A method for deriving topography of an object surface, the method comprising: directing linearly polarized light wave (102a, 103a) towards the object surface along a z-direction and a reference surface, wherein the light is from light sources and has passed through linear polarizing filters (23, 24), wherein the linear polarized light wave directed towards the reference surface is a reference directed light wave and the linear polarized light wave directed towards the object surface is a surface directed light wave; obtaining images of a reflection (102b, 103b) of the linearly polarized light wave (102a, 103a) directed towards the object surface and the reference surface for a plurality of wavelengths, wherein the images are obtained for at least four polarizations for each of the plurality of wavelengths, wherein the reflection (102b, 103b) of the linearly polarized light wave (102a, 103a) comprises a surface reflected light wave and a reference reflected light wave; and deriving the topography of the object surface based on the obtained images, wherein a delay between the surface reflected light wave and the reference reflected light wave is dependent on a physical distance between the reference surface and the object surface; wherein a synthetic instrument that does not exist physically is formed in a computer, wherein a reference z-position, equivalent to the z-position of a synthetic reference surface, is shifted to a z-position at the object surface by subtracting the known delay, such that the remaining delay is zero and a phase at the reference z-position is zero for all wavelengths; wherein the method further comprises: choosing a x-y-position at the object surface; storing a measured phase at each wavelength as measured at the chosen x-y-position; and subtracting each of the measured phases at the chosen x-y-position from phases measured at all other x-y-positions at the same wavelengths, and wherein the delay of all other x-y-positions at the object surface is referenced to the z-position of the chosen x-y-position. directing linearly polarized light wave (102a, 103a) towards the object surface along a z-direction and a reference surface, wherein the light is from light sources and has passed through linear polarizing filters (23, 24), wherein the linear polarized light wave directed towards the reference surface is a reference directed light wave and the linear polarized light wave directed towards the object surface is a surface directed light wave; obtaining images of a reflection (102b, 103b) of the linearly polarized light wave (102a, 103a) directed towards the object surface and the reference surface for a plurality of wavelengths, wherein the images are obtained for at least four polarizations for each of the plurality of wavelengths, wherein the reflection (102b, 103b) of the linearly polarized light wave (102a, 103a) comprises a surface reflected light wave and a reference reflected light wave; and deriving the topography of the object surface based on the obtained images, wherein a delay between the surface reflected light wave and the reference reflected light wave is dependent on a physical distance between the reference surface and the object surface; wherein a synthetic instrument that does not exist physically is formed in a computer, wherein a reference z-position, equivalent to the z-position of a synthetic reference surface, is shifted to a z-position at the object surface by subtracting the known delay, such that the remaining delay is zero and a phase at the reference z-position is zero for all wavelengths; wherein the method further comprises: choosing a x-y-position at the object surface; storing a measured phase at each wavelength as measured at the chosen x-y-position; and subtracting each of the measured phases at the chosen x-y-position from phases measured at all other x-y-positions at the same wavelengths, and choosing a x-y-position at the object surface; storing a measured phase at each wavelength as measured at the chosen x-y-position; and subtracting each of the measured phases at the chosen x-y-position from phases measured at all other x-y-positions at the same wavelengths, and