Abstract: This paper presents a scalable, adaptive algorithm for direct illumination in scenes with many area light sources. Instead of presampling light sources into massive point lights like previous multilight algorithms, the algorithm builds a tree structure for all area lights using agglomerative hierarchical clustering algorithm and evaluates direct illumination by a collection of tree nodes. Each node represents a light cluster and its output radiance is computed using two estimation methods. For each pixel point, a node set is found adaptively through tree traversal guided by error estimation and overall radiance variation. As the algorithm works on area lights directly, the error incurred by converting area lights to point lights is avoided. The experimental results show that our algorithm can generate images with higher fidelity and is more efficient compared with existing point light based multi-light algorithms.