It is desirable to accurately predict the lighting of architectural objects and spaces for both architectural and engineering reasons. Using the advanced lighting simulation tool Radiance to create physically-based rendering of simulated spaces is one approach to visually and numerically predicting lighting. Architectural lighting comes from two basic sources: skies and luminaires. Radiance includes sky modeling tools and more are available from various research sources. Radiance luminaire modeling tools, however, are error-prone, requiring the writing of text files, manual calculations, and repeated test renderings. To improve this situation, LumBuild, a cross-platform graphical tool for generating Radiance luminaire models, was developed. LumBuild provides graphical methods for combining luminaire geometry and photometry, generating Radiance impostor surfaces, and binding Radiance material descriptions to geometry. LumBuild maintains a small database of Radiance luminaire models and provides simulation tools for testing the generated luminaire models. The applications of LumBuild are demonstrated through architectural examples: three luminaires are modeled and simulated in test spaces. Renderings and numerical summaries of the results are discussed.