The Diffuse Infrared Background Experiment (DIRBE), a part of the Cosmic Background Explorer (COBE), has a requirement for internal calibration of its detectors. This requirement stems from COBE's orbital motion through the Van Allen Belts and South Atlantic Anomaly. Radiation hits require the annealing of the detectors and their calibration. The calibration required stimulation at six levels covering a dynamic range of 105 to a reproducibility of 1% in any three separate calibration runs. The IRS consists of four thermal greybody sources coupled to an integrating sphere. The goal of the IRS is to provide stable, linear, uniform radiant outputs for detector calibration. The design was heavily impacted by the 2K environment of DIRBE. Important considerations included thermal cycling, power dissipation, and cool down times of less than three seconds. The major design problem for the IRS was the thermal sources. They were required to have very low heat capacities in order to have rapid rise and fall times and low power dissipation but the stability more characteristic of sources with large heat capacities. A trade-off also existed with regard to stability and dynamic range. Ideally one would desire to be in the Rayleigh-Jeans limit of the thermal sources, since the linear dependence on temperature aids in stability. However, the high temperatures required for the near infrared bands were difficult to obtain and tended to swamp the detectors with signal for feasible source sizes. The relative insensitivity of the DIRBE bolometers and the linear temperature dependence of the Rayleigh-Jeans law made bolometer stimulation over a dynamic range of five orders of magnitude difficult, These problems were solved in part by having four different sources with emitting areas varying by a factor of 108. A low heat capacity design with the necessary stability appears to be thin nichrome films on a sapphire substrate. Extreme voltage stability (to 0.01%) is required for those near infrared bands that required stimulation on the Wien portion of the blackbody curve.