In upper-room ultraviolet germicidal irradiance (UVGI) design, irradiance is an important characteristic, with two opposing dominant dynamics: high-level irradiation on the microorganism and minimum levels of irradiance on human skin and eyes. The use of high-level ray-tracing procedures is followed in establishing radiance and irradiance levels. The main constants in a room influencing these calculations are the spectral and spatial characteristics of the radiation sources in the inter-reflecting surfaces inside the luminaire, as well as the surfaces in the room. The most important characteristic to be determined for the radiation source prior to calculations is its spatial radiant intensity distribution. This characterization is performed using a gonioradiometer. The complexity of the physical construction of the luminaire will determine the extent to which measurements have to be taken. Accurate gonioradiometer readings provide the required radiant intensities in all directions for computer-aided design (CAD), and can also be used to determine the total radiant flux leaving the luminaire, as well as calculating isoirradiance surfaces around the UVGI luminaire. This study will present a laboratory experimental approach to deriving the radiant intensity distribution of a UVGI luminaire. The UVGI luminaire is then characterized in situ, and compared with the gonioradiometric output.