Laboratory measurements were performed to characterize geometrical effects in the calibration of the NASAĆ­s Cloud Absorption Radiometer (CAR). The measurements involved three integrating sphere sources (ISS) operated at different light levels and experimental setups to determine radiance variability. The radiance gradients across the three ISS apertures were 0.2-2.6% for different visible, near-infrared, and shortwave infrared illumination levels, but <15% in the UV. Change in radiance with distance was determined to be 2-20%, being highest in the UV. Radiance variability due to the edge effects was found to be significant; as much as 70% due to the sphere aperture and <10% due to the CAR telescopeĆ­s secondary mirror.