1. A method of simultaneously evaluating a plurality of parameters in a mathematical model of a sample system which describes the effect said sample system has on electromagnetic radiation which is caused to interact therewith, said method involving regression of said mathematical model onto data obtained by application of electromagnetic radiation to said sample system, at least one of said parameters in said mathematical model being subject to accurate evaluation using data obtained by applying one range of wavelengths selected from the group consisting of: RADIO; MICRO; FIR; IR; NIR-VIS-NUV; UV; DUV; VUV; EUV; XRAY; to said sample system, but not by using data obtained by applying another range of wavelengths in said group to said sample system; said method comprising the steps of: a) providing a sample system and a mathematical model thereof which characterizes the effect said sample system exerts on electromagnetic radiation caused to be incident thereupon; b) providing at least one source of electromagnetic radiation, said at least one source of electromagnetic radiation having the capability of providing electromagnetic radiation in at least two ranges selected from the group consisting of: RADIO; MICRO; FIR; IR; NIR-VIS-NUV; UV; DUV; VUV; EUV; XRAY; c) simultaneously or sequentially applying incident electromagnetic radiation from said at least two selected ranges of electromagnetic radiation to said sample system such that after interaction with said sample system via reflection and/or transmission, electromagnetic radiation from each of said at least two selected ranges of electromagnetic radiation is caused. to enter at least one detector sensitive thereto, which at least one detector provides as output data which characterizes the influence exerted by the sample system on the incident electromagnetic radiation; d) simultaneously regressing the mathematical model onto the data provided by said at least one detector to the end that the parameters in the mathematical model are evaluated, including said at least one parameter which is subject to accurate evaluation using data in one range of wavelengths selected from the group consisting of: RADIO.; MICRO; FIR; IR; NIR-VIS-NUV; UV; DUV; VUV; EUV; XRAY; but not by using data in another range of wavelengths in said group.