Lighting simulation plays a significant role in the architectural design and evaluation of performance parameters of buildings. The general method of assuming a virtual sky in lighting simulation may lead to mismatch with the real situation, which conventionally ignores or approximates the complex- ity of the real sky and surrounding landscape in luminance distribution. In this paper, we propose a compact embedded system for on-board quasi real- time lighting computation based on sky monitoring and luminance mapping. The embedded system comprises a microprocessor and a calibrated low-cost image sensor with a wide- angle lens. The luminance detection range of the de- vice, by employing the high dynamic range (HDR) technique, is able to cover 7 orders of magnitude in luminance monitoring including that of the sun, sky and ambient objects during daytime. The mon- itored luminance can be mapped to 1.23 million sky patches, achieving high resolution of the sky and ground dome. Taking the outdoor luminance map as input, the microprocessor conducts the on-board lighting computation by RADIANCE program based on ray-tracing algorithms. This integrated embed- ded system is demonstrated to accomplish an intra- scene lighting computation in a quasi real-time man- ner based on the monitored sky and ambience.