The high sensitivity of the Visible Infrared Imaging Radiometer (VIIRS) Day Night Band (DNB) high gain stage (HGS) cannot directly use on-board calibration sources such as solar diffusers, so other calibration approaches are needed. A fully automated, NIST-traceable active point source called the Accurate Active Light Source (AALS) based on a 1-meter diameter integrating sphere illuminated with High Pressure Sodium (HPS) lamps has been developed to calibrate the VIIRS DNB HGS mode. This paper provides the theory and operation of the AALS. The AALS is housed in an environmentally controlled, moveable, enclosed trailer that protects the sphere and is being deployed in South Dakota during the summer and Mississippi during the winter. AALS Lambertian source radiance is measured by a NIST-traceable spectroradiometer. MODTRAN-based top of atmosphere (TOA) radiance is calculated using nighttime aerosol estimates and point source radiative transfer techniques that account for the VIIRS finite Point Spread Function (PSF). This new active point source complements current vicarious calibration techniques, which are based on lunar illumination of extended sources (clouds, playas, etc.), and helps improve our understanding of the point source radiative transfer needed to develop nighttime artificial light source applications. An uncertainty budget for TOA radiance indicates that accuracy under clear, moonless conditions should be comparable to daytime vicarious calibration methods. This paper shows that the major uncertainty is the ground-to-space atmospheric transmission and discusses approaches to reduce this error. Initial assessment of the VIIRS DNB HGS on the Suomi-NPP and NOAA-20 satellites are presented.