Method of determining a plurality of paper sheet quality parameters of a paper sheet material based on fitting to a physical model by a non-contact sensor system, wherein the paper sheet quality parameters comprise at least sheet caliper, basis weight, sheet moisture, wherein the sensor system comprises an emitter system for emitting THz radiation, a detector system for detecting THz radiation, and a processing unit operationally coupled to the emitter system and the detector system, the method comprising: - Emitting, by the emitter system, a THz radiation signal towards the paper sheet material such that the THz radiation interacts with the paper sheet material; - Detecting, by the detector system, a time and/or frequency dependent response of the THz radiation signal having interacted with the paper sheet material; - Determining model parameters of the physical model by optimizing the model parameters such that a predicted response of the physical model is fitted to the detected response, the model parameters being indicative of transmission and/or reflection coefficients at interfaces of the paper sheet material with surrounding media; - Determining, from the determined model parameters, at least one of the paper sheet quality parameters, characterized in that the parameters of the physical model are fitted to the detected response by an iterative procedure comprising the steps: (a) calculating a simulated response based on the physical model using an initial guess for the model parameters (92, 93); (b) calculating an error function expressing a deviation between the predicted response and the detected response; (c) iterating steps (a) and (b), whereby the model parameters (92, 93) are varied until the error function satisfies a best-fit criterion, (d) obtaining the fitted parameters as the final parameters satisfying the best-fit criterion in step (c), and calculating at least one of the paper sheet quality parameters from the fitted parameters (92, 93). - Emitting, by the emitter system, a THz radiation signal towards the paper sheet material such that the THz radiation interacts with the paper sheet material; - Detecting, by the detector system, a time and/or frequency dependent response of the THz radiation signal having interacted with the paper sheet material; - Determining model parameters of the physical model by optimizing the model parameters such that a predicted response of the physical model is fitted to the detected response, the model parameters being indicative of transmission and/or reflection coefficients at interfaces of the paper sheet material with surrounding media; - Determining, from the determined model parameters, at least one of the paper sheet quality parameters, characterized in that the parameters of the physical model are fitted to the detected response by an iterative procedure comprising the steps: (a) calculating a simulated response based on the physical model using an initial guess for the model parameters (92, 93); (b) calculating an error function expressing a deviation between the predicted response and the detected response; (c) iterating steps (a) and (b), whereby the model parameters (92, 93) are varied until the error function satisfies a best-fit criterion, (d) obtaining the fitted parameters as the final parameters satisfying the best-fit criterion in step (c), and calculating at least one of the paper sheet quality parameters from the fitted parameters (92, 93). (a) calculating a simulated response based on the physical model using an initial guess for the model parameters (92, 93); (b) calculating an error function expressing a deviation between the predicted response and the detected response; (c) iterating steps (a) and (b), whereby the model parameters (92, 93) are varied until the error function satisfies a best-fit criterion, (d) obtaining the fitted parameters as the final parameters satisfying the best-fit criterion in step (c), and calculating at least one of the paper sheet quality parameters from the fitted parameters (92, 93).