Method and system for calculating physical dimensions for freely movable objects in water, by illuminating the object or projecting a known or selected light pattern (13) on the object by means of at least one light source (10) at selected wavelength or wavelengths, recording illuminated object or objects with the projected light pattern (13) by means of recording means (11, 11a-b) in the form of at least one 2D camera provided with filters (12, 12a-b) arranged to only accept light having selected wavelengths or provided with image producing sensors arranged to only collect light with selected wavelengths, including generation of a 3D model based on recorded images and/or video from the recording means (11, 11a-b) as basis for calculating the physical dimensions.
