An analysis of a novel unit destined for measuring roughness of smooth and isotropic surfaces as well optimization of its parameters has been presented. The unit is functioning on the measurement of the total integrated scatter parameter using a flat photodiode integrator rather than a conventional optical sphere or hemisphere. By that means, one can obtain much less expensive and smaller instruments than the traditional ones. Unfortunately, a decrease of the integrator dimensions could reduce its spatial frequency bandwidth causing measurement errors. Therefore, an analysis of this factor utilizing the results of the Rayleigh-Rice vector perturbation theory has been performed. For the case when smooth surfaces are investigated, the lower limit of the angle range is particularly critical and should be diminished as much as possible. This is in contradiction with the demand of a broader upper limit to have a greater roughness measuring range. Therefore, a way of optimization of unit parameters has also been described. Results of experiments concerning the validity of assumptions confirmed in the analysis as well as results of roughness measurements obtained using a tentative measuring unit are also given.