The optical system discusses is an integrating sphere based uniform source. The system consists of an integrating sphere illuminated with a tungsten halogen lamp system. The output of the integrating sphere is projected using a large single- element collimating lens. Such a system is advantageous in the testing of optical detectors since in a specific region of the projected beam the irradiance is constant in a volume around the optical axis and for a significant distance along the optical axis. This property eases the positioning requirements of devices under test relative to the lens. Relative irradiance predictions of a plane in the collimated beam made using ZEMAX Optical Design Program are presented. Measured data of the relative irradiance in this specific plane achieved with the implemented optical system are presented. Comparison between the predictions and measured data are made. Predictions agreed with the measured data to within 5%.