Non-image forming potential in urban settings – An approach considering orientation-dependent spectral properties of daylight

Abstract

This paper introduces a novel approach to integrate orientation-dependent spectral properties of daylight in urban planning. These spectral characteristics of light at façades are represented in spectral daylight potential diagrams (SDPDs). The applicability of SDPDs is subsequently discussed in the context of non-image forming (NIF) effects, comparing the NIF effectiveness of skylight at façades—represented in the melanopic daylight (D65) efficacy ratio —based on SDPDs to the generally assumed CIE standard illuminant D65 and the horizontal correlated colour temperature (CCT) widely used in existing software. This analysis comprises the impact of (i) prevailing sky conditions and (ii) obstruction levels on the spectral characteristics of skylight on different façade orientations.Our core findings suggest that not considering the directionality of the colour information of skylight from clear sky conditions might lead to underestimation (D65) or overestimation (horizontal CCT) of the NIF effectiveness of skylight offered at façades. When designing for NIF responses, the first potentially leading to increased energy consumption, the latter to a lower NIF response as designed for. These findings not only advance our scientific knowledge of the spectral properties of daylight but also provide a simplified methodological framework for assessing the NIF effectiveness of daylight in urban settings.