Signal distortion due to the non-uniform response of the detector degrades the measurement accuracy of most metrology instruments. In this Letter, we report a newly developed calibration source system for reference-based non-uniformity correction using a laser source, a fiber, and a diffusive module. By applying the Monte Carlo simulation, we show that the transmittance of the system highly depends on the cavity reflection of the diffusive module. We also demonstrate the use of this system to achieve a flat field at a very low non-uniformity (less than 0.2%) with proper illumination intensity, which most costly commercial integrating sphere systems traditionally cannot provide.