Ghost imaging is an active technique that implies using a time-varying structured illumination source to image a target without spatially-resolving measurements of the light beam that interacts with the target. Traditionally, a beam splitter is used to create two highly correlated beams, such that the signal interacts with the target and is then measured by a single pixel detector, while the reference is directly measured by a spatially resolving detector. This approach allows to implement ghost imaging in the space domain, nevertheless also temporal and frequency domains can be addressed1,2, allowing to extract the pertinent information. In particular, ghost imaging in the frequency domain has been recently applied to extract spectral information from a target object by means of Fourier Transform Interferometry3,4. In this work we illustrate and discuss the results of interaction-free measurements on an Er3+ doped …