Method of measuring and calculating a current-voltage curve and a current mismatch between junctions of a tandem solar cell, comprising at least two solar subcells, the method being performed under simulated solar irradiance according to the illumination norm AM1.5, as defined in the international standard IEC 60904-3, and comprising the steps (A-L) of: A) providing at least one first light source for providing a light beam that has a spectrum corresponding to the international standard AM 1.5; as defined in the international standard IEC 60904-3, B) providing at least two narrow bandwidth light sources (S2 1, S2 2), for providing at least two different light beams (12, 14), each light beam (12, 14) being predominately absorbed by one of the at least two sub-cells; C) illuminating the tandem solar cell with said first light source, for acquiring its short-circuit current (Isc); D) Illuminating the tandem solar cell with said first light source and at the same time adding additional light step intensities by one of said at least a second and a third light sources (S21, S22), for acquiring offset values of the change of short-circuit current ( δ Isc) relative to the acquired reference short-circuit current; E) realizing a first interpolation curve using current data acquired at low additional light intensity steps during the illumination step D; F) realizing a second interpolation curve using high additional light intensity steps during said illumination step D; G) defining the intersection of said first and second interpolation curves (I, II), said intersection defining a balance ratio Bal 12 of the first and second junction short circuit current equal to 1; H) calculating the short-circuit current Iscmatch of the tandem solar cell at said intersection I) determining the mismatch junctions (Bal12, ref) of current between the from the parameter determined in Step H; J) repeating Step D to I without illumination by said first light source and by using only said second and said third light sources (S2 1, S2 2); K) Calculating the necessary light intensities of said second and said third light sources (S2 1, S2 2), to achieve an identical balance ratio Bal12 = Bal12 and identical short circuit current Isc = Isc by using adapted gains for the light sources (S2 1,S2 2), so that the combination of said light sources (S2 1,S2 2)provide the same test results of the solar cell 100 as when illuminated with said first light source S1 only; L) Using a light source that is the optical combination of said light sources (S2 1,S2 2)according to step K, to illuminate a solar cell to be tested and providing the current-voltage curve I-V of the tandem solar cell. A) providing at least one first light source for providing a light beam that has a spectrum corresponding to the international standard AM 1.5; as defined in the international standard IEC 60904-3, B) providing at least two narrow bandwidth light sources (S2 1, S2 2), for providing at least two different light beams (12, 14), each light beam (12, 14) being predominately absorbed by one of the at least two sub-cells; C) illuminating the tandem solar cell with said first light source, for acquiring its short-circuit current (Isc); D) Illuminating the tandem solar cell with said first light source and at the same time adding additional light step intensities by one of said at least a second and a third light sources (S21, S22), for acquiring offset values of the change of short-circuit current ( δ Isc) relative to the acquired reference short-circuit current; E) realizing a first interpolation curve using current data acquired at low additional light intensity steps during the illumination step D; F) realizing a second interpolation curve using high additional light intensity steps during said illumination step D; G) defining the intersection of said first and second interpolation curves (I, II), said intersection defining a balance ratio Bal 12 of the first and second junction short circuit current equal to 1; H) calculating the short-circuit current Iscmatch of the tandem solar cell at said intersection I) determining the mismatch junctions (Bal12, ref) of current between the from the parameter determined in Step H; J) repeating Step D to I without illumination by said first light source and by using only said second and said third light sources (S2 1, S2 2); K) Calculating the necessary light intensities of said second and said third light sources (S2 1, S2 2), to achieve an identical balance ratio Bal12 = Bal12 and identical short circuit current Isc = Isc by using adapted gains for the light sources (S2 1,S2 2), so that the combination of said light sources (S2 1,S2 2)provide the same test results of the solar cell 100 as when illuminated with said first light source S1 only; L) Using a light source that is the optical combination of said light sources (S2 1,S2 2)according to step K, to illuminate a solar cell to be tested and providing the current-voltage curve I-V of the tandem solar cell.