Background/Aim: The angular distribution of solar radiance and its spectral characteristics is required for the determination of Vitamin D3 production in humans. Materials and Methods: The Vitamin D3 weighted exposure can be calculated by integrating the incident solar spectral radiance over all relevant parts of the human body. A novel instrument allowing simultaneous measurements of spectral radiance from more than 100 directions has been developed. A large solar simulator for controlled experiments is described. Results: In summer it is relatively easy to obtain sufficient Vitamin D because sun exposure times are short. In winter solstice Vitamin D3 cannot be obtained with realistic clothing even if the exposure were extended to all daylight hours. Conclusion: Improved and controlled experiments to determine Vitamin D3 production are required to assess the positive effects of solar UV radiation and to assess its natural variability.