A method for determining a degree of damage to hair, comprising: while exposing a hair sample of the hair to near-infrared and/or infrared light, recording a spectrum of at least one part of the near-infrared and/or infrared light that has interacted with the hair sample; comparing at least one part of the spectrum with a spectroscopic calibration model obtained by means of near-infrared and/or infrared spectra and degrees of damage to a plurality of calibration hair samples; and determining the degree of damage to the hair, taking into account the comparison; creating the calibration model, comprising: for the plurality of calibration hair samples: while exposing the calibration hair sample to near-infrared and/or infrared light, recording a calibration spectrum of at least one part of the near-infrared and/or infrared light that has interacted with the calibration hair sample; determining a degree of damage to the calibration hair sample using an independent analytical method; and assigning the degree of damage to the calibration spectrum; and determining a correlation between the plurality of calibration spectra and the plurality of degrees of damage, wherein the analytical method comprises determining a cysteic acid content. while exposing a hair sample of the hair to near-infrared and/or infrared light, recording a spectrum of at least one part of the near-infrared and/or infrared light that has interacted with the hair sample; comparing at least one part of the spectrum with a spectroscopic calibration model obtained by means of near-infrared and/or infrared spectra and degrees of damage to a plurality of calibration hair samples; and determining the degree of damage to the hair, taking into account the comparison; creating the calibration model, comprising: for the plurality of calibration hair samples: while exposing the calibration hair sample to near-infrared and/or infrared light, recording a calibration spectrum of at least one part of the near-infrared and/or infrared light that has interacted with the calibration hair sample; determining a degree of damage to the calibration hair sample using an independent analytical method; and assigning the degree of damage to the calibration spectrum; and determining a correlation between the plurality of calibration spectra and the plurality of degrees of damage, wherein the analytical method comprises determining a cysteic acid content. for the plurality of calibration hair samples: while exposing the calibration hair sample to near-infrared and/or infrared light, recording a calibration spectrum of at least one part of the near-infrared and/or infrared light that has interacted with the calibration hair sample; determining a degree of damage to the calibration hair sample using an independent analytical method; and assigning the degree of damage to the calibration spectrum; and while exposing the calibration hair sample to near-infrared and/or infrared light, recording a calibration spectrum of at least one part of the near-infrared and/or infrared light that has interacted with the calibration hair sample; determining a degree of damage to the calibration hair sample using an independent analytical method; and assigning the degree of damage to the calibration spectrum; and determining a correlation between the plurality of calibration spectra and the plurality of degrees of damage, wherein the analytical method comprises determining a cysteic acid content. <EOS>