The invention relates to a device for measuring an optical penetration that is triggered in a tissue underneath the tissue surface by means of therapeutic laser radiation which a laser-surgical device concentrates in a treatment focus located in said tissue. The inventive device is provided with a detection beam path comprising a lens system which couples radiation emanating from the tissue underneath the tissue surface into the detection beam path. A detector device generating a detection signal which indicates the spatial dimension and/or position of the optical penetration in the tissue is arranged downstream of the detection beam path.