The OGSE GESTA (Geometrical and Spectral Test Assembly) operates in vacuum chamber to perform the optical characterization of the IRS instrument. GESTA integrates both sources, gas cell and laser sources, in order to perform a spectral characterization and calibration.The MTF measurements are performed by measuring the signal coming from a hot pattern plate, illuminated by a stable and homogeneous optical beam. To measure the background signal, the pattern plate is removed and a cryogenic blackbody is used instead of. This paper describes this illumination sub-system used in GESTA, developed by Bertin Technologies, and using the infrared laser sources. The principle of the used solution is to apply a dynamic variation to the interference status responsible for the speckle at the output of the integrating sphere, in order to eliminate the impact of the spatial coherence of the output beam and to get a uniform radiance through temporal averaging over the integration time of the optical detector.For that, the laser beams hit a rotating diffusing disc located inside the sphere. For GESTA, the requirement is challenging as speckle contrasts need to be as low as 0.1% for a 400μs integration time. The solution developed shows good enhancements, in terms of radiance level, stability, uniformity, and is easily translatable to other spectral ranges. Final achieved performances and the related influencing parameters are reported.