ABSTRACT
Orthogonal projections belong to the few calculation tools on which the most popular chemometric methods rely. They can be found in several calibration and classification methods. However, the term orthogonal projection is more commonly associated with the field of pretreatments (or preprocessings). The reason is that orthogonal projections are very well suited for the correction of additive spectral deformations. This chapter will support this assessment. At first, the specificity of NIR spectra will be presented. They lead to specific pretreatments. Then, the mathematical and geometrical aspects of orthogonal projections will be introduced. The third part will be a description of the different pretreatments based on orthogonal projections, followed by several applications. To conclude, practical properties of orthogonal projections will be enlightened.
