The invention uses light to determine the distance from the tip of a probe to the internal wall of the canal, and based on the position of the probe, this information is used to generate information about the shape of the canal. Light with a first and a second wavelength range is directed onto a semi-transparent mirror surface at the distal portion of the probe. Light in the first wavelength range is reflected from the mirror surface to illuminate a circumferential area of the internal surface of the canal, and light in the second wavelength range is transmitted through the mirror surface to illuminate an area in front of the distal portion of the probe. The received light is analyzed in the first wavelength range to determine the distance from the probe to the internal surface of the canal at points of the circumference, and analyzed in the second wavelength range to determine the distance from the probe to an object in front of the probe. Thereby it is ensured, that the bottom surface of the canal is not touched or injured while the internal surface of the canal is measured.