Abstract: An optical measuring system to indicate or determine the relative position of an object and/or a substrate in relation to a horizontal plane comprises an at least partially transparent measuring chamber containing two non-miscible media with different densities, one of the media being at least partially transparent or translucent and at least the other being a fluid medium. The relative position is found on the basis of the position of the at least partially transparent or translucent medium in the measuring chamber, when what is concerned is an optical instrument operating on the principle of a spirit level. On one side of the measuring chamber is a light source and on the opposite side there is an optical measuring cell or a flat, light-sensitive sensor.