Fast Daylight Coefficient Calculation Using Graphics Hardware

As people increasingly work in indoor environments, the need to provide natural lighting is becoming more widely recognized. Recent modelling standards such as LM-83 require the use of climate-based metrics based on daylight coefficients, rather than illuminance-based metrics that simulate single points in time. While calculations based on daylight coefficients are fast, computation of the daylight coefficients themselves is a slow process that must be repeated whenever the scene’s geometry or materials change. Therefore, it remains impractical to obtain accurate annual daylight simulation results during early design stages when designs are fluid and quickly changing. This paper describes the development of a new, faster tool to compute daylight coefficients using graphics hardware. The tool is an adaptation of rtrace_dc, the executable used by DAYSIM to calculate daylight coefficients, written using OptiXTM, a free library for GPU-based ray tracing. The effectiveness of the new tool is demonstrated using models of a typical office space with speed and results compared to rtrace_dc. The speed of the new tool is measured both on a typical workstation graphics card and on a high-end graphics server. The results show that the new tool achieves similar accuracy to the serial version but does so in one-fifth the time.