A Variable Aperture feature is integrated into all Sol3A simulators that provides user variable partial sun illumination capability. The range of attenuation is from 0.25 - 1 Sun. Uniformity at partial illumination maintains a minimum Class B uniformity per the IEC, JIS, and ASTM standards. The simple to use dial control provides infinte control of attenuation. Spectral performance and temporal stability maintain Class A performance at any attenuation..