The quest for solutions to utilise daylight as a natural, energy-efficient resource for lighting buildings that provide a pleasant and comfortable environment to their occupants (Konis & Selkowitz, 2017; Leslie, 2003; Shishegar & Boubekri, 2017) has driven the development and refinement of fenestration techniques over the past decades. Better insulation, mandatory in face of energy targets, increased coverage of transparent façade areas, as well as the increasing frequency of extreme weather events led to an increased sensitivity of buildings to solar gains, and therefore the need for solar shading (Bellia et al., 2013; Brembilla et al., 2020; Laouadi et al., 2020; Lomanowski & Wright, 2012).
An abundant variety of fenestration techniques exist (Konis & Selkowitz, 2017; Kuhn, 2017; Tsangrassoulis, 2016) that, with different prioritisation, aim to not only protect buildings from overheating and thermal discomfort, but to balance the positive and negative effects of daylight in buildings for improved visual comfort, formalised just recently in a European standard of the same name (CEN, 2018). The latter distinguishes the needs for supply and protection, and thereby reveals the ambivalent effect of solar irradiation on buildings. In particular requirements for the supply and even distribution of daylight and a view connection to the outside often collide with the needed protection from sunlight and glare, when solar shading devices such as Venetian blinds and roller shades are employed.
The shortcoming of either supply or protection by conventional fenestration motivated the emergence of fenestration systems that are capable to not only block or attenuate solar irradiation, but modulate its spatial distribution. In its most general meaning, the term optically Complex Fenestration System (CFS) refers to “any window product that incorporates a non-clear (non-specular) layer in the glazing assembly or in its attachments” (Laouadi & Parekh, 2007) and diffusing devices. It is, however, the directionally selective admission of incident irradiation, and its controlled deflection and distribution in the attached space, that allows for the deliberate engineering of solar transmission through the building skin (Grobe, 2019a). Directional selectivity and deflection are caused by the formation of geometries, often periodical, into macro-structures that are visible from a typical viewing distance, or micro-structures that are perceived as a homogenous surface property due to their small dimensions (Klammt et al., 2012).
The effects of conventional fenestration depend on geometrical variables, such as the size, position and orientation of apertures, and are commonly addressed by design guides such as the daylight factor and the window-to-wall ratio. Contrarywise the irregular optical properties of CFS introduce effects that are often counter-intuitive. Yet, these effects should be qualitatively understood by architects and engineers, and need to be quantitatively considered in building design and the selection of technologies since they effect energy efficiency and comfort, and thereby the load on building systems. Simulation software can inform and guide such decisions, but requires reliable models and therefore the accurate characterisation of CFSs.