With their high capacity and low cost, magnetic disks remain competitive for digital data storage. The low flying heights of the read/write head in the single nm-range place stringent requirements on the disk substrate topography. Interferometric surface metrology can provide the required topography data. For the measurement of state-of-the-art disk substrates with Angstrom-level waviness, the interferometer noise needs to be extremely low. A robust and cost-effective approach uses an LED as a light source providing temporally and spatially low-coherence illumination. The low temporal coherence leads to axially localized fringes, and clean single-surface fringes and topography maps are obtained even for transparent substrates. The low spatial coherence suppresses coherent noise, or speckle noise, in the fringe intensities and topography maps. Coherent noise cannot be determined by a simple repeatability measurement, since it is stable over some time for a given disk alignment. It does not appear in difference maps, but nevertheless is present in all acquired maps. An upper limit of low-level coherent noise is determined by looking at speckle decorrelation with increasing tilt of the test surface. In this presentation, the coherence issues are discussed together with the characterization of coherent noise and waviness filtering. Disk measurement examples are shown where the data were acquired with OptoFlat, an LED-based interferometer newly developed for the measurement of flat surfaces like disks and wafers.