Method for optically checking by interferometry the thickness of an object being machined by a machine tool, said thickness extending from an external surface to an internal surface of discontinuity of the object ; the method including the steps of: - emitting low coherence incident radiations (I) by means of a radiation source ; - focusing by means of at least one optical probe the incident radiations (I) on the object being machined, said at least one optical probe being part of a detection system; - receiving and detecting, by means of said detection system, primary reflected radiations (R1) generated by a reflection on the external surface of the object , and concurrent reflected radiations; - analysing by means of a spectrometer a spectrum of the result of interference between said primary reflected radiations (R1) and concurrent reflected radiations; and - processing on the basis of the analysis of the spectrum of the result of interference between the primary reflected radiations (R1) and the concurrent reflected radiations, information about the thickness of the object being machined; the method comprising a phase of direct checking wherein said concurrent reflected radiations are secondary reflected radiations (R2) generated by a reflection on the internal surface of discontinuity of the object ; the method being characterized by further comprising a preliminary phase wherein said concurrent reflected radiations are reference reflected radiations (Rref), generated by a reflection of said incident radiations (I) on a reference surface , the reference surface being adapted to define the length of a reference optical path, and to delimit and define, together with said external surface of the object , a virtual thickness (D) related to the thickness of the object , the step of processing information about the thickness of the object being machined includes checking variations of the virtual thickness (D), the method comprising the further step of checking, on the basis of the analysis of the spectrum of the result of interference between the primary reflected radiations (R1) and the concurrent reflected radiations, when the secondary reflected radiations (R2) are received and detected by said at least one optical probe , the phase of direct checking being started when the secondary reflected radiations (R2) are received and detected by said at least one optical probe . - emitting low coherence incident radiations (I) by means of a radiation source ; - focusing by means of at least one optical probe the incident radiations (I) on the object being machined, said at least one optical probe being part of a detection system; - receiving and detecting, by means of said detection system, primary reflected radiations (R1) generated by a reflection on the external surface of the object , and concurrent reflected radiations; - analysing by means of a spectrometer a spectrum of the result of interference between said primary reflected radiations (R1) and concurrent reflected radiations; and - processing on the basis of the analysis of the spectrum of the result of interference between the primary reflected radiations (R1) and the concurrent reflected radiations, information about the thickness of the object being machined; <EOS>