The invention describes a method to eliminate instrumental offset in measurement of optically active scattering and circular dichroism. The method uses the time-average measurement of the light that is systematically transformed by a series of optical devices. The optical devises perform the function of rotating linearly polarized light, interconverting left and right circular polarized light, converting circular polarized light to rotating linear polarized light and converting linear polarized light to alternating left and right circular polarized light.