In this paper,it was derived that the modulated reflective spectrum of weak electric field is proportional to the third derivative of the dielectric functions with respect to the energy.The dielectric function spectra for GaInP and doped Si or Zn GaInP samples grown by MOCVD were obtained in the region of visible light by using the ellipsometric spectroscopy,and then the third derivative spectra were evaluated.Extending the three point scaling used in the analysis of the electric reflective spectrum,the third derivative spectra of the dielectric functions can be analyzed.The experimental results of reflective spectra of weak electric field modulation were obtained and compared with that of the dielectric function spectra,the sensitivity and resolution ratio increase remarkably.