Distance measuring method and device as well as laser lithotripsy device

A measuring device to optically determine a distance (z) of a surface of a body stone located in a medium from an end of an optical waveguide, having:

• a measuring radiation source to generate electromagnetic measuring radiation of a first wavelength (λ1) and of a second wavelength (λ2), wherein the medium more strongly absorbs the electromagnetic measuring radiation of the second wavelength (λ2) than the electromagnetic measuring radiation of the first wavelength (λ1),
• the optical waveguide to emit the electromagnetic measuring radiation from the end of the waveguide towards the surface;
• a detection device to measure a first reflection signal (I1) of the electromagnetic measuring radiation of the first wavelength (λ1) reflected from the surface, and for measuring a second reflection signal (Iλ) of the electromagnetic measuring radiation of the second wavelength (λ2) reflected from the surface, and
• an evaluation unit to determine the distance (z) from a ratio (I2:I1) of the second and the first reflection signal, wherein the determination of the distance (z) comprises determining whether a predetermined maximum distance (zlimit) is undercut and/or a predetermined minimum distance is exceeded.