Predicting the performances of light pipes using formulated modelling techniques

Light pipes are simple means of illuminating interiors with daylight, usually via multiple specular reflection through the pipe (for the case of this study, a polished aluminium surface with reflectivity if 95%). To tailor a range of scenarios, light pipes can come in, theoretically, any length and diameter, and also with elbow pieces. For lighting designers, architects and the end purchaser, it is important to be able to predict the light being emitted from any pipe(s) for a given room. By doing this, we are able to decide the optimum configuration of pipes that would light a given space, and also the additional lighting (through electric lighting or windows) that might be required to obtain a design illuminance. A semi-empirical computer model (using an MS-Excel framework and VBA) is produced that can predict the luminous flux from any pipe(s) and subsequently predict the illuminance distribution from the pipe(s). The illuminance distribution is represented by a “luxplot” – a simple 2-D colour representation of a defined horizontal plane below the light pipe. The aim of the model is to be able to produce these predictions in both an efficient and user-friendly way. Although the model is proposed for light pipes with a reflectivity of 95%, and with clear, stippled diffusers, modifications are proposed that enable the same model to be used with pipes of different material (for example using total internal reflection film, as developed by 3M). The final model is validated against original luminous flux and illuminance data to demonstrate its accuracy.