Cathodoluminescence (CL) measurements were performed on a set of thick InGaN layers covering systematically a wide range of indium concentrations (x = 0.03-0.20). These thick InGaN layers are exceptionally specular for low indium concentrations (x < 0.1), while some degree of microscopic roughness is observed for x > 0.1. While in CL mappings the size of the areas with constant emission wavelength decreases with indium content, a similar change of domain size is observed by AFM. For low indium content, statistical fluctuations of the local indium concentration lead to a Gaussian broadening of a single emission line. In contrast, for x > 0.1 phase separation results in a multimodal distribution of the peak wavelength, leading to additional low-energy peaks in CL overview spectra. In highly spatially resolved CL measurements we correlate these low-energy emissions to characteristic structural defects.