The complex effects of obstructions on horizontal illuminance in interior lighting design

The lighting community is aware of the problem of obstructions in interior lighting design and limited guidance on the treatment of obstructions in artificial lighting design is available. The majority of previous research has concentrated on vertical obstructions above the working plane. Work to isolate the effects of obstructions within the floor cavity from obstructions above the working plane has received little attention. The primary aim of this research was to investigate the potential effect that floor cavity obstructions alone would have on average illuminance throughout the floor cavity, but most importantly on working plane illuminance. The effect of floor cavity obstructions was investigated using various methods: full scale surveys of real interiors, full scale surveys in a controllable lighting facility, a lighting design software package and scale model photometry. In addition, the measured results from the full scale and model studies were compared to predicted illuminances. The results from the real and controlled surveys indicate that the presence of obstructions within the floor cavity alone can cause variations in average working plane illuminance of -9.2 to 12.6%. Comparing the predicted illuminances from solid and open-sided obstructions confirmed that open-sided obstructions within a floor cavity lead to a departure from the expected behaviour in terms of the relationship between the variation in average illuminance at the working plane with effective floor cavity reflectance. The conclusions from this research indicate that the realistic representation of the inter-reflection effects of an open sided desk cavity is intrinsic to determining an accurate effective reflectance for an obstructed floor cavity. Attempting to quantify these effects has shown that inter-reflections within floor cavities that contain obstructions of an open-sided nature, is a complex system that is not amenable to generalisations.