Linearisation of optical effects at low light levels

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As a light beam is produced by an amplification of modes of the zero point field in its source, this field cannot be distinguished; consequently a nonlinear optical effect is a function of the total field. However, we generally prefer to use a conventional field which excludes the zero point field; for a low conventional field, the total field may be developed to the first order, so that the effect appears linear. This nearly trivial remark allows a correct computation of the signal of a photocell used for photon counting and shows that the "impulsive stimulated Raman scattering" (ISRS), a nonlinear, without threshold effect, which shifts the frequencies, becomes linear at low light levels, so that the shifted spectra are not distorted.