Optical model for tubular hollow light guides (1415-RP)

Tubular hollow light guides are found in many lighting and daylighting systems to transport collected light into deep spaces of building interiors. Linear straight guides are popularly used due to their high optical efficiency, but non-linear guides with bent sections are sometimes required to fulfill some installation restrictions. The optical performance of such bent guides is, however, unknown. This article presents the development, validation, and application of an optical model to compute the transmittance of light guides with and without bends. The model is based on the ray-tracing technique and can handle segmented guides with connection elbows. Measurement of the light transmittance of a light guide with two connection elbows is conducted using an outdoor large integrating box to benchmark the model. The model predictions are in good agreement with the measurement and public data for vertical light guides without bends. The model predictions for bent light guides installed in a northern mid-latitude location show that orienting the middle pipe section of the guide toward the northern direction results in better control of sunlight and solar heat gains than other orientations.