This paper introduces the RADIANCE lighting simulation and rendering system, developed by Gregory J. Ward at Lawrence Berkeley Laboratory and the Ecole Polytechnique Fédérale de Lausanne. RADIANCE is a physically-based rendering system designed to meet the demands of lighting design and architecture, using a light-backwards ray-tracing method to efficiently solve the rendering equation under various conditions, including specular, diffuse, and directional-diffuse reflection and transmission. The system blends deterministic and stochastic ray-tracing techniques to balance speed and accuracy in local and global illumination methods. The paper outlines the system's design goals, which include accurate calculation of luminance, modeling both electric light and daylight, supporting a variety of reflectance models, handling complicated geometry, and accepting unmodified input from CAD systems. It also details the techniques used to achieve these goals, such as cached indirect irradiances for diffuse interreflection, adaptive sampling of light sources, automatic preprocessing of "virtual" light sources, user-directed preprocessing of "secondary" sources, hierarchical octrees for spatial subdivision, patterns and textures, parallel processing, and animation. RADIANCE has been widely adopted by researchers and designers, with notable success in applications like daylight modeling and energy-efficient lighting design. The system's flexibility and accuracy make it a valuable tool for professionals in the field of architecture and lighting design.This paper introduces the RADIANCE lighting simulation and rendering system, developed by Gregory J. Ward at Lawrence Berkeley Laboratory and the Ecole Polytechnique Fédérale de Lausanne. RADIANCE is a physically-based rendering system designed to meet the demands of lighting design and architecture, using a light-backwards ray-tracing method to efficiently solve the rendering equation under various conditions, including specular, diffuse, and directional-diffuse reflection and transmission. The system blends deterministic and stochastic ray-tracing techniques to balance speed and accuracy in local and global illumination methods. The paper outlines the system's design goals, which include accurate calculation of luminance, modeling both electric light and daylight, supporting a variety of reflectance models, handling complicated geometry, and accepting unmodified input from CAD systems. It also details the techniques used to achieve these goals, such as cached indirect irradiances for diffuse interreflection, adaptive sampling of light sources, automatic preprocessing of "virtual" light sources, user-directed preprocessing of "secondary" sources, hierarchical octrees for spatial subdivision, patterns and textures, parallel processing, and animation. RADIANCE has been widely adopted by researchers and designers, with notable success in applications like daylight modeling and energy-efficient lighting design. The system's flexibility and accuracy make it a valuable tool for professionals in the field of architecture and lighting design.