Disclosed is a method for measuring etch depth including the following steps: splitting a light beam into a first, and respectively second, incident beam directed towards a first, respectively second, area of a sample exposed to an etching treatment to form a first, and respectively second, reflected beam, recombining the first reflected beam and the second reflected beam to form an interferometric beam; detecting a first, and respectively second, interferometric intensity signal relative to a first, respectively second, polarisation component; calculating a lower envelope function and an upper envelope function of a differential polarimetric interferometry signal; determining an offset function and a normalisation function from the first lower envelope function and the first upper envelope function; and calculating a differential polarimetric interferometry function normalised locally at each time instant.