Measuring device for determining the thickness (d) of a layer of a material, comprising: - electrical control means adapted to deliver a sinusoidal control signal having a modulation frequency f m and a control phase φ com, - means for emitting a light beam electrically controlled by said electrical control means so that said light beam has an optical power modulated sinusoidally at said modulation frequency f m of the control signal, said light beam propagating along a first propagation path up to said material layer, - photothermal detection means adapted to detect a radiated heat flow by said material layer heated by said light beam and delivering a measurement signal which is representative of said radiated heat flow by said material layer, said radiated heat flow propagating along a second propagation path up to the photothermal detection means, - digital filtering means adapted to extract from said measurement signal a sinusoidal component modulated at said modulation frequency f m of the control signal to determine a measurement phase φ mes of this sinusoidal component of said measurement signal, and - calculating means programmed to calculate, for said modulation frequency f m of the control signal, a phase shift Δφ th/opt between a sinusoidal component of said radiated heat flow and said modulated optical power of the light beam, and to determine the thickness (d) of said material layer of as a function of the phase shift Δφ th/opt between said sinusoidal component of said radiated heat flow and said modulated optical power of the light beam, characterised in that said measuring device also comprises a deflecting element for the light beam arranged to deflect said light beam so that the first propagation path of the light beam is superposed with the second propagation path of the radiated heat flow, between said deflecting element and said material layer, and in that said measuring device comprises an offset measuring head provided with two parts (254, 255), one of which is rotatably movable with respect to the other, the movable part housing at least one reflective element and the other part of the measuring head housing the deflecting element, said reflective element being arranged to reflect the light beam so that a first part of the first propagation path of said light beam, which extends from the deflecting element to the reflective element, and a second part of the first propagation path of said light beam, which extends from the reflective element to the material layer, form a non-zero angle. - electrical control means adapted to deliver a sinusoidal control signal having a modulation frequency f m and a control phase φ com, - means for emitting a light beam electrically controlled by said electrical control means so that said light beam has an optical power modulated sinusoidally at said modulation frequency f m of the control signal, said light beam propagating along a first propagation path up to said material layer, - photothermal detection means adapted to detect a radiated heat flow by said material layer heated by said light beam and delivering a measurement signal which is representative of said radiated heat flow by said material layer, said radiated heat flow propagating along a second propagation path up to the photothermal detection means, - digital filtering means adapted to extract from said measurement signal a sinusoidal component modulated at said modulation frequency f m of the control signal to determine a measurement phase φ mes of this sinusoidal component of said measurement signal, and - calculating means programmed to calculate, for said modulation frequency f m of the control signal, a phase shift Δφ th/opt between a sinusoidal component of said radiated heat flow and said modulated optical power of the light beam, and to determine the thickness (d) of said material layer of as a function of the phase shift Δφ th/opt between said sinusoidal component of said radiated heat flow and said modulated optical power of the light beam, characterised in that said measuring device also comprises a deflecting element for the light beam arranged to deflect said light beam so that the first propagation path of the light beam is superposed with the second propagation path of the radiated heat flow, between said deflecting element and said material layer, and in that said measuring device comprises an offset measuring head provided with two parts (254, 255), one of which is rotatably movable with respect to the other, the movable part housing at least one reflective element and the other part of the measuring head housing the deflecting element, said reflective element being arranged to reflect the light beam so that a first part of the first propagation path of said light beam, which extends from the deflecting element to the reflective element, and a second part of the first propagation path of said light beam, which extends from the reflective element to the material layer, form a non-zero angle.