per second in the example chosen the electrochromic element is in its bleached state between the connections of the electrochromic element an open circuit voltage u ec is measurable with the electrochromic element in zero current state proceeding from this open circuit voltage a voltage u is applied to the electrochromic element so that a current i which leads to colouring of the element flows the voltage u is increased steadily and u2014 apart from an initial stage with increasing slope of the current voltage curve u2014 preferably essentially in a linear relation to time the current voltage characteristic is in any case regulated in each case such that no current or voltage peaks occur during this stage of increase of the voltage u the values of the voltage applied to the electrochromic element u t i u t i 1 are determined at various moments in time t i t i 1 at the same moments in time the current flowing through the electrochromic element i t i i t i 1 is measured in each case from the pairs of variates a resistance value r ges t i t i 1 is determined as soon as there is a sufficient number of resistance values to permit averaging but at the latest on reaching the final value u max the arithmetic mean is formed from the individual values r ges t i t i 1 and thus the total resistance r ges is calculated the current i flowing through the electrochromic element is measured continuously from the beginning of the colouring process the maximum value i max measured in this time is stored as soon as the voltage u reaches the final value u max the starting stage stage i in fig4 is.