed by Equation (3). To obtain a reliable estimate for the circadian effectiveness of an arbitrary light source, [PERSON] and [PERSON] [68] defined the circadian stimulus (CS) metric, which gives the functional relationship between the light source's model-weighted irradiance of Equation (1) and its theoretically provoked melatonin suppression in percent with regard to a 1 h light exposure. The corresponding equation reads  
\[\text{CS}=0.7-\frac{0.7}{1+\left(\frac{\text{CLA}_{\lambda}}{355.7}\right)^{1.1026}}. \tag{4}\]  
As further shown by [PERSON] et al.