Trees in Daylight Simulation – Measuring and Modelling Realistic Light Transmittance through Trees

ABSTRACT Trees influence daylight availability inside and outside buildings by attenuating, scattering, and transmitting light. They resemble a complex fenestration around buildings that change in form, materiality, and permeability based on species of trees, seasonal variations, and environmental or human interventions. The current practice of modeling trees in daylight simulations ignores this complexity and models tree crowns as cones, spheres, or cylinders with assumed reflectance or transmittance value. In this paper, the authors propose an open-source, low-cost method using photography and image processing to measure the on-site transmittance of a tree crown described as gap percentage. Gap percentage is used to generate a 3D primitive crown model that mimics the distribution of leaves and gaps. When used in daylight simulation platforms such as Radiance, the proposed model predicts vertical light transmittance and creates shadow patterns similar to the measured tree crown. The 3D crown model also predicts luminance and illuminance-based daylight metrics similar to a detailed morphological tree model. An open-source program is created and described to generate trees from measured gap percentage data based on this research.