Apparatus and methods are described for determining tomographic and/or topographic data relating to an object. The object is illuminated with at least one wave packet from light generated by a multi-spectral light source. The wave packet is split into two split wave packets, and an optical path difference (OPD) is introduced between the two split wave packets, that is smaller than the coherence length of the wave packet. The two split wave packets are combined to generate a 2D image of the illuminated object, and the data relating to the object is determined by processing the 2D image using long coherence phase shift interferometry algorithms, by analyzing the 2D image as if the 2D image was generated using monochromatic light having a wavelength that is equal to a mean wavelength of the wave packet. Other applications are also described.