Abstract
Retro-reflective coatings applied to blinds of reduced geometric complexity promise to provide view to the outside while effectively controlling solar gains and glare. To characterize the reflection characteristics of such coatings over the entire solar spectrum, a novel extension to a scanning gonio-photometer is developed. The extended instrument is tested and applied to measure a coating's Bidirectional Reflection Distribution Function including the region of the retro-reflected peak. The measured datasets are compiled into a data-driven reflection model for the daylight simulation software Radiance. This model is applied to illustrate the coating's effect in a comparison to purely diffuse and specular surface finishes on geometrically identical, flat blinds. Daylight supply, the probability of glare, and solar gains are assessed for an exemplary, South-oriented office under sunny sky conditions. The results indicate the potential of the coating to effectively shade direct sunlight even if applied on blinds with minimalistic geometries. The modeling technique is shown to be a general means to replicate the irregular optical properties of the coating, which cannot be represented by the standard models in daylight simulation software.
Highlights
Venetian blinds comprising profiles of often high geometric complexity address the dilemma to minimize the obstruction of Abbreviations: BS, beam splitter; BSDF, Bidirectional Scattering distribution function; climatebased daylight modeling (CBDM), climate-based daylight modeling; CFS, complex fenestration system; DGI, Daylight Glare Index; DGP, Daylight Glare Probability; DSF, differential scattering function; IGDB, International Glazing Database; NIr, near infrared light 780–2500 nm; sDA, spatial Daylight Autonomy; SHGC, solar heat gain coefficient; Vis, visible light 380–780 nm; XML, extensible markup language
The subsequent data-reduction pass with a reduction target of 95% reduces the data-set to a compact model of ≈104,858 elements: bsdf2ttree -t3 -g 7 -t 95 sample.sir >sample.xml The resulting XML file can be loaded by Radiance to define a material that can be assigned to any geometrical entity: void BSDF retroreflectiveMat
The two metrics differ in that the formulation for DGP considers the vertical illuminance Ev reaching the eye of an observer as potential cause of discomfort glare, as well as to account for adaption effects which is accounted to background luminance Lb in the formulation of DGI
Summary
The effective shading of direct sunlight is a key requirement for fenestration systems aiming for high thermal and visual performance. Venetian blinds comprising profiles of often high geometric complexity address the dilemma to minimize the obstruction of Abbreviations: BS, beam splitter; BSDF, Bidirectional Scattering distribution function; CBDM, climate-based daylight modeling; CFS, complex fenestration system; DGI, Daylight Glare Index; DGP, Daylight Glare Probability; DSF, differential scattering function; IGDB, International Glazing Database; NIr, near infrared light 780–2500 nm; sDA, spatial Daylight Autonomy; SHGC, solar heat gain coefficient; Vis, visible light 380–780 nm; XML, extensible markup language. As an innovative approach to decouple shading performance from profile geometry, the application of a retro-reflective coating to the slats of Venetian blinds, and its effect on the daylight supply to an attached office, shall be tested
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