We demonstrate a new calibration technique that can be applied to multi-spectral interferometric observations. The technique measures a fixed-pattern in squared visibility measurements across the spectral channels of each baseline. Because the fixed-pattern appears to be stable on time scales longer than one night, nightly or weekly averages can be calculated based on observations of calibrator stars. The averaged fixed-pattern values can then be removed from data of target stars. We demonstrate the performance of the calibration technique on actual observations obtained with the Navy Prototype Optical Interferometer and show that the fixed-pattern effects can be suppressed by up to an order of magnitude.