The invention relates to a multivariate calibration which can be used when the optical system used for that method does not comprise a multi-channel detector such as a CCD sensor or a line array of photodiodes. An optical system without a multi-channel detector doesn't allow to carry out preprocessing steps. Thus there is the need to carry out these preprocessing steps in another way. It is suggested to partially replace the preprocessing step by a measurement of the optical signal, whereby the measurement comprises transmitting or reflecting the optical signal by an optical element, thereby weighing the optical signal by a spectral weighing function. The advantage of the invention is to teach how such an optical system without a bulky and expensive CCD sensor can be used to carry out a multivariate calibration and preprocessing steps.