Transfer by orthogonal projection: making near-infrared calibrations robust to between-instrument variation

The ability to transfer calibrations between near-infrared instruments is important for the acceptance and spreading of the technology. A method called transfer by orthogonal projection (TOP) is proposed for deriving calibrations that are robust to between-instrument variation, and thus may be transferred without adjustment. The idea is to orthogonalise the calibration spectra to directions in the spectral space in which most between-instrument variability lies. The method is related to the idea of a repeatability file, but has advantages over that approach when factor-based methods are used for calibration. Two examples are presented. In both of these, a calibration is successfully transferred to instruments not involved at all in its derivation.