This paper discusses the first full version of IDEA-l, a new natural-lighting design tool for the early stages of the architectural design process. This computer program, which is part of the larger IDEA+ research project, allows the user to judge the impact of natural light on his architectural design. For this purpose a number of tests are provided, covering different aspects of natural lighting. This paper will discuss each of the tests and the involved algorithms. The paper will end with a reflection on the IDEA+ project, which aims at providing a new integrated design environment for architecture.