Daylight poses a serious challenge to physically-based rendering. It is brilliant, ever-changing and ever-present. At first, the daylight simulation capabilities in Radiance were modest, limited mostly to exteriors and interiors with clear windows or openings. Designers, especially architects, wanted more. They wanted to be able to simulate light through venetian blinds, intricate building facades and skylights. In 1991, the author was hired on sabbatical by EPFL to improve the daylight simulation capabilities of Radiance, and developed some of the techniques described earlier in this paper. In particular, the large source adaptive subdivision, virtual source and secondary source calculations proved very important for daylighting problems.
The simplest application of daylight is exterior modeling. Many CAD systems have built-in renderers that will compute the solar position from time of day, year, and location, and generate accurate shadows. In addition to this functionality, we wanted Radiance to show the contributions of diffuse skylight and interreflection. Figure 16 shows the exterior of the Mellencamp Pavillion, an Indiana University project that recently acquired funding (along with its name).
A more difficult daylighting problem is atrium design. Designing an atrium requires thorough understanding of the daylight availability in a particular region to succeed. Figure 17 shows an atrium space modeled entirely within Radiance, without the aid of a CAD program . The hierarchical construction of Radiance scene files and the many programmable object generators makes text-editor modeling possible, but most users prefer a "mousier" approach.
Daylighted interiors pose one of the nastiest challenges in rendering. Because sunlight is so intense, it is usually diffused or baffled by louvers or other redirecting systems. Some of these systems can be quite elaborate, emphasizing the need for simulation in their design. Figure 18 shows the interior of the pavillion from Figure 16. Figure 19 shows a library room illuminated by a central skylight. Figure 20a shows a simulation of a daylighted museum interior. Daylight is often preferred in museums as it provides the most natural color balance for viewing paintings, but control is also very important. Figure 20b shows a false color image of the illuminance values on room surfaces; it is critical to keep these values below a certain threshold to minimize damage to the artwork.