Real world complexity in reflectance value measurement for climate-based daylight modelling

The magnitude and distribution of inter-reflected light is often the most important factor in any assessment of daylight illumination. This is so for the traditional Daylight Factor (DF) method as it is for metrics founded on climate-based daylight modelling (CBDM). A recent study of classroom spaces showed how wall reflectivity is a key determinant of the outcome of a climate-based daylight evaluation. Typically, when a real space is occupied, interior walls get partially covered by fixtures, furniture, posters, etc. Hence they can become very different from the initial design assumptions used for the computer modelling of daylight. The work presented here examines the differences between assumed and real surface reflectance values, assessed on a case study by means of different methods, and the consequences that this may have for predicted measures of daylighting performance. This is of particular importance now that climate-based measures of daylighting performance are a mandatory requirement for the UK’s Priority Schools Building Programme (PSBP). One of the techniques used to determine real-world surface reflectance values is based on High Dynamic Range (HDR) photography; it allows for the measurement of luminance on a per-pixel basis and thus for the creation of interpolated reflectance maps of complex patterns. This new technique is applied for the first time on real spaces here and an initial assessment of its values and limits is presented. The resulting reflectance values obtained from the measurements on the case study are invariably different than those typically assumed when carrying out daylight simulations. The impact this can have on the outcomes determined using daylight simulation is presented and discussed.