1. A method of calibrating a multi-channel measurement system comprising: directing energy through a source channel into a target medium at a source location on the target medium; measuring the energy emerging from the target medium through a detector channel at a detector location on the target medium; processing the measured energy using an iterative proportional fitting technique to determine a relative value of energy loss in the target medium; and determining a proportional energy loss associated with at least one of the source channel and the detector channel based on the measured energy and the relative value of energy loss in the target medium.